GK 200M trouble...trouble.

[Garydean]

The original problem with this amp was that the "ECHO" would not shut completely off. I removed the echo pot, soldered in a "replacement" and touched up a few loose connections on the board.

The original 50K nylon shaft pot is no longer available through GK so I got a 16mm 50kL pot with a metal shaft. It went right in, no problem.....

but then...

I turned it on using a variac, as soon as a little bit of AC was introduced the amp started to hum. No signal was introduced at this time. I checked the original with my meter and it went from 2.5ohms smoothly to 50k. I'm going to put the original pot back in and start from scratch. Has anyone worked on one of these amps with this problem? I have no schematic....

Thank You,
Gary

[52 Bill]

Gary,
Are you saying that the amp hums as you start to increase the ac line voltage via a variac?
Did you get the line voltage up to normal or did you quit bacause of the hum?

[Garydean]

Hi Bill, thanks for responding. Yes I quit increasing AC as soon as I heard the hum. It was kind of fluttering every few seconds as well.

[52 Bill]

I'm not sure with that model of GK, but I've seen them act very wrong at low ac levels, but work fine at full voltage.

Was the amp basically working ok before the control change? If it was, I'd pull the speaker and power it up and check for dc on the output. If everything looks ok then reattach the speaker and see where you're at.

[Garydean]

Yes it was functioning very well except for the echo issue. It does have a speaker disconnect push button in the back of the unit. The new pot has a metal shaft, does that have anything to do with it?

[52 Bill]

I doubt it. The shaft will basically be grounded by the panel mounting hardware, which was grounded on the old pot as well.

When an amp goes through a start up cycle at power on, different circuit parts charge up and turn on at different levels. Normally when you turn the amp on it all happens so quickly, you don't see or hear any unbalanced circuits humming or popping, etc. So when you slowly raised the ac input, you got to a point where there was a circuit imbalance that caused the amp to hum. This is probably normal, and unless you did something to the circuit when you replaced the pot, it will probably work just fine if you just plug it in and turn it on. To be safe, I suggested powering up first without the speaker load. That way if you do find voltage on the output, you will be able to backtrack before you do any real harm.

As a matter of course, we use a light bulb limiter in our shop, but there are certain solid state amps that will draw large amounts of current at start-up and fully light the bulb, even though there is no problems with the amp.

[Garydean]

Thanks Bill. I do have a dim bulb tester on my bench as well. I'll check it with that too. So... I'll power it up without the speaker connected. How will I know if the hum is still there. Where should I be testing for AC?

Gary

[52 Bill]

If there is hum, it will show up on the output. Check across the speaker output. Does it have an extension speaker jack?

[Garydean]

Ah yes. Gottcha...60cyl hum = AC.

thank you!!

[Garydean]

Hi Bill,
I left the replacement pot in and plugged into the bulb. For the record, the light was dim. So far so good. Plugged directly into the AC outlet and everything was perfect. No hum, and the bonus was..... the echo issue was corrected. Is it possible to have a pot check fine with a DMM, yet still be bad?

thanks for the advice!!

gary