Capacitors

[neepheid]

Hi there

I landed a Hartke HA3500 head recently, and thought I had been right clever in fixing the non-working valve preamp by identifying and replacing a burned out resistor - the 200V feed to the valve wasn't getting through as a result. However, I'm getting some hum only on the valve side, so I'm thinking that maybe one of the capacitors in this section is faulty (might explain the burned out resistor also).

Here's the portion of the schematic:



It was R407 which blew. This leads me to believe that C415 is involved, but I'd probably just replace all three capacitors to be sure. None of them appear bad, no bulging tops or the like, but seeing as their purpose is to smooth the power feed, I'd say any of them could be suspect.

How's my logic going?

[Dood]

Not bad, pop another valve in there too, just to rule that out as well.

[thinman]

Could have been the other cap (C411) going momentarily short before dying open circuit taking the resistor out when it did. But hum breaking through could be one of them not doing its smoothing job properly anymore.

If the amp's of any age then it's worth replacing the electrolytics anyway - especially the smoothing ones - just done mine. Some of the higher voltage ones can be a little trickier to get hold of depending on the packaging - axial seem to be a bit more rare.

Be careful!!