I am not knowledgeable as it pertains to electronics, however I have never had issues working on this equipment. Over the years I have swapped out the selenium rectifier, replaced the pcb with a pristine original I bought from Bob several years back, as well as replacing the quad cap.
Yet, with all that said, I am at a roadblock with this current upgrade. I cannot get the amp to bias properly. I have been in contact with Roy, who has been more than kind in walking me through some basic troubleshooting to try to figure out what is going on here. I have followed Roy’s instructions that came with the upgrade, by installing just the rectifier and the two left power tubes and adjusting the bias pot by measuring the voltage across the 10 ohm resister located at pins 1 & 8 of the output socket to chassis. The first tube tested will bias to 0.40 volts with ease, but when I move to the 2nd it will bias no higher than 0.35 volts at full CW rotation, and in checking back on tube 1 I find that its bias has fallen below 0.40 volts and cannot be brought up to proper level even at full CW rotation. Same thing happens on the right side.
Roy has had me test some of the readings on the unit to see if proper voltages, etc. are within range. Additionally, I have tested some on my own, relying on postings on this forum by others with similar issues. Unfortunately, the only testing equipment I have is my Sperry DM-350A multimeter.
In no particular order, I have:
Measured voltage a pin 6 of each output socket with the tubes removed and found it to be -46 to-47vdc, within range according to Roy.
Following instructions in a post by Bob, I measured the voltage at pin 8 of the rectifier tube at 405vdc, again within range indicating rectifier and quad cap are functioning. Both the quad cap and the rectifier tube have been replaced within the last two years.
I do not have access to a tube tester, however the power tubes I am using are two year old Genalex KT-77s. I also have a quad of Rubys that I swapped into the unit and retested the bias, with no change in the results.
I swapped out the rectifier with another I had on hand. No change in bias results.
I swapped out the quad cap I bought from Kevin a couple of years ago with the original I had removed. No change in bias results.
I have sent a photo of the board and chassis wiring to Roy and his observation is the job looks good.
My next step is to send the unit to Roy and have him determine what is going on here. However, the shipping cost and risk of transit damage are troubling to me. But, probably more important to me, is the pride of ownership in doing this on my own—most likely the main reason most of us treasure these old babies and try to keep them going after all these years.
So, in the end, my question to any of you so inclined is, what do you recommend I do to further troubleshoot this problem before giving up and sending it to Roy?