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-   -   Adcom GFA4402 (http://www.diyaudio.com/forums/car-audio/161350-adcom-gfa4402.html)

djQUAN 15th February 2010 11:48 AM

Adcom GFA4402
 
ok, I have this amp for repair and been in my shop for quite a while now. couldn't figure out the overvoltage protection as the pcb traces snake under the middle heatsink. lack of time could also have contributed to that. ;)

the amp runs fine, everything is in good order except that when supply voltage goes to about 14.0V it goes into protect and stays there. it runs again once supply voltage drops to about 13.5V and the remote line is cycled off-on.

anyone got a schematic? that would help me a lot figuring this out. thanks in advance.:)

1moreamp 15th February 2010 05:41 PM

Ill look for you. I have some documents for my Adcom's, but the amps and the documents are stored somewhere here at my home......C

Ill need a few days to find these doc's, so please bear with me while I look for them...c

djQUAN 16th February 2010 01:20 PM

Quote:

Originally Posted by 1moreamp (Post 2086431)
Ill look for you. I have some documents for my Adcom's, but the amps and the documents are stored somewhere here at my home......C

Ill need a few days to find these doc's, so please bear with me while I look for them...c

the owner is in no hurry so take your time. hope you find it though. :) thanks

djQUAN 22nd February 2010 09:24 AM

bumping incase someone else also has schematics. :)

damage 22nd February 2010 08:23 PM

Just curious, but what does the current do right before this happens?

1moreamp 23rd February 2010 02:16 AM

Quote:

Originally Posted by damage (Post 2094939)
Just curious, but what does the current do right before this happens?


A very good question damage ????

I am still looking for the documents I have. I looked today some while digging through two storage areas. And I will look again tomorrow after I get some help to move 15 Audison LRx 5.1k's, and a box full of other amps I have stacked in the way. I am almost positive the docs are in the box under this stack of waiting repairs. < smaller shop space then before :rolleyes: >

djQUAN 23rd February 2010 09:18 AM

Quote:

Originally Posted by damage (Post 2094939)
Just curious, but what does the current do right before this happens?

did you mean the idle current draw of the amp?

nothing. it just stays constant and when supply voltage reaches a certain point, the amp just shuts down and some red led lights up inside the amp. :confused:

if I remember correctly, idle current draw is a little over 2amps. I had to build a new CC/CV powersupply since I don't have an adjustable bench supply that can power this thing. :D

Quote:

Originally Posted by 1moreamp (Post 2095316)
A very good question damage ????

I am still looking for the documents I have. I looked today some while digging through two storage areas. And I will look again tomorrow after I get some help to move 15 Audison LRx 5.1k's, and a box full of other amps I have stacked in the way. I am almost positive the docs are in the box under this stack of waiting repairs. < smaller shop space then before :rolleyes: >

ok, I'll wait for you. :) that's a lot of amps! I had a weird problem with an LRx before. it goes into protect with unequal rail voltages and was fixed by cleaning the PCB. it had some aluminum dust from heatsink polishing. (transistor clamping area, came from the factory that way) :dodgy: but after that, it still goes into protect in random times but when it doesn't it works just fine. just returned it instead to the owner as it seems to only go into protect when the remote line is cycled on/off quickly.

damage 23rd February 2010 02:40 PM

Quote:

Originally Posted by djQUAN (Post 2095674)
did you mean the idle current draw of the amp?

nothing. it just stays constant and when supply voltage reaches a certain point, the amp just shuts down and some red led lights up inside the amp. :confused:

if I remember correctly, idle current draw is a little over 2amps. I had to build a new CC/CV powersupply since I don't have an adjustable bench supply that can power this thing. :D

Now I'm not perry, so consider this more a mind exercise.

2amps seems like too much for idle. I have only two amps that draw that much at idle, and they make way more power.

Where the power supply fets replaced? If so, I'd first recommend looking at the gate resistors. They are probably incorrect for the replacements.

Otherwise, I might suspect a leaky output.

1moreamp 23rd February 2010 05:56 PM

The Adcom"s draw a larger then normal idle current, just like some class A designs do. They tend to high bias the outputs for SQ reasons I assume. This thing looks Like Nelson Pass himself designed it inside, except for the AB output stage its just like his class A designs over in the other forums here. And it does have a class A drive stage to the output right out of the Pass handbook IMO...

I know of one adjustment on the lower rail supply control side. The other adjustments are bias idle current per channel and a DC offset fine adjustment per channel.
They modded some of these with a small board mounted on the turn on lead connection inside the amp. This was a two transistor turn on switch cause the early Adcoms had a very high current draw on the turn on lead.

By chance what are the lower rails doing when this apex point is reached ?

The usual problems I have seen on these is failure of the input mosfets on the channels. I have seen these burn the boards up in some cases, other then that flashed power traces on the supply side from excessive current draw through the power supply. That's why I am puzzled by djQUAN problem.... more later today....:soapbox:

as for the LRx issues we can talk more later or in anew thread about that. Some have Atmels inside monitoring and sometimes causing protection issues. just depends on which model. Audison can sell you a new protection board on their later models.

djQUAN 24th February 2010 09:41 AM

Quote:

Originally Posted by 1moreamp (Post 2096195)
The Adcom"s draw a larger then normal idle current, just like some class A designs do. They tend to high bias the outputs for SQ reasons I assume. This thing looks Like Nelson Pass himself designed it inside, except for the AB output stage its just like his class A designs over in the other forums here. And it does have a class A drive stage to the output right out of the Pass handbook IMO...

I know of one adjustment on the lower rail supply control side. The other adjustments are bias idle current per channel and a DC offset fine adjustment per channel.
They modded some of these with a small board mounted on the turn on lead connection inside the amp. This was a two transistor turn on switch cause the early Adcoms had a very high current draw on the turn on lead.

By chance what are the lower rails doing when this apex point is reached ?

The usual problems I have seen on these is failure of the input mosfets on the channels. I have seen these burn the boards up in some cases, other then that flashed power traces on the supply side from excessive current draw through the power supply. That's why I am puzzled by djQUAN problem.... more later today....:soapbox:

as for the LRx issues we can talk more later or in anew thread about that. Some have Atmels inside monitoring and sometimes causing protection issues. just depends on which model. Audison can sell you a new protection board on their later models.

I'm as puzzled as you are. :confused: I never thought about measuring the rails since it was running fine at moderate supply voltages. I can take a peek with the rail voltages this weekend if you like. there is actually no turn off pop sound coming from the speakers when it goes into protection. it just goes silent with a red LED lit inside the amp.

the Lrx I had didn't have a microcontroller, it's all analog. or am I remembering the wrong model number. :D either way, don't mind it, it works anyway, the last time I left it. ;)


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