Mesa Boogie M-Pulse 600 No Audio Output

[Bass Liberation Front]

Greetings:

I have an old Mesa Boogie M-Pulse 600 bass amp that was operating normally and then suddenly stopped producing audio. I seem to recall a very slight crackle or pop occurring right before exhibiting the fault. Replacing the tubes and internal fuses obviously (based on the title of my thread) had no effect. After powering the amp off and on again, I observe that the amp didn't make the characteristic "click" of the audio protection circuit that normally occurs a few seconds after being powered on. It seems logical that I'm experiencing a lack of audio output simultaneously with the symptom described above but I have relatively little experience repairing electronics, apart from fixing a Hartke 5000 bass amp and a few analog synths, electric guitars and basses. I do realize that there are at least two other threads documenting similar issues, both referencing a faulty 15uf@15V cap (C44) located near the compressor IC. I'm wondering if the other symptom I mentioned besides the lack of audio would indicate a possibility of the same fault. Can you recommend the most thorough method for a DIY approach to diagnosing and repairing the unit? Thanks for taking the time to read this and post a reply.

[nickb]

You are not getting the click because the speaker protection relay is not pulling in. This likely means there is a problem with the power amp. You should check for shorted FETS Q1-Q12. Quite often when these blow it's pretty obvious. If one is blown you have to replace them all as they need to be matched, The BUZ types are obsolete but you can use ECF10N20 and ECF10P20 and make sure you pick the selected (matched) ones. If you add your country of residence to you profile we can direct you to a distributor in your area.

Very expensive though. You're looking at $150 just for the transistors.

Anyway we are getting ahead of ourselves. Test the transistors first.