I have a pair of GFA-565s that are in fair condition. I want to get them up to date to make sure they don't have the "DC leakage" problem.
They do work, but also make noise when powered on.
I have considered doing the mods myself, and have reviewed several good threads here about doing them. However, I seem to have trouble finding the 100V 39,000 uf caps and really don't have the time anyway. The caps referenced in other threads are either no longer available or I can't seem to find them. Did I also mention that I really don't have time to do this anyway ('
')
There are two companies I find that do mods for this amp.
Musical concepts
http://www.musicalconcepts.com/adcom.htm
and
MJR
http://www.mjraudio.com/services.php
Does anyone have experience with either of these companies doing mods/updates?
Also, the mods/updates are going to run $1000 or so (unless I do it myself) and I wonder if this is worth the money, or should I just sell them and buy a new amp?
They do work, but also make noise when powered on.
I have considered doing the mods myself, and have reviewed several good threads here about doing them. However, I seem to have trouble finding the 100V 39,000 uf caps and really don't have the time anyway. The caps referenced in other threads are either no longer available or I can't seem to find them. Did I also mention that I really don't have time to do this anyway ('
')There are two companies I find that do mods for this amp.
Musical concepts
http://www.musicalconcepts.com/adcom.htm
and
MJR
http://www.mjraudio.com/services.php
Does anyone have experience with either of these companies doing mods/updates?
Also, the mods/updates are going to run $1000 or so (unless I do it myself) and I wonder if this is worth the money, or should I just sell them and buy a new amp?
Put a meter on the outputs and measure the DC offset. Shouldn't be more than 15mV if the servo is working. If it is high, probably has open 10ohm ground resistors or a bad servo IC.
If the amps work, use them and enjoy. They are great amps and dropping $1K is a waste IMO.
If the amps work, use them and enjoy. They are great amps and dropping $1K is a waste IMO.
update
Thanks BTG. Good points. I guess I was having a "lazy" moment during my original post.
It looks like the large capacitors (100v, 35000uF) aren't bad as best I can tell. They don't have any leakage that I can see and they seem to charge up, discharge correctly.
As for the DC Offset, one amp has 0.5 vdc across the outputs, the other has 3 vdc across the outputs.
Obviously the one has a big problem, the other one seems to need a bit of work as well.
I don't have a service manual, where do I find the ground resistor?
The servo IC appears to be on the input board and I've seen reference to replacing it with an OP97.
Any other pointers? There is a long thread on here from MJRaudio with a complete rebuild, but I don't believe mine require that.
They have used in a well ventilated A/C environment for many years. All the boards look fairly clean (except for dust!) and I don't really see many problems. Probably replacing the ground resistor and op amp is a good suggestion and may be all that is required.
Thanks
Thanks BTG. Good points. I guess I was having a "lazy" moment during my original post.
It looks like the large capacitors (100v, 35000uF) aren't bad as best I can tell. They don't have any leakage that I can see and they seem to charge up, discharge correctly.
As for the DC Offset, one amp has 0.5 vdc across the outputs, the other has 3 vdc across the outputs.
Obviously the one has a big problem, the other one seems to need a bit of work as well.
I don't have a service manual, where do I find the ground resistor?
The servo IC appears to be on the input board and I've seen reference to replacing it with an OP97.
Any other pointers? There is a long thread on here from MJRaudio with a complete rebuild, but I don't believe mine require that.
They have used in a well ventilated A/C environment for many years. All the boards look fairly clean (except for dust!) and I don't really see many problems. Probably replacing the ground resistor and op amp is a good suggestion and may be all that is required.
Thanks
I would try to find out the original bias levels and see if the channels can be dialed to this, then do the offset tests again...
Anyway 3V is not a "big" problem (though it may kill speakers), full rails on the output, now that is a big problem...
Anyway 3V is not a "big" problem (though it may kill speakers), full rails on the output, now that is a big problem...
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