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Old 4th Apr 2022, 9:18 pm   #41
frsimen
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

A few more things have come to mind.

As you have already changed the resistors, have you swapped the 22k and 33k in error. That would lead to reduced output voltage, although it should be higher than you are measuring.

The voltage on the base of Q1 and the collector of Q2 should be the same (they are connected together according to the circuit diagram) and it should be about 0.6V higher than the voltage on the emitter of Q1.

Your measurements tell a different story, so could you check them again. You could try making your measurements with respect to the 0V line on the regulator PCB rather than chassis in case there is an issue with the chassis connection.

Multimeters are normally pretty reliable but is it possible that your one is playing up?

Paula
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Old 5th Apr 2022, 3:13 pm   #42
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

This may help someone find my problem. I took Rr & R6 out and measured resistance across where they had been. Where R5 was measured as open and where R6 was measure as 5.4M. Then I out them back and remeasured and across R6 measured 14.2k and increasing slowly and across R5 18k and fairly quickly increasing to 20k and maybe higher!
This was with new batteries in the mm.
It was hard to get resistance readings with the older mm that I had been using mostly - seemed to be inconsistent.
Does that help?
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Old 5th Apr 2022, 4:55 pm   #43
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

Your resistance readings indicate there isn't any leakage to worry about across the resistors, so I suspect you that there is leakage from elsewhere that is reaching the base of Q2.

The resistance measurements across R5 and R6 are what is to be expected.

When you first measure the resistance, the capacitors C2, C3 and C4 are discharged. At first they will appear as near to short circuits, resulting in a measured value formed by the some series and parallel combination of R5, R6, R7 and R8. The initial resistance that you measure across R5 or R6 will be around 13k and that will rise as the capacitors charge up. If you had waited long enough, the readings would have approached what you were expecting across each resistor.

Have you tried measuring the voltages again? Also check around Q2 for the possibility of a leakage path from the collector to the base (caused by flux etc). It's sometimes helpful to check resistance twice, with the meter connected both ways around, particularly where transistors and diodes are involved, as that may reveal some useful information.

Don't keep removing components to test them, particularly where you have already checked them out of circuit, as the PCB will be damaged sooner or later if you do.

Paula
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Old 5th Apr 2022, 7:26 pm   #44
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

Can you post pictures of the PCB topside and underside.

Can you put your scope on Q2 base, emitter and collector and look for any ac signal.

Are you confident you have put Q2 in the right way around?

Are you doing these tests with the regulator outputs connected to the amp (B2, B3 & B6)?

I am still a little concerned about Q2 transistor ratings as it is being fed from a 55V rail but is a 25V transistor. A faulty C1 (soft start capacitor) could cause excessive Vce. Do you have any other NPN transistors?

Last edited by PJL; 5th Apr 2022 at 7:31 pm.
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Old 9th Apr 2022, 9:00 am   #45
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

i'm away on holiday for a couple of weeks so will do that when i get home. I need a break from it!
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Old 4th May 2022, 12:18 pm   #46
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

Back now and have tried a few things. I resoldered all the components on the pcb but no change. Then I decided to try altering the resistors in the voltage divider. I put a 68k resistor in parallel with each in turn and..... there was absolutely no change in the voltage B2. Any ideas?
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Old 4th May 2022, 7:49 pm   #47
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

With the output of the regulator being so low, it’s likely that the voltage on the base of Q2 is insufficient for it to conduct. That being the case, a small change to the value of the potential divider resistors won’t make any difference to the output voltage.

Working from the circuit diagram that you posted in your post number 14, which I think matches what you have, check the resistance of R1 and R3. If they measure as expected, check the following:

1) The voltage drop across R1 (12k)
2) The voltage drop across R3 (8.2k).

The voltage drop you measure across R1 should be roughly 1.5 times the voltage across R3. If the voltage across R1 is much more than 1.5 times the voltage across R3, replace C1. Check the polarity of C1 carefully, as if it is wrongly connected the very high leakage current will drop the voltage reaching the base of Q1, reducing the output voltage of the supply.

Paula
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Old 9th May 2022, 7:11 pm   #48
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

Paula,
R1 and R3 both measure ok. However the voltage drops are not what you suggest they should be. Across R1 55.6 to 41.3 = 14.3v and across R3 31.5 to 15.5 = 16v. So actually the drop across R3 is greather than across R1.
I did replace C1 along with all the other electrolytics and it is the right way round. Maybe I'll try replacing it again just to see.
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Old 9th May 2022, 7:22 pm   #49
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

That can't be right! I will check again tomorrow.
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Old 9th May 2022, 9:53 pm   #50
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

OK, my copying was faulty it should be: R1 55 to 30.7 = 24.3 and R2 30.7 to 13.8 = 16.9 so the voltage across R1 is approx 1.5 times that across R3.
The caps all seem to be the right way round.
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Old 9th May 2022, 11:27 pm   #51
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

That seems a lot more sensible than the earlier result!
The current flowing through R1 seems to be the same as through R3, the resistors and C1 are fine. The current through the resistors is a little over 2mA. The voltages on the base and emitter of Q2 are at a level where it shouldn't be conducting, so that only leaves one way for the current to flow, via the base of Q1 into the potential divider resistors and the Zener diode circuit.

With 2mA of base current, Q1 should be passing current from its collector to emitter. It doesn't appear to be, so the fault is going to be one of the following:

Poor connections to R2
R2 open circuit (maybe intermittently)
Poor connection/broken track between R2 and Q1 collector lead.
Q1 open circuit collector to emitter.

Paula
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Old 12th May 2022, 5:35 pm   #52
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

R2 measures correct resistance and the connection between R2 and Q1 collector is fine. Q1 C to E does not show open circuit.
There is voltage at the collector of Q1 so it is getting through there from the diode bridge.
I'm still convinced that there is a problem with the voltage divider circuit R5/R6 as I expected the voltage between them to 2/5 of the voltage at the emitter of Q1 - but it's not
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Old 12th May 2022, 7:06 pm   #53
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

If one or other end of the 22k resistor isn't making good contact due to a faulty trace on the PCB perhaps, the output voltage of the supply will be lower than it should be. The base voltage of Q2 can still measure around 13V, so long as the output voltage is above 13V of course.

Try connecting a wire link from the 0V end of the 22k resistor to the Zener diode's anode (= 0V end) and from the 13V end to either the base of Q2 or to the 13V end of the 33k resistor. This will eliminate the possibility of a high resistance break in the track and may restore correct operation.

For such a simple circuit, this one is giving you a very hard time!

Paula
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Old 12th May 2022, 8:02 pm   #54
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

One of the problems is we don't want the regulated voltage to run too high as it may damage the amplifier or I would suggest removing Q2.
Are all the power connections made to the amplifier?
How confident are we that your meter is working correctly?
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Old 12th May 2022, 9:47 pm   #55
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

I have disconnected the B2 voltage from the rest of the boards. I have 2 meters and they agree pretty well so I don't think that is the problem.
What about putting another resistor in parallel with R6? That should reduce the voltage at the base of Q2 and, I guess, up the voltage at the emitter of Q1 - or have I got that wrong?
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Old 12th May 2022, 11:34 pm   #56
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

You have already tried parallel resistors across R5 and R6 and reported that they made no difference. If my suggestion in post 53 doesn’t help, a different strategy is needed.

The difficulty with feedback circuits like this is that any problem in the circuit will upset the voltages, as you’re finding here. The answer is to break the loop. To do this, PJL’s idea to remove Q2 is the way to go. It can be done safely if you add a 33k resistor from the base of Q1 to the 0V rail. Be absolutely certain you have a good connection at both ends.

Make sure that you have disconnected the output from any external circuitry. With this configuration, the voltage on the base of Q1 should be of the order of 33-34 volts and the emitter voltage should be 0.6V less than this. If that’s what you’re measuring, Q1 is working correctly.

Assuming it is, move on to measure the voltage at the junction of R5 and R6. That should now be around 13V. If it is, the potential divider is working as it should. If it is much higher than 13V, check around the 22k resistor and 33k resistors and look for any solder splashes around where Q2 was connected.

Next check the voltage across the Zener diode. If that measures 12-13V, the Zener diode is working properly. This leads to the conclusion that Q2 is faulty, or not wired in correctly.

If the voltage on the base of Q1 is much lower than 33-34V, disconnect C14 and try again. If it is still wrong, check the voltage drop across R2 (220 ohms) – measure that directly, rather than with respect to 0V. Expect a few hundred millivolts at most. If it is zero, Q1 has failed.

Paula
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Old 15th May 2022, 10:58 am   #57
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

OK, am preparing to do that but I need to check aout disconnecting the various supply voltages. I have B2 disconnected already; B1 is before the feedback circuit so I'm guessing I don't need to disconnect that; B6 is unused; B4 is very strange - it goes only to the 8v stereo indicator lamp so why it needs 25v is a bit of a mystery; B3 I guess I need to disconnect and B5 runs off Q3 so maybe I can leave this?
It's just that wire wrap isn't easy to undo as I don't have a tool so it's probably better to cut the wires and then solder them back together afterwards and I want to minimise that.
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Old 15th May 2022, 11:22 am   #58
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

There's no need to remove the wire wrap connections. You can lift one end of the following resistors to isolate the power supply adequately:

R7, R8 and R12.

That disconnects the B2, B3 and B5 supplies. You have already disconnected B2 by some means, lifting R7 will protect C4 should anything untoward happen.

B4 can be left connected as it is only fed from the unregulated supply.

Paula
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Old 20th May 2022, 11:03 am   #59
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

OK, I didn't have a 33k resistor so made do with 2 x 68k in parallel which measured 34k.
I then measured as follows:
Q1 base 31.9v
Q1 emitter 31.5v
R5/R6 junction 12.5v
voltage across zener 12.5v

Does this mean Q2 was faulty? I had already tried substituting another one - it was hard to find a substitute for the 2SC968 but it looked like a 2N2222 should be ok. However, it made absolutely no difference to the voltages output.
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Old 20th May 2022, 4:57 pm   #60
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Default Re: Pioneer qx-8000 : capacitor or transistor problem?

It does look like Q2 is the source of your trouble. It's slightly unfortunate that the Zener voltage and the R5/R6 voltage are the same (the R5/R6 voltage is fine for 31.5V at the emitter of Q1), just check you haven't got a short between the two points.

So, the question is, what is going on that three different transistors all seem to give the same odd result?

One possible answer is that you are wiring the replacement transistors in incorrectly. It's easily done. I suggest that you confirm which hole in the PCB corresponds to each of the emitter, base and collector leads. If there is no silk screening identifying them, mark the positions on the board yourself. On your replacement transistor, slip some coloured sleeving onto the leads, to help you identify which is which. These measures should greatly reduce the chance of fitting the transistor incorrectly.

As to which transistor to use, the main issue is the collector - emitter voltage that the transistor will see in this circuit. I've examined the circuit using LTSpice, to check what happens. At no point during the start up of the circuit does the voltage across Q2 exceed 25V and the maximum collector current will not go above 2.6mA and is usually less. The original transistor was rated at 30V Vceo max, Vcbo 50V Ic max 0.5A with an FT of 70MHz. gain of 80 minimum.

There are quite a few transistors which will be adequate in that circuit position, the 2N2222 should work but it has higher FT than is ideal here. Other transistors which could be considered include BC107, BC547, BC182, BC337 but there are many others. Do check the pin out carefully if you use anything other than a TO18 cased transistor.

There is a good chance that this power supply will work once you have fitted a replacement Q2.

Paula
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