Test Equipment Anonymous

Complete lack of DC Balance on channel A of this plug-in. Obvious items to check first are the vacuum tubes in the pre-amp circuit. V3323, V3334, and V3354 were swapped out with no change. So I figured most likely a resistor in those gain/balance circuits had gone up in value or open. So I dove into this mess...



After much poking around I came up with nothing. Decided to swap Channel A V3364 and V3374 with Channel B V4364 and V4374 and presto, DC balance issue moved to Channel B.




These two 6AU6's are tired. Luckily I have spares.



The offending Type CA plugged into a Type 547. Replaced the 6AU6's from my spare stock and now have DC Balance on both channels.



But nothing comes for free. The replacement tubes in Channel B has thrown off the compensation. Hence the spikes on the waveform. Since the Type CA is really intended for the Type 535A I'll have to drag it out of the TE closet and give this plug-in a good checkout. Luckily the compensation controls for this plug-in can be accessed by simply removing the side cover on the Type 535A. That activity for another day.

Looking good!

Does this merit the "repairs" tag?

Yes, I forgot the repair tag.

MED6753 wrote: ↑Wed Oct 26, 2022 3:40 pm
After much poking around I came up with nothing. Decided to swap Channel A V3364 and V3374 with Channel B V4364 and V4374 and presto, DC balance issue moved to Channel B.


These two 6AU6's are tired. Luckily I have spares.

The offending Type CA plugged into a Type 547. Replaced the 6AU6's from my spare stock and now have DC Balance on both channels.


But nothing comes for free. The replacement tubes in Channel B has thrown off the compensation. Hence the spikes on the waveform.

Exactly what you'd expect replacing toobs when compensation was set for the tired ones. Small price to pay IMO.

The 30MHz Type CA plug-in is best suited for the Type 535 series (15MHz) and Type 545 series (30MHz), not the Type 547 (50MHz). So today I pulled out the Type 535A to calibrate the plug-in after all the tube swaps. But ran into another problem. The vertical gain on Channel B is inadequate even with the gain control fully clockwise. Turns out the 6AU6's from my spare stock are also tired. I had 2 more and they have good gain AND don't show as much peaking as the others. And luckily the DC balance could be set. But the plug-in still needs some adjustment of the compensation as the trace demonstrates. Turns out Channel A needs some adjustment too from all the tube swaps. All the compensation adjustments are easily accessible on the right side of the plug-in. Channel A on top. Channel B on bottom. More later.

After a ridiculous amount of twiddling I finally got both channels looking good across the response curve. I almost gave up but I refused to be defeated. It's now fixed.

Beautiful, crisp looking trace! Nicely done.

-Pat