MarkBass Little Mark 800 Amp major volume loss

[BrassedOn]

Tough day for my MarkBass Little Mark 800 (9 years fairly light to moderate use, and I’m pretty sure always appropriate ohms match to cabs, etc. either one 8ohm 400w cab never maxed out, or rarely, two in parallel).

At a couple rehearsals, it unusually was low output 50% for 5 to 8 minutes then (while I’m checking cables and cords) it kicked in and ran normal. Third time, same, then after 20 minutes, down to 20%. No pop or smell that I noticed. Now it seems fan is only very low speed (cause, symptom? IDK).

As it sits now, major volume/signal loss, immediately from the unit being turned on. (And have tested/changed out all speaker cables, cords, instrument cables, cabs, bass, etc.) All volume, gain, EQ, and character pots turn with no static or noise, and still affect the sound, just at the very low output. Nothing apparently loose or rattling.

I don’t usually work on amps but I unplugged and grounded, and I opened up from the top. No smell. Looking around on I think the main heat sink (L-shape aluminum near the fan) something like white sealant or caulk is liquified. Yep, around and on electrical components that are attached to the head sink. (Image))

Maybe fan want out and this over heated, or just time, end of life?

Any ideas or thoughts?
Common issue or suspected cause?
Is there a potential fix (worth $$ bench time)?

So 9 years, not bad. If repairable I’d like to fix to have as a backup.

(Or if something I did, I’d like to NOT do it again)

Thanks for any help you can offer.

[Mick Bailey]

The white substance is heat sink thermal paste. It looks messy, but apart from that seems OK to me. Try patching the FX loop send and return to see if the volume improves.

[Enzo]

Does it have an FX loop? Or a pair of jacks marked preamp out and power amp in? Plug a space cord from send to return in either of those spots. ANy help?

Also, plug guitar into power amp in jack. (or FX return) does that come out strong?

[BrassedOn]

Thanks, yes I had seen that effect send return advice, I think for a mark III, and it “seemed” to work, then it didn’t. But hadn’t tried the power amp side. I’ll try that too thanks.
UPDATE: same issuen the powered input XLR

And as far as the thermal paste, liquid and gooey seems bad, especially dripped on the components.

What might make the fan run low? Usually the fan runs fairly strong when in use.

[Mick Bailey]

Many amps have temperature controlled fans, so a reduced signal means less heat and no need to run the fan up to full speed. Not sure about your specific amp though.

[BrassedOn]

Many amps have temperature controlled fans, so a reduced signal means less heat and no need to run the fan up to full speed. Not sure about your specific amp though.

yeah, thought that could be the case, no load no heat…..low fan.
thanks

[diydidi]

The front panel has a smaller pcb with two pots( if I remember correctly). The board is only held in place by the two mounting nuts on the pots. The pins normally break off against the PCB. Remove the board and confirm all the pot pins are solid and not broken off. You can check this by reflowing the pot pins. If any feel really loose.... there is your issue.
I have found this issue on a few 800's.
When reinserting the board, add some sillicon adhesive to the board edge for extra support.
Let us know what you find!!

[BrassedOn]

The front panel has a smaller pcb with two pots( if I remember correctly). The board is only held in place by the two mounting nuts on the pots. The pins normally break off against the PCB. Remove the board and confirm all the pot pins are solid and not broken off. You can check this by reflowing the pot pins. If any feel really loose.... there is your issue.
I have found this issue on a few 800's.
When reinserting the board, add some sillicon adhesive to the board edge for extra support.
Let us know what you find!!

maybe different year or model. At the front I see a 5cm by 25cm (~2” by 10”) PCB with all pots attached. A bit tight for a novice like me to pull out. Nothing visible for me. But good to know know that’s a trouble spot.

at the back by the outputs there is a smaller PCB 3cm by 6cm.

and using the “move a wire with a chopstick” technique,
NOW I see a small burn on the L-shaped heat sink where a red and a black wire run next to. Small 1-2mm black burn on the sink, and small white mark on the wires, though not clearly burned through insulation.

[BrassedOn]

Okay, I’m seeing at least a starting point.

Recap symptoms:
significant Volume/signal loss
Tested and swapped out all cords, cables, cabinets, and the instrument. So eliminating those.
initially this was on start up, recovered a couple times, but now just 20% or less
seems like all EQ and character pots are working, have an effect but no change in volume per se.
fan running low, maybe just because there is no signal, or no load so no heat

some history on these with similar isssues being helped by connecting the effects send return. (I think on Mark III) May have worked briefly on my 800 or maybe that was just intermittent. I’m not sure what this was meant to bypass anyway.

seeing evidence of burned wire and short. Not sure what down stream would be affected.

I’m thinking worth and hour of bench time for a pro to check out. maybe could determine if those wires were the issue, or where the signal is getting lost, like maybe at the output. But not confident.
My goal now would be to have this repaired as my backup. I liked it enough to replace with the same.

Any other thoughts, or what I should ask or tell the amp repair person?

thanks, I really appreciate the comments you’ve all had