LH1000 fuse blowing - short?

[ilyaa]

trying to figure out this hartke LH1000 that's blowing fuses - my light bulb limiter tells me its a dead short somewhere - bulb goes on real bright real fast.

i started measuring around the power section, wondering if it's the PT. if i disconnect CN2 from the board, i read a short (<1 ohm) across any/all of the leads (black or red or red). seems like that part of the PT should NOT be shorted, correct?

here's a schematic:
http://music-electronics-forum.com/attachments/6758d1256749305-lh1000-v3.8.pdf

[Enzo]

Power transformer isn't where to start, it is more where to end.

You can check it easy enough.  Are there two groups of red wires coming from the transformer to the board?  There are two on the schematic.  Or do you only get one set, and the schematic has extra information?  Either way...

Unplug them so ONLY the mains wires are connected to the tranny.  Power up.  Does it still light your bulb?   Those low voltage, high current windings will have low resistance.

I bet it is OK.  Now the power amp module has a six wire connector on it.  Unplug that, and with the power transformer all connected again, power up.  Bulb still light, or not?  My first suspect is always going to be blown output transistors.  If unplugging that power amp module stops the bulb from lighting, that confirms my suspicion.

[ilyaa]

good call -

so those secondary windings should be very low resistance?

i unhooked one of the power amp modules and short gone. now to track it down....

[Enzo]

It doesn't surprise me they read a low resistance, for the reason I stated.  However, I never bother measuring that.  A "shorted" transformer winding is usually a couple turns shorted together, and a meter will never detect that.  I know failed power transformers are rare.  The functional test of unplugging the board and powering the transformer takes mere seconds, especially since you said you are using a bulb.

[ilyaa]

alright trackd em down - just a few bad output transistor and they took the drivers with em.

replaced em - looking normal.

just need a new fuse - 15A in this amp. wow. thats high!

[Enzo]

Not really, it is a 1000 watt amp.  it is a bass amp. so lots of impulse draw from the power supply.  And no amp is 100% efficient.  And you don't fuse an amp at its maximum draw, you want headroom or you get nuisance blows all the time.