Help repairing Pioneer M3

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gde3

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Joined 2013
Mooly said:
At worst you have probably zapped the lower PNP output and maybe its driver but we now go back to basics and do a few checks.

1) With it switched OFF make sure that the rails are discharged.

2) Measure on "low ohms range" or on the "diode" range on your meter across collector and emitter of both the output transistors. If one or both read short then that transistor is zapped.

3) Do the same for both driver transistors Q20 and Q21

4) Check that all the 0.5 ohm resistors are OK.

5) Check that both the 4.7 ohm resistors (R69 and R70) that feed the output transistors are OK

6) Check that R65 and R66 are OK. These are the 22 ohm resistors that feed the driver transistors.

7) Make sure the fuses in the power supply are OK.

Report back with what you find.
Click to expand...

All the resistors and fuses are fine. One of the output transistor is broke the 93G, the other transistors seems good in both ohm and diodes mode
 
Mooly

Mooly

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It looks like there is a problem in the output stage.

I'm going to be out of time again for today so this is what you do......

Although the transistors pass basic tests and don't appear short its quite possible there is still a problem with one or more. Without experience of interpreting all the possible readings across all the junctions we are going to replace them in a specific order.

1) Replace the other output transistor of the pair and retest.

2) If the bulb still lights then you replace both driver transistors and again retest.

I would be surprised if at this point there was still a problem but if there is then wait until tomorrow and don't run on ahead :)
 
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gde3

Member
Joined 2013
Mooly said:
It looks like there is a problem in the output stage.

I'm going to be out of time again for today so this is what you do......

Although the transistors pass basic tests and don't appear short its quite possible there is still a problem with one or more. Without experience of interpreting all the possible readings across all the junctions we are going to replace them in a specific order.

1) Replace the other output transistor of the pair and retest.

2) If the bulb still lights then you replace both driver transistors and again retest.

I would be surprised if at this point there was still a problem but if there is then wait until tomorrow and don't run on ahead :)
Click to expand...

After replacing both o/p and drive transistors the board is working again :D
 
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gde3

Member
Joined 2013
rsavas said:
Check that VR1 is working properly. If you measure at Q1-B (Base) and adjust VR1, it should swing + & -
The two diodes, D2/3 clamp to ~ +/-0.6V which in turn sets the offset V.
If you measure DCV across Q1/2 collectors you should see it change from slightly positive to slightly negative as VR1 is adjusted. This indicates that VR1 is working correctly. There will be a null point where Q! & Q2 are conducting equally or the bias current is in balance.
There probably is some inter-action between adjusting VR1 and VR2
Click to expand...

DCV across Q1/2 collectors is 20mv
The DCV at D2/3 is correct +/-0.6V
 
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gde3

Member
Joined 2013
Mooly said:
This is what I suggest as the next step. The bias generator TR11 is developing 3.7 volts maximum. That is failing to turn on the combined base and emitter junctions of outputs, drivers and pre driver. That's 6 B-E junctions that need to be forced into conduction and if we say 0.65 volts is needed per junction (its around that figure give or take...) then we need nearer 3.9 volts.

So what I propose is to remove R33 and then starting with VR2 on minimum resistance slowly adjust it and see if you can get the output stage to draw current. You should then be able to get a voltage between test points TP9 and 13 (the output). Remember the voltage is small. If it adjusts OK then carefully see if it will reach around 50 millivolts. The bulb will probably begin to glow.
Click to expand...

Without R33 I was able to sety R1 around 0mv but the voltage between 9-13 is -0.5mV, if I touch VR2 9-13 will never go positive
 
Mooly

Mooly

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Sounds like we are back on track :) Wasn't that bad really :D

It still seems there is insufficient bias voltage being generated. Do you have a selection of small resistors ? I'm thinking we need to raise R31 a little in value. All depends on what you have available.
 
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gde3

Member
Joined 2013
Mooly said:
Sounds like we are back on track :) Wasn't that bad really :D

It still seems there is insufficient bias voltage being generated. Do you have a selection of small resistors ? I'm thinking we need to raise R31 a little in value. All depends on what you have available.
Click to expand...

Not much avaiable, plus today is holiday so I can't go shopping:

2x 100ohm
1x 1.3K
2x 1.5K (5W)
3x 2.2K
2x 4.7K
3x 6.8K
5x 10K
 
Mooly

Mooly

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I'm thinking... at the moment we have to work with what you have available.

Two tests. It might be worth trying this channel for real on a speaker in its present condition. You have zero volts on L1 which is good. What I would suggest is connecting a speaker also via a bulb which will protect the speaker if anything went wrong.

If thats OK (and it may sound a little distorted at low levels due to lack of bias current) then I think we should try altering the Vbe generator again.

To do this first turn VR2 to minimum resistance again.

Leaving R33 out of circuit connect your 10K resistor (measure it first to make sure it really is 10K) across R32 which is a 1.6K. Doing that will reduce the apparent value of R32 to around 1.4K

Then with no speakers connected switch on and see if VR2 adjusts so that you get a voltage between 9 and 13
 
G

gde3

Member
Joined 2013
Mooly said:
I'm thinking... at the moment we have to work with what you have available.

Two tests. It might be worth trying this channel for real on a speaker in its present condition. You have zero volts on L1 which is good. What I would suggest is connecting a speaker also via a bulb which will protect the speaker if anything went wrong.

If thats OK (and it may sound a little distorted at low levels due to lack of bias current) then I think we should try altering the Vbe generator again.

To do this first turn VR2 to minimum resistance again.

Leaving R33 out of circuit connect your 10K resistor (measure it first to make sure it really is 10K) across R32 which is a 1.6K. Doing that will reduce the apparent value of R32 to around 1.4K

Then with no speakers connected switch on and see if VR2 adjusts so that you get a voltage between 9 and 13
Click to expand...

no sound :(
 
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