Adcom GFA-585 went bad - calling Adcom experts

[hangguy]

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

[SY]

Ground wire might have come loose? Check continuity of board grounds to the star point. Same with the input socket and output returns.

[hangguy]

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

[hangguy]

Here is the input circuit

[hangguy]

Here is the power supply and output circuit

[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.

[hangguy]

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

[ingvar ahlberg]

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.

[anatech]

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

[hangguy]

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