Champ 12 issues: Won't channel switch among other things.

Wharfcreek

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I recently acquired a Fender Champ 12, and just got around to 'testing' it today. The seller clearly misrepresented it, saying it was a perfectly good 'working as it should' amp with no problems. I took him at his word, paid him his $250, and went on my way. This was perhaps 3 to 4 months ago, and shame on me for taking so long to test it out. But, life got in the way as it often does, so here I am, and sadly things are NOT as represented. To begin, the speaker was blown. VC is locked up completely! So, I had an old 12" Celestion G12 here and into the amp it went. Presto!..... got good output and reasonable tone!!

Unfortunately, still has gremlins, as the overdrive LED is illuminated on the front and the hard-wired channel switch doesn't do a thing. I have no control with the main volume knob either, which is probably a function of the amp being in 'overdrive' mode. The 'Mid-boost' pull knob on the treble control does work, but 'reverb' is almost non-existent. With the control all the way down it's completely gone, and all the way up it's more like what you'd expect from a working amp when the control was somewhere between zero and 1. In other words, you can get a taste of reverb, but that's it!

So, just looking for suggestions here. For the record I did test the 3 tubes; one 6L6 and two 12AX7s. Tubes are fine! I have a nice tube tester from Hickok and I've been using it for years doing HiFi amp work. I'm OK with things like the output section, power supply, and you simple pre-amp sections. But, most of my amp work is all done on HiFi tube power amps, with very little done elsewhere. I do NO 'radio' or 'TV', and when it comes to solid-state stuff I RUN in the other direction. And, honestly, this 'channel switching' stuff and 'overdrive' it a bit out of my wheelhouse as well. So I humbly ask for some further advice here and hopefully someone with some knowledge of this amp and it's circuitry will point me in the right direction. By the way, there's a Schematic for this amp on Doug Hoffman's EL34 world, and I've looked it over to the best of my ability. I'm a little lost in following the signal path as it relates to the 'clean' and 'overdrive' parts. So, this will be a real 'learning experience' for me.


Thanks in advance for the help and consideration!!

Tom D.
 

Skyhook

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So, just looking for suggestions here.
tumblr_inline_poqqrj1c751v4atpr_500.gif
 

Silverface

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Just a couple points-

You can’t test power tubes on a common Hickok or other tube tester. They can tell you if a tube is shorted, but not how “good” it is as they run the plates at a MAXIMUM of 185 volts. Completely useless for quality tests for tube amp use.

but that not this amp’s problem. I just was given one for free by another tech who was ready to use it as a fishing boat anchor. You HAVE to remove all the control nuts/ knobs and fold the chassis back to get at a anything - including checking discharge on the 4th filter cap; a couple…or several…wires to the tube sockets WILL pop out of the crappily soldered PCB and/or the sockets themselves (With almost no space for access to both sides of the PCB); the reverb comes AFTER the power tube and its transformer weirdly seems to also be the output transformer (there are no specific reverb driver or recovery tubes)

— and the switching problems are generally related to the LDR’s they used.

one of the worst physical layouts… and designs Fender ever came up with IMO.

oh - and the only schematic available is partly unreadable, and unless you’re experienced enough to know what some parts values need to be for it to work you are S.O.L.

Good luck! I’ll post more as (or if) I bother to waste much more time on my “gift”and don’t just yank out the Vintage 30 someone installed, tear out the electronics and use the cabinet to hold books😜
 

Wharfcreek

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Silverface and Jon S, I will get more into this in the next day or two and report back. Sadly, I'm not even sure what a 'LDR' is, or where to get one if needed. That schematic ain't the best, but I think I can see most of it if I blow it up as needed. I'm just unfamiliar with a lot of the 'Fender' lingo (LDR?), channel switching and reverb circuitry. Just not stuff I've worked on much at all. In reality this is really my first go-round with it.

Skyhook; very funny!! hahaha I get it, and it may come to that, but not there yet!
 

Lowerleftcoast

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honestly, this 'channel switching' stuff and 'overdrive' it a bit out of my wheelhouse
Relax, there is no voodoo. The switching for overdrive is handled through optoisolators (also known as LDR = light dependent resistor). PN 016282 on the schematic. Each optoisolator has a couple of photo-resistors inside a case with an led lightsource. The photo resistors have a lot of resistance when not exposed to light and little resistance when light is present. The *clean-distort* switch just operates the LED light source for the photo-resistors.

The power for these LED is derived from the voltage at the cathode of the power tube. There is a voltage divider to make the approximate 3.28VDC. Jon wants to know if the voltage is there to power the LED. IOW, is there trouble with the voltage divider or the optoisolator.

So... all these LDRs are doing is allowing signal to be reduced by giving it a path to ground (clean) or maintaining a full strength signal (distortion) by using a very high impedance to ground.

If you google the Fender part number, you will find replacement LDR - optoisolator - Vactrol VTL5C3/2. (AES lists them for $12.)
 

Wharfcreek

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LLC, Thank you so much for all that info. I was wondering why the 'dual cathode resistor' thing. I figured it had to be a voltage divider for something, but couldn't figure out what. Now it makes sense, though I consider it HORRIBLE design now that I understand the purpose. I'm thinking that if the output tube goes bad and current draw through those resistors skyrockets, that could then play hell with those 'optoisolators', yes? Maybe?

Anyway, good news: Per Jon's suggestion (and 'yes', it was on my list of 'next steps' to check), I did do a continuity test for the function of the switch as it connected to the circuit board. There was no switching taking place at the board, and a test of the switch showed it to be good. So, fault in the wire. Turned out to be the outer shielding was broken in more than one area, as I was able to isolate where continuity was restored in the wire, but it still showed some intermittent connectivity. After lopping off about 2' of one end I finally got a good length, about 4' or so, and in redoing the connections on the board and at the switch the 'Channel Switching' now works as it should. Thinking I might drill a hole in the back of the chassis and add a 1/4" Jack and just install a channel switching connection using a conventional guitar cord from there. I'm surprised that there isn't a push-pull switch on the front of this amp for that purpose as well???!!! Oh well......... Cest La Vie'!!

Bottom line to this amp is that now that it's functioning at what I believe is 'properly', I think the Reverb is pretty lame, but overall the tone and 'feel' of the amp is actually pretty good. I don't know if my replacement Celestion speaker may be contributing to some improvement over 'stock', as I never heard it with the original Fender speaker. But, being a 12' vs 8" or 10" (or even the 6" in the Champion 600), this amp delivers some really nice bottom end tone, and still retains much of the characteristic 'Champ' growl that these little SE amps are famous for. And, the 6L6 output tube clearly provides a bit more power and head room than in a basic 'Champ' amp from the silver face or black face era. Sad that the reverb is so poor..... maybe the little 'tank' in this thing is also lacking, and perhaps I can find a replacement for it. But, for now, I'm a 'delay pedal' guy anyway, so no great loss on the reverb. And, the channel switching adds a nice feature that definitely improves versatility of the little thing!! So, sorry Skyhook, but this won't come floating up to the shores of Finland.... or anywhere else, at least for now.

So to finish this for now, and in so much as I'm pretty anal about the aesthetics of my little collection of amps, does anyone know where I might find a proper replacement knob for this amp? It uses the push-on knobs, black plastic, but with a slot in them where a white plastic 'pointer' is stuck into the knob. My knob is there but the white plastic piece is missing. Any thoughts on that?

Guess that's it! Thanks again guys! My 'guitar amp' savvy is up a notch, but still a long way to go. Surprising just how 'different' the Guitar Amp world is from the HiFI amp world!!

Much appreciation, Tom D.
 

schmee

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LLC, Thank you so much for all that info. I was wondering why the 'dual cathode resistor' thing. I figured it had to be a voltage divider for something, but couldn't figure out what. Now it makes sense, though I consider it HORRIBLE design now that I understand the purpose. I'm thinking that if the output tube goes bad and current draw through those resistors skyrockets, that could then play hell with those 'optoisolators', yes? Maybe?

Anyway, good news: Per Jon's suggestion (and 'yes', it was on my list of 'next steps' to check), I did do a continuity test for the function of the switch as it connected to the circuit board. There was no switching taking place at the board, and a test of the switch showed it to be good. So, fault in the wire. Turned out to be the outer shielding was broken in more than one area, as I was able to isolate where continuity was restored in the wire, but it still showed some intermittent connectivity. After lopping off about 2' of one end I finally got a good length, about 4' or so, and in redoing the connections on the board and at the switch the 'Channel Switching' now works as it should. Thinking I might drill a hole in the back of the chassis and add a 1/4" Jack and just install a channel switching connection using a conventional guitar cord from there. I'm surprised that there isn't a push-pull switch on the front of this amp for that purpose as well???!!! Oh well......... Cest La Vie'!!

Bottom line to this amp is that now that it's functioning at what I believe is 'properly', I think the Reverb is pretty lame, but overall the tone and 'feel' of the amp is actually pretty good. I don't know if my replacement Celestion speaker may be contributing to some improvement over 'stock', as I never heard it with the original Fender speaker. But, being a 12' vs 8" or 10" (or even the 6" in the Champion 600), this amp delivers some really nice bottom end tone, and still retains much of the characteristic 'Champ' growl that these little SE amps are famous for. And, the 6L6 output tube clearly provides a bit more power and head room than in a basic 'Champ' amp from the silver face or black face era. Sad that the reverb is so poor..... maybe the little 'tank' in this thing is also lacking, and perhaps I can find a replacement for it. But, for now, I'm a 'delay pedal' guy anyway, so no great loss on the reverb. And, the channel switching adds a nice feature that definitely improves versatility of the little thing!! So, sorry Skyhook, but this won't come floating up to the shores of Finland.... or anywhere else, at least for now.

So to finish this for now, and in so much as I'm pretty anal about the aesthetics of my little collection of amps, does anyone know where I might find a proper replacement knob for this amp? It uses the push-on knobs, black plastic, but with a slot in them where a white plastic 'pointer' is stuck into the knob. My knob is there but the white plastic piece is missing. Any thoughts on that?

Guess that's it! Thanks again guys! My 'guitar amp' savvy is up a notch, but still a long way to go. Surprising just how 'different' the Guitar Amp world is from the HiFI amp world!!

Much appreciation, Tom D.
Those amps are best not using the overdrive channels anyway. I liked mine a lot, cathode bias but with more beef than a Champ. It would keep up with the band at rehearsal ....more or less. Always had a bit of hum to it though, even with new higher value filter caps.
My issue was that I wanted it for use in a duo, fairly quiet music jazzy or bluesy. I couldn't use it with the hum in that situation, but for a band mix It would have been ok.
 

Wharfcreek

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Mine has a bit of hum as well. In pulling the chassis out I noted that the 3 main power caps on the filter board had all been replaced with new. What I found odd was that while I saw a filter cap #4 on the schematic, I didn't see it on the power supply board. I think I saw a mention of the #4 cap being on the tone control board, and buried on the 'under-side' requiring removal of all the knobs and control nuts. I didn't do that, so can't say one way or another. But, if 'hum' is an issue with these, mine shares the symptom.

As to the Reverb, I think any connections are OK. Rather I'd guess it's just a lame tank/design issue. I'd love to hear another similar amp and see if mine's better or worse (in sickness and in health....lol). Either way, I'm probably not going to pursue doing anything more with my amp until I can find a similar unit to compare mine to. No sense in trying to fix what might not be broke. Tone wise, I think the little amp scores pretty high marks in my world. I like these little cathode bias SE amps.
 

Lynxtrap

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Mine has a bit of hum as well. In pulling the chassis out I noted that the 3 main power caps on the filter board had all been replaced with new. What I found odd was that while I saw a filter cap #4 on the schematic, I didn't see it on the power supply board. I think I saw a mention of the #4 cap being on the tone control board, and buried on the 'under-side' requiring removal of all the knobs and control nuts. I didn't do that, so can't say one way or another. But, if 'hum' is an issue with these, mine shares the symptom.

As to the Reverb, I think any connections are OK. Rather I'd guess it's just a lame tank/design issue. I'd love to hear another similar amp and see if mine's better or worse (in sickness and in health....lol). Either way, I'm probably not going to pursue doing anything more with my amp until I can find a similar unit to compare mine to. No sense in trying to fix what might not be broke. Tone wise, I think the little amp scores pretty high marks in my world. I like these little cathode bias SE amps.

Hum is an issue in these. The easiest remedy is to step up the filter caps. I went all the way to 220uF on C101 and C103, and 80uF on C102.

IIRC, at least one of the caps is located on the tone control board.

The reverb? My theory is they did a challenge - the first guy to come up with a functioning reverb using a couple of old springs and a piece of cardboard got that week's Friday off.
 

Wharfcreek

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Pretty funny! I'll 'up' the cap values next time I pull the chassis. It's back together right now and I'm playing it some. Not a bad little amp from a 'player's' perspective. Too bad about the reverb, but like I said, I'm a pedal delay guy anyway. Most of the Champ amps I have don't have reverb anyway, so I don't really miss it or need it. I just wanted to have it working as it should. I think it is!! Poorly!!
 




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