Pioneer QX-8000a Hiss in Left Channels

[LegginsWill]

Hello all!

I have started restoration of a Pioneer QX-8000a due to some weird sounds (high pitched oscillating sound) and hissing in the left front and rear channels, but I’ve ran into an issue. I have replaced all the blue Sanyo caps on the head amp, control, logic, and decoder board, as well as replaced the 2sc1344 transistors on the head amp. This was successful in getting rid of the weird sounds I was getting, but I still have the hiss in the left channels, the rear one is also slightly distorted. Right channels are clear as day.

Now, I have cleaned all pots many times before and after recapping the boards to no avail. I have also tried to find the source by eliminating which boards it could be.

Notes I have found trying to find which board it could be:

The hiss is controlled by the volume in both left channels, except it disappears when the volume is a little below halfway, and at full blast.

When the main and pre amps are separated by the switch, the hiss goes away. When separated and connected with RCA interconnects, the hiss returns. This lead me to believe Pre-amp was the issue.

Treble controls make the hiss worse as it increases in the respective front/rear left channels.

I am new to stereo repair and this is my first try at electrical work on a receiver. I’d like to save this receiver if I can as I’d like to get back into quad and I’d like to succeed at my first electrical repair, but I just can’t seem to solve this hiss… if anyone has any suggestions or has ran into this issue in the past, I would greatly appreciate any info on what I can try to do!

Thanks!

 

[LegginsWill]

A follow up real quick because I thought these questions might come up…

This hiss is true in all 2ch, RM, SQ, and Discrete modes.

Balance controls do not seem to make a difference.

I saw the control board has 4 2SA725 transistors, which I have heard to be a problem child with the 2SC1344s. My rebuild kit came with PNP transistors to replace whatever was in the receiver. Would these be a good item to replace to possibly solve this issue/good preventative maintenance? I’m just not sure how to test those transistors.

 

[par4ken]

A follow up real quick because I thought these questions might come up…

This hiss is true in all 2ch, RM, SQ, and Discrete modes.

Balance controls do not seem to make a difference.

I saw the control board has 4 2SA725 transistors, which I have heard to be a problem child with the 2SC1344s. My rebuild kit came with PNP transistors to replace whatever was in the receiver. Would these be a good item to replace to possibly solve this issue/good preventative maintenance? I’m just not sure how to test those transistors.

Sounds like you are on the right track, the problem is with the preamplifier section. If you have replacement transistors that you have not used yet I would try replacing them. The existing ones might still function (sort of) but have become noisy.

You can check transistors with a VOM, much like checking a diode (conducts one way, not the other). That kind of testing will not likely reveal if the transistor is noisy. Myself I use a transistor tester (BK Precision 520B). It can show functionality as well as check for leakage. Those known noisy transistors should likely just be replaced.

 

[LegginsWill]

Sounds like you are on the right track, the problem is with the preamplifier section. If you have replacement transistors that you have not used yet I would try replacing them. The existing ones might still function (sort of) but have become noisy.

You can check transistors with a VOM, much like checking a diode (conducts one way, not the other). That kind of testing will not likely reveal if the transistor is noisy. Myself I use a transistor tester (BK Precision 520B). It can show functionality as well as check for leakage. Those known noisy transistors should likely just be replaced.

Thank you par4ken! I replaced the known bad transistors on the control board. Would this board be the preamp? It says in the SM everything passes through it before it goes to the main amp. To me that sounds like a preamp. It also has the tone controls connected to it. I added a picture of the SM with the snippet of what the control board is.

 

[par4ken]

Thank you par4ken! I replaced the known bad transistors on the control board. Would this board be the preamp? It says in the SM everything passes through it before it goes to the main amp. To me that sounds like a preamp. It also has the tone controls connected to it. I added a picture of the SM with the snippet of what the control board is.

Yes, that would be a preamp. Did the transistor replacement fix the noise problem?

 

[LegginsWill]

Yes, that would be a preamp. Did the transistor replacement fix the noise problem?

Replacing my 2SA725s didn’t fix the issue, but while I was looking at everything again I noticed some 2SC1344s in the path before the treble controls, which the treble knob will make the hiss more profound. So I’m currently trying to replace these and give the HF, LF, and loudness switches another clean with deoxit as these are also before the preamp. This would leave me with two last transistors before the main amp in the treble stage of the preamp. The bass stage doesn’t seem to have a noticeable hiss. Thanks!

 

[LegginsWill]

Transistors in the control amp didn’t fix the issue, however it is only in the left channel for both front and rear. With it being a quad amplifier, this leads me to believe it is happening before the signals are separated between front and rear, which is why it appears in both Lf and Lr. But even when in discrete mode this hiss is present. I attached a picture of the block diagram but I am at a little bit of a loss for what it could be.

 

[par4ken]

But even when in discrete mode this hiss is present.

You can rule out the decoder and any pre-amp stages that come before, if the noise is present on the discrete setting. The control amp could still be suspect, however I would recommend spraying a bit of electro contact cleaner on all the switches, volume, balance and tone control pots. DeOxit is great for that purpose.

Dirty switches and pots can cause noise even if they are not being operated.

 

[LegginsWill]

Transistors in the control amp didn’t fix the issue, however it is only in the left channel for both front and rear. With it being a quad amplifier, this leads me to believe it is happening before the signals are separated between front and rear, which is why it appears in both Lf and Lr. But even when in discrete mode this hiss is present. I attached a picture of the block diagram but I am at a little bit of a loss for what it could be.

You can rule out the decoder and any pre-amp stages that come before, if the noise is present on the discrete setting. The control amp could still be suspect however I would recommend spraying a bit of electro contact cleaner on all the switches, volume, balance and tone control pots. DeOxit is great for that purpose.

From what I’ve found, the control amp comes after the volume knob, the hiss changes with the volume knob. From the diagram my last suspects are the tape monitor switches. I just sprayed these with DeOxit and I am currently waiting for these to dry. I had done this before but I still had the hiss. After trying the tape monitor switches, I will probably try the volume and balance controls again, and possibly remove them and do a deeper clean on these.

 

[barfle]

Pots and switches rarely cause a hiss like you’re describing. They can cause noises while theey are being changed, but noise coming from the contacts is extremely rare.

Not that it’s impossible, but I wouldn’t devote much more time to cleaning them, especially if they donkt produce a scratching noise while you’re actively using them.

That being said, components other than transistors can get noisy. There are low-noise resistors available, but replacing parts willy-nilly can be a rabbit hole that might not be productive.

If you have an oscilloscope and a decent understanding of transistor circuitry, it might not be too hard to isolate the offending part. If you’re using this as a way to acquire trouble-shooting skills, more power to you, but it caan get frustrating, as many DIY jobs often are.

If you’re a competent solderer, it might be worth touching up all the connections in the preamp. I’ve seen a few bad solder joints come out of factories that worked fine, but eventually corroded and either opened up or got noisy.

I’m not familiar with this particular unit, so my advice has to be pretty generic.

 

[jaybird100]

Sounds like you are on the right track, the problem is with the preamplifier section. If you have replacement transistors that you have not used yet I would try replacing them. The existing ones might still function (sort of) but have become noisy.

You can check transistors with a VOM, much like checking a diode (conducts one way, not the other). That kind of testing will not likely reveal if the transistor is noisy. Myself I use a transistor tester (BK Precision 520B). It can show functionality as well as check for leakage. Those known noisy transistors should likely just be replaced.

It could also be a capacitor breaking down. That can cause a hiss, or even a "sputtering" sound.

 

[LegginsWill]

Pots and switches rarely cause a hiss like you’re describing. They can cause noises while theey are being changed, but noise coming from the contacts is extremely rare.

Not that it’s impossible, but I wouldn’t devote much more time to cleaning them, especially if they donkt produce a scratching noise while you’re actively using them.

That being said, components other than transistors can get noisy. There are low-noise resistors available, but replacing parts willy-nilly can be a rabbit hole that might not be productive.

If you have an oscilloscope and a decent understanding of transistor circuitry, it might not be too hard to isolate the offending part. If you’re using this as a way to acquire trouble-shooting skills, more power to you, but it caan get frustrating, as many DIY jobs often are.

If you’re a competent solderer, it might be worth touching up all the connections in the preamp. I’ve seen a few bad solder joints come out of factories that worked fine, but eventually corroded and either opened up or got noisy.

I’m not familiar with this particular unit, so my advice has to be pretty generic.

Thanks for the info! I greatly appreciate the tips here. Unfortunately my knowledge of electrical circuits at this time stops and testing with a DMM and replacing anything that tests bad. I did notice some cold solder joints on the control board while I had it apart which I went ahead and repaired. I do know some repair shops in my area that might be able to track down the issue if I can’t find it.

I do have a question… I found on an older post that on these Pioneers, turning the volume down all the way eliminates everything but the control amp and the main amp, which according to the block diagram makes sense to me. If the hiss disappears with the volume at zero, would that mean my hiss is coming from before the control amp? The intensity of the hiss does change as the volume changes, which leads me to believe it could be either the tape monitor switches or the balance controls, as those are the only items before the control amp.

 

[LegginsWill]

It could also be a capacitor breaking down. That can cause a hiss, or even a "sputtering" sound.

I replaced all of the caps on the control amp but 4 Elna 100mf caps. Then all capacitors on the logic, decoder, and head amps.

Now, this leads me to another question. The left f/r channels are hissing quite bad, and there is a profound sputter in the rear mostly. Could capacitors in my power stage or power board be causing a hiss as well? I received replacements for these but I didn’t see any sky blue Sanyos or any other signs of wear on any of these.

 

[par4ken]

Pots and switches rarely cause a hiss like you’re describing.

I have an Akai reel deck with a bad switch. Punching it in and out a number of times sometimes makes it work. It produces both hiss and crackling noises when not working. Cleaning did not fix the problem so actual switch replacement might be necessary but it does show that switches can make noise.

I recently purchased a Tate II decoder, it was rather hissy until I cleaned the pots.

 

[LegginsWill]

I have an Akai reel deck with a bad switch. Punching it in and out a number of times sometimes makes it work. It produces both hiss and crackling noises when not working. Cleaning did not fix the problem so actual switch replacement might be necessary but it does show that switches can make noise.

I recently purchased a Tate II decoder, it was rather hissy until I cleaned the pots.

I had a Sansui QRX-5500 that would develop a hiss if left on accidentally. I had been able to remedy the hiss by cleaning the tone controls with DeOxit. I’ve cleaned the tape monitor and loudness switches a few times, but the only other pot I have before the control amp is the balance control, which will cause the hiss to disappear in the left channel when switched all the way to the right for both front and rear.

Yesterday I tried to take on removing the tape control switches from the board and doing a deeper clean on these, but it was challenging removing all of the old solder since the front switches are all connected with a metal bracket. So I would need to completely desolder all of the switches before they could be removed from the board.

 

[LegginsWill]

Update:

I have cleaned all pots and switches vigorously. This hiss is unfortunately still apparent. To be honest, on the front speakers it’s no worse than my Mac 4100 I use for regular listening. It’s the rear left channel that’s driving me crazy because the hiss is a little crackly, I can hear it quite more over the front speakers. I will say that cleaning did seem to lessen the hiss a bit in the front.

My next plan is to go ahead and recap the entire power stage. If that doesn’t work I will likely bring this receiver to a friend who works on vintage stereo equipment in hopes that he can track down what’s causing this hiss.

Thank you all for all your support on this project!

 

[par4ken]

Yes it could be something in the power stage. Perhaps some of those same known noisy transistor types?

 

[LegginsWill]

Yes it could be something in the power stage. Perhaps some of those same known noisy transistor types?

Luckily it appears the power stage doesn’t have any 2SA725s or 2SC1344s in the power stage. I received all caps and diodes for the power stage but I didn’t change them due to no known bad parts and fitment issues for the big caps since they’re all held on with brackets that won’t fit the new, smaller caps. But I suppose nothing I couldn’t fabricate up somehow.

I am actually currently enjoying this receiver as is right now and I have to say, Pioneer made an incredible receiver here.

Seems like tomorrow I will try the power board and hope for the best. Thanks!

 

[barfle]

Thanks for the info! I greatly appreciate the tips here. Unfortunately my knowledge of electrical circuits at this time stops and testing with a DMM and replacing anything that tests bad. I did notice some cold solder joints on the control board while I had it apart which I went ahead and repaired. I do know some repair shops in my area that might be able to track down the issue if I can’t find it.

I do have a question… I found on an older post that on these Pioneers, turning the volume down all the way eliminates everything but the control amp and the main amp, which according to the block diagram makes sense to me. If the hiss disappears with the volume at zero, would that mean my hiss is coming from before the control amp? The intensity of the hiss does change as the volume changes, which leads me to believe it could be either the tape monitor switches or the balance controls, as those are the only items before the control amp.

If the volume control controls the hiss, that means the hiss is coming before the volume control, or IN the volume control. Are the tone controls before the volume control? I recall an incident in my younger, more naive days when I was upset that the treble control increased distortion, but was taught that distortion is almost always harmonics (multiples) of the original signal, so the treble control would inherently boost distortion. Not that that’s what you’re experiencing, but an illustration of unintended consequences of some of the knobs we take for granted.

 

[barfle]

I have an Akai reel deck with a bad switch. Punching it in and out a number of times sometimes makes it work. It produces both hiss and crackling noises when not working. Cleaning did not fix the problem so actual switch replacement might be necessary but it does show that switches can make noise.

I recently purchased a Tate II decoder, it was rather hissy until I cleaned the pots.

“Rarely.”