60 hz hum on one channel

[JohnRoberts]

Does the gate still operate, with extra hum, or is it not working?

If not working investigate the side chain and gain element (probably shunt JFET). If working with extra hum, try to determine where in the path it is getting in.

JR

[enginefire]

The gate is working well, it is just one channel that gives hum on the output. I unplugged the input to break up the signal path, and the hum is always present when the output of one channel is connected.

The other channels all work and have no hum so I am assuming it ismaybe something like a local bypass cap for one if the power rails?

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[JohnRoberts]

Probably not PS... since that would generally show up in all outputs.

Sometimes output protection circuits go bad and leak hum into path.

With scope or signal sniffer, work backwards from output and see where the hum is getting in.

JR

[enginefire]

A little digging has revealed that the hum is connected to the "range" knob, which effects how much signal is muted by the gate. When it is fully to the left the hum is gone, and the gate is very traditional in function. as its turned clockwise the hum increases (the gates function changes to let more signal past the gate).

I checked the other channels, they behave in a similar fashion.

I have also noted that there is a lot of mechanical noise ans vibrations coming from this units transformer, which was already replaced.

I will unrack it after this mix is done and see if u can scope out the problem!

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[JohnRoberts]

More possible to be a common fault vector like PS...

JR

[enginefire]

So much to learn... Would changing the ps caps and diodes and regulators help this? Or would removing ps completely and reworking it into an external chassis be beneficial?

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[JohnRoberts]

Unless this always had hum... let's first figure out what has gone wrong.. and repair that..

Look for bad solder connection, perhaps old wasted cap... but PS is only suspect not smoking gun cause.

JR

[Ike Zimbel]

Is the PS ground tied to chassis ground? It should be at one point, often through a 10 ohm resistor. 60hz is often a sign that is not the case.
Best,
Ike

[brianroth]

To enginefire....are you certain it's primarily 60 Hz (typically a grounding or shielding problem) vs. 120 Hz (typically a PSU problem)?

Bri

[enginefire]

Thanks John, Ike, and Brian!
I take it my software oscilloscope will not take the voltage to test the PSU properly. I guess its time to get a real one!

Di you think one like this would do the job?
http://montreal.en.craigslist.ca/tls/3093106027.html
Or
http://montreal.en.craigslist.ca/ele/3161226428.html


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[enginefire]

Sorry Brian, to answer your question, I believe it is 120Hz, it reads a a sharp Bb -2 on my tuner

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[brianroth]

If the 2235 scope in the second listing actually works, it would be a much better choice.

Bri

[enginefire]

He said the scope works fine but he could promise that the probes read accurately... I don't know much about buying scopes, does that sound like something reasonable? that the probes might need to be replaced for the unit to work 100%?

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[Ike Zimbel]

Yeah that looks like a nice scope if it works. I have a couple of extra probes that I got in an e-bay deal last year that I could part with. Give me a shout tomorrow when I'm at the shop. These are new.
Best,
Ike