Rotel RSX-972 Pops Upon Power Down. Advice?

[W.T. Holt]

I'm working on this Rotel unit that had an output transistor blown on the right channel due to accidental crossed speaker wires. The offending transistor (and its matched partner and the emitter resistor) have been replaced. The amp now turns on and plays... However the DC offset on the output is now somewhat high - around 60 to 70 mV on the left channel, 30 mV on the right. Even more of an issue though is that the amp is sending DC to the speakers right as it is turned off, resulting in a loud pop and much stress on the connected speakers.

Trying to trouble shoot this problem has me thinking about the protection relay and wondering if replacing it will take care of the pop or if there are other components in the amp that should be looked at first. I'm hoping someone who is a bit better at reading a schematic than I am will take a look and point me in the appropriate direction.

Service manual can be found here. The RSX-927 is second from the top under the "Surround Receivers" heading.

Thanks in advance to anyone who can help!

 

[Tom B]

This is a newer style surround receiver. I looked at the schematic for you.
I don't see any true Speaker relays, but probably has a standby power relay for firing up the main power supplies. If your unit has what you think is a speaker relay, let me know what board it is on and what it is called, and the # of connections.
Newer style also means there aren't any bias adjustments or DC balance adjustments either. and they are probably using a muting circuit to shut down the outputs on power off.
So your task would be to identify the channel causing the POP on power off (Front left, Front right, Rear Left, Rear Right, Center). Best way to do this would be an analog meter on DC volts. If the POP is on more than one channel, it could be an issue with one of the power supplies, or the muting circuit.
If it is just one channel, I would expect quite high DC to cause the pop, in that 70mv shouldn't do that. When you find the problem channel, check all of the transistors diodes and resistors in that channel for shorts, open's, leaky, or out of tolerance. Also check and make sure that the board connections werent damaged during output and emitter resistor replacements.
This is all that I can suggest for you good luck

 

[W.T. Holt]

Thanks Tom. The relay in question is RLY901 on the standby board (#4002002700 on pg 21 in the service manual). The pop comes through loud and clear on all channels when a speaker is connected and the amp is turned off... could you expand on what to check with the analog meter on dc volts (faulty components on the power amp channels I assume or perhaps voltage on the speaker outputs)? I'm also not sure where to look for the muting circuit so that I can start checking it out.

 

[Tom B]

That relay is used to turn the power supplies on from standby. Its not a speaker protect relay used by some older units.
You can see what leve of DC is hitting the speakers by measuring the speaker terminals using the analog DC voltmeter. You can use a digital as well, but it can be harder to see. First measure each channel with the speaker connected. Then measure again, one channel at a time, with the speaker disconnected, and just measuring the output terminals. (You are measuring + to - for each speaker connection, or just + to chassis ground)
The muting in that unit is more of an electronic shutdown of transistor circuits to "Mute" the output. It is hard to see on the schematic other than showing muting input for each of the power amplifiers. You can try to monitor that point with a voltmeter (analog or Digital) to see if you have a voltage change upon shutdown, or no change. No change could be an issue further upstream.
double check what you did while having to replace the other parts. Did you forget to connect any connectors? did you accidently put a connector in the wrong place, or offset a pin, or backwards??
Did you leave any screws out that may have been a circuit boards ground connection??

 

[W.T. Holt]

Ok so I have double checked all the connections, solder joints and screws that I have had to adjust or move or otherwise modify for this repair... every thing seems alright.

I also tested the speakers outs in the method you described. I only have a Digital Mutimeter but according to my readings I'm getting approximately 70 mV to the left speaker terminal and 30mV to the right regardless of whether or not the speakers are actually connected. If I power the amp down with the meter connected the reading spikes somewhere above 300mV and then slowly bleeds down to zero.

When I went to look into the Mute input on the power amp it came to light that transistors Q218 and QR219 which are marked on the schematic were not on the board... What? That's giving me pause.

I did however trace the Mute input back which led to a circuit marked "Mute_VCC" in the manual which can been seen on page 23 in the bottom 1/3 of the "4002002500 Power Supply Drawing". Am I right in thinking this is the mute circuit? If so I'm very curious as to how that circuit functions and what may have gone wrong with it in this particular unit to make it not work. The issue certainly must stem from the transistors in the power amp blowing.

I'm afraid my hardheadedness is getting the better of me on this one. I've spent so long puzzling over this amp that I'd hate to give up now. However I am wondering what the harm may be in deciding to let this issue be. Obviously hearing a transient pop through the speakers is no good and I really wouldn't want to put any tweeters at risk. But if I turn the volume down on the unit before I turn it off no pop comes through... maybe I just need to come to terms with this workaround.