70 Siverface Gave up the Ghost

[Carcharodon]

Brand new to the forum, thanks for taking a look. A friend of mine in college wanted my Les Paul, so I sold it to him for cash and a 1970 Fender Champ. This was years ago, and I played the amp from time to time but never really took it out of the house. Two years ago, I got the time to change tubes/speaker bug, so I loaded it with Bendix and Sovtek tubes (still have the original RCAs which are in good working condition). The original speaker had been replaced at some point so I tried out a Weber OEM and really have been happy with the sound. Then a few months ago, I fired it up and it wouldn't fully warm. A bit of smoke later and I've produced a dead amp. My first thought was, I'll Window's it, give it a second and restart, just see if it needs to cool down... nope, wishful thinking. So, I pulled it apart. I found the schematics online and went in to check caps and resistors, and did find one resistor way out of ohm spec and burnt, will replace it but I don't think that was the only problem. I noticed that none of the pilots on the tubes would glow, and measuring voltage across the transformer, I'm not reading anything when powered up. I have a new transformer from Mojo and haven't loaded it in yet.. I wanted to check with the folks here, get some more brains on this.. My question is, if the transformer is good, and the fuse is good (line out from transformer goes to the pilot light/power on bulb and back into the trasnsformer through a fuseable link, and a line from the pilot goes to the 6 V heaters in the tubes) then I should at least see the tubes glow when the amp is powered up? Correct? Do you think that three tubes that I know are good, that refuse to glow during power up is a sign of a bad transformer?

[Robster]

I think your on the right track if you dont have 6 volts coming from the trans, you will have no heaters..Maybe tell us exactly which resistor burned up may help figure out if you have other problems.

[Carcharodon]

Originally Posted by Robster

I think your on the right track if you dont have 6 volts coming from the trans, you will have no heaters..Maybe tell us exactly which resistor burned up may help figure out if you have other problems.

The 1k ohm 1w from the filter cap (the one in series with the 10k ohm 1W), it was reading roughly 220 ohm before and after I removed it from the board. I'm thinking your are right though, no 6V at the pilot, no power from the transformer..

[Groovey Records]

Just be sure to drain those caps every time you go inside .

They can recharge themselves even after you drain them while your testing

[Wally]

Originally Posted by Carcharodon

My question is, if the transformer is good, and the fuse is good (line out from transformer goes to the pilot light/power on bulb and back into the trasnsformer through a fuseable link, and a line from the pilot goes to the 6 V heaters in the tubes) then I should at least see the tubes glow when the amp is powered up?

Welcome, Carchardon.
Reading your description of the wiring leaves me scratching my head. That is not the way a Champ should look, ime.
The wire from the transformer to the pilot light should be green. From there that wire should be connected to the heaters of the 6V6 and the 12AX7.
There should be no fuse in that circuit. The other green goes to ground. Sometimes folks rewire this heater circuit due to noise. THe two greens will go to the pilot light with 100 ohm resistors from those connections to ground....and from the pilot light connection points there will be a green twisted pair going to the heaters of the 6V6 and 12AX7 tubes.

Most of us rewire the power AC in with a 3-way cord to a safer situation, also. White goes directly to one of the black transformer primaries. Green goes to ground. The AC in black wire goes to the fuse, from there to the switch and from there to the other PT primary.

IN an amp of this age, I would suggest replacing the electrolytics before proceeding. The caps need it as of this time, anyway, imho. IF you install a new PT without ascertaining the condition of all else...especially the electrolytics...you are risking taking out a new PT.
You could lift all secondaries from the old PT and check resistances for continuity. Don't forget to check each winding's relationship to ground. The swinding can show proper resistance except a short to ground won't show up until you check for that.

All of this goes with the warning ...dangerous voltages. You have already been in there, so I am hoping that you know the safety precautions.

[Carcharodon]

Originally Posted by Wally

Welcome, Carchardon.
Reading your description of the wiring leaves me scratching my head.

IN an amp of this age, I would suggest replacing the electrolytics before proceeding. The caps need it as of this time, anyway, imho.

All of this goes with the warning ...dangerous voltages. You have already been in there, so I am hoping that you know the safety precautions.

Thanks much for the clarity, yes you are right, the way I described the circuit was indeed confusing. I guess in short what I was trying to relay is that the fuse was good, however the tubes weren't glowing, and there wasn't power at the tranny.

Here is what I know now. I replaced the 1K 1W resistor and the tubes are now aglow without having to replace the transformer. All great, however I am getting a lot of static throught the speaker that wasn't there when the resistor went. Possible that the filter cap needs replacement?

Here is a noob question, the original resistors are carbon composite and the replacement resistor I soldered in today is metal oxide. Any reason that a resistor of the same value but different material could be causing the static from the speaker? I cant think of a reason why that would be, but you never know. Possible that its just the electrolytics needing replacement as mentioned above? Maybe the soldering is bad.. I'm pretty good with that but again, who knows. I thought it's go or no go in terms of soldering, meaning it works or completely doesn't.

Thanks for the heads up on electric danger, two of many things burned into my skull as a kid, don't clear the snow blower with your hand and electricity is invisible but there.

[chabby]

Sounds more like Rivera era champ...they had two fuses inside and when you say "board" it sounds like you mean PCB board, not an eyelet noard like a regular champ. That resisitor could have gone open for a number of reason and without a DMM and some knowledge you are going to end up doing more harm than good. Take your time if you really care about that amp. Sorry, but it sounds like you know just enough to be both dangerous to the amp and possibly yourself too. You don't have to be an EE to be an amp tech, but the questions you are asking and the way you are asking them makes me worry. That amp probably needs rebuilding/refreshing in a big way. I'd replace every resisitor and cap in there. There just aren't that many of them in a champ to mess with anything else. And when you not real knowledgeable, your better of with what Mr. Weber used to call the "Shotgun Approach" just replace everything. Especially since you have a new tranny. From your description there could be so many things wrong, it's not possible to even prognosticate.
Either go buy some tube amp instruction books and devour them, or take it to a good amp tech. The Champ is a wonderful amp to start your journey toward knowledge, but take your time - don't be too impatient. I say this not to criticize you, but to help you. I myself ruined an amp or two because I knew just enough to be dangerous. Of course I could have (and did) fix everyone I messed up eventually. But your descriptions are nowhere near specific enough to do anything but throw out a bunch of wild guesses. No filaments glowing? That could be anything from tubes to tranny and some points in between. Best of luck - get a schematic, a layout and the spec for all the voltages on ever terminal and pin. Then get a digital multimeter and a bunch of books. And get a wire , some alligator clips and some 2.2K 2watt resistors to make a cap bleeder. One end to ground, the other to the positive terminals of the cap. And/or always turn the amp off standby, hten hit the power and strum guitar until you here no sound...then unplug. Of course thats providing you have a cap bleeding mechanism in there. I just leave the cap bleeder installed while I work...even though the caps will be drained in about 30 seconds max.

[Carcharodon]

Originally Posted by chabby

Sounds more like Rivera era champ...they had two fuses inside and when you say "board" it sounds like you mean PCB board, not an eyelet noard like a regular champ.

Sorry, but it sounds like you know just enough to be both dangerous to the amp and possibly yourself too. You don't have to be an EE to be an amp tech, but the questions you are asking and the way you are asking them makes me worry.

Especially since you have a new tranny. From your description there could be so many things wrong, it's not possible to even prognosticate.

Either go buy some tube amp instruction books and devour them, or take it to a good amp tech. The Champ is a wonderful amp to start your journey toward knowledge, but take your time - don't be too impatient. I say this not to criticize you, but to help you. I myself ruined an amp or two because I knew just enough to be dangerous. Of course I could have (and did) fix everyone I messed up eventually.

Rivera era, hmm, no I don't think so, 1970 Silverface with a single slo blo fuse, with the particle eyelet board. Traded out the Creme Brulet torch and tin foil for a DMM and have the operation down to a science, and you are definitely right about not needing to be an EE to work on these.. crack a Kenwood KR 10K. That's EE. I'm biochem by trade so electrolytic means something totally different to me.. you can understand I am sure.

Still alive, thanks for all the folks who have been really concerned about me killing myself is this incredibly complicated piece of equipment. Took a second jaunt through and found that the 2700 ohm resistor was also out of spec.. replaced but the static was still there.

Without resulting to Mr. Weber's shotgun method of wasting perfectly good electrolytics, I replaced my Tung Sol 5 amp tube with the original RCA and perfect tone and sparkle without static is mine! Ive got a very nice Bendix replacement as well but the original RCA is sooo nice. Amp sounds like a non-Rivera era champ all over again. Just a bit of drag at 10 in the guitar when sovtek muff driven and so sweet and sparky under EXG expander. That said, it will remain indoors.. who gigs a Champ anyway?

I didn't replace the tranny, I foolishly purchased a new one but realized I didn't need it after replacing the bad $.10 1k 1W resistor. That closed the eyelet board circuit and powered up the tubes, like originally stated. I'll pack it away for safe keeping.

Laughing a bit at the dangerous to myself and the amp comment.. reminds me of Top Gun.. That's right!! Ice.. man, I am dangerous. I think I will replace the 9V battery on the strat actives just because I am feeling dangerous.

Thanks again.. glad you were able to eventually fix the amps you messed up. This has been a learning experience for me on a few levels for sure.

[Middleman]

Seems I remember the 100k resistors coming out of the reverb section are a notorious source of hiss. Might check those as well.