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Pioneer SA-710-II troubleshooting

J

JMK

New Member
Hi
My old Pioneer SA-710-II made a lot of noise, hum and pops, even when there were no music playing. The relay were also clicking right away when it was powered up, which resulted in a huge pop, when the power was turned on. I thought the problem was some bad capacitors so I ended up replacing all the electrolytic capacitors, besides some non-polarized ones which i could not find replacement for. I have also replaced the two big 12000 uF capacitors.
There are still some hum on the AUX-channel, but I think it is better than before. On the phono-channel there are still pops. I have tried connecting my turntable to phono and the signal is amplified, so I guess the preamp works. The relay is still clicking right away.
I am wondering what I should do next? It is the first time I am trying to repair an amplifier, so I am definitely not an expert, and unfortunately i do not have an oscilloscope or dummy load. Any suggestions on what could be causing this problem?
Thanks in advance
 
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Hello. The relay clicking without a delay is not normal, I'd expect a short delay at least. I'd check the relay drive circuit, perhaps somebody jumped or hacked that circuit, or perhaps you have a shorted transistor or diode in there.

Noises at the phono preamp (and only in that function) make me think in a noisy transistor or OPamp in the phono pre (given that you already replaced the capacitors)

Also, deep clean all selectors, switches, potentiometers, specially those you never move.

Is the amp working? Clean and undistorted sound?
 
elnaldo said:
Hello. The relay clicking without a delay is not normal, I'd expect a short delay at least. I'd check the relay drive circuit, perhaps somebody jumped or hacked that circuit, or perhaps you have a shorted transistor or diode in there.

Noises at the phono preamp (and only in that function) make me think in a noisy transistor or OPamp in the phono pre (given that you already replaced the capacitors)

Also, deep clean all selectors, switches, potentiometers, specially those you never move.

Is the amp working? Clean and undistorted sound?
Click to expand...

Thanks for the reply

I found a bad transistor and zener diode in the relay drive circuit, which i have changed, and the relay works now. I let the amp idle for a minute with a speaker connected and suddenly the noise got a little worse and the relay opened again. After i let it cool down for a while I turned it on again and the relay clicked like it should.
There are stil hum and pops on both tuner, phono, tape 1 and aux and it gets worse when the volume is turned up. And it is much worse on the phono channel.

When playing music through AUX the music seems clean and undistorted, but the hum and pops are in the background.

I am considering unsoldering all the transistors so I can check if they are bad. Is there a better way to check the transistors without unsoldering all of them?

My last question is if it could be the power supply causing problems and how to check this?
 
If the noise is worst in the phono channel, try to fix that phono preamp first. Perhaps you have a short there, drawing too much current from the PS and causing the HUM.

It could be just the power supply, but seeing your comnents, noise increasing with the volume control and louder from Phono, I'd start at the phono preamp.

A shorted transistor will measure shorted in circuit , and won't have a voltage drop across it when powered (same voltage at 2 leads). A good transistor should measure around 1.4V from emitter to collector. Measuring voltages at different points of a circuit is usually the way to narrow the problem.

Are the clic and pops present with volume at zero? That would mean noisy transistors at the power amp. Check the schematic and check the thread "top ten noisy transistors" to see if your unit uses some known noisy transistors.
 
Last edited:
Thanks a lot!
Yes, the clicks and pops are also present at zero volume, but increases with volume.
It is in both the left and right channel - can that help narrowing down where the bad transistor potentially can be? Or does that just mean that there are more than one failed component?
 
Being in both channels at the same time (same noise at the same time in both channels) would point to a power supply problem, even a dirty power switch.

Do you have a link to the proper schematic or service manual?
 
I'd check the +32V and -32V from Q28 and Q29 reaching the phono preamp. Also, any noise in those rails, even coming from the phono preamp, could spread all along the +32 or -32V rails, all the way to the power amp.

A freeze spray is very useful to isolate a noisy transistor , but if you are nor a professional , the spray is probably more expensive than replacing 6 transistors in the phono preamp.

Any shorted transistor or component will modify the voltages stated in the schematic, so checking the different voltages out of the power supply can show a problem.
 
I actually tested it yesterday, and the +32v is changing all the time. It does not exceed 34 v, but I have seen it be around -1v. When the voltage drops too much the relay opens again
 
Well, that means something. Now we need to know if the regulated PS is acting up, or is something else is drawing too much current.

The regulator is simply a Zener clamping the voltage at 32.6V, and a transistor, all fed directly from the transformer and diode bridge rectifier. The voltage should never be higher than the zener voltage

You could check the voltage across the zener, and at Q28 base, to see it it stays steady, voltages before the zener, at R118. And the resistor values at that regulator, r118, 120, 122. Also, check capacitors for polarity. A capacitor backwards can introduce noises and move the voltages up and down.

If the zener stays at 32.6V and the 32V rail moves, we could suspect Q28 or a short circuit ahead, pulling too much current, or even a cold solder making intermittent contact.
 
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