The idea of this part of the website is to give would be restorers ideas on how to overcome common problems.  Some are in the form of restoration stories,  others are simple lists of things to check and "stock faults" for certain models and manufacturers.

Top Ten Restoration Tips
1 Bush 1940's octal chassis  -  fitted in just about all their radios of this time.  If the radio seems deaf for no apparent reason most likely one or both of the IFT slugs have come detached from the adjusters.   Hold the chassis upside down and listen for the knock as the ferrite hits the brass spindle,.  To repair you will need to drill out the rivets on the IFT and re-attach using epoxy resin.  Be careful not to glue everything together.   This is a common problem on DAC90, AC2, AC81, SUG3 and so on.

2  Speakers.  If you have a DAC90A which sounds like an electric sander then most likely the speaker needs repair.  This is easy to achieve using evostick.  Look at the bellows which run around the outer edge of the magnet assembly, on the voice coil side.   You will see that they have detached from the magnet end causing them to rattle.  Re-attach using a cocktail stick as a glue spreader. 

3  Tesla radios such as the Talisman have their panel lamps in series with the valve heaters.  They did not fit a shunt so if one of the bulbs blows, the heater chain opens.  The radio stops working !

4  Use Meltonian shoe polish available in many colours to put that nice finish back into your Rexene covered radio.   Leather sofa balsam is also excellent for giving your Roberts or Hacker radio that "new" look.

5  Radio knobs that will not come off even after soaking them in WD40 need a bit of heat to help.  Use a hairdryer to warm them,  often the little bit of expansion gets them free.  Be careful of thermoplastic trim pieces though !  White knobs (Roberts etc) come up like new if you soak them in a solution of washing powder.

6  When fitting a new speaker cloth most new types available today are heat shrink.  When the glue has dried give it a blast with the hairdryer to  tighten it up.

7  Radios that use UL41 or UBC41 valves (DAC90A, DAC10, Ekco U245 etc) often hum.  Most likely it's a heater cathode short which means replace the valve,  not the smoothing capacitors.  This is
not always the case though!

8  Never use the plug and pray method.  You should always do some rudimentary tests and observations before plugging in a valve radio to the mains .  It could damage the transformers,  valves and worst case burst into flames.  Beware.

9 If you haven't any good service information for a radio then take some high quality close up pictures of the chassis and wiring   -  digital cameras are great for this.  Then if (like me) you forget where the wire should go you will have a reference to look at.

10  Never clean Bakelite with detergent.  It wrecks the top layer finish.  If you must use foam cleaner,  only leave it on the Bakelite for a few seconds.   Use proper Bakelite polish or Brasso.  A thin layer of wax after polishing  gives the perfect finish.

They are great radios - well that's my opinion !  The TR82B differs only in terms of colour  to the more common TR82C,  the B set having brown sides and a darker overall colour,  with gilt trim and the C model having blue sides and chrome trim.  There was also a slightly later model the VTR103,  which had the same case,  this one included VHF FM too The restoration of one of these will be covered in another article.  To finish this off there was also an earlier model,  the MB60 which again has the same case.  This was a valve based set.  Those Bush designers liked to get the most out of an idea ! 

Generally speaking they are very reliable,  usually a good clean up inside to get rid of any leaky battery disease, service the two variable resistors on the top and clean the switch contacts on the wave change.  If anything lets them down,  it is the wave change switch.

To take the chassis out of the case,  you first have to remove the tuning knob / lens.  This sometimes can be awkward,  but on no circumstances attempt to us a screwdriver to lever it off,  you will either damage the lens or damage the case or more than likely both.   In the 'trader' data sheet it says that you should use a sink plunger to remove this  (maybe there was some truth in the 60's thing and LSD !) I can't say I have ever tried this but I do have a method that has given me a 100% success rate so far.   All I do is to thread a piece of single core (plastic coated) wire behind the dial,  loop it back so that you have a loop at one side to get your finger in,  and the two ends at the other side for your other hand.  Make sure you avoid the tuning pointer.  Now pulling  gently ease the wire forward,  rocking side to side with your hands,  until the lens pops off.  You will also need to remove the pointer. Also there are four screws which retain the chassis in the case.  They will need removing too. The complete unit should then come out together with the loudspeaker.

The particular set that I was working on,  didn't work when I purchased it,  but there was some hissing noise coming out of the loudspeaker.  That   usually means the output  audio stage is working,  at least to some degree.  When I had it on the bench I started off with my signal tracer,  first of all switched to RF.  This really does make fault finding easier,  I traced the signal right up to the output of the IF amp,  there was certainly no problems with the RF stages,  many signals were heard through the speaker on the tracer.  The next thing to check was the detector diode X1,  this looked OK on the multimeter, but to be sure, I disconnected it from the volume control,  switched the signal tracer over to AF and connected it to the output from the detector.  Again signals were heard, so after reconnecting the volume control back up,  my attention moved to the driver transistors TR4 & TR5.  With the power off I checked all the capacitors and resistors around these components,  they all looked OK,  so I powered it back up to check the voltages on the transistors.  There was something wrong here,  TR4,  an OC72 type was turned hard on,  in fact it was a very good conductor.  One thing I like about these sets from a service point of view is the lack of a pcb.  Bush have made it so easy to swap  transistors and the like.  After replacing TR4 for a new transistor (I used an OC71), I switched  the set back on to check the voltages on TR5,  there was no need,  life had returned to the radio.  Out of all the transistors in the TR82,  this one seems to be the most common point of failure,  also check that  C33 is OK,  if this fails it could zap the transistor (thanks Al), so it's worth considering if you have similar problems.

I use an old toothbrush and some RS contact cleaner /    lubricant to clean the switches.  This should be done    thoroughly if you want your restored radio to be reliable.   You can get inside the volume and tone control pot's to give them a good clean if they are causing problems. 

To clean the clear tuning lens,  and the other knobs,  use a solution of washing powder and water in an old ice-cream tub.  Do not use any kind of solvent,  you will damage the soft plastic.  One other point to mention,  if the clear lens has gone cloudy or yellow, no mater how much you clean it, it will never be transparent again.  This is because of a chemical reaction between the   plastic and sunlight  (ultraviolet) This is also true to a lesser extent with the cases.

I use foam cleaner to get the worst of the grime off,  leave it on the slats in the front for a while,  then get it under the shower to rinse out the muck,  this saves a lot of time with cotton buds and so on.  To finish off the cases we use Bake-o-Bright  plastics polish.  Some people use T Cut to finish off the cases,  but personally I've not had a great deal of success with this.  Brasso does a good job of cleaning the chrome or gilt trim.  Apply    coloured shoe polish to the  sides or neutral if the colour is already good,  being careful not to get any on your new cleaned case.

Remember to put the wave change knobs back on the switches before you put the chassis back in the case,  they won't fit afterwards - I say this from experience !

Anyway all in all I ended up with another great TR82,  the sound quality is super,  much better than the new reproduction version available on MW / LW  and  for me it's just a  pity I have to sell some  of them ! 

1959 Bush TR82B

For your interest this is the Bush MB60  -  the original version of the TR82, which used battery valves.  Built around 1957.  The Bush designers were clever chaps !

After repairing radios for everyone else for the last 6 months,  I thought it was about time I had a go at one of my sets.  As you have probably guessed from my other restoration articles,  I am into these Tesla radios in a big way !  This one was found at the large flee market in Prague,  that is held weekly on a Friday morning.   If you are there  remember to have a look,  although if you don't speak Czech, expect to pay the "tourist" prices. Luckily for me I have a friend who lives in Prague, who speaks Czech like any Czech person should! 

As I mentioned above this version is the original 305U.  After removing the chassis from the case, being very careful with the screws - I suggest using a magnetic bit for this, the differences are obvious.  First of all the IF cans are rectangular,  the smoothing / filter capacitor is mounted under the chassis and the other important difference is that all the inductors have variable trimmer capacitors instead of the 'twisted bits of wire' that are used in the later sets.  This is why the earlier radios are known as a radio restorers alignment nightmare.  I like a challenge.

With the first glance looking around the chassis,  I was pleased.  It was nice and clean,  with no sign of  molten capacitors. The mains dropper resistor had  been replaced with an original Tesla part. At some point in it's life a couple of the capacitors (grid coupling) types had been replaced,  it looked like the main smoothing can had also been changed.  The rectifier had been bypassed with a couple of diodes, so I guessed this valve was dead  (when I removed the valve,  all the pins apart from the heaters had been cut off). By leaving the heaters connected up,  the voltages for the other valves heaters would be correct.  Very clever.  I removed the rectifier diodes,  and fitted a new UY1N valve in the place of the dead one.  I like to keep my radios as the designers intended them,  silicon diodes definitely were not in use in 1948  -  even behind the 'iron curtain' !    As it all looked in such good order,  I checked the panel bulbs were OK (the valve heaters are in series with the bulbs), and connected the radio up to my ac power supply.  It worked,  well sort of.  The audio was very fuzzy,  it sounded to me like the UBL21 output valve was on its last legs.  A quick check on the tester confirmed this,  it was one definitely for the bin.   Luckily I had a spare one in stock,  so this was fitted.  Much better now,  the audio was quite good,  apart from a loud buzz on the signals.  When you have fixed a few radios,  you get to know what each 'buzz' is caused by,  this one was DC on the input to the audio amplifier.  A common fault,  easily sorted out by replacing the capacitor.   Normally at this point I would replace all the wax paper components,  but after checking with the multimeter around the chassis for more DC where it shouldn't be, I found no other faults,  so I decided to keep this one as original as possible.  If it goes faulty in the future,  I will have more fun repairing it !  After re aligning the IF and tweaking the RF side of things, it was not so bad after all - maybe luck was on my side,  I had it receiving signals all over the world on SW and more local stuff on LW & MW. 

After running the chassis for a couple of hours,  it sounded quite good,  but the audio was a bit thin for my taste.  After checking the audio output section,  I noticed that there was no tone corrector capacitor soldered across the output transformer primary.  On all the other talisman radios,  there is a 0.01 uF 500V in place here,  so I made a tiny modification and fitted one in.  I don't have a copy of the circuit diagram for this version,  so I'm not sure if one was originally intended,  and had been missed out during manufacture.  The end result was much better,  the sound was now more 'rounded'. One other difference between this version of the 305u and the later types is the loud speaker   - this one is much smaller than the later types. 

The domestic side of things was very straightforward,  a couple of coats of Bakelite polish on the case and knobs had it looking like new.  The dial glass was dusted down and then
carefully cleaned on the glass (not the printed) side.  I put the chassis back in the box,  re fitted the back panel and internal aerial wire, and put the radio on test for 24 hours.  Perfect.

The end result,  an excellent just about original example of the Tesla Talisman 305U Mk1  - Just right for listening to Radio Prague on 49M.  Later this year,  when I get some more time to have a mess,  I have another 305U to repair,  I doubt this will be  easy because it arrived  as a pile of bits.  This is the later type,  so it will be interesting to compare the end result with this radio.  The Tesla collection continues…..

1948 Tesla
Talisman 305U

This is the radio that started of the Talisman range,  the most successful,  and well known of the radios produced by Tesla.  This version is very rare,  it is the early prototype model with many electrical differences from the later versions of the 305,  and the other talisman radios.  The case is also slightly smaller,  and jet-black in colour,  later 305's were dark brown. 

The other talismans and the later 305U had pressed cardboard back panels.  This one is a bit special,  it is made from paxolin,  with a pressed metal sheet riveted  over it.  The metal panel is connected internally to the aerial socket,  so that it acts as a simple antenna for local signal reception.  I have not seen another back like this on a Tesla radio,  maybe this was an optional extra ?

1952 Bush DAC90A

This radio was filthy when I purchased it.  The owner said it didn't work and I could well believe it ! It looked inside as though it had smoked 80 cigarettes a day for the last 50 years.  For some reason, a second output transformer had been fitted in the case,  maybe to connect a second speaker to in the past,  but that's just a guess.  The rest of the set looked original, and unmodified.

When I start work on a set the first thing I do is to sort out a suitable container to collect all the bits and pieces in as I strip it down.  An ice-cream tub is good for this.

First of all take the knobs off.  NEVER use a screwdriver to lever them off the spindles,  you WILL damage them and getting replacements is almost impossible these days.  On sets with push on knobs (such as the later DAC90A's) I use a piece of wire which is threaded around the back of the knob in a sort of loop,  and then GENTLY pull the knob off with this.  If its still stuck I usually apply a dab of 3 in 1 oil to the spindle at the back of the knob if there is access.  Anyway the knobs came of this one pretty easy.  To get the wave change knob off you need to get the back panel off,  and locate the lock screw on the shaft of the knob.

When I had the chassis out of the case,  the first thing I did was to check the output transformer with the multimeter,  to make sure it was OK,  and that the second transformer had not been put in as a make shift repair.  Luckily there was no problems with the original unit,  so I removed the second transformer and it's wiring.  Strange ideas some people have !

Have a look at the  pictures,  I removed most of the cobwebs with the vacuum cleaner before I took these photos  you could not even see the valves for the muck !

On inspection you can see that some of the wiring around the dropper resistor is in need of replacement and the rest of the upper chassis will need a thorough clean.  First I removed the valves and the panel bulbs,  to avoid damaging them.  I left the clean up until last,  first of all I like to get the electronics sorted out.  I was pleasantly surprised with the underside of the chassis,  after the vacuum cleaner had removed the dust and cobwebs,  it all looked pretty clean (see pictures). Some of the wax paper capacitors looked a bit sad,  and had started to dribble wax onto the tag strip,  they would probably need replacing.  With the capacitance meter in hand I started checking the values.  The first one to always check is the grid coupling capacitor on the audio output valve.  If one is going to be faulty this is it.  Most people replace this as a matter of cause.  When the set is powered up if you can see any dc on the grid side of the capacitor,  this shows that its leaking and should be replaced.  This will protect the output valve also.  I like to use high voltage polypropylene capacitors in my restorations,  they are suitable for connecting across the mains,  they look somewhat similar to the original capacitors,  and are virtually indestructible.  The only down side of these is that they cost a little more,  but this is worth while if you want to do a quality job.   After checking the other capacitors,  they were all well out of specification,  and as this set was to be used regularly, I decided to replace the lot.  The smoothing capacitor was checked,  this seemed to be OK.  It's a good idea also to check the resistors in the anode lines to the valves,  sometimes these go open circuit. 

The next thing to do was to check and replace any faulty wiring.  On AC/DC sets, such as this one,  you can usually bet that the insulation will have perished on wires near the dropper resistor.  Look out for rubber type insulation,  this dries out and can have hairline cracks in it.  Of cause these should be replaced too.  Next thing I like to do is give all the controls a service,  so if possible strip down the volume control and clean it out,  give the wiper and carbon track a  little squirt of contact cleaner and then reassemble.  This helps to get rid of crackles and pops,  when the volume is adjusted.  I fitted the valves and bulbs back in their respective places,  and reconnected the loudspeaker.  On the DAC90 /90A the mains connection is via a 2 pin plug and socket.  It is possible to fit this the wrong way round so be careful,  always check it with your multimeter if in doubt.  If you do put it on back to front the radio will still work,  however you will be left with a potentially lethal chassis connected directly to the live connection on the mains.  BE CAREFUL.  Now for the moment of truth.  I connected the power lead up to the mains via my variactor.  This allows me to gradually increase the AC voltage,  so as not to over stress the smoothing capacitor.  Success, it worked !  Well to some degree anyway.  It picked up plenty of stations,   on both wavebands,  but there was a lot of hum and distortion.  I also noticed that the panel lights were very dim,  although the valve heaters were glowing correctly.  The first thing I carefully checked was the smoothing capacitor,  to see if any AC was on the DC side of the power supply.  This was negligible,  so the problem was elsewhere.  The RF side of the circuit was working well,  so it must have been in the audio section.  Luckily I had a UL41 output valve spare,  so I switched off the set,  disconnected the mains and swapped this valve.  That's more like it,  the hum and distortion had gone away and it sound more like a Bush radio.  This is usually caused by breakdown of the cathode heater insulation and unfortunately is quite common in output valves.  The reason for the dim panel lights was that the incorrect current rating bulbs had been fitted.  This set uses 2 3.5V at 150ma bulbs,  300ma types had been fitted.  The current value is the important variable to consider in series circuits. I usually leave the chassis running for a while on the bench to make sure nothing untoward happens when they warm up.  Now for the messy bit.

First of all I removed the speaker and tuning dial from the Bakelite case,  the insides were cleaned out using foam cleaner and an old toothbrush to get into the corners.  I then rinsed out the case under the shower and left it to dry.  Back to the chassis.  I removed the valves again and cleaned them as you would a pair of glasses,  taking care not to remove the soft lettering on them.  On the chassis I like to plug up the valve bases and any other holes where cleaning agents could get in with bits of kitchen towel.  Valve bases can be a real pain to replace if they get contaminated.  Time for the foam cleaner again this time applied carefully to the chassis with a cloth.  I then use an old toothbrush (always save your toothbrushes) to get the dirt out of the corners and in-between components.  When most of this is removed,  then it's time for some metal polish to get the last of the grime off. I use Brasso,  it does a good job of cleaning IFT cans and so on.  Be careful not to get any into the tuning capacitor,  these are best left alone unless you have specialist cleaning equipment, such as an ultrasound device.  After a couple of hours work the chassis was starting to look good,  even shiny in some places.  The tuning dial was in good clean condition.  Never try to clean the printed side of the glass,  many of these have very soft markings which easily come away.  Just try to get rid of any loose dust with a dry soft cloth.  I then replaced all the components and tested the radio once  again,  to make sure I had not disturbed anything.  All was well.  On these small Bakelite models I like to fit the chassis back into the case before I do any polishing,  so this is what I did,   taking care to get everything lined up.  Here is a tip for cleaning the knobs,  get another ice-cream tub put some washing powder in it and add a little water,  pop the knobs in there for 10 minutes an then clean them under the tap again with guess what - a toothbrush!   This easily gets rid of all the ingrained dirt from the knurled edges and so on.

Bakelite is best polished with metal polish,  such as Brasso,  or we use Bake-o-Bryte Bakelite polish which is produced by the publishers of 'The Radiophile'  - see links page for details. This gives you that lovely new shine with a bit of effort.  I've found wax polishes not to be very good on Bakelite,  you end up with a 'sticky' radio that gets covered in finger prints. 

The end results are very pleasing,  and it was good that there were not too many awkward electrical problems to sort out.  This is the sort of restoration I like.

The classic Bush DAC90A was manufactured between 1950 and 1958,  having one of the longest production runs of any British valve radio.  You could get it in many disguises.   Here are a few examples that we have restored.

Back in 1946 the original version,  the DAC90 was released.  Although the cabinet styling is similar it's a completely different design internally.

1959 Murphy U102

This one had several problems to overcome,  being a Murphy that is usually the case in my experience !  When I had the traditional heavy blue painted chassis out of the case,  the first obvious problem was that quite a lot of the wiring loom would need replacing.  This was the rubber insulated type that dries out and cracks,  so I set to work stripping this out and replacing it with something safer.  This is a time consuming task but of course it needs to be done.   If you have suitable diameter sleeving to hand you can speed this process up by just unsoldering one end,  and crunching up what remains of the rubber with a pair of pliers.   Slide the new sleeving onto the exposed wire and solder  it back in place.  With half as many joints to make there's less chance of an error !

After checking around the capacitors I noted that several were  open circuit,  including one part of the HT smoothing capacitor.  These would need replacing.  Luckily I had an identical smoothing can capacitor on a scrap chassis that was in good shape,  so this was soldered into place.  I also replaced the grid coupling capacitors and the other faulty wax paper types. 

At this point the next thing to do was to replace the dilapidated mains cable for something less likely to turn into an electric fire.  With AC/DC chassis receivers you must use a 2 core mains lead.  Do not try to connect up a 3 core lead and connect the earth to the chassis - if you do this you will be making a direct connection between neutral and earth,  not a good idea !  One other important consideration here, is the fuse.  Don't use a 13A fuse, for most receivers a 3A type is suitable,  and indeed many of the radios that we supply come with a 2A fuse in the socket.

After a final safety check,  the time arrived for the mains to be connected up.  Nothing much happened.  The valves were glowing,  there was a faint crackle from the loudspeaker.  I checked the HT,  that was  OK.  Where was the audio ?  After a bit of head scratching,  I removed the grid connection from the audio driver valve,  dug out my audio signal generator and fed a 1 kHz sine wave via a capacitor to the grid of the valve.  This proved the audio stage to be healthy,  it didn't do much for my ear's though !  My next thought took me to the volume control,  which is connected to the grid on the driver valve.  It turned out that there was a direct short between the audio signal and ground.  I removed the screened cable which goes to the grid connector,  from the volume control.  This got rid of the short,  so the fault must be in the insulation between the centre conductor and the braiding of the coax. After I had replaced the cable,  the radio came to life. 

As a precaution I replaced all the other screened cables in the radio, if one had perished,  more than likely the rest are well on their way.

It was now working on all bands,  but to me the audio sounded a bit shrill, so I soldered in a 0.01 uF 600V capacitor across the primary winding of the output transformer,  this improved the audio quality considerably.  The radio was now working fine, apart from noisy controls,  which were sorted out by dismantling, cleaning and lubricating as necessary.   

The case and chassis needed little attention,  just a good clean and polish.

Short Wave performance is excellent on this radio,  together with the flywheel tuning,  it's a real pleasure to use.

Also available in brown.

1953 Pilot Little Maestro

This radio was given to me to repair by a customer.  It was the rare light green version,  that he had found at a car boot sale.  It's funny I never see any decent radios at car boot sales.  At a glance it appeared complete,  this was the ac only version, the power cable was rotten and the rest looked in original condition.  I took on the job to do the electrical repairs,  he was going to finish off the cosmetic details.

Anyway I removed the chassis, and gave it a good dusting down to remove the fossilised spiders and so on.  First off I replaced the rotten two core mains lead for a modern three core type.  As this set has an isolating transformer,  you should always replace the two core lead for three type,  and earth the chassis.    The audio grid coupling capacitor had as usual dribbled wax onto the chassis,  so this was replaced.  Most of the other capacitor's were OK,  so these were left alone.  My customer had mentioned that he wanted to keep the set as original inside as possible.

After doing the necessary repairs I decided to plug it in and see what happened.  Well it worked,  but not well.  The audio sounded like it was 50 feet underwater,  and there was a loud hum from the speaker.   Although it tested OK on the capacitance meter when I had checked it earlier,  this distortion is a classic example of smoothing capacitor failure.  At the time I didn't have any replacement can types available,  that would fit this tiny chassis,  and I didn't want to fit modern electrolytic capacitors because I was trying to preserve the originality.  Time for drastic action. 

I removed the can type capacitor,  got out my junior hacksaw,  and cut the base of the capacitor off at the crimp groove on the bottom.  This is a messy job getting all the innards out, I recommend wearing some rubber gloves too.

Finally I ended up with an empty can.  Next I sorted out some suitable high ripple current radial electrolytics (400V types) and soldered these to the original tags on the (internal) side of the base.  Then standing the can on its top,  I filled it up with epoxy resin (you could also use potting compound).  The next bit is the tricky one,  tying not to get the sticky stuff all over the place.  The base together with the new capacitors is fitted back in to it's original position.  When the epoxy has gone off and set,  the base should be held firmly in place.  When the 'new' capacitor is fitted back into the chassis clamp, it should look original.  A bit of time and effort is required,  but if originality is your thing this works really well.

After the cap was replaced back into the chassis,  it was time to try it out again.  That's better,  no more gurgling and humming noises,  however there was still a definite crackle as though something was arcing.  So I shut the curtains,  turned off the lights,  and found my way back to the radio.  I had a close inspection of the valves,  my suspicions were correct.  The rectifier valve was arcing over internally,  just a tiny spark, but anything that sparks is bad !   After replacing the rectifier valve,  the noise was cured.  All that  remained was to sort out the noisy volume control and put the set back together again.

I was really impressed with the audio quality of this receiver.  It's such a small set,  built in an acrylic cabinet,  but the sound coming out of it was excellent. 

This was the last but one Pilot Little Maestro,  which first came out in 1939.  Dozens of wood and Bakelite versions were produced,  some AC only, AC/DC, and line cord dropper sets.

A French radio with history.  This came in for restoration from a customer who's mother owned it during the second world war.  She was a member of the French resistance and kept this little Bakelite radio hidden away from the occupying Germans under her bed.  Apparently it was used to listen to instructions sent from London.  Wonderful !   The amazing thing is that it has remained pretty much intact until recently when unfortunately it was dropped.  As you can see though we put it back together and the only damage is underneath where you can't actually see it.    The French new a thing or two about building radios  -  at some point I hope we can offer certain models for sale.  Just for a bit of variety.

During and before the war France operated mains electricity at 120V,  I'm not sure when they made the switch to 220V but judging by radios I've seen it must have been sometime in the mid to late 1950's.  So the first thing to do was arrange a suitable power supply. 

There is an easy solution if you don't have a 110V adaptor,  and that is to use a mains voltage light bulb in series with the device you are using.   Using simple ohms law the idea is that it works like a potential divider,  two equal "resistances" in series effectively divide the total supply voltage by two at the point where the radio and bulb join together. You need to know the approximate wattage of the radio,  and by knowing this you can select a matching bulb.  With this radio a 100W bulb in series gives 117V at the radio which is near enough.  Using a smaller wattage bulb you can reduce the supply voltage to the  radio  -  so not only do you have a cheap 120V supply you can also use it as a variac !  Its a good idea to bring the voltages up slowing when first testing so start off with a 40W bulb and work your way up.  Note you should always check the supply voltage at the radio

1939 Radialva
Super Groom

As you can see from the simple diagram there is no earth connection.  I didn't include this as it was intended for running AC/DC radios with no mains transformer.  If you want to fit an earth connection  -  which of course is highly recommended for all radios with an isolating transformer,  connect a wire from the mains input earth to the earth pin of the output socket.

Back to the restoration.   Well  apart from the 110V supply there was nothing out of the ordinary to report here.  Electrically it's very similar to British designs of around 1939  -  it's actually based on the USA 6K7,6Q7 range of valves.  The cabinet was repaired underneath using epoxy putty and transparent araldite.   It needed quite a few of the original capacitors replacing and the valve bases were loose.  Pliers and a bit of patience required here.  The field coil speaker had previously been replaced with a permanent magnet type,  the winding being rerouted to a miniature choke.  This is a much better way to replace field coils  -  big wire wound resistors generate lots of heat,  and are likely to increase mains hum in such a small space.   The red lines were repainted with emulsion (tester pot from Wickes!),  the excess being polished off using Brasso.

1939 Pilot Little Maestro

Like most of the repair and restoration work I do these days this one once again was for a customer.   It arrived with a note saying that it had a loud buzz on the audio,  having been purchased from an eBay seller as "working".  The line cord had been replaced and well,  I was feeling brave.  I connected it up to the variac and brought the HT up to about 150V.  No flames apparent so I gradually increased the supply to full potential.   It buzzed.  Louder than any buzz I've heard before.  Think of the racket that you get when you take the top cap off an EBC33 and put your finger on the grid.  But this wasn't audio buzz,  it was RF interference.  Interesting!

After taking the chassis out and re-adjusting my chair (I nearly fell off it laughing)  I located the source of the noise.  Guess what some lateral thinking electronics expert had done.  Well you're not going to believe this.  They had fitted a domestic light dimmer in place of the heater ballast (which would have been the line cord dropper) bolted to the speaker next to the chassis.  Well yes it would drop the excess volts but what about the noise they generate ?   It's bad enough having to listen to dimmer switches from sixty feet away but to put one inside a radio cabinet right next to the high gain stages is crazy. It takes all sorts but at the end of the day this sort of thing pays our bills! 

The Little Maestro is a bit awkward in that it uses a tapped line cord,  with the ht supply being  taken someway down the line.  The reason for this is that the total voltage required for the heaters is only 67V  hence they needed to get rid of 173V.  By putting a tap in the line cord closer to the supply it lets you have a respectable ac voltage to drive the rectifier,  whilst the full drop is available for the heater chain.   I don't like to use dropper resistors in this sort of radio as the amount of heat and hum generated is just not acceptable.  So time for a cool capacitive dropper,  with a twist.

Calculations for capacitive droppers are arduous to say the least.  That was until Paul Stenning wrote an excellent calculator spreadsheet for MS Excel which takes all the maths out of it.   I suggest you get a copy from his website at www.vintage-radio.com.

Capacitive droppers are only any good for driving resistive loads,  for example valve heaters, panel lamps and alike.  On a conventional radio where the HT is taken at the same point as the heater chain  you could just use a straight capacitive dropper to replace the line cord.  However you cannot use a capacitive dropper to drive inductive loads such as the rectifier anodes directly without having an additional resistive component.  In this case that would defeat the object,  so the best thing to do is to use the capacitive dropper to drive the  heaters etc,  and a wire wound resistor connected from the supply to drive the rectifier anodes.  This works well in practice as the rectifier anode current is very low in comparison to the heater current,  therefore the amount of heat dissipated in the resistor is negligible. 

To use Paul's spreadsheet you need to know the total heater voltage required,  the heater current and the supply voltage.  Leave the surge drop voltage at the default setting.  For this radio the heater voltage is 67V,  current is 0.3A  which calculates out at a capacitor of 4.2uF.  You need to use class X2 or better mains suppression components as they will be directly coupled to the mains supply (after the switch) anything less rugged and you  are asking for trouble.   In series with the capacitor use a 3 watt 33 Ohm resistor which limits the turn on surge.   

Pilot Data
Click here to download

The completed capacitive dropper fits in place just behind the rectifier.  Try to leave as much room as possible between the valve and capacitors.

As you can see from the circuit (download the trader sheet from the orange tab for reference) several changes are made to the power supply.  The 120 ohm 7W resistor is specifically over rated in terms of power dissipation so that it runs cool.  This replaces the tap on the line cord dropper.  It's best to use vitreous enamel wire wound resistors for this purpose.  The 4.2 uF capacitor is made up from four 1 uF components and one 0.22 uF all connected in parallel with each other.    The extra 100K resistor is there to discharge the capacitor block when the power is switched off.

Room is tight on the Pilot chassis and the only place the capacitor block can be mounted is on top of the chassis.  I chose to fit it in the same place as the last "repairer" fitted their light dimmer,  at the back of the rectifier.  If you look at the rear view you will see that it's well hidden when the valves are back in place.

The rest of this repair went well,  replacing the usual grid capacitors and in this case the rectifier valve.  I guess the abuse got to it eventually.

It ran fine on the above dropper circuit with only a very slight amount of mains hum breaking through on to the output  -  so little that you couldn't hear it when a weak station was tuned in.    Another one finished,  time to print an invoice…

All back together and working well.  The block is well hidden from view.  No more buzzing! 

1960's Roberts and Hacker Radios.  The curse of the AF117

How to deal with the dreaded transistor.

Well if you are very lucky they will still be working.  Most likely though you'll have noticed that battery consumption is up or operation is intermittent.  Often a sharp tap on the metal can of the AF117 or its relatives will bring the radio back to life.  Why is this ?   Due to whiskers growing inside the transistor slowly  they are literally self destructing, shorting themselves out.  Usually either the collector or base grow their way out to the metal screening can.  That's the fourth lead.    Tapping the can  is one way of locating a faulty transistor without finding out the multimeter.  It's a bit hit and miss but does save time.

Now one way which works sometimes is just to cut the screen lead off.  In most situations apart from VHF use there is no benefit to having the transistor screened.  Assuming that the metal can is not touching any other metal work it does not matter that it's directly connected to the collector etc.  However this is not a reliable fix,  and quite often the short circuit whiskers are between the collector and base or base and emitter so no matter how many legs you snip they are never going to work.  We don't do this on any of our repair work.  However if you like cheap bodges,  good for you!

The next way is to replace all the transistors,  or at least the faulty ones.  The direct equivalents such as the AF125 and AF127 are still available from CPC although they are getting expensive.  My suggestion is to shop around and look for some of the specialist surplus suppliers.   AF125 etc do not suffer from the whisker growing syndrome.   AF200's work well in the sealed Roberts modules  (if you find a supplier for them do let me know !)

Another possibility is to get radical and go the silicon route.  Here is a trick of the trade.  Try using a BF450.  Although silicon it will operate at germanium bias and in most radios (Roberts R200, 300, Bush TR130 etc) its quite happy with the existing resistors.  Specification wise it has the same frequency characteristics as the AF117.  Hacker Heralds though need a bit more work and a few bias resistor tweaks to get them working reliably.  When using BF450's it's very likely that you will need to re-adjust the IF stages of the radio.  Oscillator tuning also has a tendency to alter so its best to go through the entire set.

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