PC2002M taking a while to come out of protection. What to check?

DWJames

DWJames

Active Member
Hi guys,
My Yamaha PC2002M power amp which has been fine for the last 6 months has just started to take a while (like up to a minute) to get both channels out of protection mode. It's just started this week and was previously taking around 2 seconds. After that it seems fine but something's obviously not right.
Can someone help me out with troubleshooting please? What should I check first? I have the service manual and a voltmeter handy.
Thanks.
James
 
DWJames said:
Hi guys,
My Yamaha PC2002M power amp which has been fine for the last 6 months has just started to take a while (like up to a minute) to get both channels out of protection mode. It's just started this week and was previously taking around 2 seconds. After that it seems fine but something's obviously not right.
Can someone help me out with troubleshooting please? What should I check first? I have the service manual and a voltmeter handy.
Thanks.
James
Click to expand...

Check your DC offset per service manual.
 
IIRC, DC offset is controlled by servo. Excessive DC is caused by the driver boards becoming conductive over time, again IIRC. There is a fix that involves lifting a component from the servo circuit but I can't recall which so that's not much help to you. Perhaps the more tech minded like Avionic can look at the schemo and nail this down.
 
Before we go lifting anything .Lets just see what the servo is doing. I suspect its working OK. And theres a problem in the protect circuit...
 
Thanks guys, I got the chance to do some measurements today.

DC Offset - Nevermind mV, measuring from cold the DC offset started around 18V on both channels! This reduced by roughly 1v every 3seconds. When this dropped to around 3v each channel came out of protection.

Once this had happened once, subsequent turning off and on of the amp gave the regular 1-2second time to come out of protection.

The regular +-80v voltages on the DC board were pretty spot on at +80.1v and -80.4v

Idling current adjustment was set approx 6 months ago at 11mv but now sits a little low at approx 7mv after 5 minutes although it's cold here currently and it does seem to be still climbing slowly at that point.

Clearly something's not right, but does that help pinpoint what?

cheers,
James
 
just rechecked after it's been on for 15 mins or so and the DC Offset is now down to 0.5mV DA Board and 1mV DB Board which is within specs and everything seems normal.

It's as though it gets a load of extra voltage leaked in from somewhere whilst turned off and can't resume it's normal operation until it's purged this extra. (do excuse the layman's terms, I enjoy soldering things and poking about but I'm no electronics expert.....)

When the 18v DC Offset was measured, it had been 24hours since last switched on.
 
According to the service manual..DC offset should be 0.00 +/- 10mv after a 10 minute warm up.
 
Do both channels "click" about the same time from a cold start.
 
I'm thinking the regulated 12 vdc power supply may have a old lazy cap or two.
 
avionic said:
I'm thinking the regulated 12 vdc power supply may have a old lazy cap or two.
Click to expand...
Thanks, that's really helpful :)
I'm rubbish with schematics, are you referring to the 3 items circled in red on the DC board here?
pc2002m-12vdc-caps.gif
 
DWJames said:
DC Offset - Nevermind mV, measuring from cold the DC offset started around 18V on both channels! This reduced by roughly 1v every 3seconds. When this dropped to around 3v each channel came out of protection.

Once this had happened once, subsequent turning off and on of the amp gave the regular 1-2second time to come out of protection.
Click to expand...

Avionic, this is exactly what was happening with mine before you fixed it. Wasn't the answer to get new dual-JFETs from Japan?
 
DWJames said:
Thanks, that's really helpful :)
I'm rubbish with schematics, are you referring to the 3 items circled in red on the DC board here?
pc2002m-12vdc-caps.gif
Click to expand...

Those two plus the one marked XC..The film cap by the fuse is not encluded.Just the electrolytics.
 
Avionic looking at the second picture, I think there appears to be brown glue residue, if that is indeed glue DWJames might be staring at a recapping project, and removal of said glue since could be conductive over time.

Also DWJames might want to check for corroded traces.
 
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