Valve gear renovation
I have been restoring/renovating some olde worlde electronics recently, so I thought I should write a blog post about it (now Phil has explained how our new website works!)
This stems from the process of slowly clearing out my late Father-in-Law's farm sheds… We keep finding old, rusty things; some of which just scream at me to be repaired!
#1 The American Army Radio
Project #1 was a famous WWII radio receiver; the RCA AR88D, as seen in all good warzone bunkers from about 1942 onwards. This particular model is quite early and flaking with rust from the 50Kg steel shell. Yes, 50Kg! I can barely move it. This turns out to be a good thing as it appears to have saved all the valves from high-G shock loads which is generally what kills them.
Suffice to say, once the rusty case was levered open I found a remarkably well preserved chassis, with even some pencil marks to note servicing and repair work. Nice. At this point I have no idea if any of the electronic components are serviceable - I do the stupid thing and wire a plug onto it to try it - and surprise myself by it NOT emitting magic smoke. Infact the lamps come on! But nothing else…
So after surviving doing the one thing you should not do with old valve gear, it was time to get out the voltmeter and start checking values against the circuit diagram… Unsurprisingly many components had been damaged by decades of damp ingress.
Every single old oil filled paper+foil bypass capacitor was blown; leaky, unmeasurable capacity, etc. These 'bathtub' style components house 3 wound cylinders in a soldered metal can and are bolted to the chassis. I decided to replace the innards and keep the cans for a more authentic appearance. Chopping my way in and scooping out the innards was messy but satisfying :)
I also found there were capacitors marked 'Micamold' that were infact just paper and foil devices encased in bakelite. These also had all failed - cue more replacement work.
Finally a lot of the carbon film resistors had gone 'high' and also needed replacing to get the valve terminal voltages within spec. A few had blown entirely; mostly due to capacitor leakage (see above!)
On the upside, all the valve heater filaments seemed to be intact! This is extraordinary! I thank both the designers for specifying metal cased valves and the weight of the chassis in damping any possible shocks.. I was hopeful we had a full set of working valves :)
Also all the wound components seemed to have survived OK along with the oil-filled power supply capacitors that are much larger and tougher than the bathtub types…
Time to make a CPC order for a handful of new ceramic 630V capacitors and get soldering!
After a short wait and some painful soldering (access is NOT the best!) it was time to power it up again! Quite unexpectedly the damn thing actually WORKED! Testing alongside a 1990s Yaesu we find the frequency is ‘good enough’ on the manual dial and the sensitivity just as good as the modern receiver, after some gentle twiddling of the controls. Amazing bit of kit..
Some more pictures during the restoration..
More info from the Radiomuseum here
#2 The Kitchen Radio
This is a pre-war Bakelite cased 'kitchen radio' with a glass backlit tuning scale. Station names from the past are inscribed in the paint across three bands: long, medium and short wave.
On releasing the rusted tight case screws (mind the Bakelite!) we find a seized tuning mechanism and a very rusted chassis. Several components have burst open from the damp ingress, and black dirt encrusts everything. The corroded valves are certainly 1930s era designs with top caps and octal bases. Several wires immediately break and fall off where the solder has corroded away and the copper gone green with verdigris.
The speaker is seized and rusty; no chance of this ever working again. All the components likely need replacing, and we’ve no idea if any valves may have survived, but it seems improbable :(
The value in these is mostly cosmetic - they were made cheaply and in large volumes, parts are hard to come by and there are no popular radio stations on these bands anymore!
So I decide this one is to become a display item with new innards. A visit to CeX returns a £20 Bluetooth speaker in a nice round tubular case. Much cleaning and a swift application of two cable ties later, and the new speaker is strapped just behind the rear grille, allowing one to still operate the buttons through the gaps. Nice.
Now we just attach a trailing USB charger cable, and we’re done!
#3 The Dancette
I have just started on a Pilot TRG108 portable gramaphone and radio unit. The autochanger's mechanical parts have been freed up and seem like they should work - turning the platter by hand results in the movements we expect so that’s positive :)
Again this thing has no real value, but it's a fun toy to play our 78rpm records :) If I can get it going for little to no investment, I will. The unit uses a BSR8 record deck - very common and needles are still available at reasonable cost. The electronics use four valves; two for the radio section, one audio amplifier and one power supply rectifier. We need to be VERY CAREFUL as the chassis is wired directly to the mains plug (!!) thus saving costs by using an autotransformer for voltage changing. It also uses Uxx series valves which have a common 0.1A heater filament at various voltages. These run is series from 125V AC source. Hmm.
Unfortunately of the four valves installed, one was smashed and two have broken heater filaments :( I had a box of valves donated to me by a friend many years ago - mostly TV ones. By pure luck I found an equivalent for the AF valve (UCL82 -> PCL82) and a rectifier valve (EY85->EZ80) I could substitute! The main issue is the heater filaments are different current / voltage ratings.
So far I have managed to replace all the leaky capacitors, a few duff resistors, the rectifier and AF valves. The rectifier is a half wave originally (urgh!) so the full-wave replacement is connected with both diodes in parallel to ensure I retain enough curernt rating. Seems to work ok.. The AF valve is an equivalent so no hacking required :)
I have managed to swap around the autotransformer to obtain outputs close to the filament ratings I needed; by running the EZ80 filament off the 6.3v lamp output and the PCL82 between the 220/250 step down taps in series with one lamp, we have a solution!
I have disconnected the filaments & HT of the radio section. There is nothing to listen to on MW/LW anyway :) I may want to re-instate some radio function later.. for now we just abandon this part of the circuit.
Unbelievably, the speaker cone is NOT seized up! I think I got the audio section working so I can play records on it!! More to come.. woodworm, rusted case hinges, lots of cosmetic fix ups to do!
More info from the Radiomuseum here
#4 The Ones That Got Away
Several other items have been salvaged from the scrap pile; notably two 1950s era top quality Hi-Fi systems and a car radio unit common in Jags at the time! These all went to happy enthusiasts on eBay for a respectable amount of cash :)
A Radiomobile model 4010
This unit is 6V positive earth, a variation of the more widely known model 100 used in upmarket 1950s Jaguar models. It includes a ‘vibrator PSU’ - essentially a mechanical oscillator, transformer and rectifier valve to provide HT. There is considerable rust and decay of the outside, but inside is not so bad! Even the push buttons kinda work.. but not the tuning. The control is seized and the cable snapped immediately on removing the case!
I have no use at all for such a radio given it would only work in ancient cars.. so off it went to a Jaguar restorer for parts.
An RCA New Orthophonic High Fidelity system
This system is a classic of it’s time, considered an equal to Quad and similar high-end manufacturers. We have here boxed pre-amp and tuner head units, and an unboxed power amp. The system is mono; stereo came later! As you can see, the power amp chassis has suffered a lot of surface rusting. Underneath the components are all splitting from damp ingress and the cables are rotten. The head units fared much better; protected by the wooden box there is little decay and clean insides. The unit runs a pair of KT66 output valves in class ‘B’ giving a comfortable 12 watts r.m.s.
I decided I did not really need a mono amp, even one as rare and nice as this could be! Also I had no way to really sort out the surface rust and decay to make it look nice - most of the appeal today is cosmetic. So off it went to a Hi-Fi restorer who could give it some love :)
More info from the Radiomuseum here
A Pye Mozart stereo Hi-Fi system
This Pye HFS-20M system is surprisingly a similar vintage to the mono RCA above, but here we have a smaller footprint and a more modern appearance. The unit is less powerful, and runs a pair of EL34 output valves in class ‘A’ to obtain 2 x 4 watts r.m.s. stereo. (A mono unit was also sold.) The head unit is again quite clean inside having been boxed, whilst the power amp is filthy; thick with dirt and rust. I didn’t spend any time checking if this could be restored as it’s only desirable in good cosmetic condition! It is however, complete and valves appear undamaged.
Off it went to the same buyer as the RCA system above :)