Adcom GFA-585 went bad - calling Adcom experts - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 20th December 2004, 05:39 PM   #1
hangguy is offline hangguy  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Temple City, CA
Default Adcom GFA-585 went bad - calling Adcom experts

My Adcom GFA-585 went bad two weeks ago and I cannot figure out what went wrong. In the process of going bad, apparently it blew the woofer of my right speaker. So I suspect that the amp put out some DC and destroyed the woofer.

What happened was I was playing some music but all of a sudden there was some feedback like hum so I went shut the CD player off, but the hum did not go away. I then shut the amp off and I supposed it was too late. Putting the amp on the bench and without connecting anything to the input, there is a very slight hum from both channels, as soon as I connected it up to a preamp, the hum became very loud, it sounds like a 60Hz hum and the music from the preamp did not come through at all.

Tried measuring some DC voltages at different places, the left channel + output is measuring 6-7V, while the R + measuring 0.5V. The input +/- is at about 5V and R +/- is at 0.2V. Looks like the op-amp for the DC servo went bad. I tried replacing them but that did not work and everything measured the same with new opamps. The weird thing is, even the input shield(-) is measuring at 5V. The rail voltages are sitting steady at +/- 80V. Both channel cannot play and have the hum.

Some inspection was done and did not reveal any obviously burnt components.

I will try to scan the schematics in later today and hope someone has some idea of what else to look.

Thanks in advance.

Mike
  Reply With Quote
Old 20th December 2004, 05:48 PM   #2
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Ground wire might have come loose? Check continuity of board grounds to the star point. Same with the input socket and output returns.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 20th December 2004, 07:16 PM   #3
hangguy is offline hangguy  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Temple City, CA
Correction to my first posts: Output -ve are connected to the power supply ground and they are reading zero V.

According to the schematics, the signal ground is not tied to power supply ground and I made sure that there is no loose connection on any wire.

I have the schematics scanned but they are too big. I will reduce the size and post them shortly.

Mike
  Reply With Quote
Old 20th December 2004, 08:12 PM   #4
hangguy is offline hangguy  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Temple City, CA
Here is the input circuit
Attached Files
File Type: pdf gfa-585 input.pdf (93.9 KB, 199 views)
  Reply With Quote
Old 20th December 2004, 08:12 PM   #5
hangguy is offline hangguy  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Temple City, CA
Here is the power supply and output circuit
Attached Files
File Type: pdf gfa-585 output.pdf (98.8 KB, 148 views)
  Reply With Quote
Old 20th December 2004, 08:28 PM   #6
diyAudio Member
 
ingvar ahlberg's Avatar
 
Join Date: Feb 2004
Location: Kungsbacka on the Swedish westcoast
Hi Mike
Sounds like blown output transistors if there´s dc on one channel.
Did You test those? Protection circuit would cut output signal but mains hum would still pass through.
__________________
Ingvar
  Reply With Quote
Old 20th December 2004, 11:51 PM   #7
hangguy is offline hangguy  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Temple City, CA
Quote:
Originally posted by ingvar ahlberg
Hi Mike
Sounds like blown output transistors if there´s dc on one channel.
Did You test those? Protection circuit would cut output signal but mains hum would still pass through.

Hi Ingvar,

How would I test the output transistors? I measured the signal going from the input board to the output board, it's measuring the same DC voltage, ~5V on one channel and ~0.2V on the other. The real problem is I am getting hum on BOTH channels.

Mike
  Reply With Quote
Old 21st December 2004, 08:55 AM   #8
diyAudio Member
 
ingvar ahlberg's Avatar
 
Join Date: Feb 2004
Location: Kungsbacka on the Swedish westcoast
Hi Mike
The most common failure in output transistors is fusing between base and emitter giving low resistance between theese, often to collector also. This is measurable on board in most amps with a dvm. Another reason for hum would be a defective cap in powersupply, Meassure ac component on psu rails, also make sure that +/- rails are in balance.
__________________
Ingvar
  Reply With Quote
Old 22nd December 2004, 01:45 AM   #9
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi hangguy,
I would check the power supply for the DC offset servos. You only have around 5V on the outputs, so you don't have shorted outputs I don't think. I seem to recall small 10 ohm resistors to the signal ground from power ground. I am not sure on that. If they open you would normally get a big hum. This could cause your offset. Measure the resistance to the main ground. There must be a connection of some kind. Possibly up to 100 ohms, but the input must be referenced to the system ground somehow. If you measure open on the 1Kohm scale, you have a problem there.

Are the distortion alert LED's on?

Unfortunately, 5V is enough to end some woofers.

-Chris
  Reply With Quote
Old 22nd December 2004, 05:16 PM   #10
hangguy is offline hangguy  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Temple City, CA
Ingvar,

You may be onto something. It looks like there is something wrong with the main power supply. I disconnected all circuit from the power supply and took some measurements. B- measures -83V DC and 0V AC, so this looks okay. B+ measures +83V DC but 165V AC!! The weird thing is, this is on both channels (2 independent supplies). I suspect there is a bad diode in the rectifiers. I will take some measurements tonight on the rectifier and the B+ caps. Any other suggestions?

Hi Chris,

Yes, there is a 10ohm resister between signal ground and power ground, at the output of the DC servo opamp. I measured that resistor and they are not blown. The distortion LED never came on.

Guys, thank you very much for your suggestions. I will continue to work on this.

Mike
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Calling all Rek-O-Kut Experts ! ! ! Bamalama Analogue Source 16 14th April 2014 08:32 PM
Calling all Soundstream experts chemhaz Car Audio 4 29th April 2008 02:01 AM
Calling all ATC mid experts! TwangBar Multi-Way 6 21st April 2007 02:13 AM
Calling all Heatsink Experts! vsr123 Pass Labs 3 28th August 2003 10:21 AM


New To Site? Need Help?

All times are GMT. The time now is 10:15 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2