Markbass Combo Head II Issue - HF on output

[nothingMaker]

Hi
I have a Markbass Combo Head II with a well know problem.
The 2 x 0.1uf 400v block caps blowing out, Replaced them, the amp working, but there is a lot of HF on output, seems to self-oscillate.
Capacitors 100nF and Resistors 4.7ohm/5W become hot.

Anyone have some ideas?

[DrGonz78]

Do you have a schematic? It looks like Stay Puft Marshmallow man was on the assembly line.

[Jon Snell]

Component number/identity may help. I presume it is the little mark II?
Nave you checked your work ... dry joine ... broken interconnecting link between both sides of the PCB?

[Greg Robinson]

Check my fix here.

[nothingMaker]

Check my fix here.

Thanks for the reply, I read your post.

I've already replaced C86 (680nF 400V), C87 (330nF 250V) and C88 (100nF 400V). Resistors R53 and R54 measures ok and I left them on board.
Unfortunatelly I still have a HF emission on output, not depending from Volume o Gain.

How can I check the Inductor?

Do you have other indication?

Thanks a lot for the support

[Greg Robinson]

LCR meter for the inductors, or else just inspect them for any scorch marks where they might've shorted turns, though that's highly unlikely. Voltages aren't high enough to cause insulation breakdown, would have to be burned off by adjacent components overheating for that to happen.
I'd otherwise guess that you might've damaged through-plating when removing the faulty parts, Markbass use through-holes barely larger than the leads on a number of components, this along with the crappy lead-free alloy that they use makes this a common issue. I've repaired damaged through holes caused by other repair attempts a number of times for these amps.

[x-pro]

Frequency and amplitude of this HF ?