Marshall 2205 doesn't switch out of the clean channel

[kevania]

Gentlemen, I just inherited a 2205 head. It's dated July 1987 and has the later style pcb with the IC for channel switching. The amp sounds good in the clean channel and the reverb can be selected with the footswitch which sounds ok as well. Main problem is that it won't switch into the boost channel. When there is no footswitch pedal plugged in it is running on the clean channel. When the boost channel is selected it gets louder but doesn't get any overdrive or distortion. Also, the red led on the front never comes on. (I thought it would run on the boost channel even without the pedal plugged in......) 1) I'll be diving in this weekend and was wondering if any body here has been down this path already with a 2205/2210? 2) I ran across the Circuitrix site which offers a tweaking kit for this amp that uses a relay for the switching as well as offering a change to the gain structure in both the clean and boost channels. Has anyone tried his kit?

I can add pictures over the weekend

[iknowjohnny]

Sounds like it could be a bad diode, as it uses diode clipping. Anyways, should be easy to troubleshoot. Just hope it's not that IC.
Heres the schematic...
http://www.schematicheaven.com/marshall ... w_2205.pdf

[kevania]

Well, I cleaned the contacts and the channel light now comes on but still wasn't getting any sound out of the boost channel. I started playing with the knobs to double check settings and found that pulling on the gain knob brought the boost channel to life. It moved a lot too, like an eighth of an inch. most of the pots have quite a bit of play in and out....probably from someone pulling the knobs off.

Thanks for taking a look at the post and thanks IKJ for the schem. Once I get the basic maintenance done I'll start looking to tweak the tone a little......see you then.

[Structo]

I would look inside and make sure the pots are not coming apart.
Sometimes when you have to push a knob on hard to get it to go all the way on, it can bend the tabs holding the pot together.

I've had one come completely apart once when trying to push on a knob.

I think it is a metric/ standard thing.

Best solution is to measure the shaft and the hole in the knob.

A lot of those aftermarket Marshall knobs need to be drilled out with 1/4" drill bit before they will go on standard sized shafts.

[kevania]

Did you get the pot back together or replace it? This one is a double pot.

[Structo]

I replaced it.

Another time I was tightening down one of two bias pots in my amp.
These are high quality PEC pots.

They have the tab that is supposed to anchor them when you tighten them but I didn't feel like drilling the small hole for the tab so I just bent the tab out of the way.

Well, I was tightening the nut and the whole pot turned, pulling on the resistors I had soldered to it. I tried to fix it but the resistance didn't track right after that.
Basically it ruined it.
So rather than take a chance I ordered two new ones and this time drilled the hole for the tab. They put those there for a purpose which I had ignored before.

If your pot just slightly bent the tabs holding it together you should be able to squeeze the tabs back into place securing the back into place.

Again, check the fit of the knobs, that is the biggest reason when the pots push apart.