Crate GTX120 residual hum issue

[booj]

This amp belongs to David Dee

YouTube - David Dee @ Big Muddy "Going Fishing Baby"

I'm measuring 6.7 MvAC @ 120 HZ at the output which is just enough noise to be a not so quiet amp. It came in with a squeal, which I fixed, but he does not like the noise it's making now. Bias is correct at 6.9 mv. across R83. I get similar radings on all the .47 ohm resistors.
I swapped out the 4700uf/50v caps with 6800uf/50caps with no difference.
I had previously changed the outputs attempting to fix the squeal. I put the old outputs back in trying to deal with this issue with difference of zip.
Plugged in a stereo unconnected plug into the insert jack. No difference.
Preamp power supply voltages are +16.8 and -17.2
Power amp rails are +38.1 and -38.2
I can hear this amp hum across the room easily. Is there a fix? Any ideas?
Thanks!

279XXSCH_3.pdf

765SCH.pdf

[Enzo]

Just a whim - make sure R97 is not open. The 10 ohm in your zobel across the speaker.

OK, so you plugged the insert and no help, meaning hum is in the power amp.

Try backing off the idle current a little, any effect? I am not a fan of resistor voltage drops as a bias measure like their spec. I would rather watch mains draw or scope for crossover.

ANother whim. Look on the preamp page, by the insert jack. The input to the power amp is there. Q16 is a mute. Does shorting D to S stop the hum? Shorting across R48 instead might be more convenient. For that matter, we could also try removing Q16, if it were leaky, problems ensue.

Turn the amp 90 degrees on your bench, to see if it might be picking up a stray field in your shop.

[booj]

Q16 is a mute. Does shorting D to S stop the hum? Shorting across R48 instead might be more convenient. For that matter, we could also try removing Q16, if it were leaky, problems ensue.

Enzo, you'll love this.
>Shorting R48 made the amp quiet, meaning the pwr amp is ok.
>Replaced Q16. No change
>Suspected C38 was bad. There was a C38, however it was actually the C39 on the scat. The C38 on the scat did not exist.
>R52 was there but it was 10k, not 680k.
>To add to the confusion, the Q16 designator was mislabled, reading SGD instead of GSD.
>For kicks, I put a 10uf NP cap across R52, and the noise level dropped considerably. I tried a 22uf polarized, but it was not as effective. I tried a 100uf, and the noise got louder. A 4.7, and again not as effective.
>Soldered in the 10uf NP. Case closed. Of course I have not a clue what is going on here.
>One last info-bit. My post is mislabled. It's a GTX212, not a GTX120. Sorry!

[lionsfaninaz]

I have a GTX212 with a hum noise as well. The hum goes away if I plug a stereo plug into the external speaker jack and the hum gets louder when I plug the guitar into the input (still quiet if plug is in external speak jack at the same time). Was the noise in yours present in all three channels? I can repair PCB's, but I do not have access to most of the testing equipment I learned with in the Air Force. The 10uf NP cap you installed, was it a 50v? I know this post is old and I do not have the knowledge most of you display, but I am limited in what I can do. My amp is a project and I am just looking for some direction, I am not afraid to do research based on suggestions. Thanks for any advice.

[booj]

As agent 86 said. "I'm glad you asked that question!"
> The cap being 50 volt should work.
>The noise issue was from the beginning the phyisical placement of the cable connector that ran across the board from front to back and consisted of a flat harness of individual red wires. Unless that cable assy. was positioned exactly right, the amp would hum. When placed correctly, the amp was silent. When I found the "sweet spot" I tacked it in place with silicone glue.

[gerald]

Enzo, you'll love this.
>Shorting R48 made the amp quiet, meaning the pwr amp is ok.
>Replaced Q16. No change
>Suspected C38 was bad. There was a C38, however it was actually the C39 on the scat. The C38 on the scat did not exist.
>R52 was there but it was 10k, not 680k.
>To add to the confusion, the Q16 designator was mislabled, reading SGD instead of GSD.
>For kicks, I put a 10uf NP cap across R52, and the noise level dropped considerably. I tried a 22uf polarized, but it was not as effective. I tried a 100uf, and the noise got louder. A 4.7, and again not as effective.
>Soldered in the 10uf NP. Case closed. Of course I have not a clue what is going on here.
>One last info-bit. My post is mislabled. It's a GTX212, not a GTX120. Sorry!

did u ever get the hum to stop?

[booj]

Yes, the issue was resolved. YOu must have come in at an earlier point in the post. The answer is......

>The noise issue was from the beginning the phyisical placement of the cable connector that ran across the board from front to back and consisted of a flat harness of individual red wires. Unless that cable assy. was positioned exactly right, the amp would hum. When placed correctly, the amp was silent. When I found the "sweet spot" I tacked it in place with silicone glue.

[thetech_III]

I got a fix for this issue

The trouble is low power at 12V in power supply, 12V drop at 8V

Soltuion:
Defective transformer... Just add a new transformer ( Digikey 237-1901-ND )
and plug the secondary on the orange cable position
Remove R10 and place a 12V regulator ( 7812 ), and the pin ground on a good ground somewhere on the board... Thats all, simple

Best regard Jack
Amplifier Thechnician for aniel radio tv, St Hyacinthe' Qc Canada