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FDP Forum / Fender Amps: Vintage (before 1985) / Fender champ 12 switching problem help!

mixtery

USA/NYC

Oct 10th, 2008 06:36 PM   Edit   Profile   Print Topic   Search Topic

I found my old Fender champ 12 from the early 80's (it's a black knob version with hardwired footswitch). The red channel won't switch over to the clean channel which is the one I want. I hear a slight volume drop but it's still on the dirty channel. I put new 12ax7's and a new 6L6 in there but the problem is still there. Any ideas? thanks in advance!

VintageJon
Contributing Member
**

USA/Austin Tx

Old World Craftsman vs. New World Order
Oct 10th, 2008 10:18 PM   Edit   Profile   Print Topic   Search Topic

The etches are fragile on the "control PCB" and are suspect.
Also the switch is suspect. Clean it. It's a complex Rivera-Era circuit and as such relies on the switching to respond to very specific voltage levels.
Jon

Steve Dallman
Contributing Member
**

USA

Oct 11th, 2008 11:18 AM   Edit   Profile   Print Topic   Search Topic

I have one of these. Actually, the switching isn't as complicated as some red-knob Fenders. There are two dual Vactrols (an LED and two photo resistors in each) that do the switching and the switching voltage comes from the cathode of the power tube. I'll attach the schematic.

It will take opening it, running it with the amp open, and measurements taken and compared to the voltages on the schematic. If you have not worked on a tube amp, then someone with more technical knowledge will need to look it over.

Champ 12 Schematic

VintageJon
Contributing Member
**

USA/Austin Tx

Old World Craftsman vs. New World Order
Oct 11th, 2008 08:02 PM   Edit   Profile   Print Topic   Search Topic

Steve,
Better schematic than I had! Thanks for the link.

Best,
Jon

mixtery

USA/NYC

Oct 13th, 2008 09:45 AM   Edit   Profile   Print Topic   Search Topic

great link thanks! I've opened up the switch and cleaned it with contact cleaner and that didn't work. I took the amp section out to look at connections inside and they seemed solid. Today I'll be taking a multimeter to the switch wires to see if I get continuity. I think that's the best of my ability. I can do measuring and soldering but I'm not completely comfortable running the amp live and poking around in there.

jazzguitar
Contributing Member
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Delicious tone !

Looking for the perfect job
Oct 13th, 2008 03:42 PM   Edit   Profile   Print Topic   Search Topic

I've had occasions where these Vactrols failed (eg in a Boogie) so I would not rule that out.

Steve Dallman
Contributing Member
**

USA

Oct 13th, 2008 03:55 PM   Edit   Profile   Print Topic   Search Topic

The problem is in the amp, not the switch, in all likelihood.

mixtery

USA/NYC

Oct 13th, 2008 06:04 PM   Edit   Profile   Print Topic   Search Topic

yep, I tested the signal from the switch and got good continuity. So it looks like it's in the amp. These Vactrols, I guess it's time to have the amp worked on. Too bad, it's barely worth the bench time.

(This message was last edited by mixtery at 06:09 PM, Oct 13th, 2008)

VintageJon
Contributing Member
**

USA/Austin Tx

Old World Craftsman vs. New World Order
Oct 13th, 2008 10:17 PM   Edit   Profile   Print Topic   Search Topic

What Jazz and Steve said...

The Vactrols fail occassionally in these. Measure to be sure the anode and cathode of the LED section change with the switch.

Check Q1 as JFETs are even more likey to fail. The 20 VDC at the Q1 Drain that Steve mentioned is a good place to start.

Best Luck,
Jon

t-yard

usa

Aug 16th, 2017 07:26 AM   Edit   Profile   Print Topic   Search Topic

Ancient thread bump. Any idea what vactrols were used in these amps? I'm having switching problems as well and would like to source some replacements.

willie
Contributing Member
********

Too Near Atlanta GA

Amp Tech Emeritus
Aug 16th, 2017 11:59 AM   Edit   Profile   Print Topic   Search Topic

Well...the schematic shows the part number (which is relatively meaningless today) as 025945. The parts list I have on file for the Champe 12 (21-6000) shows the part as 016282. Cross referencing this to my old 2000 FMIC parts book it lists the 016282 as "Optoisolator VTL5C3/2" and it once sold for $8.87. That's all I have on it...

willie

t-yard

usa

Aug 16th, 2017 12:48 PM   Edit   Profile   Print Topic   Search Topic

Thanks!! VTL5C3/2 is what I needed!

willie
Contributing Member
********

Too Near Atlanta GA

Amp Tech Emeritus
Aug 16th, 2017 12:49 PM   Edit   Profile   Print Topic   Search Topic

Glad I could be of service. ;-)

w

Stratopastor

Stockton-on-Tees

now even awesomer
Aug 17th, 2017 02:26 AM   Edit   Profile   Print Topic   Search Topic

for once in human history VintageJon (RIP) got something wrong... this amp came just after the Rivera-era.

willie
Contributing Member
********

Too Near Atlanta GA

Amp Tech Emeritus
Aug 17th, 2017 08:06 AM   Edit   Profile   Print Topic   Search Topic

I think what Jon was referring to was likely the technology in the Champ 12 being decidedly that of Paul Rivera. Though the Champ 12's production years were after what is referred to as the "Rivera Era", his innovations did continue after his time at Fender. In this case the use of VacTec Led/ldr switching was indeed a Paul Rivera thing.

willie

Stratopastor

Stockton-on-Tees

now even awesomer
Aug 17th, 2017 03:34 PM   Edit   Profile   Print Topic   Search Topic

Fair enough.

FDP Forum / Fender Amps: Vintage (before 1985) / Fender champ 12 switching problem help!




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