Troubleshooting and solving signal noise on a 1989 JCM 800 2205

[Jquijano]

my 1989 jcm 800 2205. The amp grasps the noise signal from the ac current and amplifies it. In the overdrive channel it becomes more audible even with the guitar volume knob at zero. All of my filter caps where replaced in 2020 so something else must be going on. Anyone?

[Tom Phillips]

my 1989 jcm 800 2205. The amp grasps the noise signal from the ac current and amplifies it...

Are you saying that the noise is an AC hum? Is it continuous or does it start and stop over time?

...In the overdrive channel it becomes more audible even with the guitar volume knob at zero...

Please tell us how the amp volume and tone controls affect the noise without the guitar plugged in.

...All of my filter caps where replaced in 2020 so something else must be going on.

That makes it less likely that the problem is caused by a filter cap but does not rule out a filter cap problem or a bad solder connection. A part can fail at any time.

[TomCarlos]

Just curious.... this amp has channel switching, correct? It is done using the CA3846 IC. I have read where that IC (and the channel switching) can be problematic. Regardless, my question is, does this "noise" happen when you switch from one channel to another? You mentioned the Overdrive channel, but I would like to know if you have the same issue when in the "Clean" channel.

Schematic and check to make sure it is correct. jcm800_schematic_50w_2205.pdf

[Jquijano]

Amp has the same issue in both channels. Clean channel its barely audible. However on the overdrive channel when you start raising the gain and volume it is really noticeable.

[TomCarlos]

And if you connect your guitar to the Effects Return, do you get the same noise?

[Jquijano]

And if you connect your guitar to the Effects Return, do you get the same noise?

No noise whatsover connecting the guitar at the return jack. Just normal low hum.

[pdf64]

Has a bad valve been eliminated? There are several unbypassed cathodes in the stages around V1&2, which leaves them vulnerable to buzz from poor heater to cathode insulation in their valves.

Does the heater circuit have a DC path to 0V circuit common (eg the chassis metalwork)?

Is the reservoir cap C9 for the channel switching supply definitely good?

https://el34world.com/charts/Schemat...h_50w_2205.pdf

[Jquijano]

Let me open the amp as the only parts i changed in 2020 were the filter caps.

[Chuck H]

In the overdrive channel it becomes more audible even with the guitar volume knob at zero.

What happens to the noise with the same settings if nothing is plugged into the amp?

[Jquijano]

What happens to the noise with the same settings if nothing is plugged into the amp?

No noise with nothing plugged. Its hard to describe as it is something that comes from the AC Wall and its only generated when you plug the guitar to the amp front input. Its in the background.

[Tom Phillips]

No noise with nothing plugged. Its hard to describe as it is something that comes from the AC Wall and its only generated when you plug the guitar to the amp front input. Its in the background.

That sounds like a common occurrence.
Check that the guitar cord is good. Especially the ground connection. Try a different cord and a different guitar if you have one available.
Does the noise go away when the guitar volume control is turned all the way down?

[Jquijano]

Guitar cable is brand new and cost 30 bucks gold plated. You can even hear it with the guitar volume down and the gain up on ch2 overdrive.

[Tom Phillips]

Guitar cable is brand new and cost 30 bucks gold plated...

"Sigh"

[TomCarlos]

V1 to V4 are ECC83, 12AX7 tubes. It is always good to have an extra one in your emergency kit. I would get a new tube, and start swapping out tubes, one by one, to see if this is tube related.

See PDF64 reply to this thread.

[g1]

Guitar cable is brand new and cost 30 bucks gold plated.

You must know that sometimes people have to return brand new cables because they are bad. He asked if it was good, not if it was new. Have you tried other cables?