Mountain King Megalith

A

ac30irons

Active member
Hey all, I have recently done a board for the mountain king megalith. Iv just got around to knocking one up and have noted quite a high pitched whine/oscillation. The only time it is not present is when the Output is full up.

My initial thoughts was the gain on the transistors was too high ! Swapped them out for lower gain and still present. Faulty Output pot ! No ! Revisited the board layout and schematic - all is good there. All other basic check ls complete.

Has anyone experienced this or thoughts ?
 
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If you haven't already, I would go ahead and take a reeeeal close look at your transistors, their pinouts, and their orientations as installed.

Other than that, it's the troubleshooting order of operations. Visual check>refdes check> voltages/continuity/audio probe.

Could be some power leaking into the signal somewhere? I don't see the 100u cap to ground in the power section. Is that just assumed? PCGBM schematic, right? Apparently I need more coffee today.
 
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I would guess you have some coupling of a trace on leg 1 or 3, depending on the layout.
Try a miller cap, 20-100pf, between collector and base of each transistor. Just touch the legs and see if it kills the oscillation.
 
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jwin615 said:
I would guess you have some coupling of a trace on leg 1 or 3, depending on the layout.
Try a miller cap, 20-100pf, between collector and base of each transistor. Just touch the legs and see if it kills the oscillation.
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Bit late back to this, had a busy few old days. 220pf on Q2 dampens things down somewhat. Every transistor in existence has been in those sockets. The most frustrating thing is that as a layout, it’s a basic as basic can be. The board is 86mmx50mm so as you can imagine everything is super well spaced. Iv got near 100 layouts behind me and this bugger is causing me grief !
 
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Still present with more engaged and as I would accept with the additional gain increasing with pot, gets louder.

Iv tried battery and it’s still present. I think I’m going knock another up and see if it’s present. It won’t cost me anything significant.
 
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