messa mkiv and capacitor problems

[mellowamoya]

Hi cyclone,
Yes I got the to5 last Friday and spent the weekend cleaning it up and calibrating it had to replace the 0.02 and the 2 if caps and it calibrated up very nicely so very pleased. I'll get back to the Messa later this week
Thanks
Dennis

[J M Fahey]

I don't have access to those cap testers, but I built a simple cheap voltage tripler which can be attached to any tube amp I have on the bench (I'm a great believer in "borrowing/stealing" supply voltages from anything avalable rather than building a seldom used dedicated supply) which usually gives me some 1500V DC , GREAT to test capacitors and winding-to-core insulation in transformers.

4 crocodile clips connect it to what's needed: 2 across some HV transformer winding and ground, 2 supply 1500V to whatever is tested.

For quick tests I go through a 100k resistor and a neon bulb, as simple as can be.
If precision is needed, I add a resistor from cap to ground and measure leakage current across it.

[cyclone]

Wow that is a staggeringly simple solution. I love it!! I have a couple of home built pieces of test equipment, so cool. I have a book somewhere that has all kinds of projects like that. I will dig it up and post a link

[mellowamoya]

finally managed to get this horrible amp working had to rebuild a bias supply separately from the main board as there is damage underneath that i cant get to, that worked an have 34ma and 32ma on the tubes which i think is ok maybe a little cool. it worked for 5 minutes and then a very loud hum would appear let it cool down and then after 5min hum back again, this time found one of the new jj 12ax7 tube to be at fault after replacing thew amp is now working but i find it difficult to get a clean tone, if i inject a pure sine wave it sounds pure and the scope shows a good waveform but when i put a guitar into it, it just isn't clean as i would have liked. does anyone know it a mainboard would be available for replacement.
anyone have any thoughts on this

[Chuck H]

If you're talking about that awful crackle on heavy, bassier notes, even playing clean, I've noticed that a lot of Mesa Mark amps do that. I don't know why and I've never been able to stop it with circuit value changes.

I'm just posting to offer empathy regarding the bad tube. I've built amps, all new tubes in the circuit, fire it up for the first time and MMMMMMMMMMMMMMMMMMMMMMMMM!?! Of course the first thing one suspects is a ground fault. Half a day of focused examination and one headache later I found out it was a bad tube. New out of the box and bought from a reputable distributor (if one can say that about any of them anymore). I always suspect tubes first now. Even new ones and even when there's a real possibility of a circuit error. And I always buy extra 12ax7's now because I fully expect one or two to be unacceptably noisy or headed for a quick death. This works out to be the case too often. Wouldn't it be unacceptable if every time you bought a bunch of carrots, two were rotten?!? Or if you had to buy seven quarts of oil to do a change in your car because you know two of the quarts will be full of water?!? Seriously! I hate that we have to accept this shameful service from tube vendors. The problems for me have graduated to power tubes as well as preamp tubes. I'm spending the extra money anyway because "I" am tossing tubes. Just test the damn tubes and send me good ones. Testing a product for relative goodness absolutely should be the responsibility of the vendor. Especially in a business where there's a time lag waiting for shipping. It's impossible to keep a schedule if don't have immediate control and you can't trust a product. Shameful.

[mellowamoya]

one last stab at this amp it is working but has a lot of crackling and pop,i removed V5 and the crackling is still there so it has to be in the output stage. i changed the plate and grid resistors suspecting a noisy resistor, no change. i thought i could hear a noise from the board, i turned all the lights out in the room and then i could see the problem, on two areas of the board i could see tracking that coincided with the noise. i cleaned the board off as much as possible with electrical cleaner but it is still crackling.
I think the board is a write off does anyone know of a replacement board just hoping.

[The Dude]

Is there enough room between the arcing traces to dremel out the conductive board?

[g1]

If there has been arcing, you can't clean it off with electical cleaner. Every bit of carbon from the arc must be scraped off. Or you may have to cut away parts of the board like Dude mentioned.
If there is not room to dremel out between traces, you may have to cut out a chunk of the board and run jumpers where the traces were.
But try scraping away any signs of carbon before doing any surgery.

[The Dude]

^^^^^^What he said. And I'll add: It may not be necessary to dremel through the board. Often times you can remove only the burnt area. I've often simply carved a notch with an exacto knife.

[Leo_Gnardo]

one last stab at this amp it is working but has a lot of crackling and pop,i removed V5 and the crackling is still there so it has to be in the output stage. i changed the plate and grid resistors suspecting a noisy resistor, no change. i thought i could hear a noise from the board, i turned all the lights out in the room and then i could see the problem, on two areas of the board i could see tracking that coincided with the noise. i cleaned the board off as much as possible with electrical cleaner but it is still crackling.
I think the board is a write off does anyone know of a replacement board just hoping.

I won't be much help except commiseration - I've been suffering with a Mk IV doing much the same over the last couple weeks. I think someone spilled something on the amp, it smells really weird when warmed up, best guess is perhaps phô with a good shot of nguoc-mam in it. After swabbing the board with ethanol-soaked cotton swabs, everywhere I could reach and a few places I couldn't (unsoldered some R's C's and photocells & put 'em back after cleaning) there's some improvement but still crackles especially in the first 5 minutes of warmup.

Replaced a roasted 39 ohm resistor first, part of the switching circuit.

As in yours C243 & C244 were suspect as well. I replaced one and that let the bias even-up on the output tubes as well as remove some crackle noise. Another crackler was the 0.1 uF 400V orange drop serving the bass control on the rhythm channels. I'm starting to suspect besides the conductive stinky greasy debris left behind by the soup spill, maybe there's a general breakdown of 0.1 uF 400V OD's in these amps.

Some other things I did to get around the board connections, I picked up one end of each C243 and C244 off the board - the side serving the output tube grids - same with the 220K resistors bringing bias voltage to the output grids - and "air boarded" that is connect 'em above the board so any leaked voltage would not affect the bias. Then straight to the output tube sockets, circuit board no longer involved.

That MkIV board is cram jammed. I don't want to have to replace the whole thing and you don't either I'm sure. And trouble shooting is a nightmare. I'm losing time=money on this one, big time.

Some years back Mesa offered a flat rate amp refurbish deal. Customer paid shipping plus $250 flat fee for parts/labor, amp got thoroughly rebuilt no matter how trashed it was. Wonder if they still offer that deal, because I've been tempted to do it with this amp. Of course it's not practical to ship yours back & forth from Trinidad to Petaluma.

[g1]

He-who-shall-not-be-named used to insist it was common for resistors etc. to arc to traces passing under them in modern mesa product. It would not be visible till the offending component was lifted. In spite of the dubious source of the info, it does sound plausible.

[The Dude]

He-who-shall-not-be-named used to insist it was common for resistors etc. to arc to traces passing under them in modern mesa product. It would not be visible till the offending component was lifted. In spite of the dubious source of the info, it does sound plausible.

.......... and it's all China's fault.......

[Leo_Gnardo]

He-who-shall-not-be-named used to insist it was common for resistors etc. to arc to traces passing under them in modern mesa product. It would not be visible till the offending component was lifted. In spite of the dubious source of the info, it does sound plausible.

B'lieve me that was on my mind... though he mentioned "arching". Inspecting in dark or lighted conditions revealed no sparks, no arching, no jughead nor betty nor veronica. Anyway while he was still in his short pants playing with boogers not Boogies, I developed the habit of leaving some space under any PC mounted components I'd replaced or removed for inspection. Not that I'm any fan of Mesa's cram-jam-blivet pc board construction style. OTOH I did have a positive run-in with a Mk IV earlier this year, just needed speaker reconed & when done it sounded a treat, unusually good to my ears. Which goes to show they can sound terrific when working right, & I'm sure even better if someone with some real talent was playing, not yours truly mister fumble-fingers.

The amp in question has been warming up a couple hours in my livingroom with radio playing thru it. Sounds OK, strangely the footswitch will select channels but the rotary switch on back won't. Still got that fish-o-baby fume cloud surrounding it. No it wasn't in use at the local strip club.

[mellowamoya]

thanks for all the input and advice the one arcing is fixable as it is next to c453 but the other one apears to be on the other side of the board somewhere near the pad of c243 the end that conects to v5 pin6 c243 is soldered to the pin6 of v5 as that is where the original problems started as a high resistance track from v5 pin6 to the pad of c243, if i could remove the board i would be able to cut out the bad tracks and clean the board up and respray it but it looks just not worth the effort im too much in a hole on this one with little chance of recovering what has been spent so far, as they say you win some and you loose some.
there is just too much packed on the board and i thing the spacing is very minimal for the voltages involved.
cheers
dennis

[Leo_Gnardo]

it looks just not worth the effort im too much in a hole on this one with little chance of recovering what has been spent so far, as they say you win some and you loose some.
there is just too much packed on the board and i thing the spacing is very minimal for the voltages involved.
cheers
dennis

You & me both dennis. Even if Mesa would sell us each a circuit board it would be a long long fezzle swapping it in. And who's to say it would be an improvement. Too much stuff stuffed into that box. Then they came up with the new improved Mk V, 47 relays! Hevvin help us!