Seeking MarkII tech advice

[Cheapshoes]

Hello all; I'm new here and totally new to Mesa-Boogie amps. Very glad to see there is an active forum for these. I ran accross this very road-weathered Mark IIb (1980 on the chassis), and the amp has a few technical issues, but I saw it's potential as the tone is in there - just need to fix a few things. I'm asking for some kind help with technical advice. I took it in for repairs locally before doing anything, and although they fixed the reverb, they missed some huge things which is surprising. Volume 1 knob is still very sensitive (after about 6), but worse, there is a ugly crackling on certain chords and settings that is accompanied with a variable loss in fequency and volume that comes in-and-out rapidly while holding the chord. If this is as simple as bad resistors, or exhausted caps I'm game to order the parts and fix myself and save the hassle and money. Though - I think the tech thought the filter caps were still good. I haven't changed the 6L6s, but these are Sovtek WXTs, which should be built like a tank.

I'm not formally schooled in electrical tech, but understand the basic workings, have done several mods to other amps following guidance. So - what do you thinks this is? At first, I was thinking bad tube connections, but at least the 12AX7s seem tight. Also, curiously this uses 12AX7s in all preamp slots, no AT7 listed on the chassis for any slot. Appreciate any advice/simple diagnostic tests, suggestions what to fix. I also noticed the schematics on the web are all for the 100W +EQ version - mine is the 2x6L6, +reverb, no EQ version.

A lower priority are the cosmetics. The "cream" tolex is trashed and can be just peeled off, but leaves a greenish glue underneath. I think I saw on one of these posts just doing a heavy sand-job and varnishing the birch plywood as-is. That seems very appealing low -tech solution. If I want to put on new grill cloth - do those four screws in the front release the baffle so I can do that?

I know this is a lot Thanks!

 

[guitarzan1143]

your power tubes may be going. Thats what the crackling/fading in and out problem probably is, either that or your speaker is having issues or your OT is going. Get some mesa tubes, i think reds are what the mesa tech recommended me for early Mark series amps, your tone will be much better with mesa tubes regardless.

 

[acorkos]

the problem you are experiencing could be due to any number of things. first thing i would do is clean the effects loop jacks, or simply connect a jumper cable from the effect send to the effects return jacks. next, tubes are the easieist to rule out....swap in a new set and see if the problem is gone. but before you spring for a new set, make sure the power tube sockets are tight...flip the cab upside down, pull the power tubes, make sure the caps are drained (you have a DMM, right?) and tighten each socket pin element. reinstall your tubes and see if the problem remains. after that, i would try another set of power tubes. the problem you are describing could be due to one bad solder joint inside the chassis, but like i said, anything's possible.

you need to look this over, too:

http://www.geofex.com/ampdbug/ampdebug.htm

 

[acorkos]

12AX7 tubes are specified for all preamp tube positions, but a 12AT7 can be subbed in V2 (second from the right looking from the back) for cleaner operation

IIB Maintenance and Repair manual

http://www.srl.utu.fi/~tke/MesaBoogie/Manuals/Maintenance_and_Repair-1.gif

http://www.srl.utu.fi/~tke/MesaBoogie/Manuals/Maintenance_and_Repair-2.gif

http://www.srl.utu.fi/~tke/MesaBoogie/Manuals/Maintenance_and_Repair-3.gif

http://www.srl.utu.fi/~tke/MesaBoogie/Manuals/Maintenance_and_Repair-4.gif

 

[acorkos]

remove the 4 grill screws and the grill comes off

 

[Cheapshoes]

Thanks, lots to look at over during the next week, will report back when I look over these links and know more. What does the effects loop test do? CS

 

[acorkos]

What does the effects loop test do? CS

your guitar signal runs through a switch on the effects return jack. if the switch contacts on the jack are dirty/intermittent, you lose signal/volume and get noise and static. connecting a cable between the 2 jacks makes solid contact and eliminates the loop jacks as a source of your problem (or confirms that they are the problem)

 

[Cheapshoes]

Well, yesterday I checked the effects loop and that wasn't the issue. However, one of the 6L6s was definately not tight fitting and made sure it was in there good and didn't experience the problem. Could be a coinidece, but I'll definately tighten the tension on the sockets - that one in particular. I asume one can just bend the contact points with a fine tool inward a bit? Good idea to spray some contact cleaner/lube or leave alone?

I noticed some of the 30uF/500V caps are buldging and cracking at the ends. I ordered these from Mouser: Sprague 75-53D265 (240uF/300V), and 75-TVA1907 (30uF/500V) - closest I could find to replace the original Mallory 220uF/285V and 30uF/500V original caps. I hope these are decent. The caps were not cheap: $12-13 a piece, and there's five on the main channel, and one over near the input jack.

As far as tubes, the Mesa site doesn't have any indication or mechanism to order a particular color, it just says yes or no place the order. If I go to the Tube Store instead, what would you recommend? Or is it better to go with the Mesa 6L6STR-440 (with no "color" option)? :?

Also, a question about all the handwritten sharpie markings. From what I gather on the forum, that's a neat bit of historical context, but I don't know what they mean exctly. "M3" is initialled underneath, "SR" in big letters on both sides of the chassis, and "R" is checked on the boards. Are all these the QC check, the builder, or code for something about this amp? Serial is 6xxx, if that helps.
Thanks.

 

[dodger916]

Well, yesterday I checked the effects loop and that wasn't the issue. However, one of the 6L6s was definately not tight fitting and made sure it was in there good and didn't experience the problem. Could be a coinidece, but I'll definately tighten the tension on the sockets - that one in particular. I asume one can just bend the contact points with a fine tool inward a bit? Good idea to spray some contact cleaner/lube or leave alone?

I noticed some of the 30uF/500V caps are buldging and cracking at the ends. I ordered these from Mouser: Sprague 75-53D265 (240uF/300V), and 75-TVA1907 (30uF/500V) - closest I could find to replace the original Mallory 220uF/285V and 30uF/500V original caps. I hope these are decent. The caps were not cheap: $12-13 a piece, and there's five on the main channel, and one over near the input jack.

As far as tubes, the Mesa site doesn't have any indication or mechanism to order a particular color, it just says yes or no place the order. If I go to the Tube Store instead, what would you recommend? Or is it better to go with the Mesa 6L6STR-440 (with no "color" option)? :?

Also, a question about all the handwritten sharpie markings. From what I gather on the forum, that's a neat bit of historical context, but I don't know what they mean exctly. "M3" is initialled underneath, "SR" in big letters on both sides of the chassis, and "R" is checked on the boards. Are all these the QC check, the builder, or code for something about this amp? Serial is 6xxx, if that helps.
Thanks.

I ordered tubes over the phone and got the color I wanted.
The "M3" initial is probably really MB, for Mike Bendinelli. The large "SR" on both sides of the chassis should refer to the model you have. "SR" would indicate a 60 watt with reverb; no graphic eq.

 

[acorkos]

if you are comfortable modding amps and performing cap jobs, then buy yourself a nice pair of SED =C= 6L6GC's and swap the bias resistor as needed. a fine pointed metal probe to slightly tighten the metal tube socket elements and a shot of contact cleaner and you are good.

 

[Cheapshoes]

Thanks Dodger, yep, that's the amp. Two 6L6s, reverb.

Acorkos: I'm comfortable only to a point; the two big 470ohm carbon resistors of one pin - right? Each are holding at about 510 ohms - would they need to be changed for the SEDs? Otherwise, I'll probably just let-be, and order the Mesa tubes and not mess with a groovy thing. I'll call Mesa and mention what amp and see what they say. I misread their pricing earlier and thought it was $42 a tube which was outrageous if these are made in China, but now see it's $42 a pair.

 

[Boogiebabies]

Well, yesterday I checked the effects loop and that wasn't the issue. However, one of the 6L6s was definately not tight fitting and made sure it was in there good and didn't experience the problem. Could be a coinidece, but I'll definately tighten the tension on the sockets - that one in particular. I asume one can just bend the contact points with a fine tool inward a bit? Good idea to spray some contact cleaner/lube or leave alone?

I noticed some of the 30uF/500V caps are buldging and cracking at the ends. I ordered these from Mouser: Sprague 75-53D265 (240uF/300V), and 75-TVA1907 (30uF/500V) - closest I could find to replace the original Mallory 220uF/285V and 30uF/500V original caps. I hope these are decent. The caps were not cheap: $12-13 a piece, and there's five on the main channel, and one over near the input jack.

As far as tubes, the Mesa site doesn't have any indication or mechanism to order a particular color, it just says yes or no place the order. If I go to the Tube Store instead, what would you recommend? Or is it better to go with the Mesa 6L6STR-440 (with no "color" option)? :?

Also, a question about all the handwritten sharpie markings. From what I gather on the forum, that's a neat bit of historical context, but I don't know what they mean exctly. "M3" is initialled underneath, "SR" in big letters on both sides of the chassis, and "R" is checked on the boards. Are all these the QC check, the builder, or code for something about this amp? Serial is 6xxx, if that helps.
Thanks.

Call Mesa and order them. They sell BMI filter caps and they are great.
As an alternate, I would use F&T from Tube Depot.

http://tubedepot.com/electrolytic.html

 

[lshred365]

First off change all tubes Pre & power. Since Boggie doesn't have STR-415 anymore just get the best matched set you can afford. And yes the capps need to be checked, clean potts & don't forget to spray the RCA connects for reverb.

 

[Cheapshoes]

Thanks for everyone's advice. I've changed the filter & bias caps, new reverb tank, sprayed Fader lube in all pots, new tubes throughout, dusted off the speaker, and at least tried to retension the tube sockets. Sounding much better - However, it still has an unpredictable intermittant nasty crackle that appears to be vibration induced (certain resonant notes, and bassy chords at volume). Not every time, not every note. Should I place the greatest suspect on the 6L6 sockets and clean better with a toothpick & rubbing alcohol, and try more assertively to get those contacts tighter? The screen resistors appeared sound, and I didn't notice anything abviously burned or cracked, but didn't know where to focus more intently. :wink:
Thanks

 

[acorkos]

if the problem is coming from a power tube socket pin, you probably would be able to reproduce the problem noise by wiggling the power tube(s). you may have a bad solder joint somewhere on the PCBs....if you're game you can try to locate it by running the amp (speaker connected) with the chassis open and poking at every solder joint with a wooden stick...touching the bad joint should generate the problem noise.

 

[Cheapshoes]

I tighened the 6L6 sockets a bit, and the crackle is definately not from the tube sockets anymore. Tried the wooden spoon handle test suggested by acorkos, and the only thing happening was the Volume 1 pot was very microphonic. Some connections picked up every tap like a microphone -- I reheated those solder joints, reassembled and fired it up. The other thing I did was insert an "open" guitar cable plug into the socket for the gain boost footswitch after talking to Mike at the Mesa shop; easiest fix for the gain boost is to take it out of circuit if you don't need it. This got me 95% of the way there, but got a litttle crackle once or twice, but I really wound it up and sounded great and didn't hear more.

I need the confidence I've eliminated all crackle - so the real solution is to replace or rebuild both the Volume 1 and Gainboost pots. Mesa said the push-pull pots don't stand up over an extended time to the vibrations in these combo amps, and so they avoid them now and do not carry replacements. Volume 1 has funky resistance going on too (goes up to over 2M in the middle, then jumps back to 330K or so at max. Would be nice to find stout 1M push-pull pots rather than try my shaky-hands fiddling with thin fragile carbon disks and bushings or whatever is involved in a rebuild (Mesa suggested a rebuild if I was patient).

 

[Restless Rocks]

That volume pot needs replaced. Try here: http://www.mojotone.com/amp-parts/CTS/CTS-push-pull-1MA-Potentiometer-Volume-Tone