81 Mesa Mark IIB troubleshooting, changed PT, tubes, caps

[bluelair]

I bought a nonworking Mark IIB that was blowing fuses. Immediately determined the PT was bad, replaced with a Mercury 80142 recommended by Mesa.

1. Powered on (with Variac) power on and amp produced sound, but it was sounding very poor sounding, sound was scratchy, harsh, would oscillate, and needed high volume and strong strum on the guitar to make any.

2. Next changed out the six large caps with Mesa and Sprague caps, powered on (always use a variac). (this is a before pic).

3. No improvement.

4. Next changed all six tubes (2 6l6GC, 4 12ax7) with known good ones.

5. No improvement.

6. Checked grid resistors on the power tubes, they tested within spec on the schematic (470 and 2.2k ohms)

7. Other observations: Pots do not crackle, they are smooth and silent. Ground switch is set to center setting.
I also noticed when I am powered up at 120 vac, then I use the variac knob to power down, at around 30-40 volts on the variac (on the way down), the amp wakes up and sounds like it should for about 3-4 seconds, I get perfect clean tones and it sounds great, as if in that "zone" of shutting down it's getting exactly what it needs. Does this tell us anything? I only noticed this after I replaced the tubes. I can do it repeatedly, power on slowly with variac, come up to power, get the harsh, grating sound, then back down the variac, at around 40 volts, you hear the normal hum from the speaker intensify and you can strum and hear what it should sound like normally, then that sound fades away as if it's discharging after about 3-4 seconds.

What should I try next?

 

[b00g13mk2B]

Easy thing to try is a guitar cord between the pre amp/ power amp junction underneath the chassis to the power amp in on the back. Check all the push-pull pots and the footswitch. Do you have reverb?

The large light blue cap on the preamp board might be going bad. If you have a boost pedal or a preamp out from another amp, drive the MKIIB power amp in with it

 

[MK IIB]

Another thing to try(although it wouldn't explain the amp coming back to life when powering down)is to run a small patch cable between the effects send and return jacks as these are known to get dirty or corroded and can cause problems.

 

[McBarry]

a few thoughts..
have U verified ALL voltages against schematic are correct and stable ?
Physical layout - has anything been re-routed - lots google pics of chassis shots that can help U..
Is oscillation pre-amp or power amp?
Consider that something caused the original p/trans to fail.. Speaking of which, what actually failed in it - prim or secondary..? May help diagnosing.

Nice amp though.. congrats.. !!

 

[woodbutcher65]

See those black devices on both sides of V1? Those should be a TCR 5302 and TCR 5304 current limiting diodes.

Replace them with 1.5K resistors. That'll tell you if those diodes are bad, and why those diodes were there to begin with is pretty much anybody's guess. I don't see the point of them, given that they're not in anything else. It wasn't in the Mark 1, or any other version of Mark II, or the Mark III, so I say you don't need those diodes.


If you had an oscilloscope handy you could find out where the signal goes south in no time.


Other tests: Using another amp if you have one available, you can check the amp at the preamp out/power amp in jacks. See if its preamp out works when connected to your other amp.

Then, do the reverse, connect your second amp's preamp out to the Mark II's power amp in. This way you can split the troubleshooting process in half.

As for caps, I recommend getting all replacement caps directly from Mesa. They're going to be fresh, on spec, and they're cheap enough that there's hardly any point in trying to shop around.

Do replace that purple electrolytic on the preamp board.

 

[lovetoboogie]

See those black devices on both sides of V1? Those should be a TCR 5302 and TCR 5304 current limiting diodes.

Replace them with 1.5K resistors. That'll tell you if those diodes are bad, and why those diodes were there to begin with is pretty much anybody's guess. I don't see the point of them, given that they're not in anything else. It wasn't in the Mark 1, or any other version of Mark II, or the Mark III, so I say you don't need those diodes.


.

Bad Info...

Those black passive components on both sides of V1 are the high quality dry tantalum cathode bypass caps. They are already paralleled with 1.5k resistors. They are fine and as long as both cathodes (pins 3&8) measure roughly 1500 ohms to ground, that whole matrix should not be touched. No diodes in this V1 section...

Good info...

Isolating either preamp or power amp problems utilizing the preamp send or power amp in...

 

[woodbutcher65]

Well, it's been forever since I've been inside a Mark II.

Originally I thought this was a IIA, my mistake, I should have had my glasses on when reading.

There should be TCR 5302 and TCR 5304 current limiting diodes in a Mark IIA and they're only there to make the circuit compatible with fetrons. Remove them if you'll never use a fetron. They serve no purpose without them. They're not present in a IIB.

Those "high quality" tantalum capacitors are still dry tantalum capacitors and dry tantalum capacitors most certainly have a limited service life. When they go bad, they become a dead short. Meter them out. If they're shorted, yank 'em.

I've been servicing electronics for well over 30 years. I've repaired stacks of Hewlett Packard test equipment that had failed because of tantalum capacitors shorting out. Yank and replace, verify proper operation, bill the customer. Routine business for me when I was working full time at a repair shop.

One of my cardinal rules as a tech is "Never trust a tantalum capacitor. Period."

The first test is, as I suggested before, isolate the problem to the preamp stage or the power amp stage.

This requires another amp to work with, or at the very least, some effects that can be inserted in a loop.

Either the trouble is in the preamp stage, or it's in the power amp stage, or if it's both, then it has to be a power supply system issue.


At this point with the original poster's amp I'd troubleshoot by isolation. Apply a clean low level input signal and follow the signal chain, and scope out the signal from stage to stage. I recommend a simple 1 KHz sine wave. Set up for clean. At what point does the signal's expected level change drastically or distort?

Given the symptom I'd say the trouble starts early, probably within the first or second stage. I would again make it a point to check out those tantalum capacitors. I'd just unsolder one end of them and lift them off the board and test. The amp will run without them but the bass level will change. Fine for testing.

If the power transformer was bad and has been changed, is the condition of the output transformer known? Is IT good? The sounds reported by the OP could certainly be said to match up to the sound of a bad output transformer, too.

That's why preamp/power amp isolation and testing is absolutely task no. 1 to perform. No sense spending hours tracing a preamp problem when it's the output transformer that's the real issue.

 

[lovetoboogie]

...That's why preamp/power amp isolation and testing is absolutely task no. 1 to perform. No sense spending hours tracing a preamp problem when it's the output transformer that's the real issue.

Couldn't agree more. Hopefully the original poster is making some headway with this. It's hard to do the troubleshooting over the internet based on the symptoms. Skeptical if the replacement PT was wired correctly or is the correct one. It would help if we had B+ voltage readings from the rectifier downstream. Sounds as if something is getting starved.