Pop in Pioneer SX-950

Posted previosly in Vintage solid state.
I just finished recapping my two SX-1280's with MARK's help and I'd like to give it one more shot. I had a technician (that specialize in vintage audio!!!!)to look at it, but he could not fix it. So, here are the symptoms:
When powering the unit ON, with or without any input, tone amp switch ON or OFF, filters ON or OFF, volume control HI or LO there is NO pop. Immediately after powering, turning the tone switch ON or OFF, it results in loud pop in both speakers. At the same time, with the tone amp switch OFF, turning the LOW filter ON or OFF will also result in pop, but not as loud. The pop is louder when turning both switches OFF!!! After about one minute of a warm up time, OR flicking the tone switch ON/OFF, there is NO pop!!! Turning the unit OFF, with or without any input, with volume all the way down, there is NO pop. However, with the volume up, there is a pop in both speakers. So far I changed the position of caps C29 and C30 on tone amp AWG-039, cause on the board they show different hook up then in the schematics (pin #1 and #3 goes to plus in the schem. and to minus on the board). The schematics of SX-850 also shows pins #1 and #3 connected to plus side. So I did what the schematics shows. Not much inprovement. Unless the schematics are wrong, all the caps are hooked up accordingly, using Nichicon UKL's in all sensitive places.
There it is, any ideas?
Thanks

Boiling it down, if the volume control is turned all the way down there is no pop? No exceptions?

If not, stick to these conditions: tone off, filters off - this is the most straight through path.

Then pull one pre-power jumper, see if the other channel pops.
Then replace that jumper, and pull the other jumper and see if the first channel pops or not.
We are trying to see if both channels pop, by only listening to one at a time.

As an explanation, DC is probably present before the tone amps and the filters, and the tone amp and filter when engaged REMOVE the DC causing the pop. The volume control killing the pop is an interesting symptom.

Volume control atw down eliminates the pop only when turning the unit OFF, with or without anything else present (input), or TONE control switch and LO filter ON or OFF. Volume control does NOT affect the loudness of the pop, when after powering the unit ON, I turn the TONE control or LO filter ON/OFF.
Performed the task You asked. Results: left jumper removed, pop in right channel only. Right jumper removed, pop in left channel only.
I tried something on my own, it might help to ID the problem. I used the SX-1280 to do this: I removed jumpers from both receivers. Then I used power amp from the SX-950 and hooked up the pre-amp from the SX-1280. NO pop at any time. Then I switched it around, using the SX-1280 power amp and the SX-950 pre-amp. Result: loud pops switching the tone amp ON/OFF, also turning the unit (950) OFF.
Hope You will be able to make heads or tails out of this.
One more thing. Turning the unit OFF with full volume and balance control atw to right, only the right speaker will pop. Same with the left side.
Thanks

Is there any DC or AC voltage at the pre- power-amp jumpers (referenced to ground) with the volume down and unit on? Following Mark's theory ( I think) the suggestion is that DC might be present at those terminals.

I believe there should be a pair of 'lytics on the flat amp board to kill any DC out to the power amp. If those are intact the voltage at the pre-out jumpers should be nil.

Voltage at the jumpers (to ground): NO AC voltage present on left or right. Turning the unit ON, my DMM in DC voltage mode goes to overload, then levels off at +6.0mV, both channels; turning it OFF, it goes to overload in minus then slowly recedes to 0V, both channels. Voltage at pre-amp terminals, jumpers removed: power ON: DMM goes to overload, levels off at +9VDC, then slowly recedes to 5mV, both channels; power OFF: DMM goes to overload, levels off at -7.2VDC, then very slowly recedes to 0V.
All this with everything OFF. When TONE amp is ON, DC voltage goes to +0.5mV, with or without the jumpers.
I have couple more details. When after turning the unit ON, no input, volume atw down, with tone amp switch in ON pos. and turning it OFF immediately, it will cause the protective relay to disconnect; (the pop being extremely loud in both speakers). Repeating it with the tone amp switch in OFF pos., LO filter ON and turning it OFF immediately after turning the unit ON, will cause same results. Doing it in reverse order,(tone amp OFF and turning it ON) will not cause the protective relay to disconnect, but the pop will be there, not as loud.
Mark,
You said that DC voltage may be present before the tone amp. What would cause it to be there? Where would it come from? Is there anything else I can try or do?
Thanks

I have the feeling that I have done something wrong, cause I have not heard from anybody, specifically from Markthefixer for a while. If I have, please let me know what it was, so I will not do it again. It is important to me, cause collecting and fixing vintage Pioneer receivers became my hobby after I retired. And without You guys I would be lost.
Thanks

I doubt that you did anything wrong, but it's apparent that some of the in-demand members here are VERY busy by looking at their post times and volumes.

If you keep the thread alive with feedback it's more likely to get noticed.

If I understand correctly, the symptom occurs when the Tone switch is off only. If that's the case, there are two 47µF 50V caps on the Flat Amp board (Pins 19 & 21) which should block DC from the flat amp. Those are supplanted by caps on the Tone Amp Board when the Tone switch is on. I'd check those on the Flat Amp to be sure.

I know this is very confusing, because it involves three different switches. Main tone switch (S3), low filter switch (S4) on the tone amp (AWG-039), and main power switch (ASK-080). The main power switch creates a pop in both speakers only when turning it OFF, and only with main volume (VR2, AWG-038) high, (with or without any input). Switches S3 and S4 create a pop in both speakers when turned ON or OFF (louder when turned OFF) right after turning the main switch (ASK-080) ON. After the unit warms up, S3 and S4 operate without any problem.
Originally I connected caps C29 and C30 (10uF/25V, AWG-039) as shown in P.C. board patterns. But after checking the schematics I reversed them, + side connected to pin #1 and #3, which did not result in any improvement. I downloaded my documents from HiFi Engine, and found lots of mistakes, (especially comparing to SX-850, which has the same two boards, AWG-038, AWG-039).
So, that is the mess I am in.
Thanks for the encouragement
One more thing. The only caps before pins #19 and #21 are C23 and C24 rated 4.7uF/50V; (in my schematics, AWG-038). They are connected negative towards the pins. Voltage on the plus side is 17V DC, 7mV DC on the negative side, (or pins 19 and 21).
Thanks again

Hi,
Since I have not heard from anyone for a while, I did some "experimentation" on my own, knowing it could backfire and I could be left with black chassis full of soot. I have spent enormous amount of time trying to find what I did wrong, (in my basement shop), that my wife already threaten me with divorce. It became an obsession.
Anyway, this is what I did:
First. I disconnected the signal wire on pins #1 and #3 on AWG-039 (tone amp), that comes from pins #21 and #19 of AWG-038 (flat amp). After power ON, results were same as before, switches #3 (tone ON/OFF) and #4 (low filter) making loud pop.
Second. I disconnected the signal wire on pins #7 and #5 (pre out on tone amp), and using jumpers connected them (wires) to pins #21 and #19 on AWG-38 (flat amp). After power ON, again, results were same as before. Turning the receiver OFF with volume control HI, resulted in a loud pop.
I am not a genius by any means, but I would guess that the problem could be in power supply, or protection circuit; OR, could it be, that both boards (AWG-038 and AWG-039) are wrongly connected (the capacitors)?
Could someone give me a hint, please!, please!, please!!!!!!
THANKS in advance

Couple of things. I don't have time to pull the schematic this evening however I am starting to suspect (1) the small signal transistors on the tone board which when failing will cause the pop you are hearing, fix is to pull and change them out, (2) the tone board has a pair of coupling caps at the end of the signal path through the board and as long as you have the tone board pulled I would change them to Nichicon muse non polarized caps, and (3) while you have the board out look for any cracked traces. Mark is very time bound in terms of forum tine at the moment so be patient. Fyi many Pioneers will exhibit this same sympton when those pesky transistors start to fail and as a matter of course I swap them out on every rebuild I do.
-Lee

karel.b said:

Hi,
Since I have not heard from anyone for a while, I did some "experimentation" on my own, knowing it could backfire and I could be left with black chassis full of soot. I have spent enormous amount of time trying to find what I did wrong, (in my basement shop), that my wife already threaten me with divorce. It became an obsession.
Anyway, this is what I did:
First. I disconnected the signal wire on pins #1 and #3 on AWG-039 (tone amp), that comes from pins #21 and #19 of AWG-038 (flat amp). After power ON, results were same as before, switches #3 (tone ON/OFF) and #4 (low filter) making loud pop.
Second. I disconnected the signal wire on pins #7 and #5 (pre out on tone amp), and using jumpers connected them (wires) to pins #21 and #19 on AWG-38 (flat amp). After power ON, again, results were same as before. Turning the receiver OFF with volume control HI, resulted in a loud pop.
I am not a genius by any means, but I would guess that the problem could be in power supply, or protection circuit; OR, could it be, that both boards (AWG-038 and AWG-039) are wrongly connected (the capacitors)?
Could someone give me a hint, please!, please!, please!!!!!!
THANKS in advance

Click to expand...

markthefixer said:

Starting to see more PM's from people with threads looking for help when they don't get a fast enough response. EVERYBODY gets treated equally. My slower response pace will just have to be acclimated to. I'm sorry if there are a bunch of posts and a few fall through the cracks when I do a bunch of answers. IF that occurs it was either buried and overlooked and is an accident, or I just ran out of time. Just bump it up again into visibility.

I posted an explanation that had a political basis (my 20+ years activism in a controversial subject), and after some consultations with the mods (I REPORTED MYSELF), we decided that the thread could get ugly fast, and that thread was removed by MUTUAL consent. Locking the thread was not a real option because anyone could restart it in a new thread.

I never mentioned my activism here because of
1. not wanting to offend anyone of the opposite philosophy, or preach to the (possibly mis-informed) fence-sitters
2. it IS political and that belongs on another board.

So, please do not jump on the mods or admins for the thread disappearing, and DO NOT restart the political subject, especially here, and I will not discuss it here or on any other forum on Karma, I just don't have the extra time and there are better sources of information on it than me...

Like popeye says:- I yam what I yam, and I ain't gonna change.

Click to expand...

Recap the flat amp, replace the 2sa725 transistors, and the 2sc1885

you don't even differentiate if the pop is the left or right or BOTH channels.

and if you are concerned about the power supply or protection board, search out previous posts on the expected power supply and protection pins voltages. Then post yours.

hint - look up the board's numbers and do a google search in audiokarma.org for them.

Hi Mark,
It would be the last thing I would want to do, to start anything at all, political or any other stuff. You have no idea (figure of speech) how grateful I am for the help I received from You and others. What is realy happening is, that I am disgusted with my self for recapping this unit without marking it or taking pictures at first. I apologize, if it sounded any other way.
THANK YOU

Important Point™

markthefixer said:

you don't even differentiate if the pop is the left or right or BOTH channels.

Click to expand...

No NO, the quote of the post was ONLY informational, the starting something was aimed towards those that saw the original post (about 50 views) and it's non-partisan wording on an issue - BUT the thread COULD have become partisan - the risk was too great and I just didn't want anybody jumping on the mods because the changed post was a MUTUAL decision where I bore more blame for bringing the subject into play.

As for the problem, I FORGOT that the unit had been recapped already, thus rendering my advice incorrect... I just don't have the time right now to reread and analyze the whole thread.

post #9 probably has some strong indications of the problem, but now is not my time to figure it out.

I have worked on this unit (on/off) for over a year and I can wait until You are good and ready and willing to help me. This unit was my pilot project and I learned quite a few do's and dont's, but this problem is way over my head.
Thanks

You need a scope to trace the signal through its paths, in order to narrow down the source or the area where it is introduced. A scope allows you to see what is going on and measure signals, bias points as you trace the signal through the paths. Use sine waves to trace a signal, using music as a source is not as good.
Always place the scope ground at the common point ground.
I agree it is probably a DC issue to resolve but that could manifest itself into a leaky ecap, transistor bias issue etc.
Another time I recall you had a hum issue on one channel, it turned out to be an open ground return from the PA brd back to the common point ground. So it is always something or other, the fun of it all.

I have been looking for a scope for a while already, but they are pretty expensive;(on ebay $400-600) plus shipping;($100-150). The main thing is, I do not know how to use it anyway. In another words You are dealing with an amateur.
I have come across exactly the same problem with the SX-950,(searching on the net); I think it was something like "ask the expert", where the "expert" suggested to use a lower impedance speakers. Da??? It sounds like a lot of bull....!
Like You said, it is a lot of fun.
Thanks

karel.b said:

I have been looking for a scope for a while already, but they are pretty expensive;(on ebay $400-600) plus shipping;($100-150). The main thing is, I do not know how to use it anyway. In another words You are dealing with an amateur.
I have come across exactly the same problem with the SX-950,(searching on the net); I think it was something like "ask the expert", where the "expert" suggested to use a lower impedance speakers. Da??? It sounds like a lot of bull....!
Like You said, it is a lot of fun.
Thanks

Click to expand...

If you need modern digital scope, get Rigol 1052 or 1102 directly from China. You should be able to have it for $400-450 delivered.

I'd still follow MTF's advice and replace the suspect 2SA725.2SC1885,s on the flat amp and the caps. What happens of the tone is off, low AND hi filters off? Those switch positions essentially bypass the tone amp completely and isolate the problem to a different area.

Forget the scope, re-cap & change Q1,2,5,6 in the flat amp AWG-038, then re-test.