I am presently doing the chassis restoration of my STC 632.

I have the circuit from AORSM 1939 p270 but this does not have the component voltages and wattages.

Also, the cathode bias arrangement of the mixer and 1st IF valves are different from this circuit. Normally I would restore it as per circuit. However this part of the wiring appears original as it does not appear to have been resoldered, and the variable resistor varies in value from that given in the circuit.

While I don’t see this as an insurmountable problem it would be helpful to know if anyone else in the Forum has tackled one of these.

I also need to repaint the rusted chassis which is finished in the usual STC reddish silver (or is it reddish gold?). I am going to play around with silver, gold and red paint to try and come up with a match. Has anyone tried this match before?

Steve Savell used to sell spray packs of STC paint. He may still have some available.

http://radiorevivalparts.com/.

Thanks Brad. Had a look but no paint is on offer at the moment.

I am doing an STC 620A at the moment, I would be interested in what you come up with. have you tried a good automotive paint supplier, they could probably eye match the paint if you have any original paint left on chassis or a valve cover.

Have plenty of original to compare to, but had an old tin of aluminium paint. I always suspected that the finish was based on silver paint with red or gold added. That didn't work out. However I had an old tin of bronze paint and tried that - bingo. But that was only small amounts on the silver paint tin lid top with bronze added by the drop.

Will work out the right proportions when I mix up the paint finally, and let you know.

Is the 620-A the same valve line up as the 620-1?

OK with the paint, will be interested in knowing your formula.
I do not have any info on the 620-1 only the 620A & 620B.

6U7G 6A8G 6U7G 6B8G 6F6G 5Z4G

Have found that the output transformer primary of my 632 is open circuit. I guess this is related to the fact that the 50K tone control, which is connected to the plate of the 6AG6 output valve is literally burnt out.

(Web says that 6AG6 is the exact equivalent of the KT61.)

Unfortunately the transformer case is rusted and no type coding is evident, though the number 1868 is stamped on it in ink. From patent sticker Rola can be seen, but little else. The external dimensions indicate (referring to Rola data on the net) indicate it to be a type K which should have primary of 6000ohm for KT61. The speaker is K8 on its STC sticker.

Since it is not a potted transformer, and having suitable 0.125mm wire, have considered rewinding the primary.

The reference for this is an article in March 1991 Electronics Aust which describes how to estimate the turns required. A simple method of winding using a hand drill is given.

This requires the secondary turns to be counted. Dismantled the trans and got 70 turns. Also a measure of the primary impedance or output valve impedance. The circuit says 8000ohm for the trans primary; EA article says 7000ohm for 6AG6. And a measure of the speaker voice coil impedance, which is given as 1.3 X DC resistance of the voice coil.

Turns ratio = sqrt(circuit data for secondary impedance/(DC resistance of voice coil X 1.3)) = sqrt(8000/(1.8ohm X 1.3)) = 58.5.

(Assuming voice coil means speaker voice coil and not trans secondary resistance of 0.6ohms).

Number of turns = number of secondary turns X turns ratio = 70 X 58.5 ~ 4100 turns.

I am about to try to count the secondary turns by counting the turns in the first layer and multiplying by the number of layers (if I can see them) as a check on the calculation.

In the mean time does anyone have any comments on the calculation, or specific knowledge of the output transformer for the STC 620.

Note: correction made to turns ratio calc. 1.8ohm X 1.3 from of 1.8ohm X 2.3

Regarding paint match for STC chassis:

Have found the best match to be about 75% bronze, 25% silver. This matches best with an unexposed area under the speaker output transformer bracket that hasn’t seen the light of day since the factory. If anyone tries this, start with less silver and work up slowly to the right mix for your own case.

The best preserved exposed areas were the valve shields and under the chassis. These seem to be a little more silvery and slightly redder/coppery.
In the case of the valve shields this is partly because the paint is sprayed on so thinly the aluminium shows through.

Another difference is that the particles in the original paint appear to be coarser than in the modern paint. These particles give the original finish more of a sparkle than the modern paint.

I will send Brad three photos to attach, with the 75/25 mix hand painted onto card shown against each of the areas mentioned. A different surface texture might be expected with sprayed paint.

Not a perfect copy of the original finish but looks OK at a distance.