Note the resistances & Valve data for #57 I pointed out. You will need around 1-1.5 V swing to drive the OP valve.

Mr Thom & Mr Smith, also had bad habits with wire gauge. Not only was it rubber but in many cases its failure was accelerated by it getting warm via its under rating. Rewired a lot of them.

Some of the valve car radio's used a Germanium diode detector.

Marc

Fred, thanks for the comprehensive reply

The only germanium diode at Jaycar is a BAT46/BAT48 SCHOTTKY DO35G with a forward voltage of 3.15V or the one Marc suggested a 1N914 which has a forward voltage of 0.72V. I will try both.

This Tasma is a nightmare to work on, most components are mounted across a formica board and the board wedged vertically between the tuner and transformer and the rear of the chassis. The chassis is about 6" high.

I am starting to suspect the mica capacitors in the second IF transformer for the falling volume. They are mica sandwiched between two plates with a tension adjusting screw. If I dismantle them will they fall apart? Can I buy mica sheet? Probably better to measure their capacitance and sub in some adjustable caps.

I'll give what you said a go Fred I have nothing to lose. As you say "what could go wong"

Thanks Marc, I missed your post on the next page. What do you mean "You will need around 1-1.5 V swing to drive the OP valve"? Sorry, I'm a bit slow.

Thanks, David

Fred... I tried what you suggested and it did make the sound clearer but... after a minute or so the volume fades off. With the oscilloscope connected to the grid of the 57 I can see the signal strength dropping as the radio fades. If I turn it off for a few minutes and back on the volume returns but fades again.

It doesn't seem to make a difference which direction the diode goes.

Which side of the diode does the 1meg resister to ground go?

Pretty much ready to just leave it and move onto something else.

Tippy, the resistor has one end to the grid, the other end to ground. That way the grid can leak to ground (cathode circuit) and not build up a charge that biases it off.) Try also reversing the diode if the effect still persists.
With the CRO connected to the grid even that should bleed any charge build off.
The signal dropping off may have nothing to do with the diode/resistor but something else shifting value.
That gets you back to your original problem, why does the signal level and volume drop?

Something is changing from cold to hot and dropping the gain.
As the fault is repeatable I would suspect a valve shifting gain, and if a spare 57 and 58 available replace each valve one at a time.
That's the "valve jockey" approach and may fix the problem in one go.

If the sound still drops with different valves, then look for a component in the front end of the set being the fault.
I am assuming the fault is in the front end as you observe the 57 signal changing.
As the set does not go off tune you can assume the problem is not in a AM or Osc tuned circuit.
That leaves not too many parts as suspect.

The guys that fix these things always point out how the alinement trimmers in the IFs can be suspect.
When the sound drops does resetting any IF trimmer bring it back up again?

Replace the cathode bias resistors and caps with NEW on the 57 and 58.
Put a NEW .25 bypass on the HT line.
Ditto the screen bypass, if that is faulty the gain will vary.

Something is shifting with temp/charge and screwing you about!
Lets hope its not a dodgy IF winding!

Never give up hope, its only a simple electronic device, if a man made it a man can fix it...……………..
Depends on how much time and patience you have and now you know why you don't fix radios for a living!!!

Find a valve car radio on Kevin Chants site. There is bound to be one with a diode detector.

I have never seen that sort of issue with a top trimmer IF but the wafer can be damaged or get crap in it. The Plate side winding can be damage by shorting to ground as its trimmer is alive. I have only ever in Australian sets had two mechanical failures (broke free) with caps in IF's

If you change to a diode detector you rewire the valve as an amplifier. The signal amplitude applied to the OP tube to enable it to get to full volume, is a product of the voltage applied to the first AF its coupling cap and where it sits relative to the chassis.

As you are changing things you may have to adjust the voltage on the 1st AF if you drop the cathode resistance. Often the plate resistor of the First AF is around 200K (& faulty).

Having had to fix a set where didums (trained tech) tried to remove the volume from Cathodes to Grid of OP in an STC chassis 59 I would suggest leaving it well alone. It works there.

I kept the bundle of 5Watt resistors he was using to try & get the bias right: Original control & DX switch missing. Albeit that I had to use a 5K pot not 6K. I tamed it by putting it back to roughly original, with tweaking. That was the one where I added more bias to the 6D6.

Using WW resistors in RF indiscriminately can cause instability.

Note: If the frequency shifts when it plays up it could be a make & break sort of fault

Firstly, thank your immense help and patience, I really appreciate it.

I found last night that I could get the volume back to normal by turning the primary adjustable mica cap of the first IF transformer a good half turn. I had previously tried doing this but not to this turning degree. I dismantled the transformer and found the mica cap shorted at about the point where it peaked during adjustment. I replaced the mica sheet and reassembled it and the low volume problem hasn't surfaced again... as yet. My first instinct was that a cap was at fault particularly as the symptom could be removed by causing a crackle through the set using a multimeter probe at which point I duly replaced the remaining micas but to no avail. I've had a similar problem before.

I incorporated the diode detector mod as suggested, the sound is improved but it won't win any HiFi awards but a neat idea for future projects.

I have read and digested all your comments and stored them to memory. Thank you all

Tippy you are now a fully qualified and certified old radio mechanic.
If the volume change thing has indeed gone away you qualify, but you lose your qualification instantly if the problem returns!!!
As Marcc and other guys that do this stuff for a living know there can be 10 things part wrong in an old radio just busting to go further wrong and invalidate your qualification!!
You just have to plod on and eliminate each horror as it comes up. This is why experienced technicians just replace every cap and resistor in a "repair". Its bad enough having 70 year old valves and coils in there so replacing the little horrors under the chassis makes sense to me on a commercial basis.
Well done and that's some experience to log up on you personal satisfaction list.
Fred.

Thanks Fred,

My new qualification still holds, the problem hasn't resurfaced and I'm confident that it's been dealt with. I still can still say I've repaired every fault that has presented itself in the radios I have dealt with, I thought I was going to lose that bragging right.

It's so good that people like you jump in and help even after it drags on like it did. This one was getting me down a bit and I have a lot of patience normally and love a challenge but I couldn't see the light at the end of this tunnel. So much so my wife hid all our sharp knives

I learn so much with each radio. The Tasma is quite stable now and the sound is so much improved. (it's 85 years old by the way)

Thanks so much for you and Marc's and others who helped and I look forward to my next adventure.

Tippy
Try cleaning the valve pins ( send paper) then tightening the valve sockets. A spray of CRC is good as well. I have had issues very similar to this . Do this with all the valves.

Hi Carl, how's it going?

I did all that early on, the issue was not with the valves or the sockets. I think most of it was to do with 85 year old adjustable mica caps. It's working now and I'm happy to leave it. It's great for listening to the cricket though, just waiting for Bradman to bat on this wonderful old radio!

Hi Fred, I hope all is well after your surgery

Cheers, Tippy

" It's great for listening to the cricket "

Agree with that. Yesterday though was outside doing a job so wasn't able to go in and turn on my STC 830 as don't leave it on unattended. Why should I turn it on you wonder. Well, it is known to get opposition wickets when turned on at critical moments, and also to protect our tail-enders! Might have made the difference in the India - Australia test at Adelaide.

Regarding technical qualifications, heard on the radio a few days ago, the required languages for a tech repairer. It went something like this:

-your native language
-the required technical language (jargon)
-bad language
-profane language (blasphemy)

Only too true.

STC820... you had better warm up the STC for Perth

I am proficient in all four languages and in that exact order, just ask my wife