banging my head on this one

[selloutrr]

I rebuilt a fender champ 5F1 a couple months back. I had the customer drop it by for a quick look at how the caps restoration was holding up. ( sprague epoxied inside the vintage shell) he says yeah it sounds great i moved a wire that was long now it squeels. (WTF!) ok so i plug it in turn the volume pot it warms up an squeals. I have tested each part traced wiring and tested voltage as well as swapping the preamp tube. testing the 6v6 and rec tube on the amplitrex. moving wire dressing with an orange stick with squealing and checking the inputs for shorting. I'm at a loss. I'm going to give him a call and see exactly why he was digging around inside it. i have a feeling it got dropped.

any brain storming for the next plan of attack i'll figure it out by tonight I want to go fishing tomorrow !!! i'll let you know if i make any break threw

[selloutrr]

i think i made a break threw

i disconnected the wire from the transformer circled in green

the client resoldered the orange wire at pin 8 of the preamp tube
that connects between the 1500ohm the 22K resistor

the 22k resistor tested good when out of the circuit but i'm thinking under current it's a dud.

with the OT wire disconnected it's silent.
( still passes audio at the input jack )

INPUT?

[RB]

What you have done is remove the negative fedback and that stoped the oscillation. the big queston is what made it start oscillatioing or was it always doing it. You can swap the OT primary leads and hook the 22k back up and see if it has stopped. I dont think the resistor if failing as there is not much current going through it. What wire did you shorten?

If swapping the OT primary leads fixes it, It begs the question how did it get switched to cause this condition that should have existed from the beginnig if nothing was actually changed since it was deleivered to the client?

Good Luck

[selloutrr]

I'm still confussed as to what the client did exactly the wires weren't long he said he resoldered it. I'm just not sure why, i'm wondering if something happened like abuse. The transformers red and blue are hooked up as they came from fender and worked when delivered to the client. though i will try swapping them anything is worth a shot. what's getting me is the amp worked for months and just prior to him fiddling with it.
How much damage if any will keeping the feedback wire disconnected if swapping the OT polarity doesn't fix the squeal?

[RB]

When the NFB circuit is hooked up it oscillates. That means the phase is positive not negative. A resistor will not alter the phase so forget the 22k as suspect . You could hook 3 caps up in series and get it going positive as a cap causes a 60 degree phase shift. Thats why you find 3 in trem circuits providing the 180 degree phase shift to get the positive feed back for the oscillator. So that leaves the OT wired backwards or possibly the OT is shorted in such a way that it is passing signal out of phase compared to a fully functioning OT. I personally have never seen this but that is the only way I can see the NFB feedback being positive unless there is a wire hooked up from somewhere else you cant see

[selloutrr]

thank you i'll swap and see what happens being that everything else is acounted for i'm on board for swapping the OT phase. I wouldnt be surprised if the OT is going though.

[selloutrr]

FIXED!!!

swapped the OT phase and reconnected the neg feedback wire.

it's still boggling how it operated for that long happily.

thank you for the help!

[RB]

After rereading the entire thread my best guress is that there was a bad solder joint at the junction of the 22k,1k5 and the wire going to the cathode and you never had NFB working. These little amps sound good with NFB or with out. When the owner resoldered the wire it let the 22k make contact for the first time and you had feedback and the problem with the OT primary leads reared its ugly head. Or then we could just be orperating in some kind of a space time warp and none of this actually happened. Who knows for sure? I run acroos this type of stuff evry once in while and am amazed it can go undetectd for so long. I had a tweed delux in for a cap job that had all the cathode bypass electrolytics wired backwards and the owner said it had been working fine like that for the last 20 years since he bought it second hand. Tube electronics - is it science, art, or just plain old smoke and mirrors.

[dehughes]

I had a tweed delux in for a cap job that had all the cathode bypass electrolytics wired backwards and the owner said it had been working fine like that for the last 20 years since he bought it second hand. Tube electronics - is it science, art, or just plain old smoke and mirrors.

That's just nuts...man. Ron (rawnster) rightly said that troubleshooting is the hardest part of amp work. NO question. It's what separates the men from the boys, for sure.

[6G6]

I think RB nailed it.
It's a case of one mistake having been covering up another for years.

[Michaelp]

I just wanted to post to let you guys know this thread helped me overcome a feedback problem I encountered with my first tube amp build.

I bought a 5F1 Champ kit from Triode. After building it, I began the phased in power up, tube by tube. Rectifier-preamp-power. After clearing up a wiring oversight, my power tube power up produced a high frequency squeal and a lower frequency howl.

Doing a google search, I found Amp Garage. Then searching in the board, after checking out other threads, I open up this thread and learned about reversing the phase of the OT to the output jack.

Thanks, problem solved. Been playing through the amp for some hours now.
I think I'm good to go.

[Cliff Schecht]

The worst two Fender factory mess-ups that I've been told:
1) Fender 5E3 that had every wire up to a certain point moved over by one turret. Don't remember the bloody details but somehow the amp still managed to make sound, albeit bad sounds. The guy that did the repair says the solder joints were factory and the insides of the amp had already been modified (i.e. complete recap, a few resistor changes). The repairman found the problem after measuring voltages at the socket and seeing that one of the grids was getting a B+ voltage (IIRC)..

2) Don't remember the model but there were two amps worked on, maybe a year apart, that had this same issue. The amps would work for a while but eventually made an intermittent crackling noise. It was only after retouching all of the joints on the first amp that it was found that one of the factory joints looked good but was in fact bad. The resistor lead had some oily residue on it that prevented the solder from ever even tinning the part. One this was fixed the issue never arose again, until the same model from the same era came in (I think both were blackface amps) and was making the same intermittent crackling. Ended up being the same problem.

I'm sure other techs have similar stories too. This isn't a knock on Fender either, they made great amps, but is more just a side-effect of making a lot of stuff by hand like they did back then. I'm surprised at how many ratty, beat to crap Gibsons still fire up and make sound, let alone old quality amps like Fenders..

[selloutrr]

fender has a history of making mistakes.

More then once they have cut the leads on capacitors a 1/4" to short before measuring the turret spacing. Since they are useless they just dumped them in the trash.

The new PCB amps are just sad in the quality control department. I coulod almost make a living on resoldering boards and fixing cold solder points that cause intermitant problems.

[surfsup]

That's just nuts...man. Ron (rawnster) rightly said that troubleshooting is the hardest part of amp work. NO question. It's what separates the men from the boys, for sure.

Uh, yea. I'm still a kid. You guys are amazing. I hope to get there.

[labb]

aaa