tommytones
Well-known member
Hey all. Hoping for some insight into an issue I'm having with 1 of my 438c's. Just did a thorough run through of a pair which I recently picked up. They'd been sitting for a while so did the usual recap, etc... Both units where working perfectly so I closed them up. When I powered them on again, low and behold, one of them does not zero anymore. The meter rises up and sits at about 2db compression. The only 'mod' on these is a 100ohm trimmer instead of the 34ohm (R16 on schem) nothing else. I can trim the meter down but it will only go as high as 2db compression.
I've think I've tracked it down to the plate voltages of the 6bc8 but for the life of me I cannot find the cause of this offset. Schem shows 60v - I have 57v on one side and 69v on the other side. On the working unit I've got 56V on both sides (equal as it should be). I've eliminated the 6bc8 as the issue by replacing it with 3 others and by placing it into the other unit where it works as it should. I've changed out R9, R10, R17, R18, R22 to eliminate them from the equation and have replaced all the film caps including C10. I removed the trimmer and put in a resistor without any change. Threshold and release pots are working as they should and all other voltages are where they should be. I've cleaned the tube socket and made sure there are no loose pins. I tried swapping meters and I get the same result so it's not the meter itself. I even changed out all the tubes to see if it might be coming from elsewhere but still no joy.
The unit works otherwise. It passes audio and compresses as it should. I could just live with it I suppose but my ocd won't let me look at the meter that way. Especially since I know there is an issue. It was working fine hours prior. I'm stumped on this one... Any insight is welcome.
I've think I've tracked it down to the plate voltages of the 6bc8 but for the life of me I cannot find the cause of this offset. Schem shows 60v - I have 57v on one side and 69v on the other side. On the working unit I've got 56V on both sides (equal as it should be). I've eliminated the 6bc8 as the issue by replacing it with 3 others and by placing it into the other unit where it works as it should. I've changed out R9, R10, R17, R18, R22 to eliminate them from the equation and have replaced all the film caps including C10. I removed the trimmer and put in a resistor without any change. Threshold and release pots are working as they should and all other voltages are where they should be. I've cleaned the tube socket and made sure there are no loose pins. I tried swapping meters and I get the same result so it's not the meter itself. I even changed out all the tubes to see if it might be coming from elsewhere but still no joy.
The unit works otherwise. It passes audio and compresses as it should. I could just live with it I suppose but my ocd won't let me look at the meter that way. Especially since I know there is an issue. It was working fine hours prior. I'm stumped on this one... Any insight is welcome.
