SA-710 Rumbling and protection.

I've done a little work on this amp, replacing some of the capacitors and baked resistors - mostly in the driver stages and phono stage.

Everything sound great, but sometimes on powering up, it won't come out of protection, or the speakers start to rumble and the meters flicker up and up until it goes into protection or I turn it off. Sometimes if I power it back on it will work perfectly, other times not. Because of this I'm not really using it but is this a common symptom of something maybe a capacitor going bad?

Connect a dc volt meter to the speaker connectors and take readings on bias.

If it's not coming out of protection, stuttering to come out of protection, it's possible either the bias is too high (borderline) or the protection circuit has problems namely the relay transistor is going bad. Start by taking readings at the speakers and then on the protection section. Check Zener D13, Q27, and C75 and the resistors around them.

I'll try to put it on the bench tomorrow and get some readings. I have recently done work on the amp - I recapped it except for the main filter caps and NP caps and replaced some most of the low value resistors in the bias circuit and other areas where they had baked from age. I have the schematics.

I don't think the issue is with the protection circuit - it only goes into protection when the meters start to rise up and beyond 10 Watt. Sometimes the amp will work without fault for days and then start doing this again. Usually if I turn it off and back on, it stops doing it. Both channels do it but at different rates so I imagine its a common component?

did you eyeball each and every component coming out of the unit for a match to the component going into it's position?

was it like this prior to working on it ?

Yes, I checked and re-checked everything and checked resistor values against the schematics. I didn't replace any transistors. I did swap the old 5 legged feedback loop transistors but swapped them back to the originals when I didn't see much difference in DC offset. I think at that point the problem started to occur occasionally but it might have been there before - I hadn't used the amp too much before I started re-capping it etc.

I've taken some readings around the power supply.

The main power supply voltage at the capacitors (C79, C80) is 45.2 and -45.2 VDC.

I then checked the voltages across Q28 and Q29

Q28 gave me a good output of 31.8VDC
Q29 however, was all over the shop. I measured the voltages at each post.
C: -45
B: drifted a lot varying from -32 to -17
E: also drifted a lot - starting at -14 and rising to -22

I also connected my scope to various points on the amp - to the power caps, outputs and driver stages and I kept finding a lot of erratic DC signals in the audio path but not on the power supply to the outputs and drivers.

I thought I would also check R118 and R119, which both measured 22R.

The amp is more unstable than I remembered and sometimes only comes out of protection for a few seconds before the relay clicks. There is a huge amount of DC on the speaker terminals which corresponds to the meters flickering. 80mV+

Am I right in suspecting the Zener diodes?

On testing further, I connected my scope up to either side of the transistor - the collector and the emitter. This showed me that the Zener diode is working correctly at steady 31V but the poor voltage is coming from the emitter. With peaks and dips - never managing to get over 27V.

I've ordered the equivalent transistors after looking them up on other threads and I guess the smart thing to do would be to rebuild this part of the power supply.

When replacing the zener diode, I was hoping to test the new one is regulating correctly by measuring the voltage with the transistor removed. Is this safe/possible?

Well, the transistors arrived and I've swapped over both the voltage regulator transistors - Q28 and Q29 - I replaced both as a precaution.

I powered up and voltages are much better.

Q28 E: 30.5V
Q29 E: -31.5V

I let the amp warm up for 10 minutes and idle current is 10mV on left and 7mV on right.

I checked DC offset at the speaer terminals and its sitting at roughly 30mV on both channels - is this ok for this amp? shouldn't it be much closer to 0mV? Would replacing the dual transistors Q13 and Q14 help with this value?