Pioneer qx-8000 : capacitor or transistor problem? - UK Vintage Radio Repair and Restoration Discussion Forum

jmcilkley · 8th Jan 2022, 2:03 pm

I have a Pioneer qx-8000 quadraphonic receiver which is dead on both front right and back right channels. There are 2 amplifier boards - one serves both left channels and the other both right ones. So one board works fine but the other on neither of front or rear right channels.
But now the strange thing - when I power the unit off, after a second the sound comes back on all channels at good volume and then dies away. The same on power up - It works on all 4 channels for a couple of seconds while the caps are charging and then cuts off on the same 2 channels.
My first thought was faulty caps but I couldn't understand what fault would cause these symptoms. So now I'm wondering if it's faulty transistors.
Has anyone had experience of these symptoms before?

Kentode · 8th Jan 2022, 2:30 pm

I've just had a quick look around the web, try taking out the fuses and cleaning the holders and fuses.

The service manual is available online.

Hope this helps.

jmcilkley · 8th Jan 2022, 2:40 pm

That sounds a useful suggestion - I will try and report back. I have the service manual, thanks.

jmcilkley · 8th Jan 2022, 3:01 pm

No, the fuses are ok, I awapped them around but the fault still on the left channels.

Kentode · 8th Jan 2022, 4:10 pm

I don't know if this particular model has any protection relays but these can go "sticky". You can use the working side and compare the voltages of the non-working side of the PCB as well.

Kentode · 8th Jan 2022, 4:27 pm

Also, after a quick look at the search engine on this forum, Pioneer equipment seems to have a lot of minor problems related to intermittent channels. Check the headphone socket and the on/off switch after you've had a good read up top.

theredhouseinn · 8th Jan 2022, 5:12 pm

Just had a look at the s.m cct. and no sign of a protection relay. Speakers are fed from the amp via caps and switch. Put the pre-out main-in switch into seperate and feed in a signal to all four inputs to prove the output stages, this will tell you if the fault lies in the pre-amp/sq circuits or the o/p stages, and go from there.
John.

jmcilkley · 9th Jan 2022, 12:28 am

It's the fact that the left channels come alive when powering off and the voltage drops a little that should give a clue. Any puzzle solvers out there?

Lucien Nunes · 9th Jan 2022, 1:07 am

Quote:

There are 2 amplifier boards - one serves both left channels and the other both right ones. So one board works fine but the other on neither of front or rear right channels.

What makes you suspect the power amplifiers? Have you done the test suggested by John two posts up? Have you taken any DC voltage measurements to prove general amplifier functionality? It seems to me unlikely that the power amps are at fault because the front and rear channels are completely independent within each board, even down to the power and ground connections. Therefore either both front and rear amps are faulty in exactly the same way on the R board only, or more likely the fault is elsewhere further up the chain where just one right-channel signal is being affected, before splitting to front and rear.

Test backwards from the pre-out / main-in jacks.

jmcilkley · 10th Jan 2022, 11:17 pm

I haven't had time yet to check any more but whichever way you look at it there must be two problems at least. Because in 4-channel mode all 4 channels are kept separate and there is still the fault on both left channels.
I'm still wondering what could suddenly start working when the power goes off and the voltages decrease.

Diabolical Artificer · 13th Jan 2022, 12:55 pm

If in doubt obey rule one of electronic fault finding - Thoult shalt check voltages. It's highly unlikely we can remotely fix your amplifier or correctly diagnose the fault from 100's of miles away, your going to have to help us help you.

Vintage amplifiers or any old gear will need switches cleaning, solder joints checking and basic stuff like a good visual inspection. Another good diagnostic tool is a stick (non conductive for the prodding of), give it a gentle prod, EG prod the PCB's, connectors etc.

Andy.

jmcilkley · 20th Jan 2022, 12:46 pm

OK, found a bit of time to look a bit more. With the scope I found that there is an input signal to the main amp on both rh channels but not on either lh channels. So the fault is before the main amp board. I have cleaned all the switches but the symptoms don't really fit with a switch problem.
With the scope I was able to see on the faulty inputs to the main amp board that when I power the receiver off, after a couple of seconds, the trace on those channels appears. So something is only working at a lower than normal voltage. I checked the voltages and all seems good. So I'm suspecting faulty transistors somewhere in the preamp.
Will delve a little more.....

Lucien Nunes · 20th Jan 2022, 1:14 pm

Faultfinding hint - you didn't need a scope for that! Cross-patching left and right with a phono lead at the pre-main loop-through sockets and seeing if the problem moved from one side to the other would have been my first test to eliminate the power amps before even taking the cover off.

Yes, DC voltage readings needed. The signal path appears unbroken but the DC conditions are wrong. I'd expect to find a broken ground connection, shorted cap or something that is pulling a signal node to a rail, which might be blatantly obvious with 2 minutes of DC readings.

You have the option of tracing the signal with the scope, which will tell you which stage it is not getting through. I'd probably look for that by tapping signal nodes with a screwdriver held between my fingers as an interference source. The DC tests will tell you not only which stage is bad but also what is wrong with it.

jmcilkley · 17th Mar 2022, 3:05 pm

I took the advice given a couple of weeks ago on here and disconnected the load on the 25v output (B2) of the power supply and the voltage was up to 23.6 (not far off the rated 25v). Then reconnecting one half of the control amp the voltage went down to 21.6. With both halves connected it went down again to 20.1 and with also the Q-unit connected it went down to about 18v.
I decided to check the load and on each half of the control amp it was drawing only 4.1mA.
So why isn't the voltage holding up with such a small load? I've changed the electrolytics and the 2 transistors involved but that didn't seem to make any difference so now I'm stuck and need an expert to help me out - please!

Herald1360 · 17th Mar 2022, 3:41 pm

Well..... the voltage at at Q1 emitter should be around 36V (1S338 is a 14V zener) so with no load on B2 I would expect to see about that, not 23.6V. Otherwise I would expect to see a drop of 330mV for every mA of load on B2.

Something is still wrong with that AWR-004 psu!

snowman_al · 17th Mar 2022, 5:07 pm

Well that is one confusing schematic. I have no experience of the unit so comments are purely for interest.

I assume E1 and E2 are at the same potential via different connections to the chassis? If you measure the voltage between E1 and E2 what do you get?

The B2 volts sag with a small load, what do the other outputs measure? Can you list them?

The manual I found on line has some voltages and currents listed, that might help.

Alan

Herald1360 · 17th Mar 2022, 5:34 pm

That manual is for the QX2000A. The QX2000 is similar but has different component values for the power supply PCB though it has the same part number reference.

I found a much clearer version online than the snippet attached which I couldn't read well enough to even find B2 with any confidence.

jmcilkley · 17th Mar 2022, 10:31 pm

Thanks for the circuit with voltages - mine has no voltages shown. I will check tomorrow and report back.
I notice that your schematic has different values for R5 and R6 and a different diode. Otherwise it is the same.

Herald1360 · 17th Mar 2022, 11:00 pm

Quote:

Originally Posted by Herald1360

That manual is for the QX2000A. The QX2000 is similar but has different component values for the power supply PCB though it has the same part number reference.

I found a much clearer version online than the snippet attached which I couldn't read well enough to even find B2 with any confidence.

Oops! Where did 2000 come from?

jmcilkley · 18th Mar 2022, 10:42 am

OK, more info. I measured voltages with no load and then again with one half of the control amp connected and finally with both halves of the control amp. All the outputs and also b, c & e of Q2
B1 B2 B3 B4 B5 B6 Q2b Q2c Q2e
no load 54.9 23.4 14.4 22.6 11.6 23.4 12.7 23.7 12.5
half c.amp 55.2 21.5 14.4 22.7 11.5 23.2 12.6 23.8 12.6
all c.amp 54.9 20.2 14.3 22.4 11.5 23.3 12.6 23.7 12.4

B6 is, of course, directly from the collector of Q1. Clearly the output of Q1 is way too low - should be 48v with 55v going into the collector which is correct. I'm still wondering about the R5/R6 voltage divider where I reckon the junction should be at 2/5 of the Q1 emitter and 2/5 of 23ish is about 4.6v which is a long way from 12.6 as measured. I have tested R5 and R6 out of circuit and they seem ok.
I did check the 2 grounds and they seem to be connected as diode check between them gives a beep.

8th Jan 2022, 2:03 pm	#1
jmcilkley Tetrode Join Date: Jan 2020 Location: Ilkley, West Yorkshire, UK. Posts: 92	Pioneer qx-8000 : capacitor or transistor problem? I have a Pioneer qx-8000 quadraphonic receiver which is dead on both front right and back right channels. There are 2 amplifier boards - one serves both left channels and the other both right ones. So one board works fine but the other on neither of front or rear right channels. But now the strange thing - when I power the unit off, after a second the sound comes back on all channels at good volume and then dies away. The same on power up - It works on all 4 channels for a couple of seconds while the caps are charging and then cuts off on the same 2 channels. My first thought was faulty caps but I couldn't understand what fault would cause these symptoms. So now I'm wondering if it's faulty transistors. Has anyone had experience of these symptoms before?

8th Jan 2022, 2:30 pm	#2
Kentode Octode Join Date: Mar 2015 Location: Yorkshire, England. Posts: 1,301	Re: Pioneer qx-8000 : capacitor or transistor problem? I've just had a quick look around the web, try taking out the fuses and cleaning the holders and fuses. The service manual is available online. Hope this helps. __________________ Regards, Ken. BVWS member

8th Jan 2022, 2:40 pm	#3
jmcilkley Tetrode Join Date: Jan 2020 Location: Ilkley, West Yorkshire, UK. Posts: 92	Re: Pioneer qx-8000 : capacitor or transistor problem? That sounds a useful suggestion - I will try and report back. I have the service manual, thanks.

8th Jan 2022, 3:01 pm	#4
jmcilkley Tetrode Join Date: Jan 2020 Location: Ilkley, West Yorkshire, UK. Posts: 92	Re: Pioneer qx-8000 : capacitor or transistor problem? No, the fuses are ok, I awapped them around but the fault still on the left channels.

8th Jan 2022, 4:10 pm	#5
Kentode Octode Join Date: Mar 2015 Location: Yorkshire, England. Posts: 1,301	Re: Pioneer qx-8000 : capacitor or transistor problem? I don't know if this particular model has any protection relays but these can go "sticky". You can use the working side and compare the voltages of the non-working side of the PCB as well. __________________ Regards, Ken. BVWS member

8th Jan 2022, 4:27 pm	#6
Kentode Octode Join Date: Mar 2015 Location: Yorkshire, England. Posts: 1,301	Re: Pioneer qx-8000 : capacitor or transistor problem? Also, after a quick look at the search engine on this forum, Pioneer equipment seems to have a lot of minor problems related to intermittent channels. Check the headphone socket and the on/off switch after you've had a good read up top. __________________ Regards, Ken. BVWS member

8th Jan 2022, 5:12 pm	#7
theredhouseinn Hexode Join Date: Jan 2013 Location: Kelvedon, Essex, UK. Posts: 273	Re: Pioneer qx-8000 : capacitor or transistor problem? Just had a look at the s.m cct. and no sign of a protection relay. Speakers are fed from the amp via caps and switch. Put the pre-out main-in switch into seperate and feed in a signal to all four inputs to prove the output stages, this will tell you if the fault lies in the pre-amp/sq circuits or the o/p stages, and go from there. John.

9th Jan 2022, 12:28 am	#8
jmcilkley Tetrode Join Date: Jan 2020 Location: Ilkley, West Yorkshire, UK. Posts: 92	Re: Pioneer qx-8000 : capacitor or transistor problem? It's the fact that the left channels come alive when powering off and the voltage drops a little that should give a clue. Any puzzle solvers out there?

10th Jan 2022, 11:17 pm	#10
jmcilkley Tetrode Join Date: Jan 2020 Location: Ilkley, West Yorkshire, UK. Posts: 92	Re: Pioneer qx-8000 : capacitor or transistor problem? I haven't had time yet to check any more but whichever way you look at it there must be two problems at least. Because in 4-channel mode all 4 channels are kept separate and there is still the fault on both left channels. I'm still wondering what could suddenly start working when the power goes off and the voltages decrease.

13th Jan 2022, 12:55 pm	#11
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,661	Re: Pioneer qx-8000 : capacitor or transistor problem? If in doubt obey rule one of electronic fault finding - Thoult shalt check voltages. It's highly unlikely we can remotely fix your amplifier or correctly diagnose the fault from 100's of miles away, your going to have to help us help you. Vintage amplifiers or any old gear will need switches cleaning, solder joints checking and basic stuff like a good visual inspection. Another good diagnostic tool is a stick (non conductive for the prodding of), give it a gentle prod, EG prod the PCB's, connectors etc. Andy. __________________ Curiosity hasn't killed this cat...so far.

20th Jan 2022, 12:46 pm	#12
jmcilkley Tetrode Join Date: Jan 2020 Location: Ilkley, West Yorkshire, UK. Posts: 92	Re: Pioneer qx-8000 : capacitor or transistor problem? OK, found a bit of time to look a bit more. With the scope I found that there is an input signal to the main amp on both rh channels but not on either lh channels. So the fault is before the main amp board. I have cleaned all the switches but the symptoms don't really fit with a switch problem. With the scope I was able to see on the faulty inputs to the main amp board that when I power the receiver off, after a couple of seconds, the trace on those channels appears. So something is only working at a lower than normal voltage. I checked the voltages and all seems good. So I'm suspecting faulty transistors somewhere in the preamp. Will delve a little more.....

20th Jan 2022, 1:14 pm	#13
Lucien Nunes Rest in Peace Join Date: Apr 2008 Location: London, UK. Posts: 2,508	Re: Pioneer qx-8000 : capacitor or transistor problem? Faultfinding hint - you didn't need a scope for that! Cross-patching left and right with a phono lead at the pre-main loop-through sockets and seeing if the problem moved from one side to the other would have been my first test to eliminate the power amps before even taking the cover off. Yes, DC voltage readings needed. The signal path appears unbroken but the DC conditions are wrong. I'd expect to find a broken ground connection, shorted cap or something that is pulling a signal node to a rail, which might be blatantly obvious with 2 minutes of DC readings. You have the option of tracing the signal with the scope, which will tell you which stage it is not getting through. I'd probably look for that by tapping signal nodes with a screwdriver held between my fingers as an interference source. The DC tests will tell you not only which stage is bad but also what is wrong with it.

17th Mar 2022, 3:05 pm	#14
jmcilkley Tetrode Join Date: Jan 2020 Location: Ilkley, West Yorkshire, UK. Posts: 92	More on the Pioneer QX8000 power supply! I took the advice given a couple of weeks ago on here and disconnected the load on the 25v output (B2) of the power supply and the voltage was up to 23.6 (not far off the rated 25v). Then reconnecting one half of the control amp the voltage went down to 21.6. With both halves connected it went down again to 20.1 and with also the Q-unit connected it went down to about 18v. I decided to check the load and on each half of the control amp it was drawing only 4.1mA. So why isn't the voltage holding up with such a small load? I've changed the electrolytics and the 2 transistors involved but that didn't seem to make any difference so now I'm stuck and need an expert to help me out - please! Attached Thumbnails

17th Mar 2022, 3:41 pm	#15
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: More on the Pioneer QX8000 power supply! Well..... the voltage at at Q1 emitter should be around 36V (1S338 is a 14V zener) so with no load on B2 I would expect to see about that, not 23.6V. Otherwise I would expect to see a drop of 330mV for every mA of load on B2. Something is still wrong with that AWR-004 psu! __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

17th Mar 2022, 5:07 pm	#16
snowman_al Octode Join Date: Dec 2018 Location: Llandeilo, West Wales, UK. Posts: 1,092	Re: More on the Pioneer QX8000 power supply! Well that is one confusing schematic. I have no experience of the unit so comments are purely for interest. I assume E1 and E2 are at the same potential via different connections to the chassis? If you measure the voltage between E1 and E2 what do you get? The B2 volts sag with a small load, what do the other outputs measure? Can you list them? The manual I found on line has some voltages and currents listed, that might help. Alan Attached Thumbnails __________________ Never Leave Well Enough Alone... Last edited by snowman_al; 17th Mar 2022 at 5:22 pm. Reason: More info added

17th Mar 2022, 5:34 pm	#17
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: More on the Pioneer QX8000 power supply! That manual is for the QX2000A. The QX2000 is similar but has different component values for the power supply PCB though it has the same part number reference. I found a much clearer version online than the snippet attached which I couldn't read well enough to even find B2 with any confidence. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

17th Mar 2022, 10:31 pm	#18
jmcilkley Tetrode Join Date: Jan 2020 Location: Ilkley, West Yorkshire, UK. Posts: 92	Re: More on the Pioneer QX8000 power supply! Thanks for the circuit with voltages - mine has no voltages shown. I will check tomorrow and report back. I notice that your schematic has different values for R5 and R6 and a different diode. Otherwise it is the same.

18th Mar 2022, 10:42 am	#20
jmcilkley Tetrode Join Date: Jan 2020 Location: Ilkley, West Yorkshire, UK. Posts: 92	Re: More on the Pioneer QX8000 power supply! OK, more info. I measured voltages with no load and then again with one half of the control amp connected and finally with both halves of the control amp. All the outputs and also b, c & e of Q2 B1 B2 B3 B4 B5 B6 Q2b Q2c Q2e no load 54.9 23.4 14.4 22.6 11.6 23.4 12.7 23.7 12.5 half c.amp 55.2 21.5 14.4 22.7 11.5 23.2 12.6 23.8 12.6 all c.amp 54.9 20.2 14.3 22.4 11.5 23.3 12.6 23.7 12.4 B6 is, of course, directly from the collector of Q1. Clearly the output of Q1 is way too low - should be 48v with 55v going into the collector which is correct. I'm still wondering about the R5/R6 voltage divider where I reckon the junction should be at 2/5 of the Q1 emitter and 2/5 of 23ish is about 4.6v which is a long way from 12.6 as measured. I have tested R5 and R6 out of circuit and they seem ok. I did check the 2 grounds and they seem to be connected as diode check between them gives a beep.