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Old 5th September 2005, 10:37 AM   #11
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Photos of shcem for a start
first half (paint is probably best)
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Old 5th September 2005, 10:39 AM   #12
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second half
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Old 5th September 2005, 01:53 PM   #13
anatech is offline anatech  Canada
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Hi andjur78,
Their quality has gone up. That is good. The same factory can make several brands, but only the build quality will be similar. The schematics and layout are done elsewhere.

Thanks for the schematic. Now to look at it.

-Chris
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Old 5th September 2005, 11:19 PM   #14
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sorry about the low res schems.
I'm Trying to find a scanner to get a clearer loaded pic online...

...unless anyone out there has a clear pic ready to upload?
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Old 6th September 2005, 08:45 PM   #15
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I am far from an expert but I'd personally expect something temperature/connection related. Perhaps a bad connection breaks after some warmup time. If the breaks happen more often with output loading look towards ouput stage, if not, focus on driver stages.

The positive pulse might just be the feedback system reacting to the initial negative pulse, so I'd focus on things that might cause a swing to the negative rail.

I know it sounds odd, but I'd wiggle and bend (just a bit) the transistor leads to change the pressure that they put on the connections. You could also power up the amp and tap transistors/resistors/caps with a plastic pen to see if you can evoke the response. If you can then you know which connections to reheat.

If you make your living fixing amps then you certainly know more about amp reapir that I do. So these suggestions might be a bit too obvious....

Is the pulse really loud and sudden? I am sure someone thought the amp was possesed!

Keep us updated, I'd like to know what the problem turns out to be.

Hope this helps,

Doug
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Old 11th September 2005, 12:06 AM   #16
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Hi Doug and thanks for your reply.
I have done pro audio amp repairs for the past 10 years.
I'll never say "I know it all" as this one has me stumped!
Still persistance...

Update so far- I have removed every electrolytic from the pcb and are awaiting on a new 100R bias pot.(This pot had had a small crack on it's plastic slotted part)

One stand out thing is that the brown glue they use to mount the electrolytics seems to "eat away" other components around where it has been placed. One small diode had a very corroded lead.
I will re assemble the unit with new electro caps this week and soak test again.

Will keep you all posted. Cheers Andjur78.
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Old 11th September 2005, 01:45 AM   #17
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Have you tried to use circuit coolent spray to locate the faulty component (if this is the problem)? I found it quite effective to locate weaken components.
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Old 11th September 2005, 02:36 AM   #18
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I've seen the brown adhesive become conductive. Touching the scope lead to it while the amplifier is in operation will tell you if it's happening in your case.

The corrosion sounds more like leaking electrolyte. If that's the case, you need to thoroughly clean the board with a solvent (acetone) before replacing the caps. Since it's a double-sided board, you need to make sure that any feed-throughs under the cap are still intact.

Many defective semiconductors are sensitive to temperature change. The cooling spray is a good idea. Placing your soldering iron very close to (or touching) small driver transistors will heat them up. Most of the semiconductor manufacturers use a plastic with a melting point above what your soldering iron can produce but a few (philips) sometimes use a plastic with a low melting point so be careful if you actually touch the plastic case of the drivers with your iron.
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Old 11th September 2005, 09:54 AM   #19
mzzj is offline mzzj  Finland
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Quote:
Originally posted by Perry Babin
I've seen the brown adhesive become conductive. Touching the scope lead to it while the amplifier is in operation will tell you if it's happening in your case.

The corrosion sounds more like leaking electrolyte. If that's the case, you need to thoroughly clean the board with a solvent (acetone) before replacing the caps. Since it's a double-sided board, you need to make sure that any feed-throughs under the cap are still intact.

Many defective semiconductors are sensitive to temperature change. The cooling spray is a good idea. Placing your soldering iron very close to (or touching) small driver transistors will heat them up. Most of the semiconductor manufacturers use a plastic with a melting point above what your soldering iron can produce but a few (philips) sometimes use a plastic with a low melting point so be careful if you actually touch the plastic case of the drivers with your iron.
I have seen that brown stuff also, can get down to k-ohm range when it gets old enough, usually its in megaohms.
(just stuck your multimeter leads to it and see)
I remember at least one piece of VCR where corrosion was for sure because of that glue, not electrolytics.(none of them anywhere near corrosion spots)

For leaking caps boards I use hot tap water+dishwashing soap+toothbrush. followed with compressed air, flushing with IPA and compressed air again. Works like charm, just dont *****ing smoke at the same time when blowing IPA off with air Acetone is not so good as most of acids and salts have good much better solubility on water and in tricky case IPA or Acetone wash alone can leave some salty residues on board that are still causing leaking. (alltough some 100meg leakage resistance is not so big problem in audio circuits as some others)

Corrosion caused by cap electrolyte or that glue can be a real pain in a *** to repair, as it eats thin traces intermited without visual change. Only way is to measure them to be sure. (having repaired more than 100 piece particular model of smps power supplies) (and dont ask me what I think about Panashitronic el-caps after replacing more than 900 pieces of p*ssing HFZ series)
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Old 11th September 2005, 10:32 AM   #20
dnsey is offline dnsey  United Kingdom
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How about disconnecting the PSU, and monitoring it for glitches.
At the same time you could power-up the amp via an external PSU and monitor that too. Next stage, I suppose, would be to disconnect each stage in turn (I'd start with the driver stage), and see where the pulses are introduced.
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