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re-Freshing an Adcom GFA-545ii
re-Freshing an Adcom GFA-545ii
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Old 4th December 2007, 03:34 AM   #1
WithTarragon is offline WithTarragon  United States
diyAudio Member
Join Date: Jun 2007
Default re-Freshing an Adcom GFA-545ii

I am refreshing an Adcom GFA 545ii amp. This will be used to drive the mid-range horn of a system I am building & bi-amping.

I had a look at the amp & got a copy of the service manual (Adcom will e-mail these for free). The schematic looks like it is correct for my model. This unit is from 1990, and appears to be in decent shape. It is currently in good working order, but I would like to do some slight upgrading.

1. This one does not appear to be the version that had bypassed the electrolytic power supply caps with caps in the 22uF to 47uF range (however, it does have some 0.22uF caps - C905 through C908). So I will add four of these - one in parallel with each 10K uF cap (C901 through C904). I will use a 22uF per DJK's previous advice

2. Are there any obvious things I should re-fresh while I have the amp opened up? Since this will be driving some fairly efficient horns that do not present a low impedance to the amp, I see no strong reason to beef up the power supply. However, I am worried about 17 year old parts that are nearing the end of their expected life. My DIY skills are okay but not strong.

Attached Files
File Type: pdf schematic gfa-545 ii.pdf (93.5 KB, 220 views)
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Old 5th December 2007, 05:34 AM   #2
caesar148 is offline caesar148  United States
diyAudio Member
Join Date: May 2004
Location: S. California
Here are a list of things you can do:

1) Replace all output emitter resistors with Mills wire-wound resistors if the existing resistors are ceramic or carbon resistors.
2) Replace bulk capacitor at power retifier with Nichicon Gold Tune capacitors.
3) Replace all resistors in the signal path with Holco or PRP resistors. If budget and space allows, use Caddock TF020 ultra-precision film resistors.
4) Replace all DC blocking capacitors in the signal path with metalized polypropylene capacitors like TRT Dynamicap, Auricap or Solen capacitor. These are bulky so need a lot of space. Use Black Gate polarized or bi-polar capacitors if space is tight.
5) For signal filter capacitors replace with polypropylene, polyester or thin film capacitor if existing capacitors are ceramic capacitors.
6) Rebuild the power retifier bridge using International Retifier or IXYS soft recovery diodes.

IMO the first 5 task should give you 90% of the improvements.
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