Canadian General Electric Model J-125 Restoration Gerry O'Hara Nov 2009

	Restored CGE J-125.

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's day - a 12 tube superhet with an RF stage, amplified AGC, push-pull driver/output stages (claimed to deliver up to 20W), a 10Ó 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.

This J-125 is being restored by Pat Jones (cabinet) and Gerry O'Hara (chassis) at the SPARC radio museum in Coquitlam, BC. Currently, this article describes just the chassis (electronics) restoration. 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') - some photos will be added when this has been completed.

On arrival at the museum for restoration, the power supply chassis was in a very sorry state. The power transformer was missing (judging by the tar spattered under the chassis it had a meltdown) and someone at sometime had fitted a metal rectifier wired directly to the power cord - what a deathtrap . A cursory glance at the state of the chassis indicated that maybe restoration to a working condition would not be economically feasible. Looking more closely at it though, it was 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 / Audio Chassis

The power-supply was tackled first - the schematic/manual was consulted to see what voltages and currents this needed to supply - it was deduced that the radio needed a power 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 reasonably close to this (350-0-350v at 90mA, two 2.5v, centre-tapped at 8.75A and 3.5A, and 5v at 2A) - but this is an amp short on the main 2.5v secondary, and has a 5v winding instead of 2.5v for the rectifier heater.

Beneath the power supply chassis before re-wiring (a bit of a state!).

The re-wired power supply chassis.

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. It is unlikely that the radio would be played this way in its new life, so fitting either silicon rectifiers or an alternate rectifier tube was considered (with different heater requirements) to minimize cost for new transformer(s). Fitting silicon rectifiers to vintage radios is not such a good idea, so it was 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. By paralleling the two 2.5v windings on the RDX-200, this would provide 12.25A for the tube heaters on 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.

	Asbestos!

Once the transformers arrived, work started on the power supply chassis - it was in a worse state underneath than it was above. Most of the original wiring was stripped out (what was left of it) and it was re-wired 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 the transformer problem, asbestos packing materials were encountered (these had to be carefully removed and disposed of appropriately). An exact replacement push-pull driver transformer could not be found, but a Hammond balanced microphone transformer having similar impedances and DC resistances was found in the SPARC parts stock, and remarkably, it fit into the same housing. 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. It was surmised that this was due to the replacement transformer secondary (high voltage) winding being less than the original. The original J-125 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. A 2.2uF (450v) capacitor was fitted to the choke input and up came the voltages to 400v and 230v - right on specification. Fuses were installed into the power transformer primary circuit and into the high voltage secondary centre-tap to provide protection for the new transformers (the Radio Daze transformer is also fitted with a thermal cut-out for added protection).

Power supply chassis after repairs.

Note new transformers and 5Z4 rectifier (a glass 5Z4G could be used to look more authentic).

Completed power supply chassis.

Power supply chassis in the cabinet.

The Main Chassis

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.

It was decided to clean-up the chassis before doing any more work and it was noticed that the RF section of the tuning gang was full of fluff and dust - this was cleaned out while cleaning up the rest of the chassis. On switching the set back on, the motorboating had stopped - it appears that the fluff and dust somehow caused the instability, or maybe it was the grime on the tube sockets which had also been wiped away? - whatever it was, it was cured. One of the grid wires to the top cap on the converter tube was replaced and two more re-insulated with heat-shrink tubing (the rubber insulation was starting to perish). More heat-shrink tubing was used to tidy-up frayed cloth-covered 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 to the chassis on this set.

The main chassis from above.

Underneath the main chassis (note the large tuning gang - RF section at the front).

Testing

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.

Restored CGE J-125.

Rear.

Dial and controls.