November 30, 2009 at 5:34 am #1538Gerry O’HaraKeymaster
Here is an interesting radio – a Canadian General Electric Model J-125 from the 1932-33 model year (same chassis as used in the Victor Model R22/R22A). This must have been quite a high-end/state-of-the-art set in it day – a 12 tube superhet with an RF stage, amplified AGC, push-pull driver/output stages (claimed to deliver up to 20W), a 10” speaker, circuitry to accommodate a shortwave converter and gramophone, and it weighs-in at a mighty 168lbs. The chassis is in two parts: a huge power-supply/audio output/speaker assembly and a separate main receiver chassis. The J-125 I have been working on is owned by a friend of a friend at the SPARC radio museum in Coquitlam, BC.
On arrival at the museum for restoration, the power supply chassis was in a very sorry state. The power transformer was missing and someone at sometime had fitted a metal rectifier wired directly to the power cord – what a deathtrap (I have no idea what they were doing for the 2.5v heater supply). A cursory glance by others at the state of this chassis indicated that maybe restoration to a working condition would not be economically feasible. Looking more closely at it though, I decided repairs could be effected – the main chassis looked complete and un-molested, the speaker was in good condition and the choke, driver and output transformers were all present. The power-supply was tackled first – I checked the schematic/manual to see what voltages and currents this needed to supply – I figured the radio would need a transformer with at least a 375-0-375v at 75mA and three 2.5 volts secondary windings (3A for the #83 rectifier, 9.25A, centre-tapped, for the 9 tubes/pilot light on the main chassis, and 3.5A, centre-tapped, for the two #46 output tubes on the power-supply chassis) – not too many of those transformers around these days… However, Radio Daze make a transformer (Type RDX-200) that is close (350-0-350v at 90mA, two 2.5v, centre-tapped at 8.75A and 3.5A, and 5v at 2A) – but is an amp short on the main 2.5v secondary, and has a 5v winding instead of 2.5v for the rectifier heater. The set had been designed with a #83 rectifier (mercury vapour) to assist in providing a stable high voltage supply during large current draws when the push-pull output stage was blasting out at near full volume. I thought it unlikely that the radio would be playing in this mode in its new life, so I considered the options of fitting either silicon rectifiers or an alternate rectifier tube (different heater requirements) to minimize cost for new transformer(s). I am not keen on fitting silicon rectifiers to vintage radios, so I decided to compromise and fit a 5Z4 rectifier (5v at 2A heater) and buy the Radio Daze transformer noted above, plus a small 2.5v (6A, centre-tapped) heater transformer (total cost around $63). By paralleling the two 2.5v windings on the RDX-200, I could have 12.25A available for the main chassis, 5v at 2A for the 5Z4 heater, and use the separate heater transformer to supply up to 6A at 2.5v for the output tube heaters. With the total current draw on both transformers when used in the J-125 being significantly less than these, it would mean that the transformers would run fairly cool.
Once the transformers arrived, I started on the power supply chassis – it was in a worse state underneath than it was above. I stripped most of the original wiring out (what was left of it) and re-wired it from scratch, replacing the 4 pin tube socket for the rectifier with an Octal one for the 5Z4 and installing the two new transformers in the space vacated by the original one. The original choke, speaker field coil/voice coil and output transformer tested ok, but the push-pull driver transformer was found to have an open-circuit primary winding. The output and driver transformers are housed in a metal can – on opening this up to investigate I encountered asbestos packing materials (see my posting on ‘Asbestos in Radios’ for further info and photos of this abomination). I could not locate an exact replacement push-pull driver transformer, but found a Hammond balanced microphone transformer that had similar impedances and DC resistances in the SPARC parts stock, and remarkably, it fit into the same housing (all the asbestos having been removed). The two filter capacitor cans were left in place for above-chassis cosmetics and new filters capacitors were fitted beneath. Following re-wiring, the power supply worked ok, though the high voltage supply was a bit on the low side (supplying around 320v to the output tubes and 185v to the main chassis) – the manual notes this should be 400v to the output tubes and 230v to the main chassis. I figured that this was due to the replacement transformer secondary (high voltage) winding being less than the original. I noted that the power supply design was a choke-input circuit (‘swinging choke’) – as was often used with mercury rectifiers. This arrangement gives better regulation, but a lower DC output voltage than the more common capacitor-input filter circuit. So, I tried connecting a 2.2uF (450v) capacitor to the choke input and up came the voltages to 400v and 230v – right on specification! (what a coincidence). I also installed fuses into the power transformer primary circuit and into the high voltage secondary centre-tap (the Radio Daze transformer is also fitted with a thermal cut-out for added protection).
Moving on to the main chassis, amazingly all the capacitors checked out ok (using an ESR tester, ohmmeter and capacitance meter). Most of the bypass capacitors are sealed into metal cans, but several are open-style with little wax covering the foil/tissue roll inside – amazing these are still ok. On powering-up, the radio played first time, then it was noticed that it was ‘motorboating’ slightly when tuned into a weak station (but not on strong stations). It was apparent that the problem was in the RF stage and only occurred when the RF amplifier tube was operating near maximum gain (AGC bias at its lowest). After much testing – substituting capacitors (as this type of fault is often a due to a faulty by-pass capacitor) and testing resistors, the fault proved elusive. I decided to clean-up the chassis before doing any more work and noticed that the RF section of the tuning gang was full of fluff and dust – I cleaned it out while I was cleaning up the rest of the chassis. On switching the set back on, the motorboating had stopped – I can only surmise that the fluff and dust somehow caused the instability (can’t think why it would), or maybe it was the grime on the tube sockets which I had also wiped away? – whatever it was, it was cured. While I was at it, I replaced one of the grid wires (to the top cap on the converter tube) and covered two more with heat-shrink tubing (the rubber insulation was starting to perish), as well as using some more heat-shrink to tidy-up frayed cloth-covered wire ends on the dial light wires. A squirt of de-Oxit on the tuning gang contacts and into the tone control, and a drop of Rocol high-viscosity grease on the tuning shaft (gives a nice smooth feel to a slightly-worn bearing) and that was it as far as repairs went. The set was left on ‘soak test’ for several hours over two days and played flawlessly – and can it push out some audio! The modified power supply does a good job, with hardly any fluctuation in the voltage on audio peaks – this is probably due to the high voltage winding on the transformer being of a higher current rating than the original, even though a mercury rectifier/’swinging choke’ circuit is now not used.
The cabinet is still being restored (mainly re-finishing the top, the remainder just having minor scratches and scuffs worked on to leave the general 75 year-old ‘patina’) – I will post a photo or two when completed.
I am really looking forward to hearing the set once the chassis and speaker are re-installed in the cabinet!
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