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Old 10th November 2006, 06:57 PM   #1
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Default Speakers for Adcom GFA 585?

Hi Folks,

Well here's my first post after being registered here for ages!

A good friend recently donated their broken Adcom GFA 585 and I finally have the time available to fix it*. According to the owners' manual it is rated for 250 W into 8 ohms, and 400 into 4 Ohms, with almost monoblock construction - only the power transformer is shared, and that is a real beastie. I also have a GFA 535 that came along for the ride, so a bi-amped setup could be realised.


So, the theme here is big HiFi on a budget (amp has cost me $50 so far). Assuming the restoration is sucessfull, the next step is to find speakers to partner this monster. I'd love to make the most of the current available to drive some Magneplanars, but am happy to consider your recommendations. The listening room is at least 20x20 feet and has sloping ceiling and non parallel walls, no carpet.


Cheers & thanks for a great forum,

Ed



*It looks like an electrolytic capacitor has leaked (seemingly a recurrent problem with this brand) over the input boards - this has disabled one channel which now has a massive DC offset. Hopefully it will be a straightforward clean-up and fix, since no devices appear to be damaged. I have the service manual on order for the sake of completeness.
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Old 12th November 2006, 11:56 AM   #2
K-amps is offline K-amps  United States
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Default Re: Speakers for Adcom GFA 585?

Quote:
Originally posted by Ed Holland
Hi Folks,

Well here's my first post after being registered here for ages!

A good friend recently donated their broken Adcom GFA 585 and I finally have the time available to fix it*. According to the owners' manual it is rated for 250 W into 8 ohms, and 400 into 4 Ohms, with almost monoblock construction - only the power transformer is shared, and that is a real beastie. I also have a GFA 535 that came along for the ride, so a bi-amped setup could be realised.


So, the theme here is big HiFi on a budget (amp has cost me $50 so far). Assuming the restoration is sucessfull, the next step is to find speakers to partner this monster. I'd love to make the most of the current available to drive some Magneplanars, but am happy to consider your recommendations. The listening room is at least 20x20 feet and has sloping ceiling and non parallel walls, no carpet.


Cheers & thanks for a great forum,

Ed



*It looks like an electrolytic capacitor has leaked (seemingly a recurrent problem with this brand) over the input boards - this has disabled one channel which now has a massive DC offset. Hopefully it will be a straightforward clean-up and fix, since no devices appear to be damaged. I have the service manual on order for the sake of completeness.

I have some Adc'mon spare caps 100v 22,000uF used in the GFA-5802. Will fit the 585 I think.
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Old 13th November 2006, 05:09 PM   #3
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K-amps,

Thanks for the reply. I'd have to check on the suitability of the caps. The PSU is fine though. The problem appears to be 220 uF decoupling caps, mounted on the input boards. These have leaked all over the servo & voltage reference section on the left channel. A little trawling of the net reveals this to be a known problem with the 585 and 565 amp models that use this board. The spilled electrolyte caused complete upset, resulting in the left output being driven fully negative, which blew the O/P fuse. Sadly this was too slow to prevent damage to the speakers belonging to the previous owner.

Anyway, I plan to give both input boards a thorough clean, as there are visual signs of the same problem on the board for the right channel. Then all the electrolytics on these boards will be replaced - just to be safe. Next a trip will to the surplus electronics store for the parts to build a dummy load for this monster before I do a full setup and soak test.

Perhaps one day I'll even get to listen to some music

Cheers,

Ed
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Old 13th November 2006, 06:50 PM   #4
K-amps is offline K-amps  United States
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You know what, if merely cleaning the boards fixes the issue, please let me know. I have a misbehaving 585 and two 565's lying somewhere needing help.

:-)
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Old 13th November 2006, 10:50 PM   #5
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Well, wish me luck... I'll post my progress here.

An effective cleaning technique is going to be the key thing, as the electrolyte is vile, persistent stuff. In one part of the board, I can get readings anywhere down to around 1 megohm just touching the fiberglass! No wonder the biasing was upset...

I just nipped out during my lunch hour to buy replacement caps, and bits for a dummy load, but am awaiting the service manual in the post before.

Cheers,

Ed
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Old 14th November 2006, 12:17 AM   #6
K-amps is offline K-amps  United States
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Good Luck !!!

I have a 6v dc offset on the output of a 565.... Lets see !
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Old 15th November 2006, 09:13 PM   #7
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Progress report.

Over the last couple of evenings, I pulled out both input boards, and removed all the electrolytics. The 220 uF 25 V (3 per board) were the obvious failures, but I removed the two 100 uf 100 V for the sake of completeness. The left channel had large quantities of electrolyte spread across its top surface.

The boards were scrubbed with neat "Simple Green" and rinsed thoroughly in running water, then dried with a hairdryer. After this they had a near new appearence. REplacement caps were soldered in and the boards replaced.

I also made up a dummy load for each channel, with about 80 W handling capacity, ready for testing. All was connected up and power applied*

The result? SUCCESSS!

It was late, so there was only time for brief measurements (no listening). Both channels measured at <20 mV DC offset and very little AC noise, as judged by my Fluke meter. Both channels responded to inputs and all seems well.

K- amps - I would definitely inspect your Adcoms for leaky caps - from what little I have found on the internet and one Adcom approved service dealership I spoke to (They refused to take the amp in for repair!), this appears to be a recurrent problem with the 585 and 565 models - they use these same input boards.

Most of the work time was taken up making careful notes and labeling of the connections to each board - There are some connectors, but also a handful of leads soldered directly with not much spare room to work.


I can't wait for a listening test now

*DO NOT run the unit up on a variac - there's a surge control circuit built in to the AC line circuit. The service manual warns that a resistor in this circuit will burn out if power is applied gradually.
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Old 15th November 2006, 09:13 PM   #8
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The service manual makes for some interesting reading. These are some of the points that made me sit up and take notice, and may be of interest to others:

Whilst the topology is essentially a differential input stage (allowing for feedback application), voltage gain and emitter follower complementary output stage, there is some real sophistication in the design.

Quote "The amplifier employs a descrete dual-differential cascode class A input stage followed by a dual cascode class A voltage gain stage..."

Quote "The output stage is capable of greater than 60 amps into low impedance loads."

With all this current available, the biasing networks of these stages cleverly incorporates thermal protection and power supply failure protection to shut the amp down, protecting the output and, presumably anything connected to it. This did not save my friends speakers when the circuit was swamped in electrolytic capacitor juice!

A DC servo OPAMP is used to correct imbalance in the output stage.

The input stage has a freq response 0.7 Hz to 700 kHz, with roll off applied for an actual -3 dB point at 80 kHz.

Some selected (factory) specs:

Power output 250 W into 8 Ohms, <0.02% THD
Power output 400 W into 4 Ohms, <0.02% THD

@1W into 8 Ohms, frequency response of 10 Hz to 20 kHz =), -0.5 dB
20% to 80% rise time for 120V p-p 5kHz square wave, 2.9 microseconds

Damping factor (20 Hz to 20 kHz) >=600

Weight, 45 lbs ...I know, I mistakenly took a flight with this thing as checked baggage.... kerching! Overweight bag...


Of course, none of this tells me what it will sound like - I'm still in the dark here. At least I'm in a position to go speaker shopping now...

Cheers,

Ed
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Old 16th November 2006, 01:08 AM   #9
K-amps is offline K-amps  United States
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Yes Successs !!!! Borat Style.
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Old 16th November 2006, 04:00 PM   #10
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I have been going nuts trying to get the DC offset down on a 585. I have cleaned the boards with a conservative amount of alcohol & flux remover, but I'm going to try this simple green and a more vigorous cleaning.

Had the same problem as Ed Holland - leaky bypass caps (also change the VBE multiplier cap as it's the same type/brand), blown zener & DC servo. All replaced, but I still have an offset of .7v and 1.2v.

Sound-wise, the 585 has the Adcom 5xx characteristic but more refined over the 555 or 555II across all frequencies with excellent speaker control and sweeter top end. The 565 monos have more guts, but also have twice the output devices. This amp is an excellent chioce for Maggies.

Cheers,
-JamesW-
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