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Mesa Mark V Head, Pedal doesn't work

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  • Mesa Mark V Head, Pedal doesn't work

    Sometimes I dread seeing Mesa amps in the repair pile. This one was the stand-alone head, rather than the small tightly packed Combo that weighs a ton. The amp on its' own works fine, but, with the foot pedal, having eight buttons, three being momentary that change the input channels. Powering the amp up with the pedal attached via 8-pin DIN cable, the LED's briefly light, then go out, with no function at all.

    The only documentation I had was for a Mark IV, though it's pedal is a bit different, and uses a 6-pin DIN cable. Inside that pedal, the schematic shows CMOS circuits running on +15V, with a 5V regulator inside to run additional logic at that voltage. I opened up the Mark V pedal, finding no 5V regulator, and seeing SN74 series logic chips, that meant 5V coming out of the amp to run the pedal. Finding the connection points on the 8-pin DIN cable, I connected it with the PCB out of it's housing to check if I had the proper voltage. Nope. 0.6VDC, with the SN7406N IC getting hot. I removed it, though not having any spares on hand. Powered it back up, but still had 0.6VDC on the supply output. And with the pedal disconnected, still only 0.6VDC output.

    So, had to remove the chassis from the cabinet, or......put the pedal back together and send the amp to Mesa Hollywood. I extracted the chassis, which was a PITA. There's no edges at the bottom of the chassis to allow supporting it with the lab jacks or other chassis service cradle. So, I had to install 'wings' using the hanging hardware of the cabinet to sit it onto my lab jacks. Mesa makes use of screw heads to couple their chassis to the aluminum shielding foil attached to the inside of the cabinet, and, in dragging the chassis across the thin wooden spacers, the screw heads fill up with debris, so you can't put a Philips screwdriver into the head of the screw to unthread it. Stop to fetch a scribe to dig out the debris in order to use a tool to remove those screws. Almost got the chassis out, but, they hid a cable clamp screwed into the side of the cabinet that has to be unscrewed....all while trying to hold the chassis from falling, and little hand access to achieve this operation. Finally got the chassis out, the wings mounted, and set down onto my bench lab jacks to see what further adventures await me.

    Click image for larger version  Name:	Amp Chassis -1.JPG Views:	0 Size:	1.54 MB ID:	909902 Click image for larger version  Name:	Amp Chassis -3.JPG Views:	0 Size:	1.44 MB ID:	909904 Click image for larger version  Name:	Amp Chassis -4.JPG Views:	0 Size:	1.57 MB ID:	909906 Click image for larger version  Name:	Amp Chassis -6.JPG Views:	0 Size:	1.52 MB ID:	909908 Click image for larger version  Name:	Amp Chassis -7.JPG Views:	0 Size:	1.31 MB ID:	909910

    After sizing up the rear panel PCB assy, where a 7805T IC regulator is installed, behind the 8-pin DIN connector, there's a ton of rear panel connectors, switch hardware, pots and rotary switches whose mtg hardware would have to be removed. But, if the thought was to remove that long skinny PCB in order to replace the IC.....forget it. The thin skinny power tube PCB is right behind that rear panel PCB to prevent it from coming out. So, lots more assemblies to be removed. And, of course, cables attaching it are dressed tightly, so the likelihood of getting that board out of the way in order to remove the rear panel made me want to pack the whole thing back up and send it to Mesa Hollywood.

    But, there was the IC standing there, tightly soldered into the PCB staring at me. Of course, the solder pads used were 0.080" dia ROUND pads, not Obround. And, the IC was pushed down to where the leads taper from wide to narrow to fit into the small PCB mtg holes. Could I cut the body off of the leads, then unsolder each lead individually from the top of the board? Tempting.

    Click image for larger version  Name:	Amp Chassis -13.JPG Views:	0 Size:	1.54 MB ID:	909912

    When will I learn to go with my first instinct......wash my hands of the matter and make it somebody else's problem? Tool access to chop the body off the soldered leads left me with little lead length left in the soldered holes. I had already attempted to de-solder the leads using solder wick, so now, starting with the output lead, gripping it with an appropriate tool, I got that lead unsoldered, but, with it came the top of the solder pad, now separated from the plated-thru hole. Sigh.......... I was more successful with the other two pads. Then, fetching a 7805T, I found all I had was a used part. Got it installed, but, no connection to the output circuit. There was an adjacent solder pad with a trace that had attached to the now-missing output pad, so I installed a component lead into that, folded it over and tack-soldered it to the output pin of the replacement regulator. Then, checking continuity between the DIN connector and the regulator, I had GND and Output present. The input lead appeared to be ok. Powered it up to see if I had +5V to power the pedal. YES.

    But, the pedal still didn't work. So, replacing the three logic IC's on that board.....had to order those. Would have been much easier to send it to Mesa Hollywood.
    Attached Files
    Last edited by nevetslab; 07-25-2020, 12:21 AM.
    Logic is an organized way of going wrong with confidence

  • #2
    This man's take on this situation exactly reflects mine:


    • #3
      This is exactly why good engineers are not cheap and are far and few between. A good visual is a clue mostly if not ...
      With all due respect, never forget; check your power supply voltages, if low/zero where the voltage is going before anything else.
      Support for Fender, Marshall, Mesa, VOX and many more.


      • #4
        The Logic IC's came while I was home waiting for the Covid-19 Test date to arrive. Got the IC's installed yesterday, and confirmed the amp & pedal are all working again. Put it all back together this morning, and other than the torn leather covering on the cabinet handle, all is back to normal. Did what I could using black lacing tape to clean up the tear at the hoop end of the handle, but, the only real solution on that is to replace the handle. Another part to order.
        Logic is an organized way of going wrong with confidence


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