Hi, a friend asked help with her Rotel RB-980BX amp. Left channel works fine, right channel sounds like it’s constantly overpowered, clipping. It is not louder than left btw. I checked power supply voltage, is OK, same as left (53V I measured). After a while I noticed the cooling body of the left channel warmed up but the one of the right didn’t. I tried to do some measurement on the power transistors (the ones on the cooling body), didn’t really have enough time but one of the transistors deviated from left. My theory is, one or more of the power transistors is gone, therefore the end stage’s settings are off, and the power dissipation is down and therefore no warming up and clipping.
Any thoughts on whether this theory could be right? Or could the problem also lie before the 6 transistors on the cooling body? Getting them out won’t be easy btw.
Any thoughts on whether this theory could be right? Or could the problem also lie before the 6 transistors on the cooling body? Getting them out won’t be easy btw.
Look at the driver stage close. (The Transistors not hooked to the heatsink.) One of your transistors feeding the output transistor (Q602, Q604, Q608, Q610, Q612,Q614, Q616 Or Q618) is probably starting to break down. If you have some freeze spray you can freeze the transistor individually and possibly find the issue. Also Have you checked it for bad solder. Not often Have I seen an output transistor start to fail. They usually are either good or blown. Freeze Spray is a real good friend when trying to debug these issues.
Freeze spray is good for finding several types of problems in a circuit. It works especially well for locating issues with the junctions breaking down in Transistors, Diodes and IC's. It also is very useful in tracking down bad solder joints that are hard to find or just about impossible to see.
Thermal (Hot and Cold) Stressing components is excellent for showing the weaknesses in them. Over the past 30 Years. I have found thousands of weak or failing parts using a can of freeze spray or the heat from a soldering iron tip. When using a soldering iron tip. You have to be very careful not to short out the leads or leave the tip on the case of the part too long.
Thermal (Hot and Cold) Stressing components is excellent for showing the weaknesses in them. Over the past 30 Years. I have found thousands of weak or failing parts using a can of freeze spray or the heat from a soldering iron tip. When using a soldering iron tip. You have to be very careful not to short out the leads or leave the tip on the case of the part too long.
Freeze spray can help with intermittent faults but doesn't offer much for finding burnt semis, open circuits or other failures. If the output stage is not warming up, it is obvious that there is little or no bias current flowing through the output transistors. This could be a fault anywhere as kilgorq suggests. It would make sense to do some quick multimeter checks of all those transistor basic electrical operating conditions first by testing their Vbe, which should be about 0.65V in all BJTs (somewhat less for the output transistors) and shorts between C and E. These can be compared with the good channel to identify any significant differences
The service manual also shows expected key voltages in the circuit so check them too for clues in this area. With so many tests, be careful not to slip with meter probes and cause more damage - its so easy to do. Using a hook probe, clip lead etc. is a good plan for ground referenced measurements at least. http://bwgroupsupport.com/manuals/rotel-service
The service manual also shows expected key voltages in the circuit so check them too for clues in this area. With so many tests, be careful not to slip with meter probes and cause more damage - its so easy to do. Using a hook probe, clip lead etc. is a good plan for ground referenced measurements at least. http://bwgroupsupport.com/manuals/rotel-service
I think SalveTitus was sayin it was getting hotter on the right channel.
I Agree with Ian 100%.
In most cases with Push Pull Amps if a driver transistor fails (Shorted Or Open) It will be a catastrophic failure. The Amp will swing either High or Low and start blowing fuses or worse yet shorting the Ouptut transistors out.
This sounds like a case of a junction getting noisey. That is the reason I suggested the Freeze spray. They order I would proceed in debugging this would be.
Inspect and repair any bad solder connections
With Power on and signal applied Check Thermals (Freeze Spray)
With Power off check Transistors B to E (~0.6V) B to C (~0.6V) and C to E (Open) Using diode check.
WIth Power on and no signal Check voltage on base of the output transistor. Compare to other channel. (Minor differences are OK.)
Work you way upstream on the voltage tests. Comparing Left and right Channel.
IF you still have no success then it is time to start removing transistor and checking them.
Check Each Transistors B to E (~0.6V) B to C (~0.6V) and C to E (Open) Using diode check. If you find a bad one please do not assume it is the only one. Check them all 1 at a time. Usually if one goes it damages another. This is the nature of Push Pull Amps.
A Curve tracer will become your best friend at this point. However a lot of times you do not get to this point. Usually you will find the issue in the first couple of steps. There are a lot of techs that just replace the transistors at this point. I prefer to find the ones that are actually causing the problem.
I Agree with Ian 100%.
In most cases with Push Pull Amps if a driver transistor fails (Shorted Or Open) It will be a catastrophic failure. The Amp will swing either High or Low and start blowing fuses or worse yet shorting the Ouptut transistors out.
This sounds like a case of a junction getting noisey. That is the reason I suggested the Freeze spray. They order I would proceed in debugging this would be.
Inspect and repair any bad solder connections
With Power on and signal applied Check Thermals (Freeze Spray)
With Power off check Transistors B to E (~0.6V) B to C (~0.6V) and C to E (Open) Using diode check.
WIth Power on and no signal Check voltage on base of the output transistor. Compare to other channel. (Minor differences are OK.)
Work you way upstream on the voltage tests. Comparing Left and right Channel.
IF you still have no success then it is time to start removing transistor and checking them.
Check Each Transistors B to E (~0.6V) B to C (~0.6V) and C to E (Open) Using diode check. If you find a bad one please do not assume it is the only one. Check them all 1 at a time. Usually if one goes it damages another. This is the nature of Push Pull Amps.
A Curve tracer will become your best friend at this point. However a lot of times you do not get to this point. Usually you will find the issue in the first couple of steps. There are a lot of techs that just replace the transistors at this point. I prefer to find the ones that are actually causing the problem.
Hi it took a while but yesterday I started with the freeze spray (or actually, compressed air can upside down which freezes quite nicely). Alas! none of the transistors responded. I checked all B-E voltages with power on, all where 0,56V, one 0,76 (Q632). The only new piece of information is that I have the impression the distortion is triggered by lower frequencies: low piano frequencies are the worst, higher cello sounds quite right. So I thought, maybe a condensator issue? Sprayed them as well but again no luck.
Can somebody tell me, why do Q602/Q604/Q606/Q608 look so strange, with a waxy pointy thingy on them? Maybe those where immune to my spraying. Anyhow suggestions are very welcome. Apart from the distortion, the strangest thing is the end stage cooling frame does not heat up.
Can somebody tell me, why do Q602/Q604/Q606/Q608 look so strange, with a waxy pointy thingy on them? Maybe those where immune to my spraying. Anyhow suggestions are very welcome. Apart from the distortion, the strangest thing is the end stage cooling frame does not heat up.
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