Pioneer SA-606 Blows mains fuse

[HamishBoxer]

Spent time to no avail on this but cannot locate as to why the protection circuit kicks straight in and blows the 1.25A mains fuse.

Main elec,s check ok no diodes appear short and the output transistors read fine.

The triac protection circuit fires if it receives -+ 8V according to the circuit.

Any thoughts folks?

Thanks.

[Grubhead]

Is there a voltage regulator circuit? That would cause the fuse to blow if shorted.

[TonyDuell]

How about disabling the protection circuit, firstly by lifiting the input resistors, and if that doesn't do it, by removing the triac, and then powering the amplfier via a lamp limiter. You would then be able to see if either or both channels have a DC offset on the output, and attempt to find out why.

[HamishBoxer]

Thank you for the replies, no there is no voltage regulator and I was certainly thinking about removing Triac on protection circuit and running up on a Variac.

Really thought unless the protection circuit itself is at fault I would find shorted o/put transistors etc.

Back to the bench I think.

[vidjoman]

I’ve had Pioneer amps with lots of dry joints on PCBs around the output transistors that caused the protection to trip and sometimes blow fuses.

[HamishBoxer]

Thanks for that,though I have been round the panel with my head magnifier .

They certainly blow the mains fuse within 2 seconds if the protection kicks in,but I will have another look as I needed this up and running for the NVCF to return to the owner.

[HamishBoxer]

It seems the Triac is at fault as pretty sure these do not read very low all ways.

Type NTE5629 or BCR3AM-4 400V 4Amp

[Radio Wrangler]

If we assume the protection circuit is OK and is pulling the supply due to a legitimate output voltage problem, then something is driving the output +ve or -ve

Watching with a scope will give a clue as to which way and will help diagnosis.

If it's an output transistor or darlington, then the rest of the amp will try to combat the output swing. Taking the protector off could let something get hot.

If it's in the voltage amp stage, then the amp won't fight itself and the output will sit swung over. Fault finding gets easier.

The protector is really to protect the speaker when something in the power amp is already dead. With no speakers connected, the relative risk of disconnecting the protector is small and it's what I'd do next - switch on with a voltmeter across the output to see which way the wind is blowing. be ready for a quick off.

David

[Radio Wrangler]

OOps, we crossed!

The triac may be duff, but a dead triac crowbar if it's a simple crowbar (like Quad) doesn't cause DC from the amp if the amp's OK and the offset voltage is low

David

[HamishBoxer]

Thanks David,it was reading the circuit description that said if it trips then the mains fuse goes quick.

Think I need another Triac before going on any further possibly.

[julie_m]

It's not uncommon for triacs to fail short-circuit under more than moderate overload conditions. If you leave the triac out, the circuit will be unprotected -- but if you don't connect any speakers, you will be able to spot any anomaly that might have triggered the crowbar. Be prepared to cut the power at once if the transformer shows any sign of distress (or just use a lamp limiter).

There's a good chance the fault is only present on one channel, so you may get lucky comparing DC voltages between sides. Is there another line of defence before the crowbar?

[HamishBoxer]

No Julie there does not appear to be any other protection before the Triac.

I did wonder wether the overload had caused it or it had just failed itself.

For now it has to go back and the person will fit a Triac and we will see.