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Rotel RA820A stereo amp no right channel

Bluejets

Oct 5, 2014
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And I promise I'll never put a oversized fuse in again!
Don't worry too much about it, for all the hallaballoo, it's not going to kill it with a 6A fuse.
Being a stereo amp can be helpful when tracking faults if one side is still working ok.
With details from the working side being used as a comparison against the non working side.
What would be handy would be a signal injector to plonk some amount of signal , again one side, then the other to see where your dodgy side stops.
Or conversely, again plonk a signal in the front end and using the other half of a simple signal signal injector ( the amp) see where that starts and stops.

Note:- an old transistor radio is ideal to make such a mother.
Split the signal and the amp at the volume control, one end for signal, t'other for amp.
 

juz571

Oct 8, 2023
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Hi Bluejets, Unfortunately I don't have a signal injector or an old transistor radio to make one. Looks like the service manual gives a few expected voltages at the transistors (I'm assuming that's what they are?) So I guess I could verify on the good side, then compare with the "dead" side?
 

juz571

Oct 8, 2023
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Check the +37 and -37V rails then measure at either end of F601 and F602, report all you find.
So I've just done some checks. I have +38.5v and -38.5V at the rectifier output. Then 0V at both sides of F601 and -36.9V at both sides of F602. So despite what the service manual says, it looks like F601 actually does the right side.
 

kellys_eye

Jun 25, 2010
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Theoretically, with no signal input, the voltage at the fuses is 'zero' so if you are measuring -37V you have a shorted output transistor to the -ve supply line (Q622).

That device needs testing/replacing but there's also the possibility that a transistor driving that device has shorted, causing it to go 'hard on' (giggidy) and creating the fault in the first place. Test Q618 as well.
 

juz571

Oct 8, 2023
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Theoretically, with no signal input, the voltage at the fuses is 'zero' so if you are measuring -37V you have a shorted output transistor to the -ve supply line (Q622).

That device needs testing/replacing but there's also the possibility that a transistor driving that device has shorted, causing it to go 'hard on' (giggidy) and creating the fault in the first place. Test Q618 as well.
Ahh I see. I will get them both out and tested and try and source some replacements today. Thank you again.
 

juz571

Oct 8, 2023
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Hi all,

I have removed both Q622 and Q618 and put them in my "cheap" transistor tester.

I get as follows:

2SB817 (Q622)
PNP
123 BCE
hFE 98
vF 544mV

2SB631 (Q618)
PNP
123 ECB
hFE 274
vF 613mV

Data sheets attached. I can see that the hFE on the 2SB631 is quite a bit higher than the minimum of 60, but I don't have a great understanding of this to know if it's a problem or not!

Do they look ok?
 

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kellys_eye

Jun 25, 2010
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Put Q618 back in circuit then power up and measure the voltage at Q618 emitter.

Where are you putting your test meter negative lead when measuring those voltages?
 

juz571

Oct 8, 2023
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Put Q618 back in circuit then power up and measure the voltage at Q618 emitter.
Q618 back in. Voltage at emitter -38.6.

negative lead is on the E terminal (the black) coming from the transformer. Just below F901 and F902
 

kellys_eye

Jun 25, 2010
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I should have asked for 'all' the voltages around Q618. Point 'E' is correct for the test meter though. What's the base voltage at Q618 and, further 'back' in the circuit, the voltages around Q614?
 

juz571

Oct 8, 2023
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I should have asked for 'all' the voltages around Q618. Point 'E' is correct for the test meter though. What's the base voltage at Q618 and, further 'back' in the circuit, the voltages around Q614?
Ok, so Q618 E-37.2, B -37.9
Q614 B-37.5, C-35.8, E-38.0

Looks like the flood gates are fully open!
 

juz571

Oct 8, 2023
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And I'm telling you do not power that unit up! Just take the photos. Solder side please...
Not as bad on the solder side! Still gunky though!
 

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Delta Prime

Jul 29, 2020
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Further inspection is required.
What looks like a blown out trace may just be flux.
It can be repaired.
How are your soldering skills?
What say you! @hexreader..




photo_1697228143486.pngphoto_1697227943191.png

photo_1697227622196.png
 

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Bluejets

Oct 5, 2014
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Yeah right!
Well, once agin it shows your ignorance.
Even in a mains situation, if there were any fuses ( or circuit breakers) present, the difference would be in laymans terms " bugger all" and basically irrelavant.
 

juz571

Oct 8, 2023
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Further inspection is required.
What looks like a blown out trace may just be flux.
It can be repaired.
How are your soldering skills?
What say you! @hexreader..




View attachment 61040View attachment 61041

View attachment 61043
My soldering skills are ok. I’ve still got a steady hand! I think a lot of that might be flux. Around the transistor bank at the end it looks like someone’s dog has been using it as a back scratcher I haven’t cleaned this sign yet btw, I’ve only recently opened it to get the transistors out.
 

juz571

Oct 8, 2023
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I've just measured every transistor on that side of the board and compared them to what the service manual says should be there.

Some fluctuations as my test meter wonders a bit depending on how hard I'm pressing the probes to the solder. I still have Q622 removed. Looks as if at the moment Q604, Q606 and Q610 are behaving as they should, but the rest aren't. I've noticed that Q602 has a -37.0vV reading on the collector, when it should be 36.1v? (unless it's just a mark on the schematic)
From manual
transistortypebasecollectoremitter
601/6022SA10160v36.1v0.6v
605/6062SA101635.8v0.6v36.4v
603/6042SA10160v-37v0.6v
609/6102SA101636.3v35.7v37v
611/6122SB63135.7v1.2v36.3v
613/6142SD600-0.45v1.2v-1v
607/6082SC1941-36.1v-1v-36.7v
615/6162SD6001.2v37v0.6v
619/6202SD10470.6v37v
621/6222SB8170.5v-37v
617/6182SB631-1v-37v0.5v
901​
2SC157020v33.8v19.4v
902​
2SA1016-20v-33.8v-19.4v
Actual reading
transistortypebasecollectoremitter
602​
2SA10160.1v-37.0v0.8v
606​
2SA101637.2v0.8v37.5v
604​
2SA10160.3v-38.2v0.8v
610​
2SA101637.7v37.1v38.2v
612​
2SB63137.0v-35.7v37.8v
614​
2SD600-37.2v-35.4v-37.8v
608​
2SC1941-37.2v-38.0v-38.0v
616​
2SD600-35.5v38.3v-36.1v
620​
2SD1047-36.0v38.3v-36.5v
622​
2SB817removed
618​
2SB631-37.9v-38.5v-37.3v
901​
2SC1570
902​
2SA1016
 

juz571

Oct 8, 2023
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My mistake. Q602 is as it should be, I put the wrong value in the from the manual.
 

juz571

Oct 8, 2023
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So my thoughts are along these lines. (Please be gentle I'm learning!)
Has C610 gone short and letting -37V through? Because nothing in that top half should have the -37v present. And they all have it along that line from C610, as well as the base of Q618 and the collector of Q608.
 
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