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Old 27th August 2008, 08:51 PM   #1
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Default One faulty output TR,change all 4?

Hey people
I suspect I have an intermittantly faulty (usually when warmed up) power output transistor in the left channel of my old(but great) Rotel RX-400L amp.
Do I change it or all four at the same time or what?
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Old 27th August 2008, 08:56 PM   #2
jaycee is offline jaycee  United Kingdom
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It would probably be best to change all 4.. but at least change the outputs in that channel with a new set.
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Old 27th August 2008, 09:08 PM   #3
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The service manual has the letters"O.Y" beside these output transistor names(Q613 is the suspect).Other transistors in the rest of the amp have letters beside them too such as"C","KL" and"RS".
What do these numbers mean?Could they refer to a certain gain group?
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Old 28th August 2008, 01:17 AM   #4
jaycee is offline jaycee  United Kingdom
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Yep, they are the gain group recommended
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Old 28th August 2008, 05:47 PM   #5
Mooly is online now Mooly  United Kingdom
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Why do you suspect an output ?
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Old 28th August 2008, 05:56 PM   #6
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Left ch sounded very distorted.I put sine test tone in and positive half cycle clipped,negative half too "pointy".
Got service manual,checked bias voltage.Right ok,left no bias voltage at all( is measured across Q613 emitter resistor R623,should be 4.4mV,with no signal input)
Everything else around circuit looks OK and because fault happens only when hot or even "normally" warm I took some freezer spray and took potshots at varoius parts in the circuit to no effect untill I cooled Q613 and bias comes back immediatly and distortion goes away.
Therefore I am sort of guessing that this transistor is dodgy.
Does this make sense to you guys?
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Old 28th August 2008, 06:07 PM   #7
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Hi,
That's fairly conclusive I guess. However are you sure your not cooling anything else. With it faulty measure the base emmiter volt drops on the driver and output. That can be revealing if one is going O/C. Don't measure from ground, measure across the junctions -- anything over 0.7 to 0.8v and it's duff.
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Old 28th August 2008, 06:10 PM   #8
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Should have asked as the circuits not complete, is this an AC coupled amp.
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Old 28th August 2008, 06:19 PM   #9
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The output goes through an inductor paralelled with a resistor(zobel?) and straight to the speaker.(is this AC coupled?)
Tone controls are taken from just before the zobel and are part of the feedback to Q603.
I will post more complete circuit soon if it helps.
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Old 28th August 2008, 06:33 PM   #10
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If there is no large capacitor in series with the output to the speaker then it's DC coupled. If it uses a "split" supply ie it has a positive AND a negative rail then AGAIN it's DC coupled.
You may well be right with the output transistor theory, it's just nice to prove it
When it does go faulty check no DC voltage appears on the output to the speaker as that can be damaging.
When it's faulty measure as I suggested, the base/emmiter volt drops on all but Q601 and Q603 in your picture. They should all be in the region of 0.7volts. Any that are significantly over and the junction is open circuit.
You can always swap the suspect one with the other channel to prove it.
Compare the voltages on the transistors with the other good channel when it's in it's faulty state and if you find anything post the readings here.
Will look in again tomorrow -- Good luck.

Edit-- Actually swapping transistors may not be a good idea -- the act of removing them can often temporarily cause them to "heal up"
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