Tube-Tech SMC2B

[tadejm]

Hello!

I have Tube-Tech SMC2B Multiband compressor here.
The unit works normally for approx. 2 hours, then the stereo image starts to hang to the right. Even with BALANCE, you cannot straighten it to the center (the difference is approx. 5dB). When I turn it off and it cools down (for approx. 10 minutes), it works normally again for a while.

I opened the unit and changed the tubes between the channels (L / R). And the same thing happens after about 2 hours, the image starts hanging to the same - right side.

Then I measured voltages, which are stable with very little deviation between channels.

I then looked at the unit with the IR camera as well. I found out that on the power supply part one resistor (I think 470 Ohm) is heating up abnormally! The temperature is over 150 C! Also, the diode bridge has a temperature of around 100 C. Isn't it rather too much? (I don't know if this is normal?!? or if this is the cause of the "stereo image" problem after a certain time?)

If I remove the tubes, it does not heat up at all.

Does anyone have a schematic or any ideas?

Thanks!!
Tadej

 

[TLRT]

That is normal,those looks 3W Vishay wirewound resistors, check the load on them(probably they dissipate around 2watts and go hot) You can upgrade them to 5-10watt,then they won't get so hot. I also had a unit which had a ringing microphonic effect on one channel, not fixed yet.(if I swapped the tubes the ringing stayed on that channel

 

[Rob Flinn]

if you remove the valves there is practically no load on the PSU so there is no reason for it to get hot. I would be mesuring voltages on the left side & comparing them to the right side when you get the signal level drop. That should enable you to work out the area of the fault.

 

[rock soderstrom]

Does anyone have a schematic or any ideas?

Then I measured voltages, which are stable with very little deviation between channels.

Are you sure that all voltages remain stable when the device warms up after prolonged operation?

It looks to me like one voltage drifts away due to thermal overload.

This unit has 4 tubes per channel apparently.

How is the power supply unit organized? It seems to be subdivided like this:

You will need to create a schematic sketch. At first glance, it appears that the voltages B+, F+ and Auxilliery are generated together for both sides. Somewhere along the line they are probably separated into the left and right channel, which could be the origin of your problem! Find the place where the splitting (especially of B+) takes place and monitor the current flow and the resulting voltages.

I believe that B+ or a part of it drifts away in the affected channel...

Edit: please snap some better photos of the whole PCB.

 

[rock soderstrom]

Regardless of the PSU voltages, I would also take a look at the situation on the two PCBs behind the front panel. I would check whether you have good contacts everywhere, especially the gold contacts that connect the two PCBs, and clean them if necessary.

Is anything in this area getting warmer than expected? Can you change the situation when the fault occurs with mechanical pressure (use wooden chop sticks) against the two PCBs behind the front plate?