Yet another Adcom GFA-565 problem

[Lockport]

Hi, all I am new and pretty limited to using a multimeter (although I can disk-level edit a partion table in little endian format - like there was another way) and thanks for your patience.

I bought a pair of these black beauties on Ebay a year ago and they have been working fine ever since. They are mated to my Sony STRDG-1000. Until a few days ago I noticed barely any volume coming from the front speakers, but still some. I attempted a DTS calibration using the Sony mic, but received an error that the fronts were absent or the mic was not plugged in correctly.

The amps arrived with a receipt for repair of the DC Offset issue performed by a Setauket, NY firm and I am trying to determine exactly what was done on these serial #'s but believe, according to the website, it was:

"GFA- 565 Monoblock:
These units develop a DC offset problem that leads to a loud turn on thump and eventually will burn out a speaker or lead to major failure of the output transistors. The cause is poor quality electrolytic caps on the driver board that leak onto the board and cause corrosion. We clean off the board, replace all caps and resistors , replace the driver transistors and adjust the bias."

I notice a thump powering on but began wondering if the problem is the line output from the Sony and not the amps at all as I find it odd they would both begin exhibiting problems at exactly the same time. How can I determine the root of this? The Sony was new and bought for the purpose of scting as a 7.1 processor and preamp for the blocs.

Note: The caps appear to be Adcom 35,000 and In think that would meke them the originals (or s bsd replacement.)

Thanks in advance

[Zen Mod]

have you possibility to inject any other signal ( source , signal cable ) in monoblocks ?

one solution - headphone output ( with pot ) from CD or boombox , with appropriate signal cable

or any other (volume pot equipped) source , for which you have signal cable , possible to connect to monoblocks .

[spind]

Welcome to the forums Lockport (I'm in Wpg). I think your reasoning is sound--if you lose sound from both amps at the same time maybe it's the preamp. I'm not a pro, but I have some test equipment and could help troubleshooting if need be. Have you tried what Zen Mod suggested? Alternatively, do you have an integrated amp or a receiver with pre-outs and main-ins that you could try connected to the preamp and to the amps?

Steve.

[Lockport]

Originally posted by Zen Mod
have you possibility to inject any other signal ( source , signal cable ) in monoblocks ?

I hooked my laptop to one via the audio out (headphone) jack and the volume has to be far too low for them to be outputting anything near the area of 300 watts!

It just seemsodd to me that they would both go at once. The large cans inside, these are the caps that were famous for leaking, right? Mine are branded Adcoms 110v, 35,000 - would these have been replaced with a different brand if the legitimate fix was done, or is irtpossible they are later caps that were corrected?

[Lockport]

Originally posted by spind
Welcome to the forums Lockport (I'm in Wpg). I think your reasoning is sound--if you lose sound from both amps at the same time maybe it's the preamp. I'm not a pro, but I have some test equipment and could help troubleshooting if need be. Have you tried what Zen Mod suggested? Alternatively, do you have an integrated amp or a receiver with pre-outs and main-ins that you could try connected to the preamp and to the amps?

Steve.

Cool! Same name, too!




Yes, I have an older Denon AVC2000 with pre-outs I just hooked up and ran my laptop to its Aux (everything to the newer amp is HDMI), and, although I have audio, it is not the booming, overwhelming effect of mono-blocs.

I sent an email to the firm that did the work (according to a receipt I have from the seller 18 months ago) to correct the DC offset and validate the inked in serial numbers. I am curious that the caps still are Adcom branded. I hope to hear from them tomorrow, in hopes they have time to look up a work order/invoice from that long ago. The services they do are posted and are $20more than my receipts but is probably just iunflation speaking and that is what was done. Would still like a parts detail from them if I can get it.

So, essentially the symptoms appear to be low volumes = low power output?

Am still moderated so timely lull between posts.

Thanks,
Steve

[Zen Mod]

Originally posted by Lockport


I hooked my laptop to one via the audio out (headphone) jack and the volume has to be far too low for them to be outputting anything near the area of 300 watts!

It just seemsodd to me that they would both go at once. The large cans inside, these are the caps that were famous for leaking, right? Mine are branded Adcoms 110v, 35,000 - would these have been replaced with a different brand if the legitimate fix was done, or is irtpossible they are later caps that were corrected?

dunno what's your laptop ; there is always chance that laptop have crappy output ;

according to explanation given in your first post - these big can caps aren't "caps on driver board" ;

if these big can caps are leaking , you'll probably hear loud hum in speakers ( disturbed PSU filtering , what's their role ) and distorted sound .
culprit is somewhere else , and it's still questionable is it in power amps .

have you any other ,even integrated , amp - to connect it instead of monoblocks , at least to test Sony STRDG ?

[Lockport]

Originally posted by Zen Mod



dunno what's your laptop ; there is always chance that laptop have crappy output ;

Its basically a line out with enough power to drive a headphone. I have heard the blocs with this setup when I first got them and I have no where near the volume.

Originally posted by Zen Mod

according to explanation given in your first post - these big can caps aren't "caps on driver board" ;

if these big can caps are leaking , you'll probably hear loud hum in speakers ( disturbed PSU filtering , what's their role ) and distorted sound .

I thought those were the infamous faulty ones. Inside of the unit, it is very clean. I see no evidence of corrosion or staining by any of the screws, on the top of the boards or anywhere else for that matter. Underneath may be different, however.

Originally posted by Zen Mod

culprit is somewhere else , and it's still questionable is it in power amps .

have you any other ,even integrated , amp - to connect it instead of monoblocks , at least to test Sony STRDG ?

I hooked up my Denon to act as preamp to the blocs as per previous post and this yielded the same result, ruling out the Sony as cause. It appears it is in the blocs now.

[Zen Mod]

unfortunately - seems that you need a tech .

[Lockport]

I checked the undersides of the boards in both units and they are spotless. The DC Voltage, however, across the speaker terminals after 1/2 hour warm up shows .996V on one and 1.212 on the other. This is .1 and .05 less (respectively) than when cold.

Time to bench check the power?

[spind]

Originally posted by Lockport
I checked the undersides of the boards in both units and they are spotless. The DC Voltage, however, across the speaker terminals after 1/2 hour warm up shows .996V on one and 1.212 on the other. This is .1 and .05 less (respectively) than when cold.

Time to bench check the power?

Nope. Looks like you have way too much DC offset (too much DC on the outputs). It should be less than 10mV on this amp. Do some searching on this forum---there is lots of info about this problem here. The driver boards will need to be cleaned VERY thoroughly in an ultrasonic cleaner and then have new capacitors installed. Maybe you could send the boards to Georgetown and Anatech (a moderator on these forums---you may want to email him) could help you out?

Steve