Mesa Heartbreaker blows fuses

[xtian]

Here is a Mesa Heartbreaker on my bench, currently breaking its owner's heart 'cause of no worky.

I pulled all the tubes, standby is on standby, and it blows 1a fuses instantaneously.

The PT is cannot be disconnected from the caps and all without being unsoldered.

How should I proceed?

[JazzGuitarGimp]

SS or rube rectifier?

[xtian]

SS or rube rectifier?

Switchable. I pulled the tubes (including the recto) and set it for SS. Blew the first 1a fuse.

After reading your message, I switched to tube rectified (still with no tube installed) and blue the second 1a fuse.

Your move!

[xtian]

I measured about 40 ohms across the HT secondary, and 20ohms each across each HT leg to CT.

[JazzGuitarGimp]

With all the tubes (including rectifier) removed, and the ss / tube switch set to tube, theres not much more than the pilot light loading the transformer....

[katopan]

Use a light bulb limiter to test any further, because every time you blow a fuse you are also causing more damage.

You really need to disconnect all secondaries to validate if the PT is gone or not. If that take soldering so be it.

[Ang3lus]

i had a problem like that once, turned out to be the standby switch itself, it was shorting to ground

[jupagblkxten]

Sounds like you have a short in either the bias supply or switching matrix. I would start by disconnecting their secondaries one at a time to try to isolate and identify where the short is.

[crbowman]

Rube rectifier:

[xtian]

Rube rectifier:

Well, THAT's your problem, right THAR!

[Phil_S]

Use a light bulb limiter to test any further, because every time you blow a fuse you are also causing more damage.

You really need to disconnect all secondaries to validate if the PT is gone or not. If that take soldering so be it.

FWIW, I think what kato says is the best way to check this out. With a bulb limiter, you may not be able to see the problem or the bulb may mask the problem. Disconnecting one secondary at a time is going eat more fuses than it is worth. When a fuse blows, there is a short. Once you disconnect the secondaries, use the bulb limiter to test the primary. If the primary is not shorted (bulb stays dim except for initial on inrush), then you can look at the secondaries.

The first thing you can check for on the secondaries is very low or 0 ohms on a winding, which may be an indication that it is shorted. It is likely you only need to disconnect only the center tap(s) and the bias supply. The rest of the windings should be open when you pull the tubes. Sure, the wire will add some Ohms to the windings, but that isn't so important. I think we are looking at desoldering only two wires. If there is other stuff in there that I don't know about like switching networks and such, well, then, sorry for the bad advice on the secondaries! You may have a few more wires to lift.

[Firestorm]

The schematic I found shows a single 3A fuse on the primary; are there additional 1A fuses in there somewhere? I also see an MOV across the primary; I doubt that it is common, but shorting is a possible failure mode for an MOV. Check the tube/SS switch very carefully it appears to be a DPDT that simultaneously switches the B+ source and the bias voltage. A short between those switch terminals (internally or externally) would be catastrophic.

Once you rule out switch, MOV or other shorts, I second the opinion that you may as well unsolder the PT secondary leads. Probably an even chance the PT is toast anyway.

[ChrisM]

Good news is you have isolated the problem to a very small area now. The potential culprits are small and should be easy to access.

[xtian]

Great posts, guys.

I discovered blown-open diodes in the HT rectifier. I just removed one, powered up under bulb limiter, and results are MUCH improved. 6.3v lamp came on, and I hear the relays switching.

Will let you know results of surgery when I can ignore my other work and finish this, far more important, job.

[xtian]

Rats. Thought I had it figured out, but when I put the tubes in, there was trouble. I noticed that the tube recto was making lots of noise, and, though the amp didn't blow the fuse, it was sucking a lot of power.

Works fine with no tubes. Voltages look good.

Testing tubes now (just have Knight tester for shorts and emissions).