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Peavey ValveKing 100 Overhaul
#21
While I had the amp open last night, I also desoldered C106 and replaced the lead tonestack's "slope" resistor with a 100k unit. Caps should get here Thursday.

Edit: Realized I didn't explain the caps bit...

One of the input filter caps had a mystery liquid leaked and dried on top of it around the top of it where the edge of the plastic wrapping is. That and another cap are beginning to bulge on top. Measuring across the input filters (with the dropping resistor to the screen node lifted to isolate them from the circuit) shows about half the expected capacitance (200uF). Removing the leaky cap and measuring again shows almost the same value (off by about 20uF if I recall). No change if I remove the bleeder resistors either. There is also a persistent hum in the output of the amp, which would seem to correspond to inadequate filtering. So I ordered three new filters to replace them all instead of playing whack-a-mole later. Oddly enough, the leaky cap appeared to measure fine by itself out of the circuit on my DMM, but back on the PCB (with the screen resistor still desoldered) where it's in parallel with the other input filter the readings are off again. Huh.
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#22
Hi Guys

Two cap-related points:

First, you cannot make in-circuit capacitance measurements if you expect accuracy.

Second: When it comes to assessing electrolytic health, the first step is visual. ANY sign of other-than-normal appearance tells you to replace the cap. These caps have been stressed and are likely old. Any electrolytic older than 14 years should be replaced for optimal performance.

Have fun
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