Question about Markbass Big Bang 500

[tim]

Hey all...it's been a while since I posted here and was very saddened to see that Enzo is no longer with us. I will miss his avatar which showed his type of humor. He was a bottomless pit of information and he will be sorely missed. He didn't suffer any fools yet he was immensely patient with those that were truly seeking answers to amp problems. RIP Enzo

That being said, I came into a Markbass Big Bang 500 recently for very short money and I knew it had intermittent noise issues going into it. I was able to suss out 95% of the issues but it has noisy pots, I guess this is not an unknown issue with these. Unfortunately, the pots are sealed and the values are scraped off . I guess MB does this on all their parts (what are they trying to hide?). Most of the issues were oxidized connectors which I was able to clean with DeOxit.

I would like to replace all the pots but I want to order them before I tear into the amp. I know I could pull the two preamp boards and physically measure the value of the pots after pulling them from the two preamp boards but I'd like to keep using the amp until such time as I can get the pots on my bench.

The best thing would be to have a schematic in hand but after reaching out to MB and a half dozen authorized service centers, I guess that's not going to happen. I think most of them were afraid of a Mafia hit if they passed one along to some peon. If someone here has a schematic, could that person pass along what value pots are where? That would be immensely helpful and I would be much obliged,

Thanks,
Tim

[xtian]

I've had some success letting DeOxIt D5 soak into sealed pots thru the shaft. Might give it a try. Orient shafts upright, so gravity helps the D5 slide in.

[tim]

I've had some success letting DeOxIt D5 soak into sealed pots thru the shaft. Might give it a try. Orient shafts upright, so gravity helps the D5 slide in.

Thanks...I used to have several adaptors I made many moons ago that screwed onto the threaded portion of different pots and then the other end was a tight fit for a D5 straw and that would probably work perfectly except....after my move from NH to MO two years ago, it has been misplaced. Unfortunately I no longer have access to a machine shop so I will either have to wait until it is located or try to make another one minus the milling machine or just try what you suggested.

[xtian]

D5 is the formulation with solvents. It has low viscosity, and seeps quickly into cracks. Give it a try!

[Greg Robinson]

I've only a couple of times needed to resort to replacing Markbass pots. Usually rotating them back and forth several dozen times clears them up just fine. Certainly worth a try before doing anything else.

[tim]

After going through most of my repair parts/equipment, much of it still in boxes from moving two years ago, I found one of my adaptors...unfortunately it fits a 3/8" pot...I need one for 7mm. I have a 7mm tap so I may try and make one...otherwise, there is a guy on ebay selling a set of these adaptors as well as selling single ones so I may end up just buying one that will fit the threads on these pots and be done with it.

[Helmholtz]

Are you sure the pots can not be disassembled?

[Greg Robinson]

Are you sure the pots can not be disassembled?

Markbass use 9mm pots which are sealed. They CAN be disassembled, but it requires drilling out swaged rivets rather than bent tabs like on 16/24mm pots, which then need to be replaced. Easiest way I've found to secure them back together is 18AWG solid wire bent over at the ends and trimmed to not intefere with the bushing mounting plane. If you can find them in the right size (long and skinny), bootlace ferrules can also be made to work and give a more "professional/stock" look, but are tricky to swage without setting up a jig, not worth the effort IMO.
Given these pots are so cheap anyway (Markbass typically uses more 50kB than any other value IME), unless it's some oddball value that you can't find or has some unusual feature like multiple detents that you can't purchase somewhere, you're better off just replacing them rather than trying to disassemble and clean them. That is if simply rotating them back and forth a few times to loosen up the gummed lubricant isn't enough.

[g1]

There were some pots that needed to be blown out with compressed air rather than cleaner, but I think those were Fishman. Not sure what exactly was so different about those pots.

[tim]

Markbass use 9mm pots which are sealed. They CAN be disassembled, but it requires drilling out swaged rivets rather than bent tabs like on 16/24mm pots, which then need to be replaced. Easiest way I've found to secure them back together is 18AWG solid wire bent over at the ends and trimmed to not intefere with the bushing mounting plane. If you can find them in the right size (long and skinny), bootlace ferrules can also be made to work and give a more "professional/stock" look, but are tricky to swage without setting up a jig, not worth the effort IMO.
Given these pots are so cheap anyway (Markbass typically uses more 50kB than any other value IME), unless it's some oddball value that you can't find or has some unusual feature like multiple detents that you can't purchase somewhere, you're better off just replacing them rather than trying to disassemble and clean them. That is if simply rotating them back and forth a few times to loosen up the gummed lubricant isn't enough.

They actually have an M7 thread on these pots, I was able to fab an adaptor and get those pots cleaned out. They are dead silent now as they are rotated. You're right, the pot style they used on this amp is riveted together...I have refurbished pots many times that have bent tabs etc that hold them together but I wouldn't attempt to disassemble one of these unless I knew it was the last one earth.

Thanks for the Mouser link you sent..., I will check it out.