Pioneer SX-950 Speaker Pop at Power-Off

[Brivan]

Hi, everyone

I have a Pioneer SX-950 that makes an audible "pop" noise through the speakers when powered off. I have no detectable noises from any of the controls in either the flat amp or preamp. I've read through many threads on AK for a solution to the problem and have implemented some of the fixes with no good results. Here's what I've observed, so far:

Pop noise occurs at the same time (approximately) the relay opens - which is a fraction of a second after power-off.
DC offset on both channels at around 1mV. Idle current at around 20mV.
The popping noise is affected by the volume control.
Removing the pre out/main in jumpers eliminates the popping noise.
Using an audio signal tracer, there is no popping noise from either the tone amp or flat amp.
Removing the audio connection between flat amp and tone amp eliminates the noise (making me think the problem is coming from the flat amp).
Voltage levels measured at the flat amp audio out pins fluctuate to around 5v at power-on and around -5v at power-off.

Things I've done:
Read through a ton of AK posts on this subject!
Replaced Q6 and Q7 on the protection board (even though they tested good).
Replaced R23 12K resistor with a 9.1K resistor and C5 4.7uf 25v capacitor with a 2.2uf capacitor (from an AK post).
Replaced C18 on the power supply board (even though it tested good) (from an AK post).
Replaced C11 & C12 with 4.7uf 50v capacitors on the flat amp (from an AK post).

From what I've done and from the AK posts I've read, the timing of the protection relay opening and the decay of voltage to the flat amp / tone amp section doesn't leave much room for error and the popping noise can likely be a result of things coming out of tolerance in either area. There are some open-ended posts that never did seem to find closure on this problem and I'd be glad to finally post something with steps taken to reach a conclusion but I need a little help, here.

If anyone would be kind-enough to lend their knowledge on this, it would be appreciated!

 

[markthefixer]

check R24 15k apparent discharge cap for C5 AC voltage detect circuit. It really is the R2 of a voltage divider R25/R24, the tip off is the voltage found at C5, without it there will be 7.5v x 1.414 = 10 volts minus whatever current is drained away.

be sure R25 is 2.2k

Changing R23 to 9k INCREASES the base current and makes it harder for the R22 (6.8k) / R29 (47k) voltage divider from 36.5v to -13.5v to counteract it.

be sure R27 IS 22 ohms

is D8 a 1n4004 versus an itty bitty glass bodied diode?

 

[Watthour]

It may be a band-aid fix, but have you tried to clip in a diode backward (cathode/band to Pin 11) across the relay coil to help suppress the CEMF from the relay? A 100PRV should work nicely.

 

[Brivan]

It may be a band-aid fix, but have you tried to clip in a diode backward (cathode/band to Pin 11) across the relay coil to help suppress the CEMF from the relay? A 100PRV should work nicely.

Thanks for the suggestion. I temporarily connected a 1N4004 diode across the protection relay coil contacts as indicated in your post, but it unfortunately didn't make a difference in the popping noise at power-off.

 

[Brivan]

check R24 15k apparent discharge cap for C5 AC voltage detect circuit. It really is the R2 of a voltage divider R25/R24, the tip off is the voltage found at C5, without it there will be 7.5v x 1.414 = 10 volts minus whatever current is drained away.

be sure R25 is 2.2k

Changing R23 to 9k INCREASES the base current and makes it harder for the R22 (6.8k) / R29 (47k) voltage divider from 36.5v to -13.5v to counteract it.

be sure R27 IS 22 ohms

is D8 a 1n4004 versus an itty bitty glass bodied diode?

Hi, Mark. I pulled R23, R25 & R27 and tested all three to be almost perfect to their stated values. I replaced all three resistors just to be sure they weren't causing problems. D8 was, indeed, an itty bitty glass-bodies diode - I replaced it with a 1N4004. Also, I removed the 9.1K resistor I had subbed in for R23 and changed it back to 12K.

Unfortunately, none of this made any noticeable difference in the popping noise (mostly heard in the speaker woofers) at power-off.

On the protection board, I still have Q6 2SC945 replaced with a KSC2383 and Q7 2SC1384 replaced with a KSA2690. C5 still has the 2.2uf cap in place of the original 4.7uf cap.
On the flat amp, I still have C11 and C12 0.1uf caps replaced with 4.7uf caps.

I'm open to any more suggestions you may have. Thanks!

 

[markthefixer]

edit - just a minor typo "Q7 2SC1384 replaced with a KSA2690."we all know it's KSC2690.


Let's go back to basics:

FU6, 1.5 amps on the corner of the AWR-101 power supply board, by pins 24, 23, & 22. Pin 23 goes to pin 12 of the protect board AWM-062

operate FU6 like an old-time knife switch, when the lights go off the protect should kick in. Does it thump?

voltages to check for the protection board,
pin 2 -13.5v
pin 10 +36.5v

also
make SURE pin 4 is grounded
check C5 negative (4.7uf) voltage to ground when on
also Q6 base to ground when protect is on, and also when protect is off.

make SURE the relay is disconnecting the amp from the speakers!! (ohmmeter check)

 

[Brivan]

edit - just a minor typo "Q7 2SC1384 replaced with a KSA2690."we all know it's KSC2690.


Let's go back to basics:

FU6, 1.5 amps on the corner of the AWR-101 power supply board, by pins 24, 23, & 22. Pin 23 goes to pin 12 of the protect board AWM-062

operate FU6 like an old-time knife switch, when the lights go off the protect should kick in. Does it thump?

voltages to check for the protection board,
pin 2 -13.5v
pin 10 +36.5v

also
make SURE pin 4 is grounded
check C5 negative (4.7uf) voltage to ground when on
also Q6 base to ground when protect is on, and also when protect is off.

make SURE the relay is disconnecting the amp from the speakers!! (ohmmeter check)

Thanks, again, for the information! I'm out of the shop until Tuesday (due to what we call Native American Day coming up on Monday in these parts). I'll get back at it and post my results, then.

 

[BobbyN]

Mark, Could it be a ground problem? I'm not a pro but can think of a couple times in the past where improving grounds (such as a star washer under a screw) has cured strange issues.

Bob

 

[markthefixer]

Not too likely.

More likely the opposite - once the protection circuit's established as operating within it's expected timing range for loss of AC in the lighting circuit
then we have to check the regulators to see that they are regulating with sufficient head room and aren't collapsing FASTER than that.

Then there's internal RC filters on some of the boards, the R 's voltage drop is an expression of the current being drawn, and the capacitor's value is a reflection of the stored charge available in the capacitor.

A triple whammy would be a fast collapsing supply just barely on the edge of regulation (OR LOWER) and an inordinate draw by a particular RC filtered circuit, that is already operating at a lower voltage and collapsing faster, then add in loss of capacitance

Ya wanna get really wild? How about a relay with a lot of extra magnetic material that takes a tiny bit longer to release? or even a weak spring?



BRIVAN re-reading the OP, leaves me with a few questions.

First, what if anything besides the protection board, C18 on AWR-101 psu,
C11, C12 on AWG-038 flat amp appear to be NOT original (refresh / recap etc work counts!!)

The popping noise is affected by the volume control.
Removing the pre out/main in jumpers eliminates the popping noise.

THAT narrows the choices considerably.

Using an audio signal tracer, there is no popping noise from either the tone amp or flat amp.
Removing the audio connection between flat amp and tone amp eliminates the noise (making me think the problem is coming from the flat amp).

THAT narrows the choices even more.


How did you test the flat amp for popping noise using an audio signal tracer?

RE: C11, C12 on AWG-038 flat amp are 0.1uf capacitors, C5 , C6 are 4.7uF / 50v..... huh??????????????????

 

[markthefixer]

this is the only active electronics BEFORE the volume control, excluding the RIAA phono amp.

It runs off the main 51.5 volts.

 

[Brivan]

edit - just a minor typo "Q7 2SC1384 replaced with a KSA2690."we all know it's KSC2690.


Let's go back to basics:

FU6, 1.5 amps on the corner of the AWR-101 power supply board, by pins 24, 23, & 22. Pin 23 goes to pin 12 of the protect board AWM-062

operate FU6 like an old-time knife switch, when the lights go off the protect should kick in. Does it thump?

voltages to check for the protection board,
pin 2 -13.5v
pin 10 +36.5v

also
make SURE pin 4 is grounded
check C5 negative (4.7uf) voltage to ground when on
also Q6 base to ground when protect is on, and also when protect is off.

make SURE the relay is disconnecting the amp from the speakers!! (ohmmeter check)

Hi, Mark. I see multiple posts from you since my last post, so I'll reply to each one separately.

When I flip one end of FU6 out of the fuseholder, I hear no pop from the speakers. I should note that the "pop" noise isn't the same low thump heard when a protection relay is bypassed and the woofers move in or out as the amp boards de-energize. There's a fair amount of mid-range in the pop noise, as well.

The protection relay was replaced because of crappy contacts causing distorted audio. I had the pop noise at power-off when the original relay was in place, as well. The new relay is an Omron MY-02-DC24 that snaps nicely into the case of the original relay if one tab is filed off the end of the relay base.

I'm confused on some of your references to the power supply / protection board voltages. Pin 2 should be 5.4v and it measures that exactly. Pin 10 measures 36.5v. Pin 4 isn't grounded - it measures 13.8v which, according to the schematic is correct.

 

[Brivan]

Not too likely.

More likely the opposite - once the protection circuit's established as operating within it's expected timing range for loss of AC in the lighting circuit
then we have to check the regulators to see that they are regulating with sufficient head room and aren't collapsing FASTER than that.

Then there's internal RC filters on some of the boards, the R 's voltage drop is an expression of the current being drawn, and the capacitor's value is a reflection of the stored charge available in the capacitor.

A triple whammy would be a fast collapsing supply just barely on the edge of regulation (OR LOWER) and an inordinate draw by a particular RC filtered circuit, that is already operating at a lower voltage and collapsing faster, then add in loss of capacitance

Ya wanna get really wild? How about a relay with a lot of extra magnetic material that takes a tiny bit longer to release? or even a weak spring?



BRIVAN re-reading the OP, leaves me with a few questions.

First, what if anything besides the protection board, C18 on AWR-101 psu,
C11, C12 on AWG-038 flat amp appear to be NOT original (refresh / recap etc work counts!!)


THAT narrows the choices considerably.


THAT narrows the choices even more.


How did you test the flat amp for popping noise using an audio signal tracer?

RE: C11, C12 on AWG-038 flat amp are 0.1uf capacitors, C5 , C6 are 4.7uF / 50v..... huh??????????????????

The receiver appeared to be untouched inside. It had the typical 40-year dust accumulation, all wires in place, original lamps, etc. I saw no solder points that looked fresh - they were all dull gray from years of oxidation.

I've installed the original 0.1uF caps back in place at C11 & C12 in the flat amp. From an AK post about clearing up a power-off popping noise, I had replaced them with 4.7uF as suggested by the author of the post.

Right now, the only things that have been changed in the receiver are the protection relay (Omron MY-02-DC24 fitted to the original relay case) and C18, Q6 & Q7 on the power supply board.

I test for audio signal coming from the flat amp, I disconnected wires at terminals 1, 2 & 3 of the tone amp board (isolating the two), then testing audio at pins 1, 2 &3 of the tone amp (pop at power-off) and flat amps pins 19, 20 & 21 (no pop at power-off).

Per your circled areas of the flat amp schematic, I'd like to try replacing the 2SC1885 transistors with KSC2383. These appear on some "shoot on sight" transistor lists I've seen on AK, so it looks like a good next step. Thanks and let me know if there's any information you requested that I missed!

 

[Brivan]

this is the only active electronics BEFORE the volume control, excluding the RIAA phono amp.

It runs off the main 51.5 volts.

View attachment 2683971

Okay, another update from my last post. I went ahead and replaced the 2SC1885 transistors with KSC2383. No change in the popping problem. I noticed you had C27 and R49 circled in the schematic snip from your last post. I pulled and tested C27 and it checked fine. I replaced it, anyway. I checked R49 in-circuit and it tested right at it's indicated value - I can pull a leg and re-check if needed. The pop noise still persists.

I checked all DC voltages from the power supply board and they were all within 1 volt of spec.

Again, I flipped up one end of FU6 with the receiver powered up. The protection relay disengaged, but no popping noise from the speakers.

 

[markthefixer]

I checked all DC voltages from the power supply board and they were all within 1 volt of spec.

Again, I flipped up one end of FU6 with the receiver powered up. The protection relay disengaged, but no popping noise from the speakers.

It is undeniably a power supply voltage collapse. This is an ORIGINAL power supply, correct? You obviously have a supply decaying FASTER than the on/off detect circuit. One of those original caps is hanging on "by it's fingernails"....

I don't think the 13.9v or 5.4v power can do this, and the +36.5v , +28v and -19v supplies have no storage capacity,
they depend upon the +51.5v and -51.5v supplies as their sources.

Thus we devolve down to C10, C11 at 470uf / 80v , C15 at 470uf / 63v and C20 at 100uf /63v.

It would be better to do them all and just not these four, because the smaller caps have less water to lose.

 

[Brivan]

It is undeniably a power supply voltage collapse. This is an ORIGINAL power supply, correct? You obviously have a supply decaying FASTER than the on/off detect circuit. One of those original caps is hanging on "by it's fingernails"....

I don't think the 13.9v or 5.4v power can do this, and the +36.5v , +28v and -19v supplies have no storage capacity,
they depend upon the +51.5v and -51.5v supplies as their sources.

Thus we devolve down to C10, C11 at 470uf / 80v , C15 at 470uf / 63v and C20 at 100uf /63v.

It would be better to do them all and just not these four, because the smaller caps have less water to lose.

Yes, the power supply is original.

I'm glad you came back with the advice on replacing the power supply capacitors. Just a few hours ago, I was staring at the schematic looking at those same caps. I should be able to get those (and others) changed out tomorrow and can hopefully report back with some good results.

Throughout this whole process, I was never sure if the popping noise (whether it makes it through the speakers or is cut off by the protection relay) was a normal side effect of powering the receiver down and, at the same time, wondering if the protection relay drop-out was fast-enough or if the popping noise coming from the flat amp was too fast for it.

Above all, I want to bring closure to this so someone else eagerly reading this thread, hoping to solve the same problem, isn't left dangling with an OP who never gets back with a resolution. I went through several threads trying to figure this out before I posted my own.

 

[Brivan]

It is undeniably a power supply voltage collapse. This is an ORIGINAL power supply, correct? You obviously have a supply decaying FASTER than the on/off detect circuit. One of those original caps is hanging on "by it's fingernails"....

I don't think the 13.9v or 5.4v power can do this, and the +36.5v , +28v and -19v supplies have no storage capacity,
they depend upon the +51.5v and -51.5v supplies as their sources.

Thus we devolve down to C10, C11 at 470uf / 80v , C15 at 470uf / 63v and C20 at 100uf /63v.

It would be better to do them all and just not these four, because the smaller caps have less water to lose.

Okay, all electrolytic caps in the power supply have been replaced and we definitely have good results. When powering off the receiver, the popping noise has been reduced to almost zero. There's still a small amount of that same noise present, but it's pretty quiet - and tolerable. From the power supply, C10, C11 and C15 all tested weak - anywhere from 330uF to 390uF. I'm happy with the outcome and am ready to start putting things back together, unless you have any ideas on eliminating that last small amount of power-off noise.

 

[Brivan]

More results to report: Upon checking to make sure all my earlier modifications had been removed, I discovered C5 on the protection board was still at 2.2uF (instead of 4.7uF) from an earlier attempt at making the popping noise go away. Wanting everything back to original values, I replaced it with a 4.7uF cap. At the next power-off, the popping noise was back. I reinstalled the 2.2uF cap and tried, again - popping noise mostly gone. For the time-being, I'm leaving it at 2.2uF.

I tried power-off with the volume control at about the half-way mark and got a pretty good pop. Noting that I always got a hum when I touched the volume control pot while the receiver was turned on (the flat amp has remained unmounted during all of this), I grounded the faceplate of the flat amp board to the receiver's frame and powered off, again: popping noise greatly reduced. After more trial and error, I discovered I could eliminate even more popping noise at power-off if I was touching the receiver's frame when doing it.

At one-third volume, everything's quiet on power-off and I would say this is acceptable, since powering off a receiver with a protection relay at half-volume is asking for problems with relay contacts later on and shouldn't be done.

 

[Scheid]

Very informative post !!!

Brivan, Did you end up leaving the 2.2uF in as C5?

I am working on this same issue and was wondering as it seemed to effect yours.

Thanks for any feedback.

 

[Brivan]

Very informative post !!!

Brivan, Did you end up leaving the 2.2uF in as C5?

I am working on this same issue and was wondering as it seemed to effect yours.

Thanks for any feedback.

Yes, I did. From my notes for elimination of the popping noise at power-off:
Replace C5 4.7uf 25v capacitor with a 2.2uf capacitor and R23 12K resistor with a 16K resistor on the protection board (protection relay engagement may be delayed when connected to a DBT, but should operate normally on mains power).

Also, it seems to me that the popping noise may have been less or completely gone when switching power sources from a dim bulb tester to mains power. I didn't make a note of this so I may not be remembering correctly.

 

[Scheid]

Thank you so much for your response.

I will definitely try these 2 parts 1st and go from there.