Hifi-trouble on Yamaha R840 module

Thread Starter

Didier C

Joined Jan 12, 2021
23
Hello everybody,

My son has a malfunction on his Yamaha Pianocraft R840 amp module, aged around 5 years; one channel is buzzing, a friend of mine told who has knowledges in electronics me that it sounds like half of push-pull amp is out.
Mys daughter who has some kind of same model had same kind of same trouble a few years ago and I red that this could happen quite often on that sort of equipment, as " usual suspects".
Does anyone know what could I do to repair? changing some pieces or even the entire board (if it is affordable of course).
Thanks for your suggestions.
Didier-France
 

drc_567

Joined Dec 29, 2008
1,154
... Sometimes it is possible to find a bad part by applying 'freeze-spray' ... (It is sold in a spray can.) to any likely components and listening for a change in tone. Output transistors, parts mounted on heat sinks, or electrolytic capacitors could be tested in this manner. ... No guarantees, but it might be worth a try.
 

Thread Starter

Didier C

Joined Jan 12, 2021
23
Bussing can come from a blown speaker too. Try swapping the speakers left/right. If the buzz follows the speaker, the speaker is the problem. Otherwise the problem is within the amp. You can get the service manual here: https://elektrotanya.com/yamaha_ns-bp300_r-840_mcr-640_mcr-840_mcr-940-.pdf/download.html

Some experience with electronics repair would be good here.
Dear abrsvc,
Thank you for your help and this suggestion, in fact checking speakers was the first thing I've tried before going further (as I've been playing guitar in rock bands for 30 years I know this quite well!). The speakers definitely cannot be suspected.
Best wishes
Didier C.
 

Thread Starter

Didier C

Joined Jan 12, 2021
23
... Sometimes it is possible to find a bad part by applying 'freeze-spray' ... (It is sold in a spray can.) to any likely components and listening for a change in tone. Output transistors, parts mounted on heat sinks, or electrolytic capacitors could be tested in this manner. ... No guarantees, but it might be worth a try.
Hello drc_657
thanks for the idea, I will try this.
Didier C
 

Thread Starter

Didier C

Joined Jan 12, 2021
23
Hello everybody,

I've made the procedure of auto-tests mentioned in the service manual, nothing seems in trouble.

I've checked the preamp part , both channels are OK on headphone

Measuring DC current at the HP connectors (no source, no charge):
- Right Channel 10mv
- Left Channel (the one in default): 108mv.
Measuring the idling current at amp unit (see doc joined)
- Right channel: 1,5mv
- Left channel: 0,00mv

Is that definitely the power transistor left channel which is out, ot can it be something else?

Didier

IC-AU.png
 

drc_567

Joined Dec 29, 2008
1,154
... The left channel resistor, R578, should have some idling current through it, as indicated by 1 mV or so voltage. So something that supplies current to that resistor is not working correctly. Have you looked in the area near the power supply for any fuse that may have blown?
... Maybe show the circuit diagram area which connects to R578.
... Where is the HP connector that you mention ... Can you show in the diagram the power transistor?
 
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Thread Starter

Didier C

Joined Jan 12, 2021
23
... The left channel resistor, R578, should have some idling current through it, as indicated by 1 mV or so voltage. So something that supplies current to that resistor is not working correctly. Have you looked in the area near the power supply for any fuse that may have blown?
... Maybe show the circuit diagram area which connects to R578.
... Where is the HP connector that you mention ... Can you show in the diagram the power transistor?
Hello,
Thanks for your reply.
The only visible fuse (on the power supply board, meaning not near the R578 resistors) is in good condition
I don't see any component subject to failure when looking at the entire inside of the apparel. (see photos joinedDiagram Power Tr-R578.pngMounting set.png)
The speaker was connected to the normal speaker out red and black connectors.
Here are some screenshots of diagram, main card, mounting set, all detailing location of power transistors (blue) and R578 (green) but of course, I don't know if they are part of the problem.
Hope this will help.IMG_9637.jpegIMG_9639.jpeg
DidierMain card.png
 

drc_567

Joined Dec 29, 2008
1,154
Q524A and Q524C transistors look like a possible candidates for the defective part.
However, it is very easy to make a mistake in these situations.
After allowing the unit to warm up for 60 minutes or so, is it possible to measure the temperature of Q524C using an infrared thermometer tool? Compare this with the temperature of transistor Q525C.
Alternatively, apply a,short burst of freeze-spray to each transistor and confirm that there is at least some heat producing current flowing through them. If Q524C is in fact defective, the frost will remain on the surface for several seconds. Compare Q524C with Q525C.
... If that does not seem like a simple procedure, then the best thing to do would be to de-solder the pins of Q524C and use the diode test function of a multimeter to verify that the transistor is defective
.. which is typically an easy procedure to accomplish.
... Test transistor Q524A if necessary as it may be the one that is defective .
... Just looking at the photos that you have provided, the removal of any of those questionable transistors by de-soldering looks like a task, so maybe consider using the freezer-spray diagnostic method as an initial assessment.
 

Thread Starter

Didier C

Joined Jan 12, 2021
23
Q524A and Q524C transistors look like a possible candidates for the defective part.
However, it is very easy to make a mistake in these situations.
After allowing the unit to warm up for 60 minutes or so, is it possible to measure the temperature of Q524C using an infrared thermometer tool? Compare this with the temperature of transistor Q525C.
Alternatively, apply a,short burst of freeze-spray to each transistor and confirm that there is at least some heat producing current flowing through them. If Q524C is in fact defective, the frost will remain on the surface for several seconds. Compare Q524C with Q525C.
... If that does not seem like a simple procedure, then the best thing to do would be to de-solder the pins of Q524C and use the diode test function of a multimeter to verify that the transistor is defective
.. which is typically an easy procedure to accomplish.
... Test transistor Q524A if necessary as it may be the one that is defective .
... Just looking at the photos that you have provided, the removal of any of those questionable transistors by de-soldering looks like a task, so maybe consider using the freezer-spray diagnostic method as an initial assessment.
Many thanks for these options!...
- measuring the temperature like you suggest seems very difficult maybe even impossible because in the operating configuration of this unit, the small board is placed upon the main board, so the transistors will not be visible anymore (see photo joined, situation with upper board in initial position)
- applying freeze spray should be the same difficult for the same reason
I will think about a way to solve that difficult
- one question: I wonder if it is absolutely necessary to unsold the transistors on the board to test them? would it not be possible to leave them mounted on the board and connect them to the multimeter with 3 small wires?

Didier

IMG_9625.jpeg
 

drc_567

Joined Dec 29, 2008
1,154
... Testing those transistors usually requires isolating them from other circuit components.
However, it may be possible to work something out.
The idea is to determine which pins are collector, emitter, and base. ... There are Chinese characters printed above the transistor pins, but that is not much help.
A bipolar transistor consists of two diode junctions, arranged back to back, with the center point being the base pin. So for an NPN transistor, such as Q524C, the diode test function on a multi-meter should see a good diode test, 0.6 volts approximately, if the test leads are connected from base to emitter, and then from base to collector. Conversely, connecting the test leads from emitter to base, and then from collector to base should indicate a reverse diode connection ...
The PNP transistor is similar to the NPN transistor, except the diode polarities are reversed, with the base pin having a common connection between the two back-to-back diodes.
... So try testing the Q524C transistor using the meter diode function. Test both polarity directions.
If that transistor tests out according to the diode function, then try testing the other transistor, Q525C which is a PNP version.
... Not quite sure where you can obtain the transistor pin-out configuration, but if there is a parts list someplace, then that list should provide a description of the transistor that can be looked up.
The number/letter code printed next to the transistors does not seem to be a useful transistor description. ... Maybe it is a part number, but not really sure about that.
 

Thread Starter

Didier C

Joined Jan 12, 2021
23
... Testing those transistors usually requires isolating them from other circuit components.
However, it may be possible to work something out.
The idea is to determine which pins are collector, emitter, and base. ... There are Chinese characters printed above the transistor pins, but that is not much help.
A bipolar transistor consists of two diode junctions, arranged back to back, with the center point being the base pin. So for an NPN transistor, such as Q524C, the diode test function on a multi-meter should see a good diode test, 0.6 volts approximately, if the test leads are connected from base to emitter, and then from base to collector. Conversely, connecting the test leads from emitter to base, and then from collector to base should indicate a reverse diode connection ...
The PNP transistor is similar to the NPN transistor, except the diode polarities are reversed, with the base pin having a common connection between the two back-to-back diodes.
... So try testing the Q524C transistor using the meter diode function. Test both polarity directions.
If that transistor tests out according to the diode function, then try testing the other transistor, Q525C which is a PNP version.
... Not quite sure where you can obtain the transistor pin-out configuration, but if there is a parts list someplace, then that list should provide a description of the transistor that can be looked up.
The number/letter code printed next to the transistors does not seem to be a useful transistor description. ... Maybe it is a part number, but not really sure about that.

I join here the page of the listing of considered components in the service manual (easily downloadable under this request: Yamaha-R-840-Service-Manual....I got mine from Manulalibs).
Here so far, I must confess that metering transistors as you suggest becomes is not simple for me, I never did that before. But I will try of course, your explanations are real clear, and I will get back to you with the result. It would be disappointing to abandon this amp just because of a default component.
For the moment, it is 1.00 in the morning and I must go to sleep.
Thanks again!
DidierList components.png
 

drc_567

Joined Dec 29, 2008
1,154
ok ... looked up that transistor pair ...
This should be the right pin out:
1611708149371.png

... Regarding the diode testing, if you have an ordinary diode available someplace, try using the multimeter diode test function on it to see how it works. ... Try a test in both directions by reversing the test leads. ... Be sure any power is turned off.
 
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Thread Starter

Didier C

Joined Jan 12, 2021
23
ok ... looked up that transistor pair ...
This should be the right pin out:
View attachment 228834

... Regarding the diode testing, if you have an ordinary diode available someplace, try using the multimeter diode test function on it to see how it works. ... Try a test in both directions by reversing the test leads. ... Be sure any power is turned off.
Here under a photo of my multimeter "customized" with the three wires to be able to reach the transistors deep on the board.
My knowledge in electronics is limited so I'm not sure to understand the "diode test" you expose, I will test with wires on dedicated EMB pins and give you the values I collecIMG_9648.jpgt.
 

drc_567

Joined Dec 29, 2008
1,154
Hi Didier ...
The meter Dial should be set to the Ohms 2k mark, that has the small diode icon.
That will provide the diode test function.

But go ahead and see what the hfe test gives you.
If the transistor is good, then a number ... maybe 100 or so, will appear.
...
For the diode test, only two leads are used, and they are plugged into the main/primary meter jacks, not the hfe holes. ... Refer to the meter instruction guide if necessary.
 
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Thread Starter

Didier C

Joined Jan 12, 2021
23
Hi Didier ...
The meter Dial should be set to the Ohms 2k mark, that has the small diode icon.
That will provide the diode test function.
But go ahead and see what the hfe test gives you.
...
For the diode test, only two leads are used, and they are plugged into the main/primary meter jacks, not the hfe holes. ... Refer to the meter instruction guide if necessary.

Hi,
Here is my cabling CBE to reach the electrodes, for the moment the HFE gives me indistinctly value 1.00, wehther I respect the choice PNP/NPN so something is wrong somewhere... it is quite hard to be sure to be correcty in contact with the wires (and you told me that component must be isolated from the rest).. I try again.
Something else confusing is that in the manual, the drawing of the board is reversed, that's weird. and makes more difficult to be on the right component,; other points: the TR named 524A in the manual is labeled 523 on the board, the 525C (manual) is named 526 (board).
IMG_9650.jpeg
 

drc_567

Joined Dec 29, 2008
1,154
... as I understand the circuit diagram, Q524C is the NPN type transistor.
And Q524A is the PNP type.
... And yes the B, C, E pins must correspond to the photo shown previously.

... I suspect that one or the other of these transistors is defective.

... some confusion on transistor numbers previously. I think this post is correct.
 

drc_567

Joined Dec 29, 2008
1,154
... looking at the photos of the board, one of the Q524 transistors will be the NPN type, and the other Q524 transistor will be the PNP type.... So try testing each of the Q524 transistors in either of the dial hfe positions. ... Maybe that will tell you something. At least one transistor should be good, and maybe one transistor is bad.
 

Thread Starter

Didier C

Joined Jan 12, 2021
23
HFE did not gave me more than what I saw before, only value 1.00. It is over my possiblity to unmount these comonentns to measure them away from the board.
Here are the results of measuring the four transistors on diode/2K position.
For 524 C and 525C, measuring beteen C and E gives a slowing growing value from 0 growing slowly (20 sec) to 2K and then 1 (I suppose that it exceeds the limit?).
This seems that the four TR are in normal condition or?
At this point, I don't know what else I could do.

IMG_9651.jpeg
 
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