... something is not right with the diode measurement test.
I tested a known good discrete diode. The red plug was on the anode end and the black plug was on the cathode end ... the one with a white stripe. This forward bias activated the diode test function on my multi-meter and showed a value of 0.613 volts ... indicating a good diode. When the plug leads were reversed, the reading was infinity, indicating that no conduction was present. This was the expected result, and should be the same when testing the back-to-back diode junctions of a transistor.
... So try using the red and black plugs of your meter, and just plug them normally into the usual jacks on your meter.
... Another point of confusion, I mistakenly described one of the transistors that were to be tested.
Only transistors Q524C and Q524A are suspect at this time. Q524C is NPN type. Q524C is PNP type.
... I was thinking that you were going to try using the meter Hfe test function, which should show a number of 2 or 3 digits, but something was evidently wrong there. Did you have the meter turned on? ... Maybe the three leads were not connected to the correct transistor pins. The Hfe measurement is made by connecting the e,b, and c leads between the transistor and the meter Hfe test holes and the turning the meter on and switching the dial to which Hfe transistor type you want. A number will appear in the readout. No rearrangement if the test wires is necessary. Just read the number that is displayed.

 

... something is not right with the diode measurement test.
I tested a known good discrete diode. The red plug was on the anode end and the black plug was on the cathode end ... the one with a white stripe. This forward bias activated the diode test function on my multi-meter and showed a value of 0.613 volts ... indicating a good diode. When the plug leads were reversed, the reading was infinity, indicating that no conduction was present. This was the expected result, and should be the same when testing the back-to-back diode junctions of a transistor.
... So try using the red and black plugs of your meter, and just plug them normally into the usual jacks on your meter.
... Another point of confusion, I mistakenly described one of the transistors that were to be tested.
Only transistors Q524C and Q524A are suspect at this time. Q524C is NPN type. Q524C is PNP type.
... I was thinking that you were going to try using the meter Hfe test function, which should show a number of 2 or 3 digits, but something was evidently wrong there. Did you have the meter turned on? ... Maybe the three leads were not connected to the correct transistor pins. The Hfe measurement is made by connecting the e,b, and c leads between the transistor and the meter Hfe test holes and the turning the meter on and switching the dial to which Hfe transistor type you want. A number will appear in the readout. No rearrangement if the test wires is necessary. Just read the number that is displayed.

Hello drc,
diode test: it seems I respected the protocol you write here. My multimeter is announced to be able to operate indifferently on polarity on leads, only the display indicates minus when it is reversed...
I mentioned on the sheet only results different from infinity.
Unfortunately I have no spare diode aside to check the multimeter and the measure apart from the amp.

HFE test: despite difficults with customized wires and access to the transistors, I made three times the measurement, I closely checked the correct plugging of wires in the multimete...(values like 0.490 or 0.770 between B and E should confirm that the wiring was operating).

My impression is that same values between all transitors says that no one of them is malfunctioning.
But I cannot go beyond this simple guess.

My wiring HFE is made from electrical twisted cabling, too big for easy plugging and positioning, I will look today for smaller electronic thinner cable to rewire easier, and I will measure one more time everything, diode and HFE.

Thanks to take on your time for assistance!

Didier

 

... Hi Didier,
It seems that there may be a misunderstanding regarding the diode test and the Hfe test.
I am not quite sure how to clarify the situation.

Here is some reference material ... Hope it is useful.:

... Here is a video of the Hfe test ... at 4 minutes 50 seconds the Hfe test is shown. However, if you can't fit the necessary connection wires to the transistor, the Hfe test may be difficult to accomplish.
Hfe test using multi-meter
.
.. Here is a link to the transistor test using the meter diode function. The transistor form that is relevant here is the TO220 form, shown in the text figure.:
Transistor test using the Diode meter function
... By using the diode test method, only the two meter needle probes are necessary to contact the transistor pins for a given measurement, so the diode test method may be more convenient to use in this situation, not having much extra space to attach wires or clips.

 

Hi drc,
I did my best for a new HFE test after seeing the video (wich confirmed I was doing right), here are photos of the"Fork" to reach the pins. Unfortunately, measuring was unsuccessful and disapointing, values were just infinite.
After having red the diode test method, I did it entirely for the 4 transistors, here a tab with all the results. Some of them are confusing, making difficult to identify the TR PNP or NPN.
Explanations of the colors on the right of the tab.
In red: values which should not happen
In green: values approchaing what w
I'm not sure if it is clear, but for me, something is clear: all of that is making me becoming crazy, slowly but surely !

 

Hi drc,
I did my best for a new HFE test after seeing the video (wich confirmed I was doing right), here are photos of the"Fork" to reach the pins. Unfortunately, measuring was unsuccessful and disapointing, values were just infinite.
After having red the diode test method, I did it entirely for the 4 transistors, here a tab with all the results. Some of them are confusing, making difficult to identify the TR PNP or NPN.
Explanations of the colors on the right of the tab.
In red: values which should not happen
In green: values approchaing what wAS EXPECTED
I'm not sure if it is clear, but for me, something is clear: all of that is making me becoming crazy, slowly but surely !

 

. .. A good try Didier, however for the diode method, the hfe socket on the meter is not utilized. Instead, the ordinary red and black cords are inserted into their usual meter jack holes, just as if you were going to measure a voltage.The meter dial is then then turned to the Ohms section, where you will see a small diode icon at the 2k mark.
... It seems that it would be easier to learn the diode method by using a loose NPN transistor that would allow you to verify the technique that is required to perform the test.
... A good effort shown above, considering your level of experience.

 

. .. A good try Didier, however for the diode method, the hfe socket on the meter is not utilized. Instead, the ordinary red and black cords are inserted into their usual meter jack holes, just as if you were going to measure a voltage.The meter dial is then then turned to the Ohms section, where you will see a small diode icon at the 2k mark.
... It seems that it would be easier to learn the diode method by using a loose NPN transistor that would allow you to verify the technique that is required to perform the test.
... A good effort shown above, considering your level of experience.

Uh, sorry, I forgot to precise that I've of course made the diode test as required, not with the "special fork" and socket HFE but with the usual leads red and black in the dedicated jack holes for resistance measures on the level 2K; sorry if my explanation was confusing on that point.
I'm going to find a transistor somewhere as you suggest.
My efforts must be in proportion with the time you spend on my problem!...

 

I've been to a store this morning, bought two cheap transistors, NPN and PNP, and will check this evening the test.
BUT!... at the same time, as I explained my situation to the seller, he told me that there is hundred percent chances that the values of measures are uncorrect when the components are still on the circuit, presence of other components interfering on the measure.
And the only solution is to desold transistors and measure them alone.

 

... Didier,
I am sorry if I have mis-informed you about the testing of the transistors. I thought that there was a chance to try to determine if they were defective.
... Is it possible to remove the transistor's circuit board from the platform or case to which they are attached? The idea is to get access to the bottom surface of the circuit board. If this step is not too much effort, then you can locate a specific transistor and heat the solder junctions. There is a very useful product called Solder-Wick. The way this works is that a very fine copper mesh actually absorbs the melted solder by a process called capillary action. So once the transistor solder joints are melted, apply a section of Solder-Wick to the liquid solder and you will have de-soldered that particular connection.
... Again, I am sorry about the error mentioned above.

 

Hi drc,
Hereunder is the result of diode test with the two standalone TR, BC 547 NPN and BC 557 PNP, the measures seem correct.
The HFE test was good too with values 0.604 and 0.300 (slightly wawing around)
Maybe then we could say two things:
- I made correctly the measuring ...!
- measuring the TR Q524/525 still solded on the board gives a wrong result (but it was quite predictable)
So now I would have to unnmount the entire board to unsold the transistors to test them alone, but I'm wondering if it is worth the effort, considering they are only suspected for the moment. If they are OK, I would be quite to the starting point, except the amp almost dismantled on the table.
What do you think?
Didier

 

... Didier,
Another thought, before you try to retailers the transistors.
... Turn on the power to the board. Then configure the red and black cords on your volt-meter
so as to take a voltage measurement. Set the voltmeter scale to the 2 volt range. The idea in this instance is to try to measure the voltage between the emitter pin and the base pin of the two sus
ECT transistors. The usual, ordinary voltage for this measurement is 0.6 to 0.7 volts. If one or the other of the transistors does not indicate this range of emitter to base voltage, then it is possible that it is defective.
... If this measurement appears to be too difficult due to confinement or space restriction, then proceed with de-soldering. It would not be helpful to accidentally short two of the transistor pins together.

 

... Didier,
I am sorry if I have mis-informed you about the testing of the transistors. I thought that there was a chance to try to determine if they were defective.
... Is it possible to remove the transistor's circuit board from the platform or case to which they are attached? The idea is to get access to the bottom surface of the circuit board. If this step is not too much effort, then you can locate a specific transistor and heat the solder junctions. There is a very useful product called Solder-Wick. The way this works is that a very fine copper mesh actually absorbs the melted solder by a process called capillary action. So once the transistor solder joints are melted, apply a section of Solder-Wick to the liquid solder and you will have de-soldered that particular connection.
... Again, I am sorry about the error mentioned above.
View attachment 229025

I just red this post after mine... don't worry, I know the copper mesh to unsold.
For info I have another case in progress, the chip (time clock) of my old direct to disk recorder Yamaha AW4416 has to be changed (the lithium battery is at its end), and I have made some research to see if I can change it by myself, including the use of mesh. This morning at the shop I spoke of that with the seller and he warned me about the danger of doing that myself, unsoldering chips (24 pins DS12887) yourself is quite risky. So I won't do it, I will just amount the oard to give him to make the change.
Electronic is a very attractive domain but need time (I don't have so much to spend), mathematics knowledges (I'm not a master in), a pretty fine viewing (mine is poor) and a lot of determination (I'm light equipped on that).

 

... Yes ... just now saw your recent post. Both of those emitter to base measurements look 'nominal' , that is good.
... more thinking required at this time.
Ok, the measurements listed above were for the new loose transistors, so the two board mounted transistors are still suspect.
... It is difficult to choose the best thing do. Often, in this situation it is difficult to say with confidence which parts are actually defective. The worst case that can happen is the scenario known as 'shot-gunning', which means that several parts are replaced, one after the other, until finally the one or two actually defective parts are discovered.
... I am not coming up with any good suggestions at this time.

 

... Didier,
Another thought, before you try to retailers the transistors.
... Turn on the power to the board. Then configure the red and black cords on your volt-meter
so as to take a voltage measurement. Set the voltmeter scale to the 2 volt range. The idea in this instance is to try to measure the voltage between the emitter pin and the base pin of the two sus
ECT transistors. The usual, ordinary voltage for this measurement is 0.6 to 0.7 volts. If one or the other of the transistors does not indicate this range of emitter to base voltage, then it is possible that it is defective.
... If this measurement appears to be too difficult due to confinement or space restriction, then proceed with de-soldering. It would not be helpful to accidentally short two of the transistor pins together.

I see ony now that I skipped this suggestion.
It seems impossible to make measurment with power on; I had to remove the small upper board to access to the main board (see photos in previous post), I suppose that it would be risky to turn the power whitout this upper board replaced. and all circuitry complete operating.
I'll try in a few days to remove first the big radiator, then the main booard, to unsold the transistors; at this point, there will be no gain to stand still.
Didier

 

... Agreed ... the sure thing to do would be to try to get 524C and 524A loose so that they can be easily tested.

 

Hello drc,

Here under the result of measuring the transistors. Unfortunately!!! they seem in good condition!... as far as I understand (from the diagram), Q524A&C belong to the Left channel (the one in default), the Q525A&C to the Right channel, so is there any interest to unmount these ones to measure? Would the 525 have really different value?
Another question: I started the repair process also on a french forum, somebody suggested that I check the polarity of the push, with checking also two thers transistors Q519 and Q520
Didier

 

... hi Didier,
The values for the base to emitter diode voltage of 524 A and C are lower than the nominal value of 0.6 to 0.7 volts.
I am not sure what to conclude at this time. ...
... Going to take another look at the circuit diagram.

 

There is a zener diode labeled D509, which controls the bias voltage of transistor Q519.
Is it possible to locate this diode on the circuit board and test it using the diode test?
Note the orientation of the diode. It must be installed in a certain direction.

 

There is a zener diode labeled D509, which controls the bias voltage of transistor Q519.
Is it possible to locate this diode on the circuit board and test it using the diode test?
Note the orientation of the diode. It must be installed in a certain direction.

Hello,
the measure of this diode (still on the board) gives:
- 0.668 in the usual direction which seems adequate
- 1.604 in the other direction which should be OL or?
Should I unmount it?

 

... I would say to unmount the diode, just to be sure.
Again, make a mark or write down the orientation of the diode, so that it can be re-installed without any problem. Maybe it would be simpler to take a picture of it before removing, for reference. ... The top surface side, that is.
... Here is a link describing how to test a zener diode, out of circuit, using the ohm scale of the multimeter.:
zener diode test
... the diode end with the black band is negative (cathode).
... the diode end without the band is positive (anode).