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?
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?
An externally hosted image should be here but it was not working when we last tested it.
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.
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.
gfinlayson said: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?
Yes, good point (and welcome to the forum). Freezer spray is very effective (although wildly environmentally unsound). Another good tool is a hair dryer. I got a very odd look from the girlie in the chemists when I bought a hair dryer. She didn't actually say, "Why do you need a hair dryer?" but I could see what was on her mind. Thumping things with a screwdriver handle sometimes pinpoints the fault.
Funny, I've had similar looks in chemists before, when someone asks "Can I help you?", and I reply "Yes, I'm looking for some hair gel".
Anyway, back to the point. It seems I may have found the problem. I've been all over the board checking in-track continuity and track to track insulation. There was a tiny little nib of solder between two tracks on the board, essentially between the collector of Q5 and the output. It measured about 60 kohm between tracks with the meter. After a quick touch with a soldering iron, it went up to about 9 Mohm. Am I right in thinking that this would be leaking current away, stopping C8 from charging, and thus depriving Q10 of base current?
Every switch on since, the voltage across R30 has popped straight up to 4.4 mV.
Thanks everyone for the suggestions.
I can now go and work that flat spot out of my forehead
Graeme
An externally hosted image should be here but it was not working when we last tested it.
Anyway, back to the point. It seems I may have found the problem. I've been all over the board checking in-track continuity and track to track insulation. There was a tiny little nib of solder between two tracks on the board, essentially between the collector of Q5 and the output. It measured about 60 kohm between tracks with the meter. After a quick touch with a soldering iron, it went up to about 9 Mohm. Am I right in thinking that this would be leaking current away, stopping C8 from charging, and thus depriving Q10 of base current?
Every switch on since, the voltage across R30 has popped straight up to 4.4 mV.
Thanks everyone for the suggestions.
I can now go and work that flat spot out of my forehead
Graeme
Oh ! If it's from farnell, sure they are genuine parts...
I saw one time in my hole life a problem like you are describing with genuine parts ! This is why I suspected about a fake one... Are you sure that ALL parts (ie. driver transistors) are genuines?
Once, I had problems with an RF transmitter, there was no fake parts, but the transistor refused to oscilate!
Then I replaced with a new one (from the same seller) and it worked perfect. Genuine parts can have problems too !
I saw one time in my hole life a problem like you are describing with genuine parts ! This is why I suspected about a fake one... Are you sure that ALL parts (ie. driver transistors) are genuines?
Once, I had problems with an RF transmitter, there was no fake parts, but the transistor refused to oscilate!
Then I replaced with a new one (from the same seller) and it worked perfect. Genuine parts can have problems too !- Status
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