Adcom GFA-545 II Schematic Needed

1moreamp · 27th October 2006, 03:48 AM

Hello, I came across a sweet looking Adcom GFA-545 II with a few minor issues.
I was hoping someone out there might e-mail a Schematic and a bias adjust spec for this little jewel.
Many Thanks in advance

email me at: ebayer0u812@sbcglobal.net

anatech · 29th October 2006, 05:01 PM

Hi justonemoreamp,
What problems are you having?

-Chris

1moreamp · 30th October 2006, 12:19 AM

Hello, I have a burnt resistor tied to the emitter of Q605 and no burnt outputs, but blown fuses so I want to completely go thru the thing before restoring power to it.
It appears that it might have frontend damage as I have tested all the outputs and driver and they all appear to just fine.

Hence my request for a schematic. Overall it appears to be a very straight forward design, but I have read it has a offset servo up front and I would rather not second guess that section of the amp.

It also has Adcom marked Ic's up front and if they have been damaged I would like to see the circuitry to allow replacements to be picked tested and installed from my selection of AD and BB chips.

I already have a partial schematic of the output s and drivers section only ( i found that here on the forum) but no front end section is to be found ( I have scotch tape if and when i find it

)

Plus I dont like to second guess and invent bias adjustments so hence my request for a bias spec. for the output stage. I normally repair amps all day long. So I can follow any instructions you might have to offer.

I just want to do this old girl justice and see the job completed properly and to spec. It will go nicely on my tech bench with my Adcom tuner preamp I use for a source everyday

Many thanks in advance for any support you might be able to render.

anatech · 30th October 2006, 12:27 AM

Hi justonemoreamp,
The 100 ohm resistor for the current source is burned???

If that's true, the diff pair is toast, and probably Q605,Q615, D605, D607 and more.

Choose a low DC offset op amp for the Adcom 3A if you need to. It's filtered so there is no "sound" from it per se. Pin 7 should be running around +12 VDC supply. Check for capacitor electrolyte on the PC board just in case.

-Chris

27th October 2006, 03:48 AM	#1
1moreamp diyAudio Member Join Date: Dec 2005 Location: Northern California	Adcom GFA-545 II Schematic Needed Hello, I came across a sweet looking Adcom GFA-545 II with a few minor issues. I was hoping someone out there might e-mail a Schematic and a bias adjust spec for this little jewel. Many Thanks in advance email me at: ebayer0u812@sbcglobal.net

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29th October 2006, 05:01 PM	#2
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi justonemoreamp, What problems are you having? -Chris

30th October 2006, 12:19 AM	#3
1moreamp diyAudio Member Join Date: Dec 2005 Location: Northern California	Hello, I have a burnt resistor tied to the emitter of Q605 and no burnt outputs, but blown fuses so I want to completely go thru the thing before restoring power to it. It appears that it might have frontend damage as I have tested all the outputs and driver and they all appear to just fine. Hence my request for a schematic. Overall it appears to be a very straight forward design, but I have read it has a offset servo up front and I would rather not second guess that section of the amp. It also has Adcom marked Ic's up front and if they have been damaged I would like to see the circuitry to allow replacements to be picked tested and installed from my selection of AD and BB chips. I already have a partial schematic of the output s and drivers section only ( i found that here on the forum) but no front end section is to be found ( I have scotch tape if and when i find it ) Plus I dont like to second guess and invent bias adjustments so hence my request for a bias spec. for the output stage. I normally repair amps all day long. So I can follow any instructions you might have to offer. I just want to do this old girl justice and see the job completed properly and to spec. It will go nicely on my tech bench with my Adcom tuner preamp I use for a source everyday Many thanks in advance for any support you might be able to render.

30th October 2006, 12:27 AM	#4
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi justonemoreamp, The 100 ohm resistor for the current source is burned??? If that's true, the diff pair is toast, and probably Q605,Q615, D605, D607 and more. Choose a low DC offset op amp for the Adcom 3A if you need to. It's filtered so there is no "sound" from it per se. Pin 7 should be running around +12 VDC supply. Check for capacitor electrolyte on the PC board just in case. -Chris

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