Paramount 300b 5670 Voltage Won't Adjust

[burgerbassist]

Hey All,

It's taken me a while, but I finished my first monoblock (in which I caused a short by accidentally getting flux between two terminals, per my last post) and got some blown parts replaced, and it's working fine.

I finished the second tonight and resistance checks looked good, I didn't have any big issues upon startup or anything, but when it came time to adjust the 5670 tube voltage with the pot on the soft start board, it held at 297.7 and the pot wouldn't adjust anything in either direction.  Upon further inspection, I noticed the D1 and D2 diodes on the B side of the board aren't lighting up, either.

Can anyone provide any insight as to where I would look to determine what might be causing these symptoms?  I'd assume it's a bad solder joint, but I'm not positive where to look.

I built this soft start board while I was building the first monoblock, before I knew not to use extra flux, but I've gone through and tried to clean off all of the flux I can with alcohol.

Thanks!
Dan

[Paul Birkeland]

What kind of extra flux are you using?  Why are you using extra flux?  (We do not recommend doing this)

What voltages do you see on Kreg on the half of the board with the blue trimmer pot?

[burgerbassist]

I'm not using extra flux anymore - when I built the first of the two monoblocks a few months back I used it, as that's how I've always soldered, but I accidentally got some between two terminals and they sparked, lit on fire, and shorted out the zener diode strong. We discussed the recommendation not to use extra flux in the thread I created about that incident, so I stopped, but I built all four boards at the same time so they were built with extra flux and I thought it was worth mentioning.

When I went to measure Kreg on the potentiometer side, I found that that solder joint was pretty bad, so I resoldered it and now it's working like it should and voltages are all in spec.

Thanks for your help! Now it's time for me to go put a on a record.

- Dan

[Paul Birkeland]

Anything you put conductive flux on (your comment about the flux shorting out two terminals indicates it's probably repair flux and certainly not electronics flux) needs to be thrown out and replaced with new parts with no flux. 

In the last five years, I have had to repair a handful of kits that got "extra flux" on them, and only one could be salvaged without being completely torn down and reconstructed. 

If that conductive flux remains anywhere in the amps, it will begin to migrate once the amps reach operating temperature, and you'll continue to have random problems that can't be resolved.

-PB

[burgerbassist]

What would that process entail, given that this is a long-out-of-production kit?

[Paul Birkeland]

I would desolder the power supply boards with a spring loaded solder sucker (the blue one), pop them off, then run them through a few dishwasher cycles.  Everything else on the chassis plate other than the iron would come off and go in the garbage (keep the 9 pin socket savers though, we don't make those anymore, also keep all the hardware for mounting the 2K resistors to the chassis), then replacement parts would be installed.