Pioneer SX-980 DC Offset fluctuation

[Ziconceo]

Hello! Been a long time lurker on here but finally encountered a problem I cannot seem to fix!

Lets start at the basics:

When I first bought this amp one channel was out.

Fixed the channel but noticed that the DC Offset would not stabilize but the Idle current for both channels is a steady 30mv.

The fluctuation is +- 15mv It will jump around like 5mv every second and go up to +15mv or down to -15mv
I am using a kyoritsu 2000A multimeter so it should be extremely accurate.

This amp is running a pair of JBL Decade 36 speakers.

Speaker wires are soldered to the speakers and attached to the amp via gold plated bananas.

I replaced the trimmers with: MFG Part No: 3386H-1-104LF Bourns 3/8" 100Kohms 10%

Still fluctuates. Same amount.

Wiggling every dial,switch,knob,connection changes nothing.Tapping on every component does not change fluctuation.

Thermal camera shows everything running cool and consistent, no hot spots. All transistors are electrically isolated from their heat sinks. Every transistor was diode tested for leakage before I put them in.

Audio that comes out is decent except there is a crackling in the right channel when say a distorted guitar note is played, the crackling is about 20% of the volume that the other sounds are. Some songs its there and other songs its not. Probably 40% of songs have crackling going on.

If I play music for about 20 minutes the crackling starts going away and sometimes I don't hear it for awhile...

I decided to do a full rebuild cause why not.

Wima caps, all new transistors (KSA,KSC), nichicon electrolytic, new main filter caps (22,000uf at 80 volts) all purchased from mouser, all transistors were Hfe matched to 1% or less where it was needed. Cracked solder joints de-soldered and re soldered. On all boards except for the tuner.

After the rebuild it sounds amazing! except there is still the same exact DC offset fluctuation and crackling in the right channel!

The fluctuation seems strongest around Q6 on both AWH-073 Boards

All resistors have had their legs lifted and tested for resistance, any that werent within 5% were swapped for 1% ones rated for 1 watt minimum or 5 watts for the big ones. All are now within 3%.

All transistors have been checked for correct voltage at the legs. All are within 5%.

Power supply Board is stable and puts out correct voltages within 5%

Protection circuit has correct voltages except the voltage fluctuation on the pins going to the AWH-073 boards

I haven't replaced capacitors with these values yet: 15pf, 56pf,68pf,10pf, 51pf,22pf, or any of the ones on the tuner circuit. But I did test them for capacitance and esr and they are all within 10% (doesn't mean they aren't bad though.)

Tried grounding and not grounding the amplifier, tried different power outlets in the house...

Any ideas on where to go from here? I'm suspecting either the smaller ceramic caps or maybe something happened to the power grid? I should probably hook an oscilloscope to the outlet and see if I'm getting an actual sine wave...

 

[mondialfan]

The 1st thing that I'd do is to isolate the amplifier section from the front end of the receiver. Unplug the pre out/main in jumper plugs from the rear of the unit and use either an external preamp or directly connect an audio player of some sort with a variable volume output to the "Main in" RCA's and give the amplifier by itself a listen. Is the crackling still there? Did the DC offset stabilize?

Did you replace the protection relay? Although it wouldn't cause the DC offset issues it could cause the crackling problems you describe.

 

[Ziconceo]

The 1st thing that I'd do is to isolate the amplifier section from the front end of the receiver. Unplug the pre out/main in jumper plugs from the rear of the unit and use either an external preamp or directly connect an audio player of some sort with a variable volume output to the "Main in" RCA's and give the amplifier by itself a listen. Is the crackling still there? Did the DC offset stabilize?

Did you replace the protection relay? Although it wouldn't cause the DC offset issues it could cause the crackling problems you describe.

Thank you for the response! So, lets see..... The crackling will still happen if I bypass the pre-amp. (Using my phone as a source with RCA adapters plugged into the MAIN IN RCA inputs). It only happens when playing music. Crackling does NOT occur unless something is playing.

The DC Offset also still fluctuates the same even if I disconnect the PRE OUT and MAIN IN jumpers in the back. It also still fluctuates if I disconnected the main output transistors on the big heat sink.The three wires from the main output transistors have been directly soldered to the board. No cracks and no change to sound if I wiggle them.

I opened the protection relay and put some fender F5 deoxit on a flat cotton swab and cleaned the relay contacts. No change sadly. Also tried swapping relays. No change. I suspect the issue is in the power amp boards. Thoughts?

 

[mondialfan]

Isolating the issue to the amplifier/output section is good because it greatly reduces the amount of components that you need to worry about. I would work on finding the crackling noise 1st. The reason is that the DC offset may be corrected once you find and fix this and although its less than ideal, a modest drift of 5, 10, 15mV offset isn't a show stopper IMO. I've listened to and tested amps with offset drift that low and they still tested well within their manufacturers power & distortion specs when tested into 8 and 4 ohm loads.

Looking at the schematic more closely I see that there are still more contacts after the protect relay, those being the speaker select switches.
Now that you've eliminated the protection relay as a source of the crackling I would try to eliminate the speaker select switches as a possible source. I would find a place that the channel output is easily accessible after the amplifier board but prior to the protection relay. If you can get an alligator clip on the output after the amp board and then connect your speaker up to this alligator clip. If the crackling is still present then the amp board is definitely the source of the problem. I should've had you do this test 1st, but changing the protect relay on a 40 - 50 yr old component is a good measure regardless of whether a problem is audibly present or not so its not bad that you replaced it.

Which transistors on the amp board were NOT replaced? I assume that the driver transistors and output transistors were not replaced, but what small signal transistors didn't get changed out?

One thing to note is that it sounds like the crackling becomes minimized after a period of time which seems to imply that heating seems to minimize it. When I've seen this type of behavior in the past I've used my soldering iron set to a low temp and carefully put the tip on individual components (transistors and film caps) to see if I could get a change in the noise level. Then if you have freeze spray you can rapidly cool the component to see if the crackling returns.

I once had a preamp that had about a +20db distortion imbalance between channels. I used my heated soldering iron to move from component to component in the affected channel while watching the distortion on an analyzer and finally found that when I heated a mica cap on the feedback network the distortion dropped to match the opposite channel. Because you can hear your problem you just need to have a speaker connected and slowly work your way though the amp board in the affected channel.

 

[Ziconceo]

Isolating the issue to the amplifier/output section is good because it greatly reduces the amount of components that you need to worry about. I would work on finding the crackling noise 1st. The reason is that the DC offset may be corrected once you find and fix this and although its less than ideal, a modest drift of 5, 10, 15mV offset isn't a show stopper IMO. I've listened to and tested amps with offset drift that low and they still tested well within their manufacturers power & distortion specs when tested into 8 and 4 ohm loads.

Looking at the schematic more closely I see that there are still more contacts after the protect relay, those being the speaker select switches.
Now that you've eliminated the protection relay as a source of the crackling I would try to eliminate the speaker select switches as a possible source. I would find a place that the channel output is easily accessible after the amplifier board but prior to the protection relay. If you can get an alligator clip on the output after the amp board and then connect your speaker up to this alligator clip. If the crackling is still present then the amp board is definitely the source of the problem. I should've had you do this test 1st, but changing the protect relay on a 40 - 50 yr old component is a good measure regardless of whether a problem is audibly present or not so its not bad that you replaced it.

Which transistors on the amp board were NOT replaced? I assume that the driver transistors and output transistors were not replaced, but what small signal transistors didn't get changed out?

One thing to note is that it sounds like the crackling becomes minimized after a period of time which seems to imply that heating seems to minimize it. When I've seen this type of behavior in the past I've used my soldering iron set to a low temp and carefully put the tip on individual components (transistors and film caps) to see if I could get a change in the noise level. Then if you have freeze spray you can rapidly cool the component to see if the crackling returns.

I once had a preamp that had about a +20db distortion imbalance between channels. I used my heated soldering iron to move from component to component in the affected channel while watching the distortion on an analyzer and finally found that when I heated a mica cap on the feedback network the distortion dropped to match the opposite channel. Because you can hear your problem you just need to have a speaker connected and slowly work your way though the amp board in the affected channel.

Thank you for taking the time out of your day to reply!

Hmm the speaker switches could be faulty, but I have sprayed them with Fender 5% cleaner and pushing the speaker switch on and off doesn't make a difference in the amount of crackling.

Interesting idea, I could try that alligator clip idea at some point!

ALL transistors except for the main 4 giant ones on the big heat sink have been replaced.

Hmmmm yea I think it would be a good idea to blow some hot air on parts with my solder melting hot air gun set to low