Protection Circuit and Speaker Relay Question

OctalSocket

Active Member
Just recently rotated my Sherwood AM-7040 back into use after more than a year of storage. I brought it up gently with a Variac, and heard an approximately one to two second"bzzzp" as I was bringing the voltage up past 50 or so VAC. I assumed it was the speaker relays buzzing until they got adequate supply power to the coils.

Once powered up, the amp was dead quiet when putting my ear close to the tweeters and woofers of my Bose 601s. Ran some music through it, and all was well. Shut it down after a while, no pops, thumps, etc.

The next time I turned it on (straight to 117V, no variac), I noticed a mild pop in the left channel speaker once the protection circuit light shut off and the speakers came online.

I have the service manual, downloaded from HiFiEngine, but am barely proficient in reading such a schematic. I know what most all the symbols are, and I assume there may be a cap somewhere around the speaker relays that may provide a soft start of sorts, but I have no idea which one it could be, if that's even it.

Any tutelage and advice would be greatly appreciated. This is a pretty darn good power amp, and I don't want to be damaging anything, especially a speaker relay that may be arcing, causing that mild pop.
 
Just recently rotated my Sherwood AM-7040 back into use after more than a year of storage. I brought it up gently with a Variac, and heard an approximately one to two second"bzzzp" as I was bringing the voltage up past 50 or so VAC. I assumed it was the speaker relays buzzing until they got adequate supply power to the coils.

Once powered up, the amp was dead quiet when putting my ear close to the tweeters and woofers of my Bose 601s. Ran some music through it, and all was well. Shut it down after a while, no pops, thumps, etc.

The next time I turned it on (straight to 117V, no variac), I noticed a mild pop in the left channel speaker once the protection circuit light shut off and the speakers came online.

I have the service manual, downloaded from HiFiEngine, but am barely proficient in reading such a schematic. I know what most all the symbols are, and I assume there may be a cap somewhere around the speaker relays that may provide a soft start of sorts, but I have no idea which one it could be, if that's even it.

Any tutelage and advice would be greatly appreciated. This is a pretty darn good power amp, and I don't want to be damaging anything, especially a speaker relay that may be arcing, causing that mild pop.
Check your DC offset in the channel you heard the pop.
 
Does it pop after it has warmed up a bit? Let it run for 10 min. Shut it off and turn it back on.
 
Thanks for the quick reply, Avionic.

Yes, even after being on for a while, if I turn it off, wait 15 or so seconds, then turn it back on, the left speaker still makes that one pop sound right when I see the protection light go out and hear the speaker relays energizing.

I did adjust DC offset for both channels back in 2014 or so, and it's been in storage more than not since then. Will check that again.
 
Yep, that was it. I forgot about the thumping symptom mentioned in EchoWars sticky when I did this five years ago.

When I checked the DC offset, the right channel was reading around 350 mV and that wasn't the bad channel. The left channel, the one that gave the pop, was reading almost 3 Volts!

Adjusting the right channel went quickly; got it down to hover around 2 mV. The left channel, however, took quite a bit of adjusting one way, then the other, then back and forth numerous times. I'd get it to read as close to 0 as possible, whether negative or positive voltage, then would let it settle for a few minutes. All the while it would slowly wander over a range of 30 or so mV. I finally decided to call it as good as it'll get when I got it to drift anywhere between + or - 20 or so mV from zero.

Reassembled the faceplate and lid, then put the amp back in the system, and now the pop is gone. Just a subtle tick from both channels when the protection light went out, just like it used to do. I also noticed the balance was more centered, rather than off-center right before. The bass, overall, feels to be better controlled. The amp sounds great again.

I still wonder if something is not quite right with that left channel, since the measured offset would not hold relatively steady.
 
Yep, that was it. I forgot about the thumping symptom mentioned in EchoWars sticky when I did this five years ago.

When I checked the DC offset, the right channel was reading around 350 mV and that wasn't the bad channel. The left channel, the one that gave the pop, was reading almost 3 Volts!

Adjusting the right channel went quickly; got it down to hover around 2 mV. The left channel, however, took quite a bit of adjusting one way, then the other, then back and forth numerous times. I'd get it to read as close to 0 as possible, whether negative or positive voltage, then would let it settle for a few minutes. All the while it would slowly wander over a range of 30 or so mV. I finally decided to call it as good as it'll get when I got it to drift anywhere between + or - 20 or so mV from zero.

Reassembled the faceplate and lid, then put the amp back in the system, and now the pop is gone. Just a subtle tick from both channels when the protection light went out, just like it used to do. I also noticed the balance was more centered, rather than off-center right before. The bass, overall, feels to be better controlled. The amp sounds great again.

I still wonder if something is not quite right with that left channel, since the measured offset would not hold relatively steady.

The pot for DC offset may need to be cleaned or replaced. Did you clean or replace the offset pots back in 2014?
 
I would also move the bias pots backwards and forwards a bit (with the unit switched off). Note the position where you started from and make sure that you put it back in the same place. The offset drifting you are seeing could be due to unstable bias voltage on the bases of the output transistors. If it stabilises I would also check the bias volt drop across the emitter resistors to make sure that it is to makers spec.
 
When I did this last time, I believe I saw a post where someone said this particular amp only has offset pots, but no bias pots.
Will check the schematic to verify.
 
Hit the pots with contact cleaner last night. It helped, both channels read about 5 mV +/- 3, so I'm satisfied with that. I like this amp well enough that I'm contemplating building a pretty competent second system. I have a couple other components for it, so it wouldn't take much. Speakers, primarily.
 
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