Ashdown ABM500 Very Loud Hum No Output

[picker]

Hello I am trying to repair my ABM 500 it got shorted with a bad speaker extension and it blew the output transistors and burnt a few resistors.

I have now replaced with new output Transistors a new bridge Rectifier all new filters caps and replaced the burnt resistors.
I have replaced most of the smaller electrolytics too

All diodes check ok and I am now I really stuck

All I get is a loud thump and hum when I switch on and the heatsink is getting hot very quickly.

Is there any quick checks I can make or something obvious i'm missing?

Thank you in advance for any help.

[52 Bill]

Welcome to the place. I'm not familiar with your amp, but from your description, it sounds like you still have dc on the output terminals. Check for this and if you do disconnect the speaker until you get the power amp fixed.

When you replaced the output transistors, did you check all of the other transistors in the power amp as well? Driver transistors will often short when the outputs go.

Do you have a schematic that you can post?

[picker]

Hi 52 Bill thanks for the quick reply I have cold checked the driver transistors they seem ok just getting a bit dispondant now.

There is dc on the output 74.2v

I do have the schematic

Ashdown ABM Schematics.pdf

Thanks

[picker]

Oh and I fitted a new fan Themistor as the other one had lost the backing off of it

[Chuck H]

I might suggest that you remove the speaker from the amp until you get this figured out. If the speaker isn't already toasted it soon will be.

[picker]

Yes it blew the speaker when it shorted I ran it once through my mesa boogie cab luckily it never blew

[Chuck H]

Yes it blew the speaker when it shorted

Almost happened to me. I was working on Kustom 250 and had it plugged into my 4x12 celestion cab. I pulled the connector lead between the preamp and power amp. I didn't know that this also removes the power amp 0V reference. Not immediately realizing the hum was DC (thinking it may have been AC) I took about ten seconds to poke around before deciding that I'd better just power down. That's when I saw the little puff of smoke drift out of the grill cloth. The speakers still work, measure correct DC ohms and sound fine. Might be on borrowed time though.

[R.G.]

Light bulb limiter.

[Chuck H]

I agree. Sometimes I can get a little cavalier. It often pays off. But boy, when it doesn't...

[picker]

How do I use the light bulb limiter please?

[pontiacpete]

check this out:
Light Bulb Limiter

[Alex R]

Solid state amps with fused outputs can be a bit of a swine to sort out (do-able but you need patience) - even if the drivers are ok there are bias set transistors etc a bit further back in the circuit, it's surprising how many of them get fried when the outputs short, just go round with a diode tester beeping them all. Do use the light bulb limiter when you test the amp though as the outputs could easily pop again if the whole amp aint right - in fact I'd check to see nothing new has shorted. Ideally you bring them up slow on a variac and monitor the DC on the speaker outputs.

[Alex R]

Solid state amps with fused outputs can be a bit of a swine to sort out (do-able but you need patience) - even if the drivers are ok there are bias set transistors etc a bit further back in the circuit, it's surprising how many of them get fried when the outputs short, just go round with a diode tester beeping them all. Do use the light bulb limiter when you test the amp though as the outputs could easily pop again if the whole amp aint right - in fact I'd check to see nothing new has shorted. Ideally you bring them up slow on a variac and look for rapid rises in current drawn by the amp and for DC on the speaker outputs as danger signs. Like I said the danger is shorting the outputs again.

[picker]

Thanks am building the limiter now thanks pontiacpete.

@Alex R thanks for the help I have been around the transistors and diodes a hundred times or more I'm starting to wonder if one of the new output transistors where bad but still they check ok no shorts. I will test with the bulb limiter later.

Thanks again

[Alex R]

well, at the moment we know it is sticking to the positive rail (if it's +70 odd volts DC on the output), so something is making the outputs on that side conduct all the time - so they ahave too much bias on the bases I guess. This isn't really my strong point but someone will maybe look at the schemo and make a suggestion. Did you check the big low-value resistors on the emitters, usually square ceramic jobs? and all diodes?

[picker]

Ok so bulb limiter glows bright at first then fades to a glow TR1, 2 & 3 measre 3.5v pin 1 7 3 BUT tr4, 5 & 6 measure 84v across pins 1 & 3

[picker]

Ok so bulb limiter glows bright at first then fades to a glow TR1, 2 & 3 measre 3.5v pin 1 & 3 but TR4, 5 & 6 measure 84v across pins 1 & 3

[Jazz P Bass]

It sounds like TR7 is shorted.
Either that or something else is pulling down the base voltage.
There should be a 1.2 volt difference between TR7 pin 1 & pin 3.
That means, as it is a pnp transistor, that it is partially on.

[picker]

Thanks for all the help and suggestions I have now sorted the Amp I replaced TR7 as suggested and the other BF422/423's all checked out ok on the meter so I can only assume one was breaking down under load.

Thanks again