Fender Brownface Princeton - Intermittent Tremolo

[judgeamps]

Hey all, I have a weird one that I've pretty much given up on at the moment. I've inherited from another repair tech a 1963 Fender Brownface Princeton with tremolo that works intermittently. It will work fine for for 1-2 hours and then stop for awhile.

The previous tech replaced most of the trem circuit except for the intensity and speed pots, but both test fine, and 2 of the .01 caps. Bias is at a good place with the fixed resistor. Voltages are all in the correct range. Plate and cathode voltage do not fluctuate when the effect doesn't work, but do when it does work. The pedal on/off jack seems fine and doesn't short to ground. There's no pedal plugged in.

Chopsticking the amp, I did find when I just lightly tapped the plate resistor for the tremolo side of the 12ax7, the trem suddenly started working on 2 different occasions. Just a light tap. I re-flowed solder at that connection, but didn't fix it. It could be a coincidence.

I tightened the pins on the tube socket and re-flowed solder at every connection in the circuit.

Any ideas? Thanks in advance.

[sluckey]

Plate and cathode voltage do not fluctuate when the effect doesn't work, but do when it does work.

This pic shows all the components that could cause the oscillator to not work. The fiber board could also be conductive. Or poor ground connections.

[PRR]

> I just lightly tapped the plate resistor ....trem suddenly started working ....

So is this a carbon-composition resistor? They go loose-connection when old.

[judgeamps]

Sluckey, thanks for this. I'm going to go through that one by one. A conductive fiber board doesn't make me too excited. This amp has typical Fender ground connections soldered to the chassis without a terminal, just a blob. I'm always skeptical if those are good after 50+ years.

PRR, its a new carbon film resistor replaced by the previous tech.

[sluckey]

Another possibility is a dirty tube socket. Spray some deoxit (or other electronic cleaner) into the socket and insert/remove the tube several times to insure good pin contact.
 

[judgeamps]

I used some deoxit on the socket, re-soldered all ground points, replaced the 2 .01 caps for the hell of it. The effect still comes and goes. Tapping around doesn't seem to make a difference. It worked for about 30 minutes and then stopped. I turned it off for about an hour and this time, it just suddenly started working while I wasn't looking, so no chopsticking. The fiberboard has a few mV DC near the eyelets.

Do I have any reason to suspect the 100k resistor going to ground from the speed pot?

Not sure if its related, but I checked the B+ and it's 410V with a 5y3GT which seems way too high to me. Schematic wants 315V

Thanks again.

[sluckey]

Do I have any reason to suspect the 100k resistor going to ground from the speed pot?

Not sure if its related, but I checked the B+ and it's 410V with a 5y3GT which seems way too high to me. Schematic wants 315V

That 100K is a suspect just as everything else in that cropped schematic. The ground connection for that 100K is also suspect and since the resistor is soldered to the pot body, the mechanical connection provided by the pot bushing/nut is also suspect. I'd loosen the nut and retighten it securely.

The oscillator requires a strong 12AX7 to work consistently, so roll a few known good ones in that socket.

I don't suspect the high B+ but it seems you may have a new production 5Y3. If so, replace with a NOS 5Y3 to lower the B+.

[judgeamps]

I moved the 100k resistor from the speed pot ground to share the same ground point as the cathode. It read as almost 120k so I replaced with a new one for now. I did try a few new JJ 12ax7's and that didn't help. The 5y3 is an old stock RCA. Will report back.

Thanks, sluckey.

[judgeamps]

I left the amp on for over 2 hours yesterday and it's still working. Turned it on today and it's still good. Crossing my fingers. I'll follow up if it stops again.

Summary: solution was to move the 100k resistor from the speed pot to a different ground point, in this case, I ran a wire to the point where the trem side of the 12ax7 cathode is grounded.

[pdf64]

I moved the 100k resistor from the speed pot ground to share the same ground point as the cathode. It read as almost 120k...

That resistor seems a likely suspect as being the root cause; surely it was an original, CC type?

[drew]

The 5y3 is an old stock RCA. Will report back.

Could someone have put a blackface Princeton power transformer in it?

[judgeamps]

I was feeling pretty good about this fix, but the customer brought it back and says its not working again. How extreme would the fiber board conductivity have to be to cause this?

PDF64, I did replace that original carbon comp resistor.
drew, the transformer appears to be original. I'll double check the code, though.

[sluckey]

Replace the 3300Ω and 25µF cathode RC with a 5mm red LED. Connect the LED cathode to ground. Any better?

[tubeswell]

... its a new carbon film resistor replaced by the previous tech.

It could still have a bad/intermittent lead connection, especially if you tapped it and it started working...

[judgeamps]

Sluckey, the LED didn't help the problem.
Tubeswell, I went back and re-soldered and checked every connection in the trem circuit, no luck.

I did notice this time that the plate and cathode voltages were off. They were definitely correct previously. Couldn't spot a problem, but I thought I saw some carbon buildup in the tube socket between pins 1 and 2, but I had never seen that much before in a pre-amp socket. I scrubbed the hell out of it and it's back to working. Not sure for how long, though.

[tubeswell]

Tubeswell, I went back and re-soldered and checked every connection in the trem circuit, no luck.

What I'm talking about is the resistor lead's connection(s) to the resistive material in the resistor (not the connection to the board eyelet at the other end of the resistor lead)

[acheld]

Is this another fibreboard failure?