Noob alert! Adcom 565 Q's

[gt4rcdude]

Hello.
I've spent hours reading the threads on this forum about the Adcom GFA 565's. I have two of these monsters. One has never given me any trouble. The other I've had repaired twice at over $250 a pop. The first repair was a 10K ohm 1/2 watt resistor, a 47 Ohm 1/2 watt resistor and a 2.7 Ohm 1/2 watt resistor. Parts $23 and $230 labor. The next repair on the same amp was part number 2SC945 transistor and 1 ohm 1/2 watt resistor. $18 in parts and $281 in labor. This done by an Adcom certified repair store.

It started both times with distorted audio. I know it's vague but I'd love to learn how to fix this myself in the future. If this sucker breaks again I'm going to either fix it myself or use it for a boat anchor.

I've noticed some 565's have a cooling fan on the top. Mine do not. They still have adcom filter caps as well. How/where do I get this fan mod? Where does it get power? Any idea what they "fixed"? Is it just because the amp is old?

BTW, serial number is 011214627

Thanks in advance. I'll keep reading and learning in the meantime. I really need to find the repair/service manual for these things.

[EchoWars]

Stock cooling is fine on these amps...the fan was for rack mounted, professional-use units. You'll never run it that hard at home.

Welcome to the site!

[Patrik Floding]

Originally posted by gt4rcdude
Hello.
I've spent hours reading the threads on this forum about the Adcom GFA 565's. I have two of these monsters. One has never given me any trouble. The other I've had repaired twice at over $250 a pop. The first repair was a 10K ohm 1/2 watt resistor, a 47 Ohm 1/2 watt resistor and a 2.7 Ohm 1/2 watt resistor. Parts $23 and $230 labor. The next repair on the same amp was part number 2SC945 transistor and 1 ohm 1/2 watt resistor. $18 in parts and $281 in labor. This done by an Adcom certified repair store.

It started both times with distorted audio. I know it's vague but I'd love to learn how to fix this myself in the future. If this sucker breaks again I'm going to either fix it myself or use it for a boat anchor.

I've noticed some 565's have a cooling fan on the top. Mine do not. They still have adcom filter caps as well. How/where do I get this fan mod? Where does it get power? Any idea what they "fixed"? Is it just because the amp is old?

BTW, serial number is 011214627

Thanks in advance. I'll keep reading and learning in the meantime. I really need to find the repair/service manual for these things.

I'm not familiar with this amp, more than what I just googled. However, if it's as good as people seem to think, and as powerful, I think the repair is probably worth it. Hopefully the repair shop put some beefier resistors in, that can handle more current without going bad. If you know a local electronics repair guy (preferably a buddy ), he may be able to compare the two amps (looking for the changed resistors) and perform preventive modifications to the other amp.

It helps if you can find the schematics for the amp as well. Also, if you are lucky, the earlier repair bills specified which resistors were replaced.

P.S: Just read a horror story of a 565s going bad, outputting DC and burning speakers.. Hopefully the authorized repair shop checked the electrolytics. Although they only had one amp to look at, of course. If DC develops slowly, then it may help to measure for DC periodically. Someone who knows more about failure modes can hopefully give more and better advice on this!

[gt4rcdude]

I think I may need to test the DC offset. The amp I had repaired always run significantly hotter than the other. I've tried switching speakers to no avail.

How do I do this? What am I looking for? If it's too high, what's wrong?

Thanks for helping a noob out!

John

[gt4rcdude]

Anybody?

[Herb S]

Have you checked the DC offset yet? If you reviewed the various threads you know that these amps have problems with leaky caps. I had to clean and replace components on my 565's earlier this year and they are fine now. If I can help let me know, I'm in Cumming so we may be near each other.

[gt4rcdude]

I'm in Snellville.

How do I test the DC offset? What am I looking for?

[Herb S]

To check the DC offset you need a voltmeter.
1. Disconnect the speakers and the signal input cable.
2. Short the input connector (you can leave the cable in and put a jumper across the other end that would connect to the pre-amp).
3. Connect a DC Voltmeter across the speaker terminals.
4. Turn on the Amp.
5. You should get a reading of a few mV. Polarity doesn't matter.

I have heard of readings in the Volt level; one of my amps read a few hundred mV. When fixed the both amps read less than 10mV.

Note, on power up the reading will start high and within a minute settle down. There will be some fluctuation with air currents and other factors as the dc servo corrects itself, but this will be 10mv or less. Higher readings indicate problems such as caused by leaky caps messing up the DC servo circuit (it is extremely sensitive).
Let me know how you make out or if I can be of further help.

[ben62670]

Hey guys I am in Conyers GA east of ATL. I have repaired several Adcom's, and The only one that has given me any headache is the 585. I am having an audio get together this spring, but locals are allways welcome at my place. gt4rcdude you are more than welcome to swing by with your amps. I can take a look at them if you would like. I have plenty of parts for those including the original Adcom/Tosh output trannies.
Ben

[gt4rcdude]

Thanks! I'll give that a try. Sounds easy enough.