adcom gfa555

[sda2mike]

hi...i have the adcom gfa555..

one side is room temp when running...the other is warm/not quite hot-no pain thoughts?

i think i need to adjust the offset..the service manual says to put the multimeter across an emmitter resistor...where do i find this?

i have a low hum caused by the amp...i think most of it is caused by my pool pump, which is on the opposite wall outside..if i go to another outlet, i need to get an extension cord...is this advisable?

thanks!
mike

[nigelwright7557]

It could be the offset or the bias or maybe both.

The emitter resistors are usually high pwoer (large) resistor that connect to the output transistor emitter connections.

An extension cord wont cause a problem unless you are taking huge amounts of power.

[WithTarragon]

First, you need to check the bias on each channel. My Adcom 555 service manual gave the proceedure. You will need some power resistors at the speaker outputs so you can bring the amp up to temperature. Then you check with a multi-meter attached to the test points (labelled "TP" on the PCB).

Second, measure the DC voltage at the speaker outputs. Is it similar on both channels? Is it more than 30-40 mV?

[sda2mike]

Originally Posted by WithTarragon

First, you need to check the bias on each channel. My Adcom 555 service manual gave the proceedure. You will need some power resistors at the speaker outputs so you can bring the amp up to temperature. Then you check with a multi-meter attached to the test points (labelled "TP" on the PCB).

Second, measure the DC voltage at the speaker outputs. Is it similar on both channels? Is it more than 30-40 mV?

hi...thanks for the responses...tarragon: if i remember, i had reading of high 20s L and high 40s R...the right side is warm to touch..btw, i tested it with nothing hooked up. with the leads at the spkr outs

thanks
mike

[sda2mike]

hmmm...my reply didn't show up...Tarragon: i was getting high 20s L and high 40s R..the right side is warm to touch....i did the test with nothing hooked up with the test leads at the spkr outs

incedentally, the amp blew 2 fuses on the R a few weeks ago

thanks
mike

[WithTarragon]

Originally Posted by sda2mike

hmmm...my reply didn't show up...Tarragon: i was getting high 20s L and high 40s R..the right side is warm to touch....i did the test with nothing hooked up with the test leads at the spkr outs

incedentally, the amp blew 2 fuses on the R a few weeks ago

thanks
mike

The plot thickens ....
It would be nice if the DC was more comparable between the two channels, but that is certainly nothing to worry about.

Next thing to do is to check the bias (use the proceedure outlined in the manual) and do not try to "high bias" the amp.

Of concern is this new tidbit of information: that it blew two fuses. Was there an explanation for this, i.e., did you do something unwise? If this was an "unexpalined event", then I would concentrate on tracking down that problem.

[sda2mike]

Originally Posted by WithTarragon

The plot thickens ....
It would be nice if the DC was more comparable between the two channels, but that is certainly nothing to worry about.

Next thing to do is to check the bias (use the proceedure outlined in the manual) and do not try to "high bias" the amp.

Of concern is this new tidbit of information: that it blew two fuses. Was there an explanation for this, i.e., did you do something unwise? If this was an "unexpalined event", then I would concentrate on tracking down that problem.

only thing i can say is: after the amp was running all day at a healthy level, i heard 2 slight bits of static. didn't think much abt it shut it down till next time..next time comes and R channel is out..do fuses just get old?..nothing was leaking or burnt inside..BTW the build date is 1986..also, it sounds really good on both channels..

what is 'high bias'

thanks!
mike

[sda2mike]

ttt

[eckmyredavid]

As most techs know the emitter resistors are typically 2 watts or higher and are located near the output transistors. It appears that the Emitter resistors are 0.82 ohms at 2 watts.At a quiescent state (without audio) put your DMM at dcV to the mm range. Clip one lead to the one pin of the emitter resistor and with the other free hands prepare to adjust the trim pot located on the driver board, and with the other probe touch the other pin of the same emitter resistor. observe your reading, and adjust the trim pot to + or - 16mV.
Because this amp depends on feedback loop to amplify, the DC offset is taken care of by proper biasing and a good driver section. I hope this gives you insight of what is at hand when setting the bias.