Hi all,
I'm diagnosing a problem with a Foreplay III. Think I have it figured out, but before I order parts and start cutting wires I could use some independent assessment.
It's a 2006 build with no real modifications and worked well for years. I recently moved, and it may have taken a hit somewhere in transit. (My other pre-amp failed at the same time! It's solid-state, and looks to be a total loss. But this is a kit. We can rebuild it. We have the technology.)
The only symptom is that when I hooked it back together, I got a very loud 60 Hz buzz. Definitely not a ground loop either. The buzz increases slowly as the preamp warms up, reaching maximum in about ten seconds. It's identical on both channels and does not change with input or volume settings. There's no obvious damage and no release of magic smoke. Took it back to the bench, all resistance and voltage measurements check, all tubes light, all LED's light. It also still passes an input signal, just nearly drowned by the buzz. In every way it looks 100% except for the buzz.
Since it was on both channels, my first thought was cold solder joints or wiring being knocked loose in the high-voltage supply. I found three -- one at station 21, which could affect B+ to both 12au7's. The others were on stations 4 and 9, where the bleed resistor spanning them had come completely loose. However, fixing these had no effect.
Next I looked at the heater supply, reasoning it was also common to both channels. Couldn't find anything. I also swapped and re-seated all the tubes, no change, tube bases and pins look good.
Finally I dug out the oscilloscope. The buzz on both channels looks suspiciously like an RC charge-discharge curve, amplitude about 0.3V, faster time-constant on the charge leg. There's no obvious 120 Hz component. There is a nearly identical waveform across B+. Just to be sure I pulled both 12au7s and checked again, and it's unchanged.
What I think is going on is that one of the two 220uf voltage doubling capacitors has given up, and it's only charging on half the AC cycle. Whatever jarred loose the bleed resistor could have broken the cap, I guess. It looks a little bulged. I also tested all diodes and they're fine.
Does this make sense? Or have I overlooked something?
Is there anything downstream that could have been permanently affected?
How closely do the doubling caps need to match? (I'm guessing not very close, just better than 100% different)
How much ripple should I tolerate on B+ when it's working correctly?
Thanks in advance... if it will help I can take more data.
