Hartke LH500 blowing fuses

[andywragg]

Hi all,

Sorry to interrupt your Sunday but the LH500 I was working on last week (protection circuit and power transistor replacement) has gone faulty again.

Here's the post with all the comments http://music-electronics-forum.com/t44272/

Before handing it over last Wednesday night I had the amp running for at least 30 minutes with music playing through it (mp3s) and it was just fine. The owner took it to a gig and within an hour it was blowing the 5 amp fuse in the mains cord.

To say I'm gutted is an understatement. Luckily the guys whose amp it is, is a seasoned gig vet and always has a standby amp to use. He's dropping the amp off within the hour.

Obviously I'll be going over the work I've done so far as that was the last component change. Anything else I should be looking for? LH1000 v3.8.pdf

[Jazz P Bass]

I would go at it as a new repair.

Keep an open mind.

[Enzo]

Including the fuse, was it the proper type?

[andywragg]

Yeah - 5amp slo blo. Tested it out with my cable and cab and it lit up my dim bulb tester first time.

Tested it with the power section unplugged and it was OK so I'm pretty sure the power stage is the problem.

All the thermal pads were good and no short to the heat sink from pin 2 of each power transistor.

The thing is this board has 2 additional diodes that aren't shown on the schematic. From pin 2 (collector) of Q317 to JP18 with the cathode on Q317, then another from JP18 to pin 2 of Q318 with the cathode on JP18.

[g1]

What is JP18?
Did you clean off all the heatsink grease before using the sil-pads?
Were the replacement transistors you used from a reputable source?

[J M Fahey]

Wait until you actually have it on your bench.
It might very well be "something else" than what you repaired.

[andywragg]

There are numerous wire links across the board. JP18 appears to join 2 halves of a common signal bus that ties all the big resistor together on each side of the board.

Yes - before I removed the board from the heatsink block I checked all the transistor legs for short to the heatsink.

As for the quality of the components. Hmmmm....
I ordered a quantity of Toshiba's 2SC5200 and 2SA1943's. I'm beginning to wonder if they are genuine as I've tested the component I replaced after the last/original one blew and it too has gone short. Only drops about 0.06 V (as opposed to 0.6 like one that hasn't been fitted yet).

I'm going to drop all the remaining power transistors from the board and test them out of circuit. Just to be thorough.

[andywragg]

OK So i know about biasing valve amps and how to do this. But....

I've read on another post that a guy was having trouble with another Hartke LH500 and someone recommended checking the bias was cold to reduce the heat related issues.

Can anyone elaborate here. I'm guessing that's what the other trim pot is for.

[Enzo]

You didn't specify which trimmer you were talking about, so I don't know which is the other.

SVR301 is the offset control, it adjusts the DC voltage on the output of the amp - set it for zero volts there.

SVR302 is the bias control. It is to be set where the crossover notch just disappears, and no hotter.

Forget the two extra diodes. if one is shorted, replace it, otherwise it is a common part of most SS power amps.

Did you buy your transistors from a real supplier or from ebay?

Testing transistors with a meter is fine to detect gross failures, but your meter never puts anything remotely stressful on the part, so the high voltages and currents of typical operation are not tested for.

[andywragg]

OK hands up - I used ebay.

I'm starting to loose the will to live with this amp. The other night, after replacing the OP transistors Q313-320 I hooked it back together and mistakenly forgot to attach the signal connector from the preamp.

R301, 302 and 303 burnt up almost immediately. Luckily I have these (albeit 1/4w) so I replaced them. I've just hooked it up again and its shorting somewhere.

Another check of the OP transistors and some are wrecked already. D310 is also bad.

I note that there's a cable soldered and glued from where R303 is and R317. Is this a shielded cable? I had to move some of the glue to get at the resistors so I'm wondering if the shield has shorted to the inner.