Sony TA-5650 loud pop at power-off

[MattiaM]

I recently refurbished a Sony TA-5650 v-fet, with a full recap, replacement of the killer diodes, addition of the voltage doubler on the power supply and replacement of the speaker relay. Bias adjusted fine at 50mV and the DC offset at the speaker output is about 2-3mv on one channel, and 15mV on the other. Not as low, but still fine, so I did not change the transitors in the differential pair.

Now, it works great but there is a very annoying loud pop when I switch it off. There are a lot of threads on this, and many say these are old amps so it will happen, and all the Sony 5650 and 4650 I have seen do pop when switched off. In my (very limited) experience, in most cases cleaning the on-off switch would reduce the problem significantly. However, for the amplifier I currently have on the bench this is not the case, and I want once and for all to be able to find and troubleshoot where the noise comes from.

What confuses me is that there is a circuit that has been specifically designed to avoid pops when switching the amplifier on or off, but it does not seem to do its job.

I attached the schematics (from the TA-4650, but components and values are exactly the same for the 5650). When the amplifier is switched off, the -highly negative -- rail raises towards 0, the base of Q406 becomes more positive (I guess through the discharge of C418?) than the emitter switching Q406 on, which shorts the B-E junction of Q404, turning off Q405 thus opening the relay (taken from the circuit description). The 'highly negative' rail is the -82V half way-rectified rail in the 5650; the less negative rail is the full wave-rectified -44V rail.

If the muting doesn't occur, I assume Q406 never turns on. Why?
I really have no idea how to systematically troubleshoot this as it all happens in milliseconds.
Any help would be greatly appreciated!

 

[spaghetti]

 

[sax6]

Is there a way to turn off the feed to the speakers before turning off the amp ?

Cap coupled amps would have tendencies to do that.

 

[audio3000]

What could have gone wrong?
1. The relay got stuck at the "on" position all the time.
- Was the relay click delayed after power on?
- If not:
- Relay mechanically stuck at the on position? May need to remove the relay to check. Also can connect it to a power supply after removed from the circuit to check if it works as it supposed to be.
- Bad Q405 and/or Q404? With SPEAKER REMOVED and power on, short C416 to see if the relay clicks. If not the transistors could be the culprit. Check them.

2. Why Q406 was not activated when AC was lost?

I was not able to find description on how this circuit works. My educated guess is that it relies on the difference in power capacitor discharge timings. I assume the -71V rail will discharge faster than the -43V. At such, Q406 base will got push up due to the charge still held in C418. In this case, I will check C418 to make sure it's still a 10uF cap and Q406 is still a good transistor.

Hope this can serve as starting steps to trouble shoot your problem.

I have a TAN-5550. But I always try to turn off the speakers during power on and off to protect my beloved speakers. During power on, I will wait until the relay clicks and no smoke before switching on the speakers. That's me...

 

[MattiaM]

Thanks for all the answers.

Is there a way to turn off the feed to the speakers before turning off the amp ?

I can manually turn the speaker switch off preventing any noise, but I really want to understand what's wrong with the muting circuit.

What could have gone wrong?
- Was the relay click delayed after power on?

The relay works fine, and clicks when it's supposed to. Just to make sure that wasn't the issue, I installed a brand new one, but that has not prevented the pop at power off.
The pair Q404-405 also seems to be working otherwise the relay would not click at power on.

What could have gone wrong?
My educated guess is that it relies on the difference in power capacitor discharge timings. I assume the -71V rail will discharge faster than the -43V. At such, Q406 base will got push up due to the charge still held in C418. In this case, I will check C418 to make sure it's still a 10uF cap and Q406 is still a good transistor.

That is also what I understood. I would not think the capacitors are the issue here, as they are all brand new (I did a full recap with high quality caps). I will try and replace Q406, just to see if that makes any difference. Does anyone know if old transistors see a worsening of their switching times?

Would you think that a leaky snubber diode D901 could also in theory be responsible for the pops?

 

[audio3000]

Was the relay clicks right after the amp was powered off? To make sure the root cause is 100% in the protection circuit, how about manually de-energize the relay before powering off the amp to see if the loud pop is still there.

 

[llwhtt]

Pull the plug instead of turning it off, still pop?

Craig

 

[spaghetti]

@MattiaM were you able to find a solution to the problem?

 

[cgutz]

Somewhat of an old thread, but for reference, I have had my TA4650 since it was new, and I have never heard a pop when powering off.

Hope the OP got this straightened out.