Mesa Mark 4 Revision B Lead Master not functioning.

[NickB]

Hi, fellow Mesa users. I'm new to the forum and this is my first post.

I've enjoyed quite a few Mesa amps and combos over the years and am currently researching the acquisition of a Mark 4 Rev B wide body combo. I actually bought one but on collection found it had two faults; the reverb was non-functioning and the lead master seemed to be stuck on full. I did not buy the amp as it was advertised as fully functioning and clearly was not. The seller was understanding, provided a refund and apologised so no issues there. On reflection I may still buy it but would need to fix at least the lead master as it's by far the loudest channel with gain and drive up so switching to it produced a crazy hike in volume, even with the other channel masters on maximum. If the reverb proves difficult I can always use a pedal in the loop, it should be possible to fix the reverb as it's often a preamp valve or the reverb tank - that will have to wait until I have the amp. If I'm fairly confident I can fix the lead master problem I would buy the amp anyway but negotiate a sensible and reasonable price reduction to pay for repairs and compensate for the risk of an expensive trip to a tech.

I've checked the circuit for the Mark 4 rev B and see that it's likely to be either the pot (Lead Master 100k) or the relay LDR13 immediately before the pot in the circuit. I know the pots are chassis mounted so that could be an easy fix (may even be a loose earth) but I have little experience with Mesa relays. Since LDR13 is in line with the pot it should be easy to locate by following the wire but would need advice on the following:

Is my theory that it must be the lead master pot or relay (LDR13) sensible? and; Are there any other reasons that the pot may not function?
Assuming it is a malfunctioning LDR13, what's it's value and component code and can I source one or a compatible part easily?
Is LDR13 PCB mounted and can it be easily replaced? (I can solder PCB components as long as the tracks are not likely to peel away and Mesa uses good PCB's).

Apologies for the lengthy first post, it seems sensible to provide as full an explanation as possible. Thanks in advance for any advice offered.

 

[Boardcertified]

From what I've read oline, it's a common problem with this unit. You can test the pot for the resistance and if it's all good, then it's probably the relay. Not sure about sourcing parts, but since people still hold on to these, I'm guessing there's a way to fix them up and get them operational.

 

[shimmilou]

I would suspect the LDR first, get the number from the LDR itself, should be easy to source an exact replacement or equivalent. Not sure if this is the one you might need, but just an example: https://store.mesaboogie.com/products/ldr-dual-cell-p873-13.html

 

[NickB]

Many thanks for the replies and suggestions. With your confirmation that it's likely to be pot or relay plus the helpful link to a possible relay replacement (gave an idea of price too) I contacted the seller to make a potential offer. He responded explaining that the amp is currently in for repair. If it comes back fully functioning I'll be willing to give it another shot (it's a 260 mile round trip though). It's just possible that the amp tech may check the pot but not the relay in which case the amp will still have the fault - if that happens I'll make an offer on the assumption that it's fixable. Given that the channel works in every other way it wouldn't be too difficult (I may come to regret acting on that thought).

Once again, thanks for the advice.

 

[voltagepathmaker]

I'm really glad the info was helpful. It sounds like you've got a solid plan in place. I'm keeping my fingers crossed that the tech will give it a thorough check. Here's hoping it's just an easy fix!