Mackie SRM450V1. No HF. Help please :-)

[MarkNZ]

Hi folks,

Looking for a bit of help regarding repair of a Mackie SRM450V1.

I picked up two non-working speakers. One was an easy fix; the other has gone past my skill level. (I have a degree in Electronic Engineering from a long time ago. I'm okay with simple fixes and have done quite a few, but my experience is out of date and my brain faded :-) ).

Any guidance and help gratefully received.

Problem: No HF. LF is okay.

Diagnosis: Horn driver was blown. I replaced it. Now horn gives minimal "farts" on the louder transients. I swapped the amp with the know good one and the horn works fine. Therefore it's the amp at fault.

Fixes: Q21 and Q3 were blown. Replaced them. Ditto D22 and D33.

Still no joy.

Amp bias on J20 was sitting at about 13v and didn't respond to adjustment.

Found R240 open circuit. Replaced. Bias now at 2mV but not really responding to adjustment.

Have checked the signal into the HF circuit at E25 and it's there but fuzzy/intermittent. (<--- This is an edit from my original post).

Signal at E2 is good. (just after HPF1 and HPF2).

Signal at E5 (just before heading to the power amp section) is farty/futzy/intermittent.

I don't really understand the filtering part of the HF section.

Stumped.

Any help and suggestions of were to go next are gratefully received.

Thanks muchly.

Mark

[g1]

Are these the schematics you are using?

[MarkNZ]

Yep, that's the one. I was just in the process of modifying my post with the schematic, so thanks for linking that up.

One other thing I probably should have mentioned... U23 is toast. Literally.

Now as far as I can tell, it's part of the HF filtering and isn't directly in the signal path. So on the basis that I get HF signal at E2, and even though it may not be exactly filtered as Mr Mackie intended, I should be good... ish. Feel free to call me a blithering idiot if I'm wrong though

Cheers