Mark III Combo passed away :-( Help needed

ivovasconcelos · Feb 28, 2024

Dear fellows,

I was playing on it and suddenly it was off.
I did check the fuse and it seems to be ok.
Here is the picture:

After that, I did remove the power tubes and tried to turn the amp on.
The pilot light didn't turn on.

I also checked the preamp tubes and they are all properly connected.
I put the power tubes back and tried again to turn the amp on, and it did not work.

Do you have any suggestions?
What else should I check before sending it to a tech?

Thanks in advance,
Ivo

domct203 · Feb 28, 2024

Did you check the fuse with a meter or just by looking? You can’t always confirm visually. Try another fuse, start with the power tubes out of the amp. If it powers up, shut it down, reinstall the power tubes and try again. If the fuse blows you have a power tube issue.

If the fuse is good and you have no power it’s a bigger issue that will need a tech.

Dom

user 41014 · Feb 28, 2024

Switches can go bad. And have serious power going to them if you're fingers short from a lug to the chassis. I wouldn't jump to that conclusion yet you said the pilot light isn't getting juice so I wouldn't rule it out.

Dom beat me to checking that fuse for continuity- just to not chase your tail even tho it doesn't look blown.

bandit2013 · Feb 28, 2024

Two things that come to mind. An aged amp with electrolytics. Hmm.
Always test the fuse with a multimeter if you have one. I usually keep a supply of extra fuses, never know when they will pop on you.

So assume you do have a good fuse, if you have bad power tubes, I doubt it would blow the fuse instantly if the amp is in standby. However, if the power caps are gone or have shorted out, the fuse will blow instantly the second you turn on the power switch. I would not rule out a worn switch or loose plug or not plugged into a power source. Not sure if the fan is on the fuse circuit or is it just parallel. If I recall, the fan plugged into an electrical outlet on the amp chassis. You can unplug it if the fan motor crapped out.

So we could be left with a few things on the list. Switch, Capacitors, tubes and the like. Then again, is the amp plugged into a power strip or direct to a wall outlet? if it is a power strip, check the circuit breaker on it. It has been a long time since I owned a Mark III, is that accessory plug on the back of the amp tied into the power switch? you can check for power at that plug, plug in a small lamp or something.

I am out of ideas.