Adcom GFA-585 went bad - calling Adcom experts - Page 2

hangguy · 23rd December 2004, 04:39 AM

Okay, this is weird. I ohmed out the diode bridges and all eight diodes measured fine. Using a DMM, in one direction it's open, one direction it's measuring ~40ohm.

Then, measured all 4 main PS caps, there is no problem as well. They are charging up nicely.

So what else could be wrong? What can give me a 160V AC on my B+ rail for both channels? This is with no load on the B+ and B- rails at all.

Mike

anatech · 23rd December 2004, 05:53 PM

Hi Mike,
Bad connection with your meter, or there is something wrong with your meter. A high level AC across a cap would draw huge current, heat the cap and pop the seal.
Take your time and make sure you have a good ground to the meter, then retake your measurements. While you are at it, measure the RCA jack gnd potential and spkr gnd potential. If you are careful then measure the emitters of the diff pairs. They should run around 0.6V from ground as the bases are ground referenced.
-Chris

hangguy · 23rd December 2004, 07:41 PM

Hi Chris,

Yes, I agree this doesn't make much sense. Too bad I don't have an oscilloscope to look at what is really going on.

I am borrowing a good DMM from work and will verify the readings and take some more tonight. All previous readings have been referencing to Power ground (center tap of the transformer secondary). Will try to measure the secondary output, and poking around more in there.

Mike

anatech · 23rd December 2004, 08:06 PM

Hi Mike,
Take your ground reference from a "quiet" ground point. Somewhere on the chassis is normally okay. Probably easier for you too.
As far as meters go, I find that Fluke models are the best. They also hold their calibration better than most. For bench meters, an HP is hard to beat. (I used to work in a calibration lab). A Fluke 87 series is one of the few handheld meters I will trust much over 60Hz on AC. Whatever the current Fluke 87 is called.
-Chris

hangguy · 24th December 2004, 08:09 PM

Yep, it was a bad meter. Using the borrowed Fluke meter, everything looks normal, no AC at all on the rail buses. Signal ground is sitting at about 0.2V for both channels.

I will go make more in depth measurements tonight.

Thanks to all for all the suggestions. Merry Christmas and Happy New Year.

Mike

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23rd December 2004, 04:39 AM	#11
hangguy diyAudio Member Join Date: Feb 2004 Location: Temple City, CA	Okay, this is weird. I ohmed out the diode bridges and all eight diodes measured fine. Using a DMM, in one direction it's open, one direction it's measuring ~40ohm. Then, measured all 4 main PS caps, there is no problem as well. They are charging up nicely. So what else could be wrong? What can give me a 160V AC on my B+ rail for both channels? This is with no load on the B+ and B- rails at all. Mike

23rd December 2004, 05:53 PM	#12
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi Mike, Bad connection with your meter, or there is something wrong with your meter. A high level AC across a cap would draw huge current, heat the cap and pop the seal. Take your time and make sure you have a good ground to the meter, then retake your measurements. While you are at it, measure the RCA jack gnd potential and spkr gnd potential. If you are careful then measure the emitters of the diff pairs. They should run around 0.6V from ground as the bases are ground referenced. -Chris

23rd December 2004, 07:41 PM	#13
hangguy diyAudio Member Join Date: Feb 2004 Location: Temple City, CA	Hi Chris, Yes, I agree this doesn't make much sense. Too bad I don't have an oscilloscope to look at what is really going on. I am borrowing a good DMM from work and will verify the readings and take some more tonight. All previous readings have been referencing to Power ground (center tap of the transformer secondary). Will try to measure the secondary output, and poking around more in there. Mike

23rd December 2004, 08:06 PM	#14
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi Mike, Take your ground reference from a "quiet" ground point. Somewhere on the chassis is normally okay. Probably easier for you too. As far as meters go, I find that Fluke models are the best. They also hold their calibration better than most. For bench meters, an HP is hard to beat. (I used to work in a calibration lab). A Fluke 87 series is one of the few handheld meters I will trust much over 60Hz on AC. Whatever the current Fluke 87 is called. -Chris

24th December 2004, 08:09 PM	#15
hangguy diyAudio Member Join Date: Feb 2004 Location: Temple City, CA	Yep, it was a bad meter. Using the borrowed Fluke meter, everything looks normal, no AC at all on the rail buses. Signal ground is sitting at about 0.2V for both channels. I will go make more in depth measurements tonight. Thanks to all for all the suggestions. Merry Christmas and Happy New Year. Mike

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