hifi engine

Adcom gfa 565 question

I recently had my 565's rebuilt. They sound and operate wonderfully now. I have yet another DC voltage question about the 565. When I do a mv check one of the amps goes to over 100 mv when I first turn it on and drops to 2 mv within 1 minute. The other amp goes to around 100 mv when first turned on and drops to around 40 mv within 1 minute. After 30 minutes, the first amp is at 0-.3 mv and the 2nd amp is down to 9mv. At the 1 hour mark, the first amp is still around .3 and the 2nd amp down to 2mv. Is it normal for an amp to take an hour to get to its lowest DC output voltage? Thanks for any knowledge on this. Doug



Re: Adcom gfa 565 question

Depending on how the bias circuits are set to thermally track, it can take a while for the amp to settle into it's "idle state". As long as you are in the millivolt range you should be OK. A couple volts at the output terminals when first turned on would be cause for concern.

Re: Adcom gfa 565 question

Thanks for your reply. Both amps work perfectly and I don't have any issues with either of them, but it's odd that 2 identical amps are this much different. One reaches its idle state in 1 minute where it takes the other almost an hour.

Re: Adcom gfa 565 question

The temp sensor is soldered on the driver circuit board and the sensor body is mechanically fastened to the output transistor heat sink. Most times, some type of heat sink compound is used on the sensor when mounted. Sometimes, the junction where the temp sensor is mated to the heat sink is not very secure and as a result it takes longer for the sensor to react to the changing heat sink temps. The one that is taking longer to stabilize may not have any compound applied to the sensor, or the sensor may be mounted loosely.

Re: Adcom gfa 565 question

I'll check that. Thanks for the help.

Re: Adcom gfa 565 question

Just wondering, is there any type of bias adjustment that can be made for this?

Re: Adcom gfa 565 question

You can download the service manual from the LIBRARY section and you will find instructions for setting the bias. Understand, the driver and output transistors along with the thermal diode and other resistors are all connected electrically and in some cases mechanically. They work in unison to set the idle (bias) current. If any one of them is out of spec, or not fastened securely, it can cause the situation you have. Be very careful when you are adjusting the bias; one slip of a probe and you might see smoke rising from the unit, along with an expensive repair bill. That amp has 10 output transistors iirc and they are pricey.

Re: Adcom gfa 565 question

Maybe I'm just being too cautious on this. If I would have never checked the output voltage id never know any difference. Both amps are in spec, but I guess it bothers me that they aren't the same. I'll check to see if anything is loose, but I doubt that I'll risk going through the bias- adjustment process. Once both amps are completely up to temperature, both heat sinks on both amps feel the same to the touch. At this point (usually after an hour after turning the amps on) both amps are reading below 3mv dc on the speaker terminals.