yamaha av receiver repair

[kelvin marsden]

yamaha rxv673 won't power up. self diagnostics indicate power transistors problem "Iprt". removed power transistors and tested on 15 dollar tester. all transistors test as diodes. i don't know what that means. does that mean all transistors are shorted? what could cause all the power transistors to shrort.
any help would be appreciated.

 

[Nigel Goodwin]

I can't see anything in the service manual about self diagnostics checking power transistors?, or any mention of 'lprt'?. I'm dubious about any such function?.

Transistor amplifiers are like rows of dominoes, anything fails and the output stage is catastrophically destroyed. This isn't a gentle occurrence, burnt components and fuses blown to bits are highly likely.

I'm also a bit concerned about randomly removing all the output transistors, the chances of much greater damage have become vastly greater.

Even assuming some output transistors have failed, it would almost certainly only be one channel - and you shouldn't touch any other channels.

What was the exact self diagnostic function that caused you to remove the transistors?.

 

[ahsrabrifat]

Test between Base and Emitter, Base and Collector, Collector and EmitterFor BJTs, you should see diode-like readings from base to emitter/collector but open circuit in reverse polarity. Any low-resistance reading across collector-emitter is a clear sign of a short.

 

[kelvin marsden]

pg 31 of service manual "Display provided when Self-Diagnostic Function started, #2. When the protection function worked to turn off the power." it directs to the P2 protection history. first result was Iprt. I took "prt" to mean protect. the self diagnostic doesn't provide much. "Iprt" indicates the power transistor issue, "current protect". #2-1 says check each transistor before applying power (won't let me do anyway).
unit is dated aug 2013. i bought unit for 10 bucks to try and learn repair. was very clean inside. no sign of any heat damage, no bulge caps, won't power up. little red light blinks a few times when you push power, then shuts down. diagnostics will come on. i've tested a few of the transistors with a multimeter and they test as working. i will test the rest. just the tester i bought indicates they are bad. tester shows them all as diodes. i tested a transistor i removed from something else and it showed as a transistor on the tester?

 

[Nigel Goodwin]

You need to fault find, randomly checking components isn't how you repair things - with an amplifier the first thing to do is check the voltages on the output transistors - in particular the voltages on the emitters should be very close to zero (within a few 10's of mV). The NPN collectors should be about 60V or 30V (depending on impedance selection), and the PNP's should be -60V or -30V.

 

[kelvin marsden]

a thousand people looked at this before i bought it. seems to be a very over complicated design. its running firmware and has a windows logo in the service manual.
doesn't it have to be able to power up before i can test those emitter values? i did the multimeter tests {unpowered} before i took it apart, but they seemed pretty inconclusive. the self diagnostics said to test each power transistor {7 pairs).
my main question is why i'm getting unexpected "diode" results from my transistor tester, for each power transistor. they all test the same. i will multimeter test the rest of them. the ones i've tested seemed to be fine.
the transistor tester indicated fine for everything else i've used it for. some caps and a transistor i took from an old microwave.
i don't understand how all the power transistors could have failed.

 

[Nigel Goodwin]

The power supply has to be running, but if you've got voltages on the collectors, then it's running enough to test the emitter voltages.

 

[kelvin marsden]

i don't think the power supply was running. according to the vids i watched, there is a standby power supply, so the remote will function, that runs diagnostics before returning a signal to power up the main transformer. i wished i had cleared the protection history, and then tried to fire it up. i may have worked. but why did the protection history show current issues initially?
the only multimeter tests i ran before tearing it apart were nonpowered tests, while still in the circuit.
i have all the power transistors out and have tested them with multimeter. tests done with diode function. they are pairs of sk a1695 and c4468 transistors, 7 pairs. all the results seem consistent. no continuity between collector and emitter. base to emitter and base to collector readings are .31 to .5.
there is continuity between collector and the plate where the mounting screw goes thru. is that normal? and they still all test as diodes on that component tester.?
one thing i forgot to mention. when i did the diagnostics, it ran "manual test, all". i had speakers and headphones connected and i got white noise thru the headphones. the unit has a internal signal generator.
thanks for your time.

 

[Nigel Goodwin]

i don't think the power supply was running. according to the vids i watched, there is a standby power supply, so the remote will function, that runs diagnostics before returning a signal to power up the main transformer. i wished i had cleared the protection history, and then tried to fire it up. i may have worked. but why did the protection history show current issues initially?
the only multimeter tests i ran before tearing it apart were nonpowered tests, while still in the circuit.
i have all the power transistors out and have tested them with multimeter. tests done with diode function. they are pairs of sk a1695 and c4468 transistors, 7 pairs. all the results seem consistent. no continuity between collector and emitter. base to emitter and base to collector readings are .31 to .5.
there is continuity between collector and the plate where the mounting screw goes thru. is that normal? and they still all test as diodes on that component tester.?
one thing i forgot to mention. when i did the diagnostics, it ran "manual test, all". i had speakers and headphones connected and i got white noise thru the headphones. the unit has a internal signal generator.
thanks for your time.

Yes, the mounting plate is connected to the collector, which is why you have insulating washers underneath.

 

[kelvin marsden]

i was able to test those power transistors and 6 of them have hfe's lower than 50, which is minimum (assuming i'm interpreting the data sheet correctly) for a sanken a1695 c4468 transistor.
could this be the issue with this receiver?

 

[Nigel Goodwin]

i was able to test those power transistors and 6 of them have hfe's lower than 50, which is minimum (assuming i'm interpreting the data sheet correctly) for a sanken a1695 c4468 transistor.
could this be the issue with this receiver?

No, faulty transistors go S/C or O/C, low gain would be EXTREMELY unlikely, and wouldn't affect operation anyway.

 

[kelvin marsden]

reviewing data, it is all the c4468 (npn) transistors that have low hfe. and it's all seven. around 39.
i understand on the "unlikely to affect operation", thought it was curious. i guess they all check out.

 

[Nigel Goodwin]

reviewing data, it is all the c4468 (npn) transistors that have low hfe. and it's all seven. around 39.
i understand on the "unlikely to affect operation", thought it was curious. i guess they all check out.

The gain of the amplifier is set by the negative feedback, and as long as the total number of stages have plenty of gain, then all is well - basically the total gain massively swamps the required gain, similar to an opamp (which is essentially what it is).

The transistors were probably out of the same batch, hence their similar gains.

 

[kelvin marsden]

i put the unit back together and it powered up about 6 times. no sound, but could change inputs. after the sixth time, it would no longer power up. i tried going into diagnostic mode and it immediately heated up one the the resistors (one of the double resistors connected to the power transistor emitter). i probably bridged something when i soldered the power transistors (center channel).
the unit is so hard to get apart, i'm probably not going to work on it for awhile, no bottom panel, so it all has to come all apart again.
thanks for your time and answers.

kelvin

 

[Nigel Goodwin]

i put the unit back together and it powered up about 6 times. no sound, but could change inputs. after the sixth time, it would no longer power up. i tried going into diagnostic mode and it immediately heated up one the the resistors (one of the double resistors connected to the power transistor emitter). i probably bridged something when i soldered the power transistors (center channel).
the unit is so hard to get apart, i'm probably not going to work on it for awhile, no bottom panel, so it all has to come all apart again.
thanks for your time and answers.

kelvin

Oh dear, you have to be extremely careful in amplifiers, you've probably blown a LOT of parts in the centre amp.

I agree that AV amps are absolutely horrible to repair, and you really need the extension leads etc. from the manufacturer to work on them - but the extensions are VERY expensive (if even available). Out of warranty, I used to just give a big estimate and hope they turned it down you're talking hours of work just getting it to pieces to a state where you can work on it.

 

[Tony Stewart]

My feeling is the Iprt was due to an oscillation and not a hard failure. These can be very troublesome in the intermediate stages driving the output stage because all the bipolar emitter followers increase Hfe with base current and then decrease at peak current. The output signal would be full amplitude ultrasonic with white noise as phase modulation at some low level.