TH30C Channel switching not working

[Jowl]

Hello! I bought a th30c used and have used it for about 3-4 hours tops. Today when I turned it on, the dirty channel worked as usual, but when switching to the clean channel, the dirty channel was still on. I tried switching multiple times, but no change. I also tried switching between half power and 4 tubes/2 tubes, tried external channel switch box, but the problem persists. Does anyone know what the problem may be? Thanks in advance!

 

Update: I also noticed now that the volume sometimes changes up and down when trying to change channels, although the channel still stays on dirty no matter what...

[WRGKMC]

Are you using the front panel switch or foot pedal?

 

Sounds like you're trying to use two switches at once. According to the manual:

 

Using the Footswitch The Switch on the front of the amp must be set to Dirty Channel for the footswitch to function. This feature ensures the user will always be able to switch channels even if there is damage to the footswitch jack.

 

[Jowl]

I never used the footswitch until I ran into this problem. I always used the switch on the amp, but when it stopped working I tried connecting the footswitch as instructed in the manual. However no matter what I do it doesn't change channels, only raises and lowers volume half of the time

[WRGKMC]

It might be the switch itself. The rockers inside the switch may not be toggling properly or making a good contact. Many of the new switches are made of plastic (instead of Bakelite) and the contacts can be melted easily when the wires were soldered to it. The contacts are imbedded in the plastic case and provide a contact for the rockers inside. If over heated they can move they're position and provide a poor contact when the switch is toggled. The symptom you have suggests the switch might be making 1/2 a contact or not fully switching.

 

What I suggest you try first is lay the amp on its back and wrap the switch with a towel. Then force some contact cleaner down its shaft while clicking the switch. The towel will prevent the liquid from getting on the face plate.

 

I normally advise using a non lubricating contact cleaner for this job, but given the symptom, the switch may be hanging up inside so a lubricating pot cleaner might work better in this case. The switch is low voltage so its shouldn't arc out. You have to spray very slowly and essentially drip the liquid down the shaft into the switch which can take awhile for it to work itself down to the contacts.

 

The only other options are to open the switch and manually check it or replace it. I'd get out an ohm meter and check to see if its working first.

With the wires connected you should read something with the switch open but it should switch to zero ohm when toggled. Its the zero ohm reading which is important. If this is a double pole single throw (as I suspect) it has two contacts so you need to measure vertically between the two.

 

Taking a switch apart depends on the type. If its a classic type, you have metal tabs which can be bent to open it. Beware. The toggle itself contains small springs and plastic tips on the ends of the springs. The rockers just sty in place by the springs. The springs tend to launch themselves across the room when opened so you only have on shot at opening it and not loosing the small parts. Once open you can clean the contacts with ultra fine sand paper then reassemble.

 

If the switch is modern its likely riveted together. Replacement is the only good option if cleaning doesn't work. I have had success where heating the contacts with a soldering iron has transmitted enough heat inside to restore. You basically heat the contact till the plastic just begins to belt then nudge it in a micron or two. This is tricky because you can easily destroy the switch, but I've had it work on many occasions as a last ditch effort before replacement.

 

If you do replace it be sure to heat sink the connections with a hemostat. the temp it takes to solder wires is greater then the temp it takes to melt the plastic around the contacts. Without heat sinking you'll never get the contacts hot enough to solder the wires without destroying the switch. Blame it on modern junk parts. The old Bakelite did melt and those switches lasted for a hundred years. The new plastic junk you're lucky if you can get it soldered in place and still work, no less last a few years.