Soviet East German Radio

[Murphyv310]

Well spotted that man......... Thank you.

[Nowhere-Man]

Was doing some work yesterday. There's a wax capacitor that could do with being changed. I was cleaning the inner board to get better visibility and testing some sockets. Drilled out the rivets and got tiny screws instead for later. I am doing bits of work and tracing HT from rectifier and on. This will be ongoing as I hope to do a feature on my website.
My hope is I can trace any switch fault from the track side and will of course look into Crackles switch link to R12, thanks. For your interest you can see some switch contact points just between the chassis rim and board. They solder onto the board itself.
I can check the tracks everywhere aren't broken.
I think I was not thinking so well before and getting ahead of myself. Only when I've explored all options will I contemplate removing the board. In such a case I'd need to make more notes and more pics.
I am not rushing as I want to also learn a bit about the circuit design so just enjoying the project.

[Nowhere-Man]

There are some points to note in the schematic I find interesting. As I gather C60 is the smoothing capacitor that supplies R12. Notice the HT line goes to the output transformer in series with R35. This 1K resistor was once off circuit so I can confirm the HT line directs across it. Further evidence is the huge shock I got from the cap (with the set unplugged months ago).
So this HT line heads to R12 which is 33 K. Now here's the punch line: If supply is 230 and the ECH81 anode 95 volts, R12 would drop 135 volts with the anode (triode) drawing 13 milliamps. So, oddly I get 0.013/135 = 10384 R. That's a fair way from 33 K. Or 34 K if you wish. My only explanation is the drop across the windings??
So far as I know the Mullard and Telefunken tubes are not different.
Given I'm hoping to learn as I go along I am trying to get a bit sharper with the schematic.
I like to ask myself these questions so I share my observations although I ought to be celebrating New Year. Anyway Happy New Year let's hope it's a good one.

[Crackle]

You have previously reported there is no voltage on the anode of the ECH81,
This may be a silly question, please excuse, but have you got the radio switched to an AM wave band when you did this test.

The black circles with a number identify the switch contacts. See inset diagram of switch at top of circuit.
Contact number 5 being the centre common contact, with contact 6 the supply to the ECH81 triode (AM mode), and 4 being the contact for the supply to the ECC85 (FM mode).
If there is a fault in the switch between contacts 5 and 6 it should be easy to determine.

Have you confirmed the resistance of R12 yet by measuring it with an ohm meter. Sometimes resistors have an invisible crack causing them to go very high value. It would be good if you could confirm if the resistor is close to its specification.

"So, oddly I get 0.013/135 = 10384 R. That's a fair way from 33 K. Or 34 K if you wish. My only explanation is the drop across the windings??"
Who is to say the triode would normally work at 13mA, that figure from the specification of the ECH81 triode is only the maximum it should work at. Have you taken into account R35. I think you may be mixing theoretical max figures and coming up with a theoretical wrong answer.

Keep at it, the answer cant be far away.

Mike

[Nowhere-Man]

Hi, Happy New Year (here it's been a rainy one). To answer the question R12 is bang on spec as is R35. I mentioned before about it being on spec.
One test I mentioned too was meter between HT and R12 with all knobs being pushed in expectation of flicker. Not sure about current on R35 but admittedly you have some drop across the output transformer as well as oscillation coils.
I feel I can fix the AM but I am working on some socket plugs and spotted a dodgy wax capacitor I'll replace. Also been cleaning up chassis and board. I want to do a special feature on the Saalburg on my website so I really want to spend a bit of time reading up on whatever I can find. Audiokarma touches on it but most accounts trail off with no final account ever published.
I still have the Astrad to mess with too.

[pwdrive]

There are some points to note in the schematic I find interesting. As I gather C60 is the smoothing capacitor that supplies R12. Notice the HT line goes to the output transformer in series with R35. This 1K resistor was once off circuit so I can confirm the HT line directs across it. Further evidence is the huge shock I got from the cap (with the set unplugged months ago).
So this HT line heads to R12 which is 33 K. Now here's the punch line: If supply is 230 and the ECH81 anode 95 volts, R12 would drop 135 volts with the anode (triode) drawing 13 milliamps. So, oddly I get 0.013/135 = 10384 R. That's a fair way from 33 K. Or 34 K if you wish. My only explanation is the drop across the windings??
So far as I know the Mullard and Telefunken tubes are not different.
Given I'm hoping to learn as I go along I am trying to get a bit sharper with the schematic.
I like to ask myself these questions so I share my observations although I ought to be celebrating New Year. Anyway Happy New Year let's hope it's a good one.

Maybe there's something wrong with the maths, a 135 volt drop across a 33k resistor = 4mA approx., if the receiver was working to spec then current through R12 would be 3.7mA approx.....0.013/135...should be the other way round...academic anyway because I think your original current calculation is wrong.

Amongst other things, R35 is part of an impedance balancing network, one section of the output transformer being the other, those two will balance the impedance in the other leg which consists of the other section of the output transformer and the output valves anode resistance (ra).....I think, but stand to be corrected.

Lawrence.

[Terry]

Take care when reading data sheets. Most give two sets of figures: one is the absolute maximum ratings for the device whereas the other will be typical figures in operation.

The Philips data sheet give the following figures for the triode when used as an oscillator, as in your set: HT 250V, anode resistor 33k, anode current 4.7mA - obviously, the actual figures will vary due to the individual designer's preferences, but should be about the same.

Also, as Lawrence said, your formula is the wrong way round. You've written it to show the current being divided by the voltage. It should be the other way round: 135/0.013 although, as previously pointed out, the calculation is meaningless in an operational context.

[pwdrive]

Sometimes data sheets can be confusing, the Philips sheet gives the typical characteristics as 13mA for 100 volts anode voltage and 0 volts grid bias,
It also shows that the maximum cathode current not to be exceeded is 6.5mA.

Lawrence.

[Refugee]

I have a feeling that a few low resistance tests have not been completed.

First switch the set off and check that the HT capacitor is discharged and then clip a continuity buzzer or meter set to continuity to the terminal on the capacitor.
Select FM and check that you get a beeper on terminals 4 and 5 on the switch. Next select AM and check that you have a beeper on terminals 5 and 6 on the switch. If this is true you have got a good switch.
Then move the switch end of the meter/buzzer to the end of the wire from pin 6 to the circuit board and make sure you have got a beeper there. Next check the track on the board between the wire connection and several points on the track until you get to the solder end of R12 or R35 or whatever.
Then do a track repair between the two points where the buzzer stops.
You should then get HT back on the AM part of the circuit.

[Nowhere-Man]

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.