Champ 30 DSP Imitating Short Wave Radio

[rjb]

I've got a Fender Champion 30 DSP that recently developed an oscillation problem.

I can't find a schematic for it, and don't have an o'scope- but I'm hoping the symptoms are so distinctive that an old-timer can tell me the cause just from the description.

Symptoms:
Channel select switch set to normal channel. No instrument cable plugged in.
As the volume knob is turned, loud oscillations emanate, sounding like a an old radio being tuned between stations.
The sound is a very high constant pitch, accompanied by a lower pitch that varies up and down as the volume knob is turned.

Turning the treble control to 0 silences the oscillation.
Inserting an instrument cable into Input 1 or 2 silences the oscillation.
Inserting an open T-S plug into Input 1 or 2 has no effect.
Inserting a shorted T-S plug into Input 1 or 2 silences the oscillation.

I have determined that Input 1 has a defective switching jack that is supposed to ground the input when no cable is inserted.
So, I'm going to order and install a new jack.

But my question is: Will the amp really be fixed?
In other words, is the weird oscillation "normal"?

Thanks,
-rb

[Steve Conner]

Well, why do you think a switching jack was used in the first place?

I've had one or two of my homebuilt amps exhibit the same problem, instability with the input left open-circuit. It is something of a nightmare scenario for oscillation. Fixing it took very careful lead dress and ultimately a 47pF capacitor between grid and ground of the first stage.

An instrument cable can squelch the oscillation as it has much more than 47pF of capacitance.

[soundguruman]

You may not need to replace the jack, the soldering connections on the jack are likely cracked...so have a look first.
However, the plastic jacks are damaged quite easily when the guitar cord is yanked out... and it's a common repair to re-solder the board...
The board being static sensitive, will require a static safe soldering station.

[rjb]

You may not need to replace the jack, the soldering connections on the jack are likely cracked...so have a look first.

I've seen that before, have even had to repair cracked pcb tracks. But not this time.

However, the plastic jacks are damaged quite easily when the guitar cord is yanked out... and it's a common repair to re-solder the board...

I can actually see how and why the switch is not closing. The jack is a "not very robust" (AKA crappy) design.

The board being static sensitive, will require a static safe soldering station.

Thanks for the reminder.

-rb

[rjb]

You may not need to replace the jack, the soldering connections on the jack are likely cracked...so have a look first.

Reporting back after repairing the amp. You were right.

Initially, the switch spring seemed weak, plus a bit of the plastic case (flashing?) blocked the contacts from closing. However, after I filed away the plastic and rebent the spring, the switch was still intermittent.

So I removed the pcb from the chassis to access the trace side of the board. I was somewhat surprised to see a gap around each of the jack's four terminals, between the terminal and the solder. I actually pulled the switch from the board without applying heat; there was *no* solder on the terminals. It took quite a bit of cleaning/buffing & flux to get the terminals to take a tin. It looks to me like the jack's terminals were oxidized when the amp was built; the jack was never properly soldered to begin with.

After tinning the terminals, I reinserted the jack, soldered it in, put the whole shebang back together, and the amp works just dandy.


Thanks for you help,
-rb