ADCOM GFA 535 - Bias Adjustments - diyAudio
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Old 25th May 2017, 01:36 PM   #1
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Join Date: May 2017
Default ADCOM GFA 535 - Bias Adjustments

Dear All,

I currently sourced an ADCOM GFA 535 MK II and am looking at adjusting and checking the Bias to see if it is perfect.
I also read that the input jacks should be shorted when adjusting the BIAS,as per the service manual.

I am quite a noob in this topic and am quite confused with the terminology used to short the input jacks.

By shorting does it mean that the RCA plug of each channel has to be shorted, the inner ring and the outer jack of each channel the left and right. Or is it just a cable that needs to be run from Left RCA inputs to the Right RCA inputs.

I am quite a noob , in this topic and am looking forward to get inputs before i even start with it.
I will be really grateful for inputs provided.

Thanks in advance.
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Old 25th May 2017, 02:51 PM   #2
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Welcome, if you post the bias procedure we can help you out more with the finer details. Shorting input means the outer ring and pin are connected. What I did was take an old RCA plug, cut it and twist the outer ground shielding an inner wire together.

Everyone has a starting point, but you need to appreciate and respect what you are dealing with. Amps can have a deadly potential when you go poking around. Plus it is very easy to screw things up when muckin around inside (ask me how I know).

Also you will need a multimeter if you didn't know that already.
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Old 25th May 2017, 05:57 PM   #3
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Quote:
Originally Posted by mjstriker5 View Post
Welcome, if you post the bias procedure we can help you out more with the finer details. Shorting input means the outer ring and pin are connected. What I did was take an old RCA plug, cut it and twist the outer ground shielding an inner wire together.

Everyone has a starting point, but you need to appreciate and respect what you are dealing with. Amps can have a deadly potential when you go poking around. Plus it is very easy to screw things up when muckin around inside (ask me how I know).

Also you will need a multimeter if you didn't know that already.
I do have a multimeter with me, i have managed to find the pin out digrams on the PCB, and have started to measure the DC Offset Voltage.
Please find the instructions for bias adjustment in the attached image.

I have have also attached my multi meter reading for you to verify.
Thanks again for helping me out !
Have a good day!
Attached Images
File Type: jpg Bias Adjustment.jpg (121.4 KB, 27 views)
File Type: jpg Multimeter Reading.jpg (338.1 KB, 27 views)
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Old 25th May 2017, 07:31 PM   #4
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Looks like you have it all squared away.

DC offset should technically be zero but it might be tough to get exactly that. Anything less than 25mV is more than fine but you probably won't be anywhere near that.
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Old Yesterday, 06:35 AM   #5
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Quote:
Originally Posted by mjstriker5 View Post
Looks like you have it all squared away.

DC offset should technically be zero but it might be tough to get exactly that. Anything less than 25mV is more than fine but you probably won't be anywhere near that.
I have read that the DC Offset should be measure in the speaker out terminals, please correct me if am wrong.
Another strange thing which i noticed when adjusting the BIAS TP1,TP2, TP3 and TP4 is that the max output of TP1 and TP2 is higer than the max output of TP3 and TP4 pins.

Lets say when we turn the potentiometers fully to send the full signal,then the max output measured across TP1 and TP2 is 30mv and the max output across TP3 and TP4 is 15mv, is there something which i should check in the circuit.? Or is this fine .

I have adjusted them as per the instructions in the service manual to 7mv.
I doubt if this could be attributed to dirty potentiometers as they are open and prone to accumulate dust, i am going to clean them and check again when i get home.

Just wondering if there are other components in the circuit that would need checking too.!
Thanks for your expertise in helping me out.
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