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Old 13th March 2006, 06:08 PM   #1
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Default Lazy transistor?

Here's one for you SS experts.

I have two monobloc power amps which are essentially Naim NAP 135 clones. One of them has developed an intermittent problem. Music is severely distorted, as in half the output isn't working sort of distortion.

Anyway, I got the lid off and checked it out. When powered up, with no signal applied, I should see about 4.4 mV (20 mA bias) across R29 and R30 (schematic below). I'm getting 2 odd mV across R29, but nothing across R30, and no base-emitter or base-collector drop across Q10, the MJE15033 driver tranny. Eventually, if left powered up on the bench for a while, Q10 springs into life and the bias on both sides settles out around 20 mA (4.4 mV give or take across R29 and R30). Once it's up and running, it sounds just fine.

The problem doesn't occur all of the time. It'll power up just fine about 5 or 6 times, and the next time it'll play up. I've checked everything I can think of, replaced all of the signal transistors as I had plenty of spares (all the ones I took out tested fine), replaced the diodes, and Q12. The only tranny I haven't replaced is Q10 as I currently don't have a spare. I took out Q10 and tested it with the diode tester on my multimeter and the base-emitter and base-collector drops were OK ~ 0.5 to 0.6 V.

Is Q10 likely to be at fault? I've never experienced intermittent operation on transistors. I thought they usually work or they don't. Can anyone offer any suggestions?

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Old 13th March 2006, 06:33 PM   #2
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Hmm. Blanket resolder and swap out C8 would be my first move.
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Old 13th March 2006, 07:38 PM   #3
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Leaky cap? Never thought of that. I've got a spare 47uF 63V I can try. If it works, then I'll order up a replacement Black Gate
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Old 13th March 2006, 08:38 PM   #4
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Ok, it's not C8. I swapped it out, and on first power up, there was no output from the MJE15033. I tested the Black Gate on my LCR meter, and it was reading 44.3 uF, which is well within the tolerance spec, so back in it goes.
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Old 13th March 2006, 08:52 PM   #5
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This is weird. The base-emitter voltage on Q10 jumps around all over the place after power up - pretty much everywhere from 100 mV to 400 mV. Eventually, it pulls its way up to over 500 mV, and the tranny kicks in. Once it's in it stays in and the voltage across R30 settles at just over 4 mV.
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Old 13th March 2006, 09:26 PM   #6
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OK, you have a problem around Q10. I suppose it's just possible that the transistor is intermittent. I've never seen that fault before, but then, I'd never seen a transistor go noisy before fixing the amplifier in my valve tester. My money would be on soldered joints or cracked tracks. You've very nearly nailed the problem. If Q10 isn't switching on, then it's either not being told to switch on, or it doesn't have any power to switch on. Rather than just checking Vbe, measure all the absolute voltages on the transistor pins. If it's a solder joint, it's essential to wick away the contaminated solder and clean the surfaces before applying fresh solder.
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Old 13th March 2006, 09:43 PM   #7
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Pinkmouse,

Are you still up for an audition of the new speakers - if I can get this amp working properly (otherwise it'll be left hand only mono)?

Graeme
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Old 13th March 2006, 09:53 PM   #8
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Sure, might be a week or so though, I'm tied up with family stuff at the moment.
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Old 13th March 2006, 09:58 PM   #9
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Quote:
Originally posted by gfinlayson
This is weird. The base-emitter voltage on Q10 jumps around all over the place after power up - pretty much everywhere from 100 mV to 400 mV. Eventually, it pulls its way up to over 500 mV, and the tranny kicks in. Once it's in it stays in and the voltage across R30 settles at just over 4 mV.
I'm with EC8010, it sounds more and more like a PCB issue, possibly settling down as something heats up. Have you tried turning the amp off and checking tracks for continuity with a meter?
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Old 13th March 2006, 10:11 PM   #10
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I've had intermittent wire-bond failures on transistors before now.

Usually findable with freezer spray. Have you tried that?
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