Mesa Boogie Nomad 55 burnt PCB

[Gerhard Grass]

Hi to all,

I have here a Nomad 55 which sounds a bit crappy and needs new tubes but works.
When I switched on the power on the other day to test the amp it blew the mains fuse.

Replaced the fuse, switche it on and blew again with an awful brzz and funny smell.
When I took the amp out of the cabinet I found
the PCB around the rectifier diodes was burnt. The diodes seem to be o.k.
according to the component-tester in my scope.
Tried to clean the PCB and started it up using a variac.
The cleaning was not successful, there was still a short.

So any hint how to repair the damage, repectively
how to clean the board is very much appreciated.

Thx in advance

Gerhard

[MarkusBass]

If the PC board is carbonized (and I see that it is), there is not other way than this: desolder the components, remove the carbonized board with dremel tool, assemble the diodes on a small prototyping board and solder it to the main board.

Mark

[Mick Bailey]

Component testers will generally test at very low voltage and this can't be relied upon as a true reflection of operational conditions in a tube amp. Having said that, a catastrophic failure in a diode at high voltage will more often result in the junction fusing and going dead-short. The cost of those components is low, so I'd replace them anyhow.

The first thing to do is to clean everything as well as you can - all the leads, components and both sides of the board. A glass-fibre pencil is useful for cleaning the board. This enables you to inspect the extent of the damage and assess the repair. You also need to determine which components have failed and eliminate any possibility of a fault further downstream causing the damage. I usually print out the affected area of the schematic and highlight the affected areas. You may or may not be able to tell what caused the failure as some Mesa amps can short spontaneously between the closely spaced tracks.

Follow MarcusBass's suggestion about removing the damaged area. There's no way to salvage carbonized PCB. Bear in mind that the dust will contaminate the rest of the board and will need carefully removing.

If you don't have enough space you may need to mount the satellite board remotely and link it in with flying leads.

There are ways to do an 'invisible mend' on a board and replace the damaged substrate and apply new tracks, but the techniques are time-consuming, require skilled application and the materials expensive so are reserved mainly for industrial repairs. In some cases a new section of board can be etched and bonded in place, the tracks being bridged in with copper foil soldered across the joins.

[Gerhard Grass]

thx for your the fast response.

As you both suggested, I removed the burnt parts and tried to figure which other parts might be damaged.

First to look at was the PT.
I isolated it from the rest of the amp and fired it up with my variac.
I put about 20V to the primary and checked the voltages on the secondary windings.
The HT windings gave a reading of about 86V !!

Maybe I am completely wrong, but that would mean a HT voltage of 990V@230V mains ???
Could a short of the primary winding explain what I found?
(Took a look at the DC-resistance and found 3,5 Ohms).

If you think, the PT could be the problem, any hint for getting a
spare part would be fine. Found similar PT at hammondmfg.com
mercury magnetics has only a PT for the nomad 45

thx in advance

Gerhard

[Gerhard Grass]

hi Mick,

I removed all the burnt parts and loooked for other damaged components.

I am afraid, the PT has a shorted primary. (Details in my post to the thread.

Any suggestions an a hint where to buy a replacement PT ?

thx
Gerhard

[g1]

I don't think you should conclude the PT is bad just from that test. The applied voltage is too low to give an accurate result.
Ramping up a PT with a variac does not always give a nice linear result at the output.
Is the HT winding center tapped? If so, measure on side at a time with 230V on the primary.

[Enzo]

Agree.

If the primary is shorted, it isn;t going to make it put out multiplied secondary voltages.


Just for science, disconnect the MOV across the primary, it could be shorted or at least MOVing at a voltage lower than the mains.


You want to know if the transformer is bad? Disconnect the secondary wires - all of them - from the circuit. Secondary wires now sticking up in the air, power it up and see if it blows fuses ( or Lights your light). Or monitor the mains current as you ramp up.

If the lone transformer now still takes fuses, I will believe it is bad.


Ah, I see you already isolated the PT. Current is still the issue, funny voltage readings at low volts are more likely red herrings.

[Mick Bailey]

When I'm testing a PT I always make certain that it either works or doesn't and there's no doubt either way. If I get it wrong, then the repair will make a loss. If it gets to the stage where I think it's most likely the transformer I'll completely disconnect it and do a ring test. If that looks ok (it only tests at about 400mv) I'll connect the primary to a terminal block and power it off a bulb limiter. If there's no drama at that stage, I then wire the secondaries onto a terminal block and check voltages.

If I'm happy with the voltages I load the HT output with a couple of appropriate power resistors. Sometimes the HT will check out fine, but will fail under load. To make absolutely certain I'll additionally load the heater supply using a couple of flying leads onto the amp's heater chain. At that point I have a loaded transformer which is operational and I'll leave it powered long enough to make sure it doesn't get hot, smoke or smell bad.

The reason I load up the transformer is I've occasionally had one where it checks out fine on the bench but fails in the amp when under load.

[Gerhard Grass]

Hi to all

did as you suggested.
Cleaned up the mess, let the dremel tool do a good job,
built up the rectifier seperately and see, it works.

thx to all

Gerhard