Does anyone have an STC 402-3 wood case radio, I have a circuit for it but the valves do not match to circuit.
The circuit says it is a 4 valve vibrator set, before my Dad passed away he had it going, but it looked like it had been modified. I am yet to get into it, had a quick look today while setting up my test bench and it no longer works. We had changed all the usual caps etc.

Might pay to list what is in there. Often during the war years, strange things happened to keep sets going.

Why? At one stage WD scarpered off with all of the 6B8's, that did cause a few problems.

Marc

Hi Marcc
Obviousy the vibrator has been replaced with another power source, I will get valve data tomorrow.

I would consider that rare but not unusual.

The vibrator transformer is likely changed out for a mains type. Some sets were made with a plug in module for 32V (lighting plants), or a module for mains depending where they were going to be used.

Marc

Looks like a small mains transformer has been fitted underneath the chassis, there is a 5Y3GT rectifier there also on top.
The schematic shows the following valves:
6D8G
6S7G
6T7G
38
Vibrator

Valves in my unit are:
6D8
6S7G
6B6G
6F5G ? need to check this again
5Y3GT
These seem OK, so problem must be else where, it worked when Dad had it, I brought it home and nothing. I guess I need time to just sit down and trace through it.
I mainly was interested if anyone else had one!!

Photos uploaded.

The stuffup may be in the area of the output tube.

Logic is that you fix the power supply first, then the audio as you go toward the antenna.

The Pentagrid is similar to 6A8 The 6T7 is similar to 6B6 and the latter easier to get at the time.

Those are pretty much pin interchangeable in a pinch. albiet there are suttle bias differences.

6S7 is a remote cut of RF Pentode which may be metal it shares the same pinout as 6U7 and it may work for test purposes.

The output is likely a 6F6 which is not anything like a 38. 38 is a 5 Pin and has a top cap (Both are pentodes)

Bias is completely different and the 6F6 will draw around 10mA more with its correct bias and loading naturally pin outs are different as 6F6 is an octal, you could change that to a 6V6 if the other is crook, as 6V6's are a dime a dozen (sorta) still made. (less bias more current draw.

I would expect a grid stopper on 6F6 / 6V6 / 6L6 50K pin 5 to 6 then the grid leak to chassis or CT. Coupling cap to pin 6.

Circuit likely is a run of the mill 5 Valve superhet.

I would get rid of the old paper & electrolytic caps & check resistors before powering it up as they are the likely killers. 500 to 600V filter electrolytic's with a 5Y3

We sit on fingers until the circuit is seen

Marc

I have a 632 (1939) with a similar cabinet, with one of the bars across the speaker opening delaminated and pieces of the finishing veneer missing. So I wonder if you have considered yet how to replace the missing bar on your radio and the type of timber in the finishing veneer.

Speaker cloth still in place though damaged; so can show what that was like if you are interested.

Nice looking test bench setup. Would like to see more of it.

That is a fairly common cabinet for STC around that time.

I actually have the circuit, which is not of a good standard.

Going by what is there, and the Philips data the 38 had a 10K speaker transformer and a grid voltage of -25. The cathode resistor of 1200 Ohm is consistent with 250V HT at the screen (Pin 4 of 6F6).

This is a circuit which assigns a number to a value and it turns up everwhere.

I would be inclined to add a grid stopper to the 6F6 but otherwise leave the rest of the values well alone, with the exeption of the cathode resistor on the 6F6.

That is supposed to have a 7K primary OP transformer and the cathode resistor 410 Ohm. (-16.5V) In veiw of the op transformer, slightly higher will not hurt.

Do check the WW resistors some of those were horrors for corrosion & going open.

Marc

Well this thread has a great response from you boys, well done.
STC830, I am going to make the missing bar, not a problem using the other as a template for the curve, my brother who loves woodwork sent me some veneer with that strange grain pattern. He did tell me what is was but my memory fails me, he will be up to visit next week. I would like to see the original speaker cloth, mine is not original.
Marcc, will take your comments into consideration when I can get to it, maybe Sunday. I have a huge collection of valves from my Dad but they are stored in over 60 boxes at present.
GTC, the test bench is my late father's, it has variable AC 0-280v, Isolated 240v, 2.5v & 6.3v filament voltage terminals, variable DC 0-500v, 110v outlets, a few other things but I am inside at present. All measurements have voltmeter and Ammeters on the panel. I will get a better photo over the weekend. Send me your email address to ploweexemail.com.au if you want photos or if anyone else is interested I will ask Brad if I can start a thread on members test benches and if he can place the photos up for us all to see. I have a test bench for sale, I will place the advert in the for sale forum.
Now I am keen to get back to this project, just have to find somewhere to put the clutter from my test bench.

Feel free to send photos. Just start another thread in this forum when ready. I'd better get my act together and tidy my workshop up.

Hello Airzone

I will send to Brad for addition here a scan from 1938 STC Sales Manual put together by Brian Smith which shows the dual wave ( four knob version) of your radio. It is called the STC Type "A" Mantel cabinet which carried AC models (500A, 520 A, 600A, 620A), Battery (400 A, 502 A, 522 A), vibrator (402 A, 524 A). The speaker cloth pattern is orientated vertically.

Also a photo of the cloth in my 1939 model 632, which is orientated horizontally, but appears the same cloth as in 1938. The colours are black and pale (gold? - faded) silky thread, woven onto a base of brown cotton. The colours are the same as in this thread, which is a different pattern:
http://vintage-radio.com.au/default.asp?f=2&th=112&offset=2.

Regarding the veneer on the grill bars, the nearest I could get on the net (years ago) was tiger or zebra maple, but still not really right.

While working on the STC 632 found a cabinet label in pieces and managed to piece it together. Never found anything like this before so thought it was worth posting.

Says Cabinet Work by EB Dearing Ashfield, NSW

Will ask Brad to put in the photo.

^ Yes, definitely worth posting.