Crate VC50 180 Hz Hum

[misterc57]

I have an VC50 that has a 180 Hz -60 dB audible hum (as measured on a spectrum analyzer app) on the clean channel only. When switching to the gain channel the hum is gone.

Happens with or without an input source. Reverb connected or disconnected. Regardless of volume settings. Regardless of footswitch connected.

The owner has replaced all filter caps, tube swaps, pot swaps, cleaned channel switch, cleaned the channel switch, and checked solder points.

I have stepped through the clean channel input signal with a listener and could not detect the noise, but it is there on the output.

Before I pull the board out I was hoping for suggestions. Thank you, MarkO

crate_vc-50_07s269-11_sch.pdf

[Helmholtz]

180Hz hum sounds odd (I don't trust spectrum analyzer apps).

Scope pic shows a mix of frequencies, the highest being around 70Mhz.
A time base setting of 5ns/div makes no sense here.
Go to 5ms/div instead.
I might help trigger stability to set trigger source to line.

[misterc57]

Hopefully these output scope images are better. First image is clean channel, second image is gain channel

[Helmholtz]

Obviously there's no visible hum with the clean channel.
The fundamental hum frequency on the gain channel is 60Hz.

If this is scoped at the speaker output I woudn't consider it a real problem, as the hum level is only around 2mVrms.

[glebert]

If it happens on clean channel only and indepent of everything I would probably start by pulling V1 and see if the audible hum goes away. If it goes away then I would start AC shorting out parts of the signal path (I have a high voltage cap with short leads and alligator clips for this). I would only reference the spectrum analyzer to confirm if that frequency component changes.

Actually, pulling V1 is probably not even necessary, but I would probably do it just anyway since it is so easy.

[misterc57]

The audible hum is so low on the clean channel that I told the owner it was an acceptable noise level. But the owner dislikes it even at that level.

What perplexes me is that switching to the gain channel the output is dead silent. And normally I would expect this to be reversed, no noise on the clean channel and some noise on the gain channel.

Tell me more about "AC shorting out parts of the signal path". I have never heard of this or done this.

Thank you

[The Dude]

I'm with glebert. First, try pulling V1 and see if the hum goes away. You could simply have a noisy tube.

[glebert]

Tell me more about "AC shorting out parts of the signal path". I have never heard of this or done this.

I started using this for troubleshooting to add extra capacitance (to ground) for power supply nodes, but then realized it can also be hooked up to nodes that would normally have audio on them. Any AC signal in the audio range should be functionally shorted to ground but not the DC. This helps to narrow down the part of the circuit where the noise is coming from. Even on nodes that don't have DC on them (like a preamp tube control grid) I find this less likely to pick up ambient noise than a straight wire jumper. Obviously safety precautions need to be followed to discharge it if you put it on a node with DC on it.

[Helmholtz]

If you hear hum on the clean channel, the scope pic must be wrong.

[misterc57]

Any AC signal in the audio range should be functionally shorted to ground but not the DC. This helps to narrow down the part of the circuit where the noise is coming from.

I am not getting this yet. Negative end of the cap is connected to ground, and the other end at different points in the signal path sending the AC signal to ground. What does this tell me as I move along the circuit path?

[misterc57]

Pulled V1. Noise is still on the output on the clean channel, no noise on the gain channel.

Pulled the rest of the preamp tubes one by one and tested. Noise is there until all preamp tubes are pulled.

Then tried a different tube in V4 (no other preamp tubes connected). Noise is back.

Again the strange thing in all this is only when switched to the clean channel there is noise.

Going to pull the board next and check solder.

[x-pro]

Using the same oscilloscope sweep (5ms/div) check the noise in the power supply at points TP26, TP29. Closed input at the probe.
D13, D14 can be noisy.

[glebert]

I am not getting this yet. Negative end of the cap is connected to ground, and the other end at different points in the signal path sending the AC signal to ground. What does this tell me as I move along the circuit path?

If you hook this to a node and the noise doesn't go away the noise is "downstream" of that node (toward the power amp). If the noise goes away then it enters either at that point or "upstream" from that point. It may not be helpful if you have some sort of holistic, distributed issue. The fact that somehow the noise comes back on the clean channel only when you put a tube into v4 might be that sort of generalized problem.

I love VC amps, but they are problematic for servicing, including cracked solder joints. Have you tried using a dowel or similar to flex the circuit board(s) to see if the noise changes? The TRS effects loop jacks are also notorious for causing problems. I used to have a TRS plug that was shorted tip to ring to put into that jack to make sure that jack isn't causing a problem.

edit: also forgot this has separate reverb returns for clean and lead channel. I would be looking to see if the noise could be coming in around Q5 or Q6.

[misterc57]

Thank you all.

After reflowing some pots and swapping one preamp tube from a 12AT7 to 12AX7 there is only the faintest of background hiss now on the clean channel and the drive channel is dead silent. Owner is happy.