MKIII Blue Stripe - Tech Help neded

[fdesalvo]

Hi, all.

Working on a customer's MKIII. Scratchy volume pot is making mew nuts.

- Electrolytics test good
- Voltage at sockets measures good
- Replaced PI coupling caps
- Replaced C505/506
- No DC on pots
- Subbed in new volume pot - no change
- FX jacks clean
- All tubes test good

Any tips? TIA!

 

[fdesalvo]

Sorted! TY for entertaining my madness. Leaking tone stack .1 cap. Was a dead short!

 

[Henz]

Sorted! TY for entertaining my madness. Leaking tone stack .1 cap. Was a dead short!

I have that same problem with my C+. My bass and mid pots have no DC ( well, .2 to .3 mv but that's no biggy). You said that you had no DC on pots. How did you discover? Did you lift one leg of orange drop and measure? The MK III schematic thats out there, how the heck can you read it!? The values are so blurry. And how accurate is it? Hey while im here, does the Volume 1on the III your working on make a slight popping sound thru speakers when push/ pulling bright? Mine does. With respect to both amps, V1A and V1B circuit are just about identical. I did everything except try another pot. I disconnected the 180pf cap thinking that was it . nope. Thanks! Jim

 

[fdesalvo]

Hey I was mistaken when measuring Initially. I measured the grid directly the second time and saw voltage rising as the volume was wound up. This rise accompanied a decrease then swell in the level of noise. Eventually passing no signal about halfway up. After lifting the leg of the .1, I measured a dead short. I’d get those creamy caps out of yours. It’s just a matter of time, like all those awful 400v of caps placed on decoupling and pi tasks.

As to the pop, that’s pretty normal even with old fenders as the cap is discharging or charging, it will pop.

The schematic that’s out there is erroneous as most are even from the factory. I found a version that was made by a dude who blue printed his mkiii. Super helpful cat. When I get home I’ll see if I can attach it here.

 

[Henz]

Hey I was mistaken when measuring Initially. I measured the grid directly the second time and saw voltage rising as the volume was wound up. This rise accompanied a decrease then swell in the level of noise. Eventually passing no signal about halfway up. After lifting the leg of the .1, I measured a dead short. I’d get those creamy caps out of yours. It’s just a matter of time, like all those awful 400v of caps placed on decoupling and pi tasks.

As to the pop, that’s pretty normal even with old fenders as the cap is discharging or charging, it will pop.

The schematic that’s out there is erroneous as most are even from the factory. I found a version that was made by a dude who blue printed his mkiii. Super helpful cat. When I get home I’ll see if I can attach it here.

Ty for your reply. You're way more advanced in electronics than i. I don't quite understand how you found it. I'm assuming it was pin 7 on V1B? Are you referring to the orange drops when you say creamy? lol. Yep! Lots of techs scoff at Mesa for using 400 v caps when a few more cents could get ya 600 v's . If I change out my tone stack caps to 600 v's, they won't fit. Too tight. Unless there's a different polypropylene available now that's smaller in size. My preamp pcb has all 400's with varying physical sizes. They are all original from 1984. Some Mallory's, some Spragues. My PI circuit has a combo of 400 and 200 v caps. Again, all original. Though I do have two CDE .1uf 6PS's 600v for the coupling to output tubes. I had them in but I put originals back in. Don't ask me why. I have bad OCD and I thought I should put originals back in. . If you were me, would you shotgun every cap and replace with higher voltages? And thank you for popping issue! That always bothered me. Now I know its normal. And I really don't need a MKIII schematic cause I don't have one. Wish I did though! Picts of tone stack and larger scan of pcb's. Thank you for your time brother! Jim

 

[fdesalvo]

Lol leaky! Darn speech to text converted it to creamy lol. I’ll make some additional comments after I get these toddlers fed and put to bed!

 

[Henz]

Lol leaky! Darn speech to text converted it to creamy lol. I’ll make some additional comments after I get these toddlers fed and put to bed!

Lol. God bless you and your family. . Take your time. I'm in no hurry. I just love to talk with techs. I learn so much!

 

[fdesalvo]

Alrighty. You can get quality polypropylene caps that are significantly smaller and sound fabulous in critical positions. I was a huge skeptic until I repaired a fender clone that was built with them. Amazing tone and led me to believe that cc plate resistors, paper interleaved output transformers, and quality speakers are the big contributors.

That said, consider where the caps are and the voltages they are supposed to withstand. While 630v caps may be overkill in tone circuits, modern ones are very compact, so why not? If I were you I’d put those 630v caps back in the PI decoupling slots. The 400v oem will fail and take your power tubes with them. You’ll end up with dc on your grids. Yikes!

 

[Henz]

Alrighty. You can get quality polypropylene caps that are significantly smaller and sound fabulous in critical positions. I was a huge skeptic until I repaired a fender clone that was built with them. Amazing tone and led me to believe that cc plate resistors, paper interleaved output transformers, and quality speakers are the big contributors.

That said, consider where the caps are and the voltages they are supposed to withstand. While 630v caps may be overkill in tone circuits, modern ones are very compact, so why not? If I were you I’d put those 630v caps back in the PI decoupling slots. The 400v oem will fail and take your power tubes with them. You’ll end up with dc on your grids. Yikes!

Ty for your reply! I'll put those back in ! Hey brother, I gotta ask ya one more question then I'll find another tech's mind to pick on . It may be a difficult question to respond to since there are so many amp designs but you mentioned putting good caps in critical spots to achieve optimum tone. Could you elaborate just a lil on that regarding a Boogie C+ ? But the old adage says "if it ain't broke, don't fix it. " I'm A perfectionist and would LOVE to tweak my amp to get the best tone! Where would these critical spots be? When you said carbon comp plate resistors, was you suggesting that they be used or not? Theres this tech i follow on YT and he despises Mesa Boogies! lol. He cant understand why Mesa uses carbon comp' s and low value caps. And I totally agree that the speaker plays a critical role in tone. That's the last frequency filter in the chain. Would Mouser have these better caps and be physically smaller? That's more than one question. I apologize!! Thank you for your time! Blessings, Jim.

 

[fdesalvo]

Lmao you must be referring to Aussie Brad!

Man, thats a deep philosophical question for certain— and no two dudes agree. In my opinion, discreet components make more of a difference in single channel amps or on clean channels, where there aren’t tons of snubber caps lopping off high end from the preamp. Still an amp is the sum if its parts, so I’d say that for my money the most sensitive cap placements are in decoupling positions.

Panasonic ECQ series are AMAZING in all positions. The little, magical, Blue Molded Ajax caps in vintage fenders are truly special. Truly. The ECQs sound JUST LIKE THEM. Additionally, they have very compact form factor! Look on eBay - you will often find sellers getting rid of lots of these for a great price. I have a biz agreement with a major parts supplier in Arizona and usually get parts for a nice discount, but still peruse the ‘bay for special deals. I gobble up all the common values I can find (.001, .003, .01, .02, .047, etc). My customers love love love them. And they are very reliable. When I sometimes have to deliver bad news about their prized pi caps, their anxiety is vanquished once they play the amp.

With respect to resistors, I like carbon film for most positions, except for plate loads, where I hugely favor carbon comps in the 1/2-1w range; there‘s a non-linearity in the way they respond to operating conditions that’s adds a little something. It’s impossible to describe, but there’s something missing when they aren’t there. When I’m doing vintage restos, I never sub in any other type for the preamp plate supply. That’s critical duty.

But - all this is just one guy’s worthless opinion.

HTH

frank
@ forgotten gear restorations

 

[Henz]

Lmao you must be referring to Aussie Brad!

Man, thats a deep philosophical question for certain— and no two dudes agree. In my opinion, discreet components make more of a difference in single channel amps or on clean channels, where there aren’t tons of snubber caps lopping off high end from the preamp. Still an amp is the sum if its parts, so I’d say that for my money the most sensitive cap placements are in decoupling positions.

Panasonic ECQ series are AMAZING in all positions. The little, magical, Blue Molded Ajax caps in vintage fenders are truly special. Truly. The ECQs sound JUST LIKE THEM. Additionally, they have very compact form factor! Look on eBay - you will often find sellers getting rid of lots of these for a great price. I have a biz agreement with a major parts supplier in Arizona and usually get parts for a nice discount, but still peruse the ‘bay for special deals. I gobble up all the common values I can find (.001, .003, .01, .02, .047, etc). My customers love love love them. And they are very reliable. When I sometimes have to deliver bad news about their prized pi caps, their anxiety is vanquished once they play the amp.

With respect to resistors, I like carbon film for most positions, except for plate loads, where I hugely favor carbon comps in the 1/2-1w range; there‘s a non-linearity in the way they respond to operating conditions that’s adds a little something. It’s impossible to describe, but there’s something missing when they aren’t there. When I’m doing vintage restos, I never sub in any other type for the preamp plate supply. That’s critical duty.

But - all this is just one guy’s worthless opinion.

HTH

frank
@ forgotten gear restorations

Ty for your reply. Yes , Aussie Brad. I just recently found him but his friend Lyle from Psionic Audio. Lyle ******* at Mesa constantly for using inferior components. When Lyle gets Boogies in, he immediately changes the carbon comp power tube plate resistors to metal oxides. And always complains as to why they didnt use 600v caps. Do different caps really make a huge difference in tone in the pi circuit? In my humble, non educated mind, from watching many videos, decoupling caps are used to block any DC to getting through to the next stage. Looking in my amp and the schematic, every orange drop is a coupling cap . Please, correct me if I'm wrong. So where would a more sensitive cap be? In an amp book that I have, the author states when tweaking your amp, leave the preamp plate resistor alone. I'm assuming he meant values. Mine came from factory with carbon films on preampies. But you like cc's there? Back to the pi. Mine has a long tailed pair circuit. Would changing to a different type of cap change tone? And, your opinion is not worthless . Blessings, Jim. This link may work, I dunno. Its my schematic.