Randall Century 170 Raw input leak to power amp?

[aczfenderstrats]

Hi all, I tried posting this on Reddit only to get crickets. Might be more complicated than I realize. Maybe someone here can help. Thanks in advance!


I've got a Randall C170 from 1992 that is acting funny. Hoping someone with more electrical engineering knowledge than myself can shed some light on what might be going on.

Symptoms: There is a semi-clean signal that is coming through the amp at all times independent of the channel selection, gain, or volume knob position. It does not get louder or quieter with volume knob adjustment. It is not the clean channel leaking into the dirty channel. You can hear that the unwanted semi-clean signal comes through on top of the clean channel when selected. It is loud enough to be a problem.

My guess: Somehow the signal direct from the input jack is shorting to ground. It's clearly not passing through the preamp gain stages. If it does not change with the volume knob, it has to be sneaking in somewhere and bypassing everything except the power amp. Does this sound at all possible?

Anyone have any ideas?

I already opened it up and used Deoxit on all pots and jacks. The amp was acting funky in a different way before I cleaned it out. It was compressing very hard (I thought I was hitting the input too hard with the stacked gain stages in front of it) and the EQ pots clearly had build up in them. The bass pot had dropouts which are now gone after contact cleaner. I do not remember the amp having this signal leak issue before that but I cannot confirm. I am adding this just to make sure you have all pieces of the puzzle in this mystery. Could I have messed up a connection with Deoxit? I was quite careful not to poke around and move any components. I am not ruling out that I could have caused this new issue. But since I have no idea where to start on troubleshooting, I have no idea if this is my fault or not.

I am skilled enough to replace any component in the amp. I do not have a scope, the expertise or other diagnostic equipment to figure out the problem on my own.



Link to the Reddit post w/ video: https://www.reddit.com/r/GuitarAmps/..._to_power_amp/

[g1]

You will need to find and post the schematic for this model. Not much can be done without it, unless you are at the skill level where you can draw it up yourself.

[aczfenderstrats]

Here is the schematic.

Another thread where someone took photos inside. If anyone needs me to take a photo of a specific section, let me know. https://music-electronics-forum.com/...amp-schematics

[x-pro]

First, check if the "Master" control and the whole termination amplifier are working. To do this, connect any signal source from any guitar console to the "Return" input.

[aczfenderstrats]

Plugging in to the return seems to be working properly. No weird noises. Just clean signal.

[x-pro]

There is a semi-clean signal that is coming through the amp at all times independent of the channel selection, gain, or volume knob position.

Then why is it not regulated by the "Master" resistor?

[aczfenderstrats]

I am not sure how to answer that question. I am still no closer to figuring out what might be going on here.

[Pixel]

I am not sure how to answer that question. I am still no closer to figuring out what might be going on here.

Now that you have the schematic you can look at every single wire in the amp and see if they are going to and from the right places. Without a scope, expertise or diagnostic equipment there isn't a lot you can do otherwise.

[g1]

I am not sure how to answer that question. I am still no closer to figuring out what might be going on here.

I think there is some confusion over which control you meant when you said 'volume knob' in the 1st post. The master volume pot is after the FX return jack. So turning the master all the way down should kill any sound from the preamp, including the 'leakage'.

[Helmholtz]

There's another "master" pot in the gain channel.

[aczfenderstrats]

I have found the source of confusion here. This schematic is labeled poorly. It is actually for the C-170 II. I have the C-170 I which does NOT have a master volume. I am trying to find a schematic for version 1 but I am not having much luck so far...

Additionally, in my testing, I am starting to think the leak is the clean channel leaking in to the dirty channel. When on the dirty channel, the leaking tone sounds exactly like the clean channel when I switch over. When I'm on the clean channel, the leak is occurring but appears to just get louder when turning up the volume pot. Tone does not change. The dirty channel has "dirty channel tone" leaking with the volume all the way down but goes away when you switch to the clean channel. It seems like the clean channel is leaking on both channel settings.

Could this be the front mounted switch? How can I completely disable the clean channel to test?

[x-pro]

The field-effect transistor after the volume control may be defective.

[Marcio_Luthier_Brasil]

Hello friends from Brazil, I'm new here, I'm trying to help my friend, I redid the diagram following and tracing the Randall 170i plate, I found discrepancies
The connection of the fets was wrong, all the modifications in relation to the original drawing are in blue, I numbered the components, I hope it helps people in the future.

[Marcio_Luthier_Brasil]

board drawing to help