Mesa Boogie 5:50

[leydenjar]

How is this thing constructed? Are ther any ribbon cables between sections? Ribbons can get plugged in off a pin. Pin headers can get bent so one is bent down touching its neighbor.


A couple questions: Is this the real commercial Mesa amp? Or is it a home made clone? Did this amp used to work and just one day found itself like this? Or has it been worked on for some other problem and during the work now has this problem too? Either at your bench or at some place else?

It's a commercial Mesa that was boogered up before I got it. Someone had been attempting to repair it. I had another post up concerning the voltage regulator in the 12vdc preamp heater circuit being loose. Power amp voltages have been fine but not the preamp.

The ribbon cables appear to be plugged in correctly. I've worked on a few Peaveys so I know to check those.

[leydenjar]

Maybe this: Pull the board, and with all the jacks and controls no longer grounded, measure node C to circuit common. (Not chassis) Now with clip leads, ground each thing in turn - the input jack, the master volumes, whatever. Which things affect the resistance from node C to common? And which don't?

Is the + lead of C54 an approprate spot to call "Node C" and the neg lead of C62 "circuit common" ?

The resistance between those 2 points is 219 ohms.

When I am grounding "each thing in turn" can I just attach that pot or jack or control to the chassis with a nut or do I need to find "circuit ground" for that part of the circuit and run a clip lead from there to the chassis?

[Enzo]

Eithr one or both. We are not chasing a failure a failure in the circuit, we are chasing some sort of unwanted "circuit" due to something shorted between two points. We cannot assume anything.

The + end of C54 is of course node C. The negative end of the same cap is common. Start right there, just measure resistance across C54 with the board out. If your low resistance is there - or some sort of low resistance - then grounding part to chassis may just be the revealing agent. Where is C62? I see C52 at node B, and the bottom of that would also be common. So 219 ohms would seem to include your short.

SO if you have already lifted C53b and C54 with the low resisitance remaining, keep looking. I know it is a pain, especially on most Mesa boards, but at this point I would probably start right at node C on the board and closely follow every trace radiating from that point to wherever it goes watching for points where it may be shorting to some other trace. Or what might be easier: since the master volume aappears to adjust the resistance, look at that point. I see two master pots in parallel, their wipers selected by a relay and fed to C24. There are four other resistors over to C20, with a relay to common and a direct connection. If those caps are intact, any DC should be confined to this area, so much less to check. Go over erevy inch of the tracework for those few parts looking for anywhere they coul touch something else.

Here is another strategy: Look at node C, coming into it is the 6k8 resistor. Unsolder it and lift it. STill have 200 ohms across C54? Follow the traces from C54, and look along each, and unsolder and lift each part you come to. ALways watching that 200 ohms. If we find some part unsoldered changes the resistance, it must be involved.

[ogeecheeman]

re;mesa express 5:50. hope this is the appropriate place for this question-new guy here-
anyone ever experienced a condition of momentary drop out when you tap the main board in area of preamp tube next to driver tube? there is a relay about an inch away.I raised he main board so it is free floating.beat the hell out of the chassis,the amp works flawlessly.tap on that board near the relay and it drops out and comes right back.like a bad relay would.just asking here but I've never seen a relay go bad in a mesa.any ideas appreciated.already resolder that board.

[Jazz P Bass]

Mesa Boogie Express 5:50 schematic

There is not a board layout on the amp..
Can you provide any info on which relay it is?

[ogeecheeman]

RYCS / THE CHANNEL SWITCHING RELAY
i took it off the board and installed jumpers wires.pound on the board,no drop out.tap the relay lightly or hard-drops out every time.I found it interesting that I have 9vdc on the relay when charged.but it drops out regardless of which channel is selected so it must be a bad relay you think?

[soundguruman]

It's good to install sockets for the relays if you can,
however Mesa Boogie Board is very challenging to work on.
Requires professional tools and soldering equipment.

Installing sockets allows changing relay without removing board, saving hours of time also, anytime a relay goes bad.
And it is no more trouble to install a socket, compared to installing a new relay.

It's good to power up the relay with an external supply, and test the contacts in NO NC with an ohm meter.
It is not unusual to have relays wear out.

I leave a space under the socket, and slide a wire tie underneath.
I use little wire ties to lock down my relays in the sockets.
I use open frame machined pin gold plated sockets, when I can get them.

The last Mesa I worked on, there were 3 bad relays.
OK that tells you something, you may have to change the relays.

[ogeecheeman]

NEC brand MR62-12FSRY right?