Soviet East German Radio

[Nowhere-Man]

Soldered the board back on now cleaned up. Radio working fine as before. Not much point posting more info till I continue but plan to recap as wax ones look past it.

[pwdrive]

Yes, interesting point. I should clarify I'm very keen on seeing how helpful basic theory is so I like to apply it to something on the bench. Here is an exam question from 1959 that reflects the standard voltage drop estimate:
"It is found from tube tables that the tube to be used should have a negative grid bias of 8 volts and at this bias the plate current will be 12 milliamps. What is the required cathode resistance."
I alway prefer to say divide current "into" volts rather than divide "A" "by" "B". So, 8 volts div 0.012 = 666. An unlucky number at that.
So, the "tables" have shown currrent at a set bias. Maybe I should find the bias of the 13 milliamp valve anode because that value was the downfall. In this case too current will rise and drop as signal pushes the DC bias more into positive (less negative).
Don't get me wrong the theory just adds a bit of interest but the applied engineering side is already done for me. 33 K is the standard value and that tests just fine.

If you are referring to the ECH81 triode section then the bias voltage for an Ia of 13mA is already given in the data sheets, it's 0 (zero) volts, which more or less tallies up with the IaVg curve in those data sheets:

http://www.r-type.org/pdfs/ech81.pdf

Lawrence.

[Nowhere-Man]

Today I printed out three more schematic pages I had lacked but now these appear to be incredibly helpful. I just tested R9 R10 and R11 with my meter and all is well with those. Really I just wanted to identify them and clean around with buds.
I noticed too a soldered, empty lug on the output transformer (very suspicious as clearly it had been in circuit). Of course my eyes are very poor and only notice at very close range.
I don't want to bore people with these points but I think it's good practice for me to better study the schematic and apply it to my meter tests. The repair shouldn't be difficult so I can change leaky capacitors too.
When it's all done I'll run a feature on my website.

[Nowhere-Man]

I soldered and cleaned the output transformer as connections weren't so good. I have retested that the oscillator and one side of the resistor connects to the oscillator anode. Also the pushbutton switch makes or breaks the feed.
The switch contacts however seem to indicate there is where the problem lies. As stated before my meter shoes no contact between supply side of resistor and HT. To make it clearer still the radio won't even work at all unless I press down on the piano key and hold it down. If I let go the sound may go off.
I could do with getting at the button mechanism but will try WD40 and constant switching. Do these piano keys get really filthy or do the contacts just warp? Where I can I'm testing the PCB side of the switches.

[Terry]

Do these piano keys get really filthy ...

Yes they do!

This doesn't just apply to piano key switches but all push button and rotary switches found in radios, etc.

The contacts are silver plated brass and if you consider what a silver pot in your house would look like after twenty years, say, if you didn't clean the tarnish off, you'll have a pretty good idea of what your switch contacts look like!

The contacts are supposed to be self cleaning but even that doesn't last for ever and, of course, if the switch isn't operated for some time - as would appear to be your case - there is no cleaning action at all!

I doubt you can get any of the traditional switch cleaners these days so WD40 might be the only option available to you.

Can you get into the switch bank at all? Could you get to the contacts using a screwdriver blade, for example? If so, with the set switched off, place a small drop of light machine oil or 3-in-1 on the tip of the blade and use it to transfer the oil to the contacts. Make sure you do this for all  the contacts on that switch as it is pointless cleaning them one at a time! Be careful with the blade as you don't want to bend or otherwise damage the contacts.

Now operate the switch vigorously for a while before switching the set back on to check for any improvement.

I'm sure that everybody who has much experience of such switches has come across some with jet-black contacts, particularly in industrial areas, which just don't seem to respond to any switch cleaner known to man but there is  a solution! First you will need to find a tin of liquid Silvo - it must  be the liquid variety and it must be intended specifically for cleaning silver only: the impregnated cotton wool type is no good - and use the screwdriver to put a drop of that onto the contacts. Then work the switch vigorously as before. 

Under NO circumstances whatsoever apply BRASSO or similar to any switch contact!

Good luck!

[Nowhere-Man]

Cheers, Terry. I will try what you suggest. It seems the give-a-way was the fact it just stopped working totally the other day. Then when I pushed the keys, it crackled and played. Just FM, of course. Now, I know the main "on" switch works as I tested the resistor R12 supply to the oscillator anode, with switch on and off. And all behaves as expected. Continuity in the "on" position. So, it seems logical to assume the condition of the switches has created huge resistance and effectively cut off HT at the other end of R12.
Access will be tricky but I will think if there is some way I can get at the contacts or even view the tabs. I'll let you know how it goes later.

[Terry]

If you do have to resort to the Silvo method, it might be a good idea to 'hose' the contacts down with WD40 afterwards. Then, when most of the white spirit component has evaporated, add a drop more 3-in-1 to the contacts. The oil film will help with the self-cleaning action and stop the air getting to the silver.

[Nowhere-Man]

First you will need to find a tin of liquid Silvo - it must be the liquid variety and it must be intended specifically for cleaning silver only:

This is stocked in a shop locally so will check it is just for silver only.

I have been trying to find a good diagram of Continental push-button knobs and am surprised there are none to be found. Even doing a search on images only got me nowhere. As to access, the best I can do is tilt the radio and shine a lamp between the switches and then operate. It is as you say very dirty, even around the surface.

I also noticed my output capacitor (or "that" capacitor) is actually electrolytic. Most I encountered before were wax paper type. Again, this is soldered to the board but when I finish cleaning the switches, I may well replace it.

[pwdrive]

I also noticed my output capacitor (or "that" capacitor) is actually electrolytic. Most I encountered before were wax paper type. Again, this is soldered to the board but when I finish cleaning the switches, I may well replace it.

The coupling capacitor to the audio output valves control grid won't be an electrolytic, or at least it shouldn't be, however, that valves cathode bypass capacitor will be an electrolytic type.

Lawrence.

[Nowhere-Man]

The coupling capacitor to the audio output valves control grid won't be an electrolytic, or at least it shouldn't be, however, that valves cathode bypass capacitor will be an electrolytic type.

Lawrence.

Then that's odd. I carefully tested both of the resistors next to it. That in itself was very tricky as there wasn't much exposed leg to connect across but eventually the readings fitted with the schematic. I did locate the cathode bypass for certain and that's electrolytic. The anode grid capacitor appeared the same as the cathode bypass but smaller. Silver coloured. I think it's a 10 nanofarad going by memory. Although having said all that, it was kind of hard to match up and I was doing my utmost to decipher the digits on the capacitor with a light and magnifying glass.