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Old 17th January 2014, 02:49 PM   #11
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Join Date: Jul 2013
Location: Denver, CO
Thumbs up Fuses were the issue but bias and DC are still a mystery.

Thanks for comments.

I checked all the fuses' values and it was a mix.
One was 5A which was blown, two were correct 7A (AGC 7A-250V/312) and one was 6A.
I replaced the two 5A and 6A with 7A and now it is working fine.

BTW, I found the test points. They are small metal bars on the vertical boards.
However, setting bias is so weird.
When I adjust the resistor, it changes value but automatically comes back to around zero soon after when no input is connected.
On right channel, it doesn't even do anything. Left channel changes value but resets to zero.
Then I connected input signal and played a track and bias went up and down depending upon the music, around 14 to 18mV.
So basically, can't adjust bias.
Also checked DC voltage and it was close to 0 fluctuating sometimes to .001 when speakers were hooked up so I guess it's good.
I read this amp doesn't have speaker protection so that's something to read next. May be external DC protection is the way to go.

Here are the test points for reference highlighted in red circle.
However, they are not numbered TP201, TP301, TP251 and TP351 as mentioned in the service manual.
The numbers were J### something.

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File Type: jpg Adcom-GFA-555-ii-test-point-P1050424.jpg (288.3 KB, 98 views)
File Type: jpg Adcom-GFA-555-ii-test-points-P1050425.jpg (300.4 KB, 98 views)
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Old 17th January 2014, 07:04 PM   #12
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Location: Denver, CO
It still bothers me that I can't adjust bias.
Turning R119 and R169 doesn't make any difference on mV readings.
I'll give it another try and see if I get somewhere.
Hmmm...
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Old 17th January 2014, 09:32 PM   #13
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They want a sizable input signal (66 watts). Use a tone generator (or recorded tone on a CD). Dial it up using the relation: Power = Volts^2/R. "R" is the power resistor (and it will get hot). The amp must be warmed up before testing the bias. Follow those instructions carefully. Adjusting those pots will alter the bias and you should be able to read this at the test points.

Please note that the traces on the Left and right circuit boards look different. The ones on the left look like thay have gotten very hot in their previous life (compare to the right side). That hodge podge of different fuses, suggest this amp has a bad history.
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Old 17th January 2014, 09:59 PM   #14
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Join Date: Jul 2013
Location: Denver, CO
Quote:
Originally Posted by WithTarragon View Post
They want a sizable input signal (66 watts). Use a tone generator (or recorded tone on a CD). Dial it up using the relation: Power = Volts^2/R. "R" is the power resistor (and it will get hot). The amp must be warmed up before testing the bias. Follow those instructions carefully. Adjusting those pots will alter the bias and you should be able to read this at the test points.

Please note that the traces on the Left and right circuit boards look different. The ones on the left look like thay have gotten very hot in their previous life (compare to the right side). That hodge podge of different fuses, suggest this amp has a bad history.
The traces are different as their arrangement is different in each channel.
The outer right board is same as the inner left board and vice versa.
Same board uses same colored wire.
This picture shows same trace on the boards.

Click the image to open in full size.

Yes, it does look little brown on that right channel board.

I'll check what to do with bias.
I couldn't find 10w 10ohm resistor at the Radioshack but I have bunch of 25w 2.7 ohm resistors that I can use three of them in series to get around 8 ohms for testing.
Last night I used two for a total of 5.4 ohms for DC test.

It's working fine so I may leave it for now.

BTW, if I want to use external inline fuse on the + speaker terminals as this amp doesn't have DC protection, what fuse size is good?
The speakers are Boston A400's, 4 ohms 150 watts.
So 150/4 = 37.5
Square root of 37.5 = 6.12
6A fuse??? That sounds very high. Click the image to open in full size.
The amp has two 7A fuses on each channel.
Thanks
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Old 17th January 2014, 10:04 PM   #15
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CAREFUL: A fuse will NOT provide DC protection
As a very, very rough check: are the heat sinks warm, and are they equally warm on each side?
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Old 17th January 2014, 10:07 PM   #16
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Hmm... so in-line fuses don't provide DC protection?
I don't want to fry my speakers as this amp is new to me and haven't build a trust yet.

I'll check tonight after I get off if it gets equally warm or not.
Thanks
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Old 19th January 2014, 02:47 PM   #17
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Thumbs up So far it's running good...

I tested the amp with some test speakers for around 4 hours continuous at moderately high levels and it felt very slightly warm on both sides.
It's not even hot at all.
So I plugged in the Boston A400's and listened all day yesterday.
I have not tested the Bias but it's more or less exactly in the middle as it was before I adjusted it.
I'm guessing it may be off by few mV's in the two channels but that shouldn't hurt right?
Thanks

Here is the setup in my living room:
Philips 312 Turntable.
Yamaha CD Player (I have a Nakamichi CD player somewhere in the basement that'll replace it soon).
Harman Kardon Signature Series AP2500 Pre-amplifier.
Adcom GFA-555 II Amplifier.
Marantz 3000 Cassette Player.
Boston Acoustics A400 Speakers.
Definitive Technology PF15TL+ subwoofer.

Click the image to open in full size.
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Last edited by Mystery123; 19th January 2014 at 02:55 PM.
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Old 19th January 2014, 04:46 PM   #18
tvrgeek is offline tvrgeek  United States
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Location: Md
Fuses are there to prevent fires, but they don't always even do that. The silicon fuses are usually quicker than the wire fuse. Speaker cones flying across the room can happen long before the fuse blows. Surprise! That's a big honking amp and it can do some damage.

Bias being low will cause lots of distortion. Being a bit high not so much. Too high can lead to stability issues. You find them easily when it sounds funny and by the time you walk over to it, the heat sink is too hot to touch and the outputs will fry when your hand is about a foot away from the power switch. This I know first hand, thank you CM Labs.

DC offset on the speaker terminals should by under 50mV or so. Things do drift with years. You may want to add some way to adjust the offset, by pot or add DC servo.

I can't remember, did the series II have one of Nelson's automatic opto-coupled bias adjustment setups? Series one used his driver current sense, but I don't know what changed.
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Old 20th January 2014, 05:07 PM   #19
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Location: Denver, CO
Quote:
Originally Posted by tvrgeek View Post
Fuses are there to prevent fires, but they don't always even do that. The silicon fuses are usually quicker than the wire fuse. Speaker cones flying across the room can happen long before the fuse blows. Surprise! That's a big honking amp and it can do some damage.

Bias being low will cause lots of distortion. Being a bit high not so much. Too high can lead to stability issues. You find them easily when it sounds funny and by the time you walk over to it, the heat sink is too hot to touch and the outputs will fry when your hand is about a foot away from the power switch. This I know first hand, thank you CM Labs.

DC offset on the speaker terminals should by under 50mV or so. Things do drift with years. You may want to add some way to adjust the offset, by pot or add DC servo.

I can't remember, did the series II have one of Nelson's automatic opto-coupled bias adjustment setups? Series one used his driver current sense, but I don't know what changed.
I have no idea if they have automatic bias adjustment.
May be someone here can answer.

I don't have expensive speakers so main issue is fire hazard so I'll make sure to turn everything off when I'm not around them.
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