Speaker pops on power up

[Lou Weed]

Howdy! I have a technics sa-5360 been using it for about 5 years with no issues. Recently it started to make a “pop” when I power it on. It’s not super loud but I can hear I and I can see the speakers push out.

I realized I can skip the speaker pop if I have the speakers switched off when I power up, I wait for the relay to click, wait about 5 seconds or so and then turn on the speakers. No pop.

I checked the dc offset at the speaker terminals and when the amp is warm it runs around 40-45mv on each channel. From what I read here not ideal but not terrible.
Basically my question is: does any of this pose a risk to my speakers?
I’m still learning and I think any repairs at this Point are beyond me. Someday I’d like to do some Recaps and learn on some old pieces I have lying around but not yet!

I’m wondering if I keep the same power on sequence for my technics is it ok to just run it and enjoy the music….. or is this popping a sign for concern and maybe I should take it somewhere to get it looked at.
Cheers and thanks!

 

[Chris Brown]

For curiosity sake you might try measuring the DC offset during power-up, just to see what kind of DC voltage is being sent to your speakers during that time. It's difficult to judge that just based on the sound of the pop. Also make sure to keep an eye on the regular (after warming up) DC offset from your amps over time and make sure it's not slowly increasing. If it climbs over 50mV I would strongly consider having it looked at.

 

[Lou Weed]

For curiosity sake you might try measuring the DC offset during power-up, just to see what kind of DC voltage is being sent to your speakers during that time. It's difficult to judge that just based on the sound of the pop. Also make sure to keep an eye on the regular (after warming up) DC offset from your amps over time and make sure it's not slowly increasing. If it climbs over 50mV I would strongly consider having it looked at.

It seems when it’s cold and first turned on it runs right around 50mv when first fired up.
If I try to measure it while I hit the on switch it pops up to around 250-300 and then settles back down within a second.
If I leave it on for an hour and measure both channels seem to be steady at 40mv.

 

[Semanon]

Looked at the schematic for that. Lots of ICs in that unit, ‘specially for 1976 or 77. Can’t see a way to adjust the offset.

 

[chazix]

Recently it started to make a “pop” when I power it on. It’s not super loud but I can hear I and I can see the speakers push out.

This must mean that the speaker protection relay has failed in such a way that the contacts are always closed - I don't think there is any other remotely plausible explanation. It is not worrisome that the amplifier produces transients on power-up - the transients should not reach the speakers because the relay contacts should be open. If a click is still heard a reasonable delay after powering up, then the protection circuit is still trying to do its job, but it can't because the relay contacts aren't actually open. I think if you just replace the relay, all will be well.

And I think it's pretty important to do so, rather than just rely on the speaker switch workaround. At this point, the relay cannot serve its speaker protection function if something goes wrong while the receiver is playing.

 

[Semanon]

This must mean that the speaker protection relay has failed in such a way that the contacts are always closed - I don't think there is any other remotely plausible explanation. It is not worrisome that the amplifier produces transients on power-up - the transients should not reach the speakers because the relay contacts should be open. If a click is still heard a reasonable delay after powering up, then the protection circuit is still trying to do its job, but it can't because the relay contacts aren't actually open. I think if you just replace the relay, all will be well.

And I think it's pretty important to do so, rather than just rely on the speaker switch workaround. At this point, the relay cannot serve its speaker protection function if something goes wrong while the receiver is playing.

You so smart….You think the relay is stuck closed? Welded due to arcing?

40mv of offset seems a little high to me, regardless.

 

[Tom B]

I agree that there may be an issue with the relay contacts frozen, or the protect circuit has been disabled by someone bypassing the relay contacts. But 40mv on both outputs seems to point to a possible difference in the power supplies. Check the +35v, and -35V to see if they are fairly close in value. Also check those 100 ohm fusable resistors which can drift in value, giving you different voltages at the early stage of the power pak IC. Lastly, they isnt any real DC offset adjustment, and the 40mv may just be internal with the IC. Those IC's stamped as STK0182 are hard to find, but can be easily substituted with a STK084.

 

[Skywatcher]

You so smart….You think the relay is stuck closed? Welded due to arcing?

40mv of offset seems a little high to me, regardless.

40mv is "normal" for this model. There is no offset adjustment to compensate for drift caused by aging components. We'd like to see it under 10mv closer to zero, but it won't hurt anything being 40mv. Some units consider anything 100mv and under to be "in spec"...

 

[ecluser]

It is true that there was no provision to adjust the DC offset in this circuit, but it is adjustable to those who have a good understanding of this circuit. If your 40mV offset is positive, increase R613, R614 from 39k to 43k. If your 40mV offset is negative, reduce these resistors to 33k.

 

[Skywatcher]

It is true that there was no provision to adjust the DC offset in this circuit, but it is adjustable to those who have a good understanding of this circuit. If your 40mV offset is positive, increase R613, R614 from 39k to 43k. If your 40mV offset is negative, reduce these resistors to 33k.

If you really want to make it easy, replace the resistor with a 50k pot, then adjust it on the fly.

 

[ecluser]

That would be oversensitive.