Sansui AU-6500 died on me :(

[TestTones]

So it powers on and you hear the click to indicate the power amp has also just switched on but then......

The speaker cone shoots forward and there is a "zoooogt" noise with each forward excursion. It does not fire the cones backward what so ever. It ran great for over a decade for me up until maybe 4 years ago.

I have no respectable experience with power amp repair, design or modification. I am 6 months into my electronic engineering degree and do not start learning about power amps for a few weeks. I have a fair amount of experience in custom preamp design but again, no familiarity with standard industry circuits. I've always taken the you design it, you build it, you blame nor thank nobody but yourself for the outcome.

My father gave me this amp nearly 20 years ago so I would really like to salvage it. I won't make any theoretical guesses about what's wrong because I am truly ignorant about these symptoms.
I will however degrime it until maybe we can get it running again!
It's unbelievable how good all the small caps look.

[wintermute]

first thing unplug those speakers!! don't plug them back again until you are 100% sure it is working properly.

It sounds like you have a fault that is putting DC on the output, checking that with a multimeter with dummy load (or even no load) attached is probably a good first step.

Tony.

[TestTones]

Originally Posted by wintermute

first thing unplug those speakers!! don't plug them back again until you are 100% sure it is working properly.

It sounds like you have a fault that is putting DC on the output, checking that with a multimeter with dummy load (or even no load) attached is probably a good first step.

Tony.

Ah yes, very good observation!
The single direction throw would be a sign of DC.
I will dummy load test them and post my findings.

Thanks man!

[jitter]

Indeed, typical symptom of DC on speaker terminals.
Service manual here.

[TestTones]

So if I have DC getting to the speakers, it means I have some bad caps that aren't blocking the DC?

[jitter]

Yes, that's a possibility, especially since the electrolytic caps in this amp are most likely in need of replacement. The speakers are DC-coupled to the output stage, so any DC-offset that gets introduced anywhere in the amp finds its way to the speakers. The "protection block" (F-2041 in schematics) should, however, detect this and disengage the speaker relay RL901.
Another possibility is a failed component (other than already mentioned elcos).

Please measure the DC-voltage on the speaker terminals of both channels. According to page 19 it must be 0 V with +/- 30 mV tolerance.

Edit: it seems that pre-amp and power amp can be disconnected from eachother by removing the links on the rear panel, see page 6 (PRE-OUTPUT and MAIN-INPUT).
If you do measure DC-offset on the speaker terminals, then remove the links and measure again. If the DC-offset disappears, the fault is not in the power-amp but in the pre-amp.

[TestTones]

Thanks for the patience and help guys.

I am getting DC!
First, I connected an 8 ohm dummy load and sure enough, DC every time the relay switched on. I'm assuming the relay is part of the protection circuit?
I see it arch and turn on then off. With each on click, 30 volts DC.

Then, I connected it without the dummy load.
The amp switches on and I get a constant 30 volts DC. No relay switching the circuit in and out.
This is is occurring on the Left and Right channels of both amps A and B.

I will disconnect the preamp today or tomorrow and we will then see if I am still getting DC. If the relay switches in and out with the dummy load on, would this indicate that it is a DC blocking cap on the power amp that has gone bad?
I am curious to know if this is a common indication or if it still involves further testing to specify where the faulty capacitor is.

I must also confess.
I have had a small handful of audio electronics die always knowing it was likely due to a part costing either a few cents or maybe a couple dollars and was ignorant so rarely did I mange to repair the item.
Now however it's just so very exciting to know I will be able to repair just about anything that gets ill, especially once I've completed my degree and have a bench full of test equipment!

[Puffin]

I love your enthusiasm. I wish I was in your position of getting the knowledge required to fix. I get by, with help from the good people on here, but I wish I had a whole lot more knowledge.

[Osvaldo de Banfield]

I sometimes use an incandescent light bulb of some great voltage than the supply rails (Ej, a 48V lamp for +-40V amplifiers), so you may work without your hand at the VOM. The current in the lamp and then its power, must be in the margin of the SOA of the amplifier, the the lowest wattage of it, the best. Obviously, as the guy said, disconnect the speaker(s).

[jitter]

Originally Posted by TestTones

Thanks for the patience and help guys.

I am getting DC!
First, I connected an 8 ohm dummy load and sure enough, DC every time the relay switched on. I'm assuming the relay is part of the protection circuit?

Yes, it is.

I see it arch and turn on then off. With each on click, 30 volts DC.

Better not do that too many times. The arching wears out the contacts.

Then, I connected it without the dummy load.
The amp switches on and I get a constant 30 volts DC. No relay switching the circuit in and out.

Hmm, the protection circuit should switch off the relay wether or not the dummy load is connected. The arching may have welded the contacts on the relay shut.
See if turning the speaker selection knob to and fro between SPKR OFF and A + B makes the relay click. If it clicks, measure if in SPKR OFF position, you still have DC.

This is is occurring on the Left and Right channels of both amps A and B.

Which may indicate a problem in a circuit common to both channels, e.g. a power supply rail. Measure if rail voltages are present (see page 15, B1..B4) and check the fuses. Given the offset voltage of 30 V, I wouldn't be surprised if the negative rail has failed (which should measure approx. -31 V).
If the fuses are still OK (measure continuity, sometimes a fuse looks to be OK, but isn't) make sure they contact the fuseholder well (measure continuity of the fuse from outside the fuseholder). If (one of) the fuses have (has) blown, things get more complicated.

I will disconnect the preamp today or tomorrow and we will then see if I am still getting DC. If the relay switches in and out with the dummy load on, would this indicate that it is a DC blocking cap on the power amp that has gone bad?
I am curious to know if this is a common indication or if it still involves further testing to specify where the faulty capacitor is.

Don't make any assumptions yet and take one step at a time. Please first check the voltages and report back.