Summer / Consolation Amplifier: 1995 Adcom GFA-5200 - Page 4 - diyAudio
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Old 15th June 2015, 06:30 PM   #31
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It's got these handy yellow flashing lights to try to protect my hearing, but they're starting to get annoying...
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Old 16th July 2015, 09:15 PM   #32
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Here's one in downtown Oakland for only $85!
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Old 15th March 2017, 06:28 PM   #33
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Default Fresh donor GFA-5200

In my attempt to corner the market on used 5200's, I now have a newer updated GFA-5200 to play with (date sticker says 3-97). It has separate windings for the input stage, so input rails are +/-54V while output stage rails are ~42V. (Maybe it can swing the full output rails?) Then it also has separate PS (bridge rectifiers and caps) for each channel, so a useful advantage over the original.
It appears to have been assembled very quickly, and the input signal cable is actually RESTING on one of the rectifier bridges, so clearly cable routing is not one bit important to these hacks. Nor is capacitance: It has only 4 puny 6800uF caps, so it needs some help getting the PS ripple down from an unacceptable ~50mV.
Now, "protection" is something they obsessed over: It has more transistors watching for faults (10) than for making sound (9). I will be referring to the schematic for the 5300 (attached) which is very close to the 5200.
It is my intention to replace the cover with a perforated aluminum cover which will be attached to the existing heatsink for maximum passive heat rejection without the use of a fan.
Stay tuned!
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File Type: jpg Adcom GFA-5300 schema.jpg (257.3 KB, 111 views)
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Old 23rd March 2017, 02:45 AM   #34
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I was inspired by your posts to get one for myself. Bit of a novice at all this, so I very much appreciate you sharing. Havent cracked mine open yet, as I wanted to get a good feel for it before changing anything.
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Old 23rd March 2017, 04:47 AM   #35
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I'm so proud to hear that, fraggler!
But, by listening to me you've taken a big step toward blowing it up, so I agree it's a good idea for you to hear it for a while first.
What are you driving with it? Have you checked dc offset and bias yet? What is your ps ripple voltage? If it's much above 50mVAC then it may need recapping.
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Old 18th June 2017, 09:45 PM   #36
srinath is offline srinath  United States
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This thread was in the back of my mind when I got a 5200 free with another purchase. This one is blowing those little fuses in the middle. In fact it had been replaced with 5 amp and I put 5's in and it also blew.

I am thinking its something in the PS side cos both fuses are going instantaneously when powered up. I think its the power supply caps.

could someone show me a circuit diagram. Mine has the 2 fuses in the middle of the board not 4. Is it same as an adcom 5002 ?

Thanks.
Srinath.
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Old 18th June 2017, 10:20 PM   #37
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Quote:
Originally Posted by srinath View Post
This thread was in the back of my mind when I got a 5200 free with another purchase. This one is blowing those little fuses in the middle. In fact it had been replaced with 5 amp and I put 5's in and it also blew.

I am thinking its something in the PS side cos both fuses are going instantaneously when powered up. I think its the power supply caps.

could someone show me a circuit diagram. Mine has the 2 fuses in the middle of the board not 4. Is it same as an adcom 5002 ?

Thanks.
Srinath.
The fuses that blow inside of the amp are the rail fuses, they are after the main filtering caps and will not blow when one of the main filtering caps fails. The fuse on the rear panel will blow when a main filtering cap goes bad.

The fact that oversized 5A rail fuses were installed means that whatever was failing is almost certainly dead now. I would expect to have to replace the output mosfets in the affected channel.

The adcom 5300 should be very similar only it has 4 outputs per channel versus the two in your 5200. The 5300 schematic can be found online.
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Old 18th June 2017, 10:31 PM   #38
srinath is offline srinath  United States
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Awesome, I will get my transistor tester and get on it sometime after some of these already cracked open and awaiting my time/space/parts projects are cleared out.
It is in mint condition. That's always a good point.

Thanks.
Srinath.
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Old 19th June 2017, 01:00 PM   #39
srinath is offline srinath  United States
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Default Both channels

Quote:
Originally Posted by Chamberman View Post
The fuses that blow inside of the amp are the rail fuses, they are after the main filtering caps and will not blow when one of the main filtering caps fails. The fuse on the rear panel will blow when a main filtering cap goes bad.

The fact that oversized 5A rail fuses were installed means that whatever was failing is almost certainly dead now. I would expect to have to replace the output mosfets in the affected channel.

The adcom 5300 should be very similar only it has 4 outputs per channel versus the two in your 5200. The 5300 schematic can be found online.
That's what got me thinking that its power supply - its both channels, and at the same time.
I have a few that are blowing 1 channel. That I know is transistors in that channel.
I guess the answer is still to test them and replace as needed.

Thanks.
Srinath.
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Old 19th June 2017, 05:16 PM   #40
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Srinath, on yours the ps is common to both channels. One fuse is for the positive rail, and the other is the negative.
Are you saying yours has visibly blown output transistors on both L and R channels?
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