Hartke HS1200

[lowell]

Schematic here:
http://music-electronics-forum.com/a...hs1200-sch.pdf

Please let me know if my approach here is legit.

The mains fuse blows instantly. I checked resistance across the flyback diodes D209/210 and get about 250k. It seems the PA may be ok. I then checked resistance from the HV supplies to ground and I get SHORT. So either the rectifier is shot or both filter caps are shorted.

Please let me know if this if I'm on the right path here. I'm going to pull the thing apart and check all this, but curious if my approach is sound. Thanks in advance.

[The Dude]

More likely shorted output transistors.

[Randall]

If one or more of the outputs are shorted one either side, it would show as a short on the HVs, would it not?

[lowell]

I don't think it's the output transistors because if they short they do not short to ground, they'd short to the output rail (between the two sides of the PA). A short to ground from the HV seems more likely to be a PSU issue than a PA issue.

[The Dude]

Are your tests with a speaker connected?

[lowell]

I meant to clarify that, good question, no speaker connected.

[The Dude]

Then I would indeed check the rectifier first.

[lowell]

Thanks for the help. Wanted to make sure I wasn't missing something.

[Enzo]

Has anyone worked on this before you got it? Any chance someone left out the insulating mica washers from under the output transistors? So their backsides (collectors) are shorting to the heat sink?

[lowell]

Thanks all. I used my bulb limiter and found a smoking component. The snubber cap across the AC pins of the bridge rectifier was shorted and smoking. Replaced it - fixed. Thanks again.

[Jazz P Bass]

Thanks all. I used my bulb limiter and found a smoking component. The snubber cap across the AC pins of the bridge rectifier was shorted and smoking. .

I have seen that cap go short more than once.

[lowell]

Used a .001uf 600v orange drop instead of lower v metal film cap. Sure it still helps switching noise somewhat...maybe not as much...but somewhat...and can take more of a beating.

[Jazz P Bass]

Switching noise?

It's across the Vac secondaries.

[lowell]

Thought that cap was for snubbing rectifiers high frequency noise when they turn on and off.

[52 Bill]

Thought that cap was for snubbing rectifiers high frequency noise when they turn on and off.

Normally that's when there is a cap paralleled with each diode. Across the ac usually is for high frequency noise rejection.