Need Adcom GFA-555 II Test Point Help... - diyAudio
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Old 11th October 2012, 07:16 PM   #1
kico is offline kico  United States
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Default Need Adcom GFA-555 II Test Point Help...

Hi Guys,

Picked up this Adcom at a local yard sale last weekend - lady said it wasn't working, she was half right; left channel is out which I will address later. No, its not the fuses.

Right now though, just trying to locate the test points. I have the service manual but can not locate the bias Test Points (TP) they show in the schematic or refer to in the bias alignment section. There are some pins sticking out from the output boards, but they are not labeled as TP's.

Again this is the GFA-555II, not the older GFA-555. The manual refers to TP201,TP301,TP251 and TP351...what am I missing?

Thanks all.
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Old 25th January 2013, 04:07 AM   #2
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Default GFA-555ii

Having the same problem down to the same channel.
Schematic does not provide test points and expected voltages i am fairly certain a test center would utilize when troubleshooting this unit.
Ordered an oscilloscope and wave generator but they haven't arrived yet.
First place to check is the output from the transformer which is feeding the channels independently. I am reading the same voltages on both so it is deeper in for me thankfully. The transformer is quite pricey for the 555ii.
Otherwise you really need a scope to see much.
With a scope you can look at the connection from the pre-amp board to the left channel board with a square wave generator on the left channel input. The output should be clean. If not you have at least traced it back that far.

You can test voltages across the transformers but again without a scope it will only clue you in if one has failed completely. Be very careful prodding around with a meter. You can easily cause more even damage with the slip of a lead.

Will post my findings once my oscilloscope arrives. But if anyone has any pointers troubleshooting the 555ii it would definitely be appreciated here as well.
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Old 10th June 2013, 06:08 PM   #3
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Last edited by Soundimpressions; 10th June 2013 at 06:21 PM.
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Old 12th June 2013, 01:39 PM   #4
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Not sure why the last upload didn't work. I guess Dropbox doesn't like to share..
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Old 11th December 2013, 03:08 PM   #5
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Just did mine. The test points are not labeled on the boards. The points are adjacent to the resistors as indicated in the schematic (ie TP251 is near R262, etc) and consist of a metal rod about 3/4" long. Just clip your minigrabber onto them.
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