Yamaha Receiver (RX-500) in protection - DC on Right Channel Output

Thread Starter

Electone_Guy

Joined Dec 19, 2021
18
Working on a nice old Yammy receiver. Relay won't click - it's in protect. Measured normal DC offset on Left channel (10mV). Right channel is showing +20vDC which is obviously why it's in protect. Pulled the two output trannys on the R channel (2SC2579 & 2SA1104) and it now comes out of protect. But, both test fine out of the unit - no shorts. The drivers also test fine.

I have worked backwards measuring the various transistors on the right channel and the incorrect voltage tracks back quite a ways. But I'll be damned if I can find the culprit. These are the voltages as per the service manual and what I'm measuring:

Manual: B / C / E // Measured: B / C / E
C202: -11 / 0 / 0 // -11 / 0 / 0
C204: 44 / 43 / 44 // 45 / 45 / 45
C206: 0 / 43 / 0.5 // 0 / 45 / 6.0
C208: 0 / 45 / 0.5 // 6.0 / 45 / 6.0
C210: 44 / 18 / 44 // 45 / 0 / 45
C212: 44 / 1.1 / 44 // 45 / 21 / 45
C214: 0.5 / 1.1 / -1.1 // 19 / 21 / 19
C216: 1.1 / 45 / 0.5 // 22 / 45 / 22
C218: -1.1 / -45 / -0.6 // 19 / -45 / 20
C220: 0.5 / 45 / 0 // 21 / 45 / 21
C222: 0.6 / -45 / 0 // 21 / -46 / 21

As you can see, things start to go off at the emitter of C206. Any help would be appreciated. Thanks.
 

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AlbertHall

Joined Jun 4, 2014
11,952
From your measured voltages, Q206 (I assume you mean Q not C in your list) must be open circuit base emitter. But that may not be the only fault.
 

Thread Starter

Electone_Guy

Joined Dec 19, 2021
18
From your measured voltages, Q206 (I assume you mean Q not C in your list) must be open circuit base emitter. But that may not be the only fault.
Yes, those should read Q. 206 was pulled and checked out of circuit, and it was fine. Actually, I've swapped most of the trannys with the left channel counterparts and the problem remains in the right channel.
 

MisterBill2

Joined Jan 23, 2018
11,556
This i where we meet the big benefits of those direct coupled amplifiers,in which any small deviation of any small part can bring about the destruction of many expensive components. And because there is also DC feedback, diagnostics can really be challenging. If there is a designated location for opening the loop that would be very useful, but the only things that I see are the two switches, SW05 and SW06, partly through the power sections. Those may be intended for servicing the amplifier. It does appear that they will allow isolating the last sections a bit, probably not enough to be useful.
A good start, possibly already done, will be to verify all of the supply voltages for both sides.
 

MisterBill2

Joined Jan 23, 2018
11,556
Switch 106 is not installed in the USA, Canada models and S105 seems to be a bass enhancer.
OK, then that suggestion does not work. As I look at the circuit it becomes clear that it is a DC coupled circuit with both AC AND DC feedback, which makes it very challenging to service,because a component failure almost anywhere will disrupt the whole loop, because of the feedback. So checking base to emitter voltages and diode forward drop voltages would be the safe way to search for an active device that was not right. Fortunately you have one side working correctly to use as a reference.
 

MrChips

Joined Oct 2, 2009
25,919
Yes, those should read Q. 206 was pulled and checked out of circuit, and it was fine. Actually, I've swapped most of the trannys with the left channel counterparts and the problem remains in the right channel.
Stop calling something a “tranny”. We don’t what that is.
It can be confused with something else.
 

MisterBill2

Joined Jan 23, 2018
11,556
Stop calling something a “tranny”. We don’t what that is.
It can be confused with something else.
A "Tranny" is the transmission between the engine and the drive shaft in a motor vehicle. Transistors are semiconductor devices. They are not at all interchangable.
A "transformer" is an electrical device that couples electrical power by magnetic coupling, between inductive windings, An "Xformer" is an actor in a pornographic production. They are not interchangable either.
 

Dodgydave

Joined Jun 22, 2012
10,230
Manual: B / C / E // Measured: B / C / E
Q202: -11 / 0 / 0 // -11 / 0 / 0
Q204: 44 / 43 / 44 // 45 / 45 / 45
Q206: 0 / 43 / 0.5 // 0 / 45 / 6.0
Q208: 0 / 45 / 0.5 // 6.0 / 45 / 6.0
Q210: 44 / 18 / 44 // 45 / 0 / 45
Q212: 44 / 1.1 / 44 // 45 / 21 / 45
Q214: 0.5 / 1.1 / -1.1 // 19 / 21 / 19
Q216: 1.1 / 45 / 0.5 // 22 / 45 / 22
Q218: -1.1 / -45 / -0.6 // 19 / -45 / 20
Q220: 0.5 / 45 / 0 // 21 / 45 / 21
Q222: 0.6 / -45 / 0 // 21 / -46 / 21

Screenshot_20211220-203957.png
 

MisterBill2

Joined Jan 23, 2018
11,556
Interesting, but the part of the circuit that has the most abnormal values is not visible. Q204 with the same voltage on all 3 leads looks like it may be shorted.
 

MisterBill2

Joined Jan 23, 2018
11,556
There is a big black strip covering one edge, and a round blue object a bbit above that, towards the right. And everything past Q207 is under that black strip. And yes, the whole circuit is shown in post #1, and when magnified enough to see the numbers it is blurred and unreadable. The view shown in post 9 has the details I seek now covered over. Q210 is not conducting at all, the same voltage on E and B. and whatever is feeding the base of Q210 is under that blue item. Or possibly C274, 680 Pf has gone shorted.
Neither Q210 nor Q212 appear to be conducting, and the voltages on Q213 are not provided in the table at all. Those transistor are what sets the bias for the output transistors, and so they are rather important. Since Q208 also is not conducting, Vbe=0, that leaves Q206, with a Vbe of 6 volts as a suspect.And measuring the voltages with a 2 digit voltmeter, referenced, I presume,to supply common, although the reference has not been mentioned.
Anyway, that is where the finger points as to the source of the offset. Either a failed transistor or a leaky or open one, and quite possibly a leaky capacitor.
Ansd note that seeing the base-emitter voltage of a bipolar transistor is a handy way to see if it should be conducting a bit.
 

Thread Starter

Electone_Guy

Joined Dec 19, 2021
18
There is a big black strip covering one edge, and a round blue object a bbit above that, towards the right. And everything past Q207 is under that black strip. And yes, the whole circuit is shown in post #1, and when magnified enough to see the numbers it is blurred and unreadable. The view shown in post 9 has the details I seek now covered over. Q210 is not conducting at all, the same voltage on E and B. and whatever is feeding the base of Q210 is under that blue item. Or possibly C274, 680 Pf has gone shorted.
Neither Q210 nor Q212 appear to be conducting, and the voltages on Q213 are not provided in the table at all. Those transistor are what sets the bias for the output transistors, and so they are rather important. Since Q208 also is not conducting, Vbe=0, that leaves Q206, with a Vbe of 6 volts as a suspect.And measuring the voltages with a 2 digit voltmeter, referenced, I presume,to supply common, although the reference has not been mentioned.
Anyway, that is where the finger points as to the source of the offset. Either a failed transistor or a leaky or open one, and quite possibly a leaky capacitor.
Ansd note that seeing the base-emitter voltage of a bipolar transistor is a handy way to see if it should be conducting a bit.
I have attached a blown up section of what you said. Hopefully it is a little more legible. I appreciate your help.
 

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MisterBill2

Joined Jan 23, 2018
11,556
With six volts Vbe on Q206 and 39 volts Vce, it is clear that it is not conducting as well as it should be. If it still checks "OK" I wonder how you are checking them.Likewise with Q 204, with zero volts between all three connections, how does it check OK? It seems like it may be leaky or shorted. What test says it is OK??
 

Thread Starter

Electone_Guy

Joined Dec 19, 2021
18
With six volts Vbe on Q206 and 39 volts Vce, it is clear that it is not conducting as well as it should be. If it still checks "OK" I wonder how you are checking them.Likewise with Q 204, with zero volts between all three connections, how does it check OK? It seems like it may be leaky or shorted. What test says it is OK??
I have pulled Q206 and tested it in diode mode on my DMM. I get .635 b-c and .638 b-e. I also swapped it on the board with its counterpart Q205. For Q204, I get .631 b-c and .644 b-e. Is the schematic incorrect as it shows there should be 44v base, 44.6v collector and 43.4v emitter? That's also what I measure on its counterpart Q203 (which I also swapped with Q204). Problem remains on right channel. Stumped...
 

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Last edited:

MisterBill2

Joined Jan 23, 2018
11,556
Reading that small a diode voltage collector to emitter voltage does not seem right. And the diode-check is not a complete check, it will find open devices and shorted devices. Reading the same voltage to some reference point on all three connections, B,C, and E, is rather suspicious, pointing to leakage or a shorted condition.
The +20 volts at the output points towards either an open on the side towards the negative supply or high leakage on the positive side. So for the next step, verify that R294 and R296, the emitter resistors for the output transistors, are still the correct values and that the connections have not failed. Also verify that the +45 and -45 volt supply are the correct voltages.

And this is one more demonstration of why direct coupled amplifiers are a poor choice!
 

LesJones

Joined Jan 8, 2017
3,701
When you say Vbe on Q206 is 6 volts you do not give the polarity. If the base is 6 volts negative with respect to the emitter the transistor will be in a non conducting state.

Les.
 

Thread Starter

Electone_Guy

Joined Dec 19, 2021
18
Base of Q206 is +6V. I have re-checked Q202 and can confirm that I'm getting +1.5V on the emitter there now. It should be zero volts. The base and collector measure as per the SM. Q202 is the beginning of the circuit. I'm wondering if that 1.5v is being amplified along?
 

Thread Starter

Electone_Guy

Joined Dec 19, 2021
18
Here's something that doesn't make sense and perhaps I'm not understanding this correctly. On Q202 (and its counterpart Q201), the collector is connected to ground. It is an NPN transistor, therefore current flows collector to emitter. If zero volts (ground) is flowing from the collector to the emitter, how can there then be 1.5V present on the emitter. Shouldn't there just be zero volts?
 
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