Adcom GFA-585 Problems/Explosions

Status
Not open for further replies.
Jump to Latest
#41
Listen to Chris not ME. For some reason I had it in my head that the 585 (since it's essentially a dual mono design with1 transformer) might not be common ground. I sold Adcom and Polk among may others back in the day late 80's early 90's. I knew some of the newer 5xxx models were not common ground but was unsure specifically if the 585 was.

So as Chris says NO need to tie the grounds on the 535, 545, 555, 585. Except if your run the 555 in mono ( you'll need two). The 555 is a much better amp in stereo than mono anyways unless it's being used for sub duty.

How's the 585 coming along Ben?

Completely off subject Chris, but are you familiar with the Adcom GCA-510. It's a 50 watt per channel integrated which Adcom calls a control amp (but it's not a pre nor does it have pre-outs) the pre-amp section is completely passive and it ends up being a high gain amplifier. Adcom describes it as a very high gain power amp preceded by a passive level controller with no seperate pre-amp stage. It has a two stage discrete front end with doubly regulated and temp compensated active current bias sources.

Any feedback on this design. I have one here that has some issues like I think the bypass caps are leaking and I'm looking to purchase another unit. These were made for only about a year or two in the early 90's. I haven't even been able to locate a service manual.

I'm going to use it for a second system. Any thoughts or experiences.

Thanks
Brock
 
#42
Hi Brock,
The GCA-510 seems to be very rare. I didn't even get a service manual for it. Given that it was built in the early 90's, you may possibly be looking at a run of bad capacitors. The Adcom engineers did everything they could to use high quality components. Too bad you're far away, I'd love to look though one of these.

BTW, you can't run a pair of GFA-555's in mono with those SDA's since you no longer have a common ground. If someone were to use a pair of mono blocks they really do need to ties the speaker commons together or the signal grounds stand a good chance of being damaged.

Another point I'd like to make is that an amplifier in bridge mode normally does not sound as good as an amplifier run normally.

-Chris
 
#44
Oh No

The buzz is back! I am assuming that the input resistors have gone bad again. Can I use 1/2 watt resistors instead of 1/4 watts? I was running the amp fairly hard at a 2 ohm load to see how she would do, and it never overheated, the distortion lights never came on. I turned it down and heard the hum again. The speaker cones were extended a little bit, and had the buzz.

Thanks Ben
 
#46
Yeah its kinda weird. When I was working on the amp I was giving good advice on what to look for. Anatech hit it right on the button when he told me to look for open 10 ohm resistors. I found them and sure enough they were open. I replaced them and the amp worked real nice and clean. I have to take it apart again and test the resistors again to see if they are open.

Thanks for your post
Ben
 
#47
Hi Ben,
Thanks.

I agree with Glenn though. There is no way you should be blowing those resistors. That indicates current in the signal grounds which may damage your preamp also.

Just measure between your RCA grounds again with no source hooked up.. You should read 20 ohms. I'm betting it's open again. 🙁

-Chris
 
#52
use a dc voltmeter and test across the speaker terminals where the speakers connect. you should have less than 100mV dc across them. most amps i've seen pretty much hover around +/- 50mV. keep the meter on there for about an hour, and make sure it stays below 100mV as the amp gets warm. to check idle current, measure across one of the output emitter resistors. you should see between 10mV and 20mV for most amps. the best way to set the bias is to run the amp at 1W/20khz into an 8 ohm dummy load with a distortion analyzer, and while watching the analyzer residual on an o-scope, adjust until the little spikes where the signal crosses zero just disappears, don't turn the bias any higher than that. the "quick and dirty" wat to set it is to measure across the output emitter resistors, and adjust for between 10 and 20mV. it helps if the emitter resistors are .1 ohm, or .5 ohm. across the .1ohm, the voltage is numerically identical as 1 tenth of the current. if you measure across .5 ohm resistors, from + emitter to - emitter, you have a total of 1 ohm, and your current is numerically identical to the voltage.
 
#54
Hi Ben,
There is a factory procedure on setting the bias on Adcom amplifiers. Follow it. I've listed it in some other threads, so try and search it out.

As for DC offset, unclejed613 has the right idea with some exceptions. These depend on how the amp is designed. In your case the amp has a DC servo. Therefore your DC offset should not exceed about 5 mV except in the first few seconds the amp has been on. This would hold true of any amplifier using a DC servo circuit.

For non servo amplifiers, you can figure out the expected DC offset from the schematic. Normally this should be less than 50 mV. Any higher and it's either cheap, incorrectly designed (same thing) or needs corrective action. Marantz amplifiers (from the 500 up) should measure less than 10 mV and typically less than 5 mV. There are adjustments for this.

-Chris
 
#57
Hi Glenn,
Possibly the op amp is pegged? 😉 It could be at it's limit of correction due to another more serious fault. Then there is the electrolyte issue from the caps that could very easily be the cause for this.

Ben,
Please confirm your readings.

-Chris
 
#58
0.9V Is the correct reading. I measured it black to red on both channells. It runs hot at Idle, and the speaker drivers are extended a little. The boards were completely washed with pots removed, and new caps were installed. I am getting 20ohms across the input grounds. Op amps are what $10 each? i wouldn't mind changing them any ways. I don't know which ones to get, or where i should get them from.

Thanks Again
Ben
 
#60
I scrubed them down with electronics cleaner with a brush, and then scrubbed them down with simple green and a tooth brush! I never even seen any trace of electrolyte on the board to begin with. I am planning on taking the boards out again any way to check on the values of other resitors on the boards so I can reclean it than.
Thanks
Ben
 
Status
Not open for further replies.
Top Bottom