Siemens Klangmeister RD30 50Hz hum on channel

[Depaj]

Hi guys,

I've got a Siemens klangmeister picked up for a few bucks on a flee market that's humming on one channel (50Hz measured with oscilloscope, about .8V peak to peak). It doesn't change with volume control, is the same with tuner or external input and increases with the "hi-fi" control activated. It had a bad output tube so one has been replaced (I tried swapping tubes but it stays on the same channel) it's also there without the output tubes but not as loud. I checked values of all resistors and their resistance to ground and I found no differences between both channels. There is no hum when I first turn the amp on and comes up gradually after a few seconds.

Here's a schematic of the amplifier stage :
View attachment RD 30 Klangmeister II 2.pdf

Could use some help, thank you

 

[Kay Pirinha]

Check the output transformers for insulation faults and for continuity in both parts of the primary winding. Do you know why one ECL86 has been replaced?
Best regards!

 

[Depaj]

What do you mean by "insulation faults" ? I forgot to say that other than the hum both channels work and sound pretty good.

According to the previous owner there were some weak spots in the soldering of the tube sockets so he decided to resolder himself but he made a mess of it and damaged the tube. I cleaned up the mess ad redid the soldering correctly.

 

[Depaj]

I just noticed R45 (on the channel with hum) at the output transformer was missing and on the other channel instead of R46 were two 680ohm resistors in series (so not the 2k announced on the schematic. I replaced both with the correct value resistors but there is no change at all.

 

[Kay Pirinha]

Lift one leg of your newly installed R45 and measure the voltage at the lower (hum compensation) end of Tr2.
Best regards!

 

[Depaj]

I measure 312.6V (exactly the same on the other channel without removing R46).

 

[Depaj]

Does this look ok to you ?

 

[Kay Pirinha]

Yes, it's ok if there's a positive voltage of about the plate voltage at the primary's lower end. I had suspected that the compensation part could be open. OTOH I haven't any further idea right now.


Best regards!

 

[Depaj]

Could it be that the hum is induced by the filament wires running to close to some other sensitive part of the circuit ?

 

[Jerry Ho]

If you took down the tube from the channel which had hum, but you still hear hum, is the electrical connection same as schematic? perhaps there had wrong connection.

 

[Depaj]

There were some bad connections at first but I did some solder work and tested continuity between sockets and components in circuit following the schematic and found nothing wrong. I tried putting a shield between the power transformer and the output transformer of the humming channel (they are already turned 90° but you never know), that didn't change anything. I did notice that the bias is higher on the channel that has hum (7.5v across R63 (220ohm) against 6.4V indicated in the schematic and also measured on the other channel (I replaced both R63 and R83 with 1% 1W resistors).
This is what I get :


-both output tubes in : right channel ok, left channel 50hz hum
-only right output tube in : right channel ok, left channel 50hz hum (less loud)
-only left output tube in : right channel very faint hum, left channel 50Hz hum
-no output tubes in : very faint hum on both channels

 

[Kay Pirinha]

Short the left channel pentode's grid (pin 8) to ground. If the hum disappears, the coupling capacitor C101 or the compensation cap C102 may be the culprit.
If not, swap tubes and report.

Best regards!

 

[Depaj]

Do I do this with tube in the socket or without ?
I swapped the tubes before without change so I believe both tubes are OK.

 

[Kay Pirinha]

Yes, the tube has to be in it's socket, and the devices is powered on.
You've swapped the tubes and the hum remains at the same side? Then the tubes most probably are ok. At least they aren't responsible for this strange hum phenomenon.
Best regards!

 

[Depaj]

It's actually worse with pin 8 shorted to ground.
Yes hums stays on the same channel when I swap tubes.

 

[Kay Pirinha]

Well, it's getting more difficult from step to step !
What's the cathode voltage with the grid grounded? Does it remain the same as before?

Disconnect C102, the compensation cap. What happens?
Best regards!

 

[Depaj]

I appreciate the help a lot, I hope you're not out of ideas yet


It remains approximately the same, it drops a few mV when I ground pin 8.
No change in hum with C102 disconnected.

 

[Kay Pirinha]

Ok, so the difference in cathode/bias voltage is due to tolerances between both tubes.
Now you need to do some bigger surgery: Swap both output transformers, i.e. disconnect both plates, and R69 and R89 as well, and swap them to the other channel.
Best regards!

 

[Depaj]

Just did that and hum moves to the other channel so there is something going on either with the output transformer or the feedback (probably why activating the "hi-fi" mode increases hum). Or maybe now that I thing about it it's actually the other way around, the output transformer is ok and the problem is still somewhere before...

 

[Kay Pirinha]

Ok, you've swapped the connections as I recommended, but you didn't swap the speakers, and the hum went to the other side, with the OT? Then the OT is the culprit .

Best regards!