Pioneer SA-520 Amplifier Faulty operation

[Afex547]

Hello everyone,
I have a 35 year old amplifier that belonged to my dad,
it had worked fine for 35 years but recently started acting up.
the audio is only playing through one speaker and its incredibly distorted,
If you press the power button while its playing it sparks back to life for a split second before it shuts off,
all connections were tested on the back and the problem is the same for all of them.
I should also add that the problem was just there one day when we turned it on , no pop or bang.

I opened it up to see if there was any obvious fault,
I cleaned out the 30 year old dust and also found a severed connection on the underside of the circuit board where the solder had broken on what looks like a transistor, black square with 3 legs,
mounted with thermal paste on that big silver heatsink in the middle amp, there are four of them in total.
after resoldering it still hadnt changed one bit.
does anyone have an idea of whats wrong?
I am no expert when it comes to electronics so perhaps the problem is a simple one, or perhaps not.
I'd appreciate the help alot ,
Cheers

heres a photo of its internals:
http://www.hifigoteborg.se/images/Pi...0%20inside.JPG

[Radio Wrangler]

OK, you've been working logically.

I don't have any particular knowledge of this model, but others will have. You will need some sort of voltmeter to get any further. What have you got available?

The big secret about transistor amplifiers, only ever told to signed-up apprentices is that several transistors are connected together so closely, that if one dies, it usually takes a few friends with it. If a repairer without this secret knowledge comes along and spots one dead transistor, and replaces it, the other dead transistors will kill it. The repairer thinks that part is OK now, after all it's new, isn't it? He replaces another device, which promptly is killed and he goes round in circles for ever more.

So the process is to find the extent of the damage, in general terms and then to keep the power OFF. Test all the devices, replace all the dead ones, check a few other likely bits that might have been hurt in the crossfire, and finally to then apply power.

A basic multimeter, a schematic, some guidance and the discipline to not apply power too soon will get you there.

Fixing transistor amplifiers is famously difficult, but that's a legend put about by people who tried to rush it, didn't know, or didn't have the self control.

David

[Afex547]

I'll pick up a multimeter tomorrow and get back to you, thanks!

[sp10mk11]

As David has said for the time being keep the power off, you could check that the soldering on all the power transistors is good (those being the 4 transistors mounted on the sliver heat sink, after that go for the fuses and the spring mounts they sit in, these can corrode at times.
Also check for other dodgy dry joints on other transistors, they normally show up as a ring aound the solder, bit like a tree ring.

Look for any scorch marks around components on the board, if these have been running hot they will definitely cause a dry joint to occur at that soldering joint.

Also a squirt of Servisol switch cleaner in all the switches and controls would not go a miss.

[Silicon]

You can download a service manual complete with schematic from the internet.

If you carefully inspect every square inch of the circuit board with a magnifying glass for 'dry joints' you might find other faults. Dry joints are often found on components that get hot. Look for dark areas of the PCB which also indicate overheating.

Transistor Q22 & Resistor R103 and also Q23 & R108 are part of the voltage regulation circuit for preamplifier stages. They can run hotter than the power transistors.

Faulty amplifiers can damage the speakers. If you wire a 50 Ohm 50 Watt resistor in series with each speaker it might protect them. I use cheap speakers for faultfinding.

Also look for 'bulging' or leaking electrolytic capacitors.

What input socket are you using?
What is the voltage across the output connectors when the speakers are not connected?

[Afex547]

Sorry for the delay lads, I've ordered the multimeter online and I'm still waiting.

Had a good look around the circuit board as you suggested and I found some very minor dry joints, will fix/test tomorrow, I dont think they are the culprit though.

As for the input socket it's the UK type and the other end is hardwired into the amp, I hope that's what you were asking.

I did notice that one of the fuses, possibly the main fuse, looks a bit 'worn out', it isn't blown though.

[Silicon]

Most Japanese manufactured amplifiers use RCA phono sockets for the input connections.

The Tuner, Tape & Aux inputs are designed for a 120 milliVolt audio signal.
What type of audio signal are you feeding into the amplifier?
Is it possible that you are overloading the input?

[percival007]

Originally Posted by Afex547

..............If you press the power button while its playing it sparks back to life for a split second before it shuts off................

This seems very strange.
Do both Channels start to work correctly when you push the Power Button?

Could it be poor contacts on the Power Button?

James.

[Nickthedentist]

I would certainly bypass (short-out) the mains switch to eliminate it from your enquiries.

[Silicon]

One possibility is that you have a fault in the regulated power supply for the preamplifier stages.

Although the faulty circuit creates an unsatisfactory voltage when it has stabilised, for a brief moment at startup it may create the correct voltage and the amplifier will appear to work correctly.

[Afex547]

Originally Posted by percival007

This seems very strange.
Do both Channels start to work correctly when you push the Power Button?

It has started behaving differently since, when you let go of the power button the left has a slower, smoother fade out, and the right sort of stutters out abruptly.

[Radio Wrangler]

It sounds like you now have two faults. Some components can fail soon after being put back into use after a long rest.

Fix the mains switch first, and then chase the second fault.

It's a reasonable amplifier, but anything from this era, freshly woken up is likely to throw up a few faults. If the sound is OK on both channels with the power button held in, then what you have is a hint that it's worth going through the thing and testing capacitors to head a few off before they fail.

David

[Afex547]

Bypassed it and the switch is fine so we can rule that out, cheers!
and I finally got my multimeter, so to review the problem at hand:
Raspy distortion in the left speaker channel, present in all inputs, no visible damage inside the amp.
now whats my next step?

Originally Posted by Silicon

One possibility is that you have a fault in the regulated power supply for the preamplifier stages.

How should I go about testing this?

[Afex547]

oh and i just realised that the headphone jack on the amp is unaffected by the distortion,

[Radio Wrangler]

Sometimes the headphone output is taken from the outputs of the power amplifiers. If so it suggests that you have a faulty switch, connection or loudspeaker.

A quick check is to disconnect the speakers and to reconnect each one but to the opposite channel

David

[Afex547]

i tested with different speakers and the distortion was still there

[McMurdo]

I'd hook it up to a signal generator, run a sine wave through it and monitor the output on a scope, the nature of the distortion on the scope will generally suggest where the problem lies, eg crossover distortion, one half the wave missing or cramped, oscillation, offset etc.

[Glowing Bits!]

Originally Posted by Radio Wrangler

Sometimes the headphone output is taken from the outputs of the power amplifiers.

If the SA-540 has anything to go on, the headphone channels are connected to the main outputs via resistors.

Taking a look at the speaker switches first before diving in on the electronics would be the best thing to do at this moment in time, failing that, by all means dive inside.

Once the amp is working, you will be very happy with it, the old Japanese amps perform very well.

[Silicon]

What is the DC voltage across the speaker terminals of the amplifier with (a) the speakers connected and (b) with them disconnected? Put the black multimeter probe on the - negative and the red probe on the + positive terminal.

Remember to turn on the speaker selection switches.
Test Left and Right channels.
There are no fuses on the amplifier outputs.

[Afex547]

Bit of a delay getting back to the amp.
I'm getting zero on the faulty channel and 8mv on the working channel.
The amp behaves very strangely if you turn it on with a source playing, I will make a recording of it rather than trying to explain it.
Possibly getting a pulse of undistorted sound from the faulty channel, very hard to tell though.
After doing a a bit research im thinking the problem could be a capacitor(s), the amp had worked fine for years except we have been using it much less the past couple of years, apparently with old caps infrequent use is what causes them to fail.

Originally Posted by McMurdo

I'd hook it up to a signal generator, run a sine wave through it and monitor the output on a scope, the nature of the distortion on the scope will generally suggest where the problem lies, eg crossover distortion, one half the wave missing or cramped, oscillation, offset etc.

Building a sine wave generator atm for fun and also for this purpose since you suggested it. Would a soundcard scope do the job?