'81 Mark IIB Troubleshooting - blowing fuses

[bluelair]

Picked up a very clean looking Mark IIb on Facebook Marketplace. Previous owner said it was blowing fuses and he let it go for $200.

Serial is 6414, confirming the model. There is magic marker build date on the chassis saying 1981.

I talked with a local amp shop technician. He wouldn't take it in (said he doesn't work on Mesa because they're so compact) but he gave me some idea on where to start.

I have worked on Tube amps for about 10 years, but this is my first Mesa. I have assembled two Marshall 18w kits, built a Bottlehead Crack headphone tube amp and have modded many pedals.

First visual, things look clean. No burns, scorch, arc marks, no smell of burning around the components. Cabinet is clean and appears to be original 12" speaker. Also came with footswitch.

Fuses are 3A slow blow (per label on back of amp.)

The tech recommended I first pull all the tubes, replace the fuse and see if it blew again without the tubes in. Got some new fuses, pulled all the tubes, tried new fuse & power on. Fuse still blew with the tubes out of their sockets. Damn.

The fuse blew instantly and I didn't notice any arc or spark in the chassis, just a mild flash from the fuse socket.

(Next, tubes still removed, unplugged, then discharged all of the capacitors with a 100k ohm/5W resistor for safety. Tested caps with a meter to verify no residual voltage.)

Mesa recommended to next test the silicon diodes, I found five on the power board where the large caps are. Tested each with a multimeter with a Diode test setting, results:

Diode#/Forward bias reading/Reverse bias reading

1/.531v/OL 2/.535v/OL 3/.536v/OL 4/.535v/OL 5/.546v/OL

So does all appear to be in order with those? This seems to be telling me the diodes are "good".

Are there any other visual inspections to perform before continuing with instrumented tests?

What would be the next thing to check? I've been told to check the power and filter caps, check the tube sockets for shorts, and that the PT might be bad.

 

[MK IIB]

Hi Congrats.That's a steal if you can get it up and running."I've been told to check the power and filter caps, check the tube sockets for shorts, and that the PT might be bad" That's definitely were you should be looking.Hopefully it's not the power transformer though?Is that extra wire shrink wrapped/taped at the ends of the taps on the PT going into the power board?They don't look like the cloth covered originals?Maybe something to look at?
Incidentally my Mark IIB was doing the same thing when I got it even with tubes removed but I was putting it in for a full recap anyway and after the new filter caps were installed and a general service it was fine so I never did find out the exact cause but I suspected a shorted filter cap although the larger 220uf caps in yours are already new/replaced Mesa caps.
Good luck.

 

[darkbluemurder]

The diodes appear to be OK.

Next things to check would be the filter caps in the power supply for shorts as already mentioned.

In order to check the power transformer you have to disconnect any secondary wires from the circuit, tape them so that they cannot short against anything, put in a new fuse and then power up. If the fuse blows again, you have a bad power transformer.

Good luck,
Stephan

 

[bluelair]

Thanks. I want to do this right. Built a current limiter and got a variac.

I connected it with tubes in and a fresh fuse via the current limiter and got it powered on. The current limiter bulb light glowed bright indicating a short.

1. Pulled tubes - CL still bright
2. Disconnected Power Transformer secondaries from circuits - CL still bright
3. Disconnected Power transformer primaries - CL bulb went out.

I think I'm on to something and it's seeming like it may be the power transformer. Anyone know where to source one?

Stamping on the top says:

MESA
60-162318
EIA606-838

 

[lovetoboogie]

I think I'm on to something and it's seeming like it may be the power transformer. Anyone know where to source one?

Stamping on the top says:

MESA
60-162318
EIA606-838

http://www.classictone.net/40-18005.html

 

[bluelair]

I think I'm on to something and it's seeming like it may be the power transformer. Anyone know where to source one?

Stamping on the top says:

MESA
60-162318
EIA606-838

http://www.classictone.net/40-18005.html

Thanks, so with this one, the total voltage is around 680v, that is different than the 448v that the original PT had, does that higher voltage pose an issue? Do I need to add anything else to the circuit?

 

[lovetoboogie]

that PT will give you 330-0-330 VAC...after its rectified it will be 450VDC... its the correct replacement...

 

[lovetoboogie]

Do I need to add anything else to the circuit?

No, its wired as the original...

 

[newholland]

i'd start with a full retube, and roll. only thing i've ever had to deal with with any of mine was worn tubes in thousands of hours, and one solder joint that i broke on a knob 'cause i overtightened a collar nut due to my own derp.

just bought an '81 iib myself in december, retubed and rolled, and not a hitch since, save for losing a fuse to crap power playing in NYC!

 

[Boogiebabies]

Bias cap ?

The power supply has two new BMI 220uf and two newer sprague and an original yellow from 81. The bias cap is original too.
I have seen these bias caps fail a few times.

 

[woodbutcher65]

Do you not see that there are caps as old as 1985 still on the cap board????

The big orange cap, date code 1985, 26th week. 8526, that's what that means.

No way it's still good!

Do you see the corruption under the recently replaced big blue 220 uF Mesa/BMI caps?

The BMI caps are probably good to go. Mesa hasn't used them for very long. But all the other power supply caps need to be replaced, and the board under it needs a serious cleaning. See all that black gunk under it? That's a yellow-green G10 fiberglas PC board and it's covered in puked cap and who knows what else? It needs a hardcore cleaning.


Forget what your DC ohmmeter reads when checking the caps. A cap can be bad to an AC source and still look OK on a DC meter.


Begin with a recap job.

I'd recommend you treat the whole chassis to a full can of GC Electronics Big Bath electronics cleaner. That thing's FEELTHY.


Once you've given the thing a serious cleaning (get a nylon bristle brush, think "jumbo toothbrush" size, and go at it with some good cleaners) and recapped it, THEN you can evaluate the transformer condition which is probably OK. Mesa power transformer failures are very rare.



Now, my opinion....a tech that won't work on Mesas because "they're too compact" wouldn't get any of my business no matter what amps I have. So what if they're compact? There's room to work and if working in tight spaces is not for you, you can't claim to be an electronics technician in my book. I do electronics repair work UNDER A MICROSCOPE AT TIMES. I fix iphones and other smartphones as one of my income sources. High density surface mount components are no problem if you have the tools and the mindset for it. And there's nothing resembling high density electronics in even a Mesa.

 

[boops]

Ive repaired a mesa mark llB in 1989. it killed fuses too it was a micro cut on trace,on the pcb. B+ on the power tubes.

 

[woodbutcher65]

So, fifteen months later, how's this amp now?