Burnt out transistors - replace with same or something better? Rotel RB-991 - Page 2 - diyAudio
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Old 19th November 2012, 07:37 AM   #11
Mooly is offline Mooly  United Kingdom
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It will be an output stage fault (and associated damage)

Are you sure your up to working on this ? Its ultimately a fairly straightforward circuit but don't be under any illusions of just swapping an odd transistor and it being fixed. Although you could get lucky its a fair bet there is other damage as well.

As a general procedure you need to,

1. Disable the vbe multiplier to reduce output stage current to zero on both channels.

2. Remove and test all the output devices and drivers as a basic first step. Whatever the outcome of the tests I would be inclined to replace the drivers anyway as they can be intermitent in failure and this can cause the problem of destroying the outputs. It's just not worth the risk of leaving them.

3. Check low value resistors that could have been damaged by transistor failure.

4. Optional step. I would then test the amp with NO outputs fitted (done by moving the feedback resistor take off point to the vbe multiplier) and confirming the DC conditions are correct.

5. Replace outputs and test and set up.

ALL the above done with a bulb tester in place.
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Old 19th November 2012, 07:57 AM   #12
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"Are you sure your up to working on this ?"

Even less sure now!
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Old 19th November 2012, 08:03 AM   #13
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Its up to you

We can talk you through what to do. Its really a case of being methodical and proving all is good along the way.

If only one output is shorted (and it all has to be proved) then just maybe you could be lucky. IF not a methodical approach finds the problems.
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Old 19th November 2012, 08:33 AM   #14
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The offer of help is appreciated.

One embarrassingly basic question. I have a RB-991 amp in bits. )All it needs is one cap to be soldered in and it is perfect for resale. However that cap has been sellotaped to the top of it for a year waiting for me to fit.)

Are the boards swapable between the amps, or is there some setup that needs to be done with each board {like swapping a transistor}
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Old 19th November 2012, 09:05 AM   #15
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I would doubt the boards are swappable from a 991 to a 993. You'd have to compare circuit details but there are sure to be differences such as differing supply voltages and consequently differing components/values used even if the basic topology seems essentially identical.
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Old 3rd December 2012, 11:40 AM   #16
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Well it almost went well.

I swapped out the transistors and checked over the board. All seems good.

I then remembered (OK got reminded) that I needed to recheck the bias.

I unplugged the inputs/speakers and turned the amp on.

Bias was fine on board 3 (which I was working on).
Bias on board 1 was 0....

Turns out I have now lost the output transistors on channel 1 now!!!

This seems a bit much to be a coincidence.

So what could take out the transistors on both sides?

This is WAY over my head.

p.s. I have 1 set of spare transistors left. If I lose another set this amp is dead (the odds of getting genuine spares on this are next to 0)
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Old 3rd December 2012, 11:42 AM   #17
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I should add that the transistors on board 1 were fine this morning and the amp has been used all morning.
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Old 3rd December 2012, 11:52 AM   #18
freax is offline freax  Australia
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you didn't by any chance swap left and right boards over and replaced the good ones with two other good transistors?
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Old 3rd December 2012, 12:02 PM   #19
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Have the transistors gone short circuit ?

Any fault finding has to start at the begining. Maybe the bias was too high and thermal damage occured. Its impossible to say at a distance.

To fix something like this isn't a quick job. It must be gone through methodically. Everything should be confirmed as good before the bulb tester is removed and speakers connected.
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Old 3rd December 2012, 12:43 PM   #20
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"you didn't by any chance swap left and right boards over and replaced the good ones with two other good transistors?" I wish it was that easy. I did have to double check.

Thermal damage.... seems likely these amps run HOT.

I checked the bias on board 3 after swap, bias was over 200 (supposed to be 7).

Bias on board 2 is 6.5, so fine. Can't measure board 1 until I swap trans.

Now also have a problem on my RB-991 too, right channel is getting no high frequency.

I have come to the conclusion that old equipment that has not moved for years does not like being touched!
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