Channel 3 not working correctly on Mark V

I sold a Mark V on ebay last week and the new owner is having issues with channel 3. I played through the amp before I sent it and it worked perfectly. My guess is maybe it took a little bump during shipping. Luckily this guy is being very patient and allowing me to help before asking for a refund and/or giving me bad feedback. Channels 1 and 2 work just fine for him. He has also put in brand new preamp tubes which did not cure channel 3. The symptoms as he describe are the amp sounds very quiet on channel 3 and there is absolutely no gain/break-up even with the gain set high.

I sent him a little trouble shooting guide based on my knowledge which included hard bypassing the loop and running the amp “bare bones” to try and isolate the problem (rule out pedals, connections, etc.). I’m worried that changing the preamp tubes did not fix it because that would have been my first guess.

I’m starting to get worried and really want to help this guy. Does anyone have any ideas of what the problem may be?
Thanks in advance!

If it's still under warrantee, have him send it to Mesa. He'll feel better if they go through it.

Unfortunately it is not under warranty. I am torn because I packed it well and verified it worked before hand. When he got it, it instantly blew two fuses, the reverb tank was unplugged, and now this issue with channel 3. The money is still in "limbo" with paypal and I don't really want bad feedback. It sounds like it went through a Hell of a time during shipping. I would genuinely like to help the guy and have a positive transaction. But I also kinda want my money. Kind of a stressful situation.

scott7d said:

Unfortunately it is not under warranty. I am torn because I packed it well and verified it worked before hand. When he got it, it instantly blew two fuses, the reverb tank was unplugged, and now this issue with channel 3. The money is still in "limbo" with paypal and I don't really want bad feedback. It sounds like it went through a Hell of a time during shipping. I would genuinely like to help the guy and have a positive transaction. But I also kinda want my money. Kind of a stressful situation.

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By the sound of it, you might be correct about it taking a hit during transit. It might be time to make a claim with the shipper. Did you ship it insured?

scott7d said:

Unfortunately it is not under warranty. I am torn because I packed it well and verified it worked before hand. When he got it, it instantly blew two fuses, the reverb tank was unplugged, and now this issue with channel 3. The money is still in "limbo" with paypal and I don't really want bad feedback. It sounds like it went through a Hell of a time during shipping. I would genuinely like to help the guy and have a positive transaction. But I also kinda want my money. Kind of a stressful situation.

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Wow...such a tough and sucky situation.
Blowing two fuses sounds like a tube or more was damaged in transit.
With that, it could cause those screen resistors to go. Others have had those
replaced with success. Good luck!!

I would call his nearest Boogie Dealer and explain the trouble. Have him send it or take it there and have the tech send you the bill if there is one. Boogie is very good about dealing with amp issues and fixing things. Even if there is a charge it will probably be very small.

Really they want to make him and you happy. Both of you are potential repeat customers. I deal with the Boogie Hollywood shop and those guys are nothing short of awesome.

The buyer has been beyond understanding. He sent it to a tech and said the cost to fix should not be major, probably less than 60 bucks. In the meantime he said he LOVES channels 1 and 2 and is excited to hear channel 3. My funds have already been released from paypal now. I will continue to support him if needed, though.

I will say the price my amp sold for was very low. It surprised me actually. That may be why he is not throwing a fit. But, he still deserved a working amp regardless of the price.

I appreciate your input, thanks guys!

Love seeing stories like these. So many people I have dealt with are genuinely interested in good outcomes for buyer and seller, not just moving the gear for a buck. There are exception of course, but for the most part guitar heads are empathetic.

I would have suggested cycling all of the rotary switches on the back of the amp. I had a sudden loss of one of the channels (no sound). I always used the footswitch and had the rotary channel selector set to foot switch. The channels did work on the individual settings and since I moved the switch the footswitch function returned to normal.

I know this post is a bit stale, but did you ever find out what the fault was? I'm having channel 3 issues as well, from a Mark V head I purchased on eBay and shipped from I think Minnesota to San Diego. I suspect it experienced some abuse, because the guy had recently replaced the tubes and v6 died within 2 weeks, and channel has been anywhere from massive and crushing to muddy and fizzy to the point of complete uselessness. 1 and 2 are awesome, and I can get plenty of high gain fun and heavenly lead tones out of channel 2 but when 3 is working it's incredible. I've swapped every tube so I'm thinking a solder joint was compromised somewhere, or possibly a blown cap or resistor. There's also the faint but unmistakable smell of burnt electronics when running EL-34's (that's with the correct bias selected) so I went back to 6L6's. They sound a bit better to me anyways.

Even if it is stale, there may be useful information available. There are many relays that switch circuits around the tubes. If the switching logic chips are not working properly there may be some issues. There are three tube triodes that are associated with CH3 only. Those triodes are in V4, V5, and V6. Since each preamp tube has two triodes, side a and side b, it may be possible that half of the tube is not working properly. Verify you have all 12AX7A tubes in stalled. Before tube swapping, remove the foot switch controller from the back of the amp. Us the selector knob on the back of the amp to select the channels. I would actually cycle a few times back and forth moving slowly. Stop at CH3 and check the sound. Still the same, adjust the reverb setting and compare. Change the toggle position for the triode/pentode switch. Also set all of the master volumes of each channel to a lower setting and turn off the FXloop (set to hard bypass) and change channels again. It could be a hanging relay or one that is not working. If no change in VH3. If you have a spare 12ax7a (even if it was one of the originals that were replaced) use it for diagnosis. Best to check it in V1 to confirm it works with the other two channels and then remove it and put back the tube that was there. Use the confirmed tube in V6, if result is same, return the tube that was there and swap V5, repeat with V4 if no change occurs. You could also try V3 since that is part of the CH3 chain but also used in the other channels. Note that even though the tube is common to all channels, there are switched resistors and capacitors used to set the gain and character of each channel. If CH3 cannot be restored to operation by simple test, Mesa or a tech will have to evaluate the preamp board. If the amp is pulled from the chassis, (warning that there may be lethal voltages present on the PCB's due to the high voltage stored in the capacitors) you may attempt to unplug and reseat the ribbon cables that run though out the amp, do one at a time so you do not forget where the cable goes. Take pictures before you remove any internal cables. Best not to venture inside the amp as there is no real serviceable parts. This may be best taken to a Mesa Authorized repair shop or to Mesa.