Fender Super Sonic Twin automatic bias failure

[Roberto Lasco]

Hi Guys,
Got this on the bench, with the automatic bias board shot, as it seems.
Stuck with the three lights (cool/normal/warm) on, controls do zilch, Vg also stuck at -74V...you can imagine how great it sounds in a cut off like this.

From what I gathered from the net, there's some issue with these SMD boards; not sure in USA, but I can't get a replacement down here in Brazil, nor I'm able to fix it (don't do SMD stuff).

My question is: what prevents me to lose that crap (I have the owner agreement) and just put a voltage divider on the raw bias supply ?
Except maybe for the "Club" setting that would become pretty overbiased...(it cuts plate voltage in half, so I doubt this sounds good anyway).

Besides disconecting the cable from the SMD board, should I lift the 68K resistors (R101-104) that goes to that board connector ? Anything else could I be missing ?

here's a link for the schematic:
http://support.fender.com/schematics...atic_Rev-B.pdf

thanks for any thoughts,
roberto

[52 Bill]

I've only had to fix one of those so far, the ribbon that connects the switch board to the bias control board was not making good contact. So pushing the buttons didn't change the bias.

Have you tried contacting a Fender dealer in Brazil?

[Roberto Lasco]

Hi Bill,

There's only one here, and his "service center" can't tell what side of a soldering iron to hold. Believe me, that's not an option.

I just tried the voltage divider idea (inserted on the anode side of D16), but I'm sure missing something. It takes about 5 minutes to bias voltage to fully rise on pin 5 of power tubes - which means trouble - especially if stdby switch was not properly used.
But in this amp bias voltage passes through 10M resistors, 22ohm resistors, individual bias caps...