Channel switching relay

[Socatevoli]

Hi all,

I recently purchased a used Mark V head.
Upon testing the amp it seems switching to channel two gets bypassed by both the footswitch and the knob on the rear. Switching from channel one sends you right to three. Going from channel three to two keeps it on channel three. I’ve contacted Mesa support and they believe it’s a bad relay for channel two. I believe I could do this repair myself, but need further information on the correct relay and location. Any help would be greatly appreciated..

Thanks

 

[IronSean]

If it's still under Warranty I'd just have them do it. No offence but if you need additional help to find it then you might not be as equipped to handle it yourself as you think. Mesa's are pretty complex inside and tight working spaces.

To start you'll want to check a schematic and try and track down the relays that handle it, but there's quite a number of relays involved in channel switching and to my knowledge there is no schematic of the switching circuit itself out there. So you'll just be firing in the dark, making educated guesses replacing relays one at a time until you find the culprit, if that's even the cause. You might be able to set up the probes to check the points that should change (as best as you can tell) when the channels switches.

PLEASE make sure you are safe, know what you're doing, don't touch anything on the amp with both hands, and discharge the caps before trying. But this is inherently more risky because to probe the relays that should be changing you'll be probing while the amp is powered on.

 

[Socatevoli]

Thank you for your reply!

The amp is dated 2010 so it is out of warranty.
I am capable of discharging caps, and have fair knowledge about tube amps, but as you stated the amp will need to be on to check voltages. This is my first time with a Mesa and the guts do seem pretty complex for my knowledge. I was hopping maybe someone encountered this before, so I’m not chasing relays. It just seems weird to me that the previous owner never had an issue with channel switching, and it seems no one else has had this issue (that I could find anyway). I can’t see something breaking free during shipping as it was packed fairly well and the quality of the circuit board work seems on point.

 

[etepbbb]

Check the footswitch cable, they're know to cause flaky switching. I'd try turning it around, plug in the cable the opposite way it currently is, also ensure the plugs are seated properly in the socket. It may be something along those lines, considering the former owner had no issues with it.

 

[bandit2013]

I would look at the back of the amp (on the inside) where the individual channel selector jacks are located. They are ground pulls so if thee is a short or some debris near the jacks where they solder to the rear panel board that would cause an issue. Also inspect the rotary switch on the back panel. I have not had the need to look for issues with the logic switch circuitry. relays may be pulled in the FETS or BJTs. There are many J175_D26Z (Fairchild /On Semi) P-CH FETS. Which one to check is the question. May not be a relay at all but it could be a FET that is driving it. Could also be the logic controller or the rotary control switch on the back.

I believe the relay is listed as RYC1A but this is marked on the 2009 schematic. References may have changed though. If the tone controls for CH2 work when switched to ch2 even though it looks like CH1 is active it is not that relay then. Not exactly sure if the LED indicators are driven by the logic controller or if they are linked to the relay circuit. The other relay that switches between CH1 and CH2 is RYC1C. The rest seem to switch between CH3 and the other two. All (if not most) of the relays have the manufacture part number on top of them, it not it will be on the side of the relay. Note the references are from the 2009 Schematic, there were some minor changes in 2010 (tone stack on CH3). Not sure if the references have changed.

It may be a mine field at this point as you would have to remove the main board to remove the relay. Before you do so, take many pictures of the wires on the board and where they go, you will have to remove them in order to get the main board out. Use a tape tag on the wires and mark them with a letter or number to reference. The ribbon cables are no big deal but take pictures anyways. Note that it is lead free solder so if you are not equipped with the correct tools good luck.

 

[Monsta-Tone]

There may be a Mesa tech in your area.
The V's board is really tight! I've worked on Mesa amps for a lot of years. I wanted to mod my V slightly and wound up burning a Molex connector for one of the ribbon cables. I need a smaller diameter iron before I try that again!

You might try looking for a Mesa tech before you attempt to do it yourself. It shouldn't cost very much and your amp should be in good hands.
I haven't seen many relays go bad on Mesa amps, usually it's the small FET's that control the relays that go bad. Pretty simple fix if you find the right one....

But.....again, it's tight in there and I gotta tell you......400V DC hurts like a mother!

 

[bandit2013]

I would agree, have a tech do the repair. This would be my advice to myself as well..... :roll:

I had to work on mine to repair the reverb circuit when it went dead about two years ago. It is extremely tight but I had the proper equipment to repair it. I used the solder station at work that I use to fabricate prototype boards (mostly surface mount assembly). I doubt most people will actually buy a solder station considering the cost of the investment. I was able to remove the damaged JFET and get the reverb working again. Also had to replace the screen resistors on the power tube board. Not an easy task and my solder iron at home would not have been able to be used for such.
This is what I have at work. I have tried the home type solder irons before, they literally suck when it comes to thermal mass of the PCB copper and are not very good with most lead free solder as it requires a higher even temperature.

http://www.howardelectronics.com/desoldering/desoldering-stations/by-manufacturer/jbc-tools/rmse-1d-jbc-tools-complete-rework-station/

 

[Socatevoli]

Thanks everyone for the replies!

So the amp sat for about a week or so while i was on vacation. I still had the chassis removed, and basically gave up because i couldn't find the culprit. I was going to see if i could find a mesa tech in the area. That board IS tight. So I came home, I looked at it and had a funny feeling...not kidding..fired it up, ran through the channels.. all the channels...

It's been about a week since then, and the amp is still working fine.

So I guess my conclusion is mesa must of put a little unicorn inside of my amp prior to shipping, and it is just running a muck bashing things with its little horn.