Hot Rod Deville what isn't working proper

[norris2002]

This really is confusing me. I went back to the beginning again. Replaaced the PI resistors again with brand new 1/2 watt's, and followed continuity on as much as I could. Re flowed lots of the pads that looked a bit dry. Checked values on components in the PI. tried a different 12ax7 in the PI after re-tentioning all the socket pins. checked the underside of the main board for bad joints. Having so many components to look through does make this much more difficult than a traditional build, but I am determined to find the problem. I did notice that I could not bias the tubes very well. When turning the trim-pot, the mV reading traveled from around 80mV down to 60mV (where I was to leave it) but then rapidly rose to around 100mV and kpt going up. At this point i quickly returned the dial to 65mV and kept the meter probes there a short while to confirm it was not changing. Could the power tubes be causing the low volume and bad distortion? I have been concentrating on the PI when looking for problems.

Neil.

[billyz]

I just had one like that, it was the Supply cap on the main board, cold solder joint. I had already resolder it too. I used some flux and got it good and hot this time. All good now.

[norris2002]

Thanks for the tip Billyz. I will have a look at the big caps, I tested them on the board, and they are all functioning, but will look at the joints. The caps have never been replaced, and my friend who owns the amp wanted to know if 16yr old caps are a candidate for replacement. I told him that 16years is nothing, but I can change them if he wants.

Thanks again for people putting up with my questions, but this really has had me stumped.

Neil.

[billyz]

Look at C43 and C38 . I had a cold solder joint on C43 even though it looked fine and had already been resoldered once . I had to clean and use extra flux and more heat to get a good connection. It could be another component is the bias circuit too. the trimmer, R76, or CR15.

Yes it could be the power tubes and if they are drawing that much current they will take out the screen grid resistors and the protection diode as well as the output Transformer . I included a schematic just in case you do not already have one. Good luck

[norris2002]

Thing is, the C- test point is giving a good -54 volts. I will still have a good look at the bias bits of the circuit. Continuity was good on the bits i checked, but pressure from the probes when checking the circuit when it is not switched on can easily close up a bad solder joint. Good job I'm not charging for this, because it is taking a long time.

Neil