71 Super Reverb Slow To Engage Vibrato

[Chuck H]

Then check the other components now. Your already halfway through them anyhow!

Check resistors to see if any have drifted way out and, as Enzo said, replace the cathode bypass cap. FWIW I've read on a few occasions that even experienced techs will come up against a trem issue in an old amp and just shotgun the circuit. There's actually a lot happening in an oscillator/tremolo circuit and it's just easier to change every component than it is to check and evaluate each one for replacement. Unless, by some stroke of luck, you happen onto the offending component early in the process. Which, as you've found, doesn't work out often.

Also FWIW the trem I just built does ramp up a little from initial trigger. I thought they all did.?. But you know if yours is behaving differently of course.

[überfuzz]

Also FWIW the trem I just built does ramp up a little from initial trigger. I thought they all did.?. But you know if yours is behaving differently of course.

Are you referring to bias tremolo or bug-tremolo? I always seem to get a 'ramping' tremolo in my bias tremolos. Light but tremolos seems more ON/OFF to me...

[Chuck H]

Are you referring to bias tremolo or bug-tremolo? I always seem to get a 'ramping' tremolo in my bias tremolos. Light but tremolos seems more ON/OFF to me...

I'd have to dig pretty deep to remember the last time I had a bug trem amp in front of me (where I actually used the trem anyway). My amp is a bias wiggle. But I figure since a bias supply doesn't (properly) develop significant current and the oscillators are pretty much the same that all things were equal. I guess not.

[Jazz P Bass]

It's relatively easy to 'see' whether or not the oscillator is running.
Get on the plate end of the 'bug' with your meter (other lead on ground) & set your meter to read Volts ac.
Varying the 'speed' control should change the reading.

[Enzo]

Just replacing every part sounds simple and appealing. But what if the problem isn't a part?

Don't forget the many times techs have found the eyelet board itself has become partially conductive on its surface. This allows stray voltage/current to flow, causing issues.

See the 10M resistor, with the yellow wire connected to the top end? Isn't the yellow wire B+? Set your meter for DC and probe the eyelet board NEXT TO that eyelet. Does ANY voltage register? Put the probe here and there all around the parts in this circuit to see if any stray voltage is on the surface of the black eyelet board material.

[Chuck H]

Surprised "I" didn't think of that!?! I think I've seen a disproportionate number of conductive black fiber circuit boards for the number I've dealt with. Definitely check THAT.

[keithb7]

OP here again. I took it too rehearsal and am home with the amp on my bench again. Taking my time here and will update as I go.
I did find a couple of problems. Yellow wire in photo is tremolo intensity wire. Only 1 single strand is still holding. All other strands were broke off. Old and rotten.
This is through my 5X loupe magnifier. Will go over rest of board too and look for other rotten connections. Maybe this is my problem? Note sure will reflow and try it.
Also found the 100K resistor beside, the 25/25 cathode bypass cap, sharing same solder points, will not settle into a reading with my DMM in auto mode. It's all over the
place. What is that, in series, I think with the cap? Will change and report back.



Kinda makes sense that the 100K resistor, which is very close, if not touching the cathode by pass cap would be cooked. That cap can get pretty hot I believe.
I actually found about another 5 crusty wires, barely left "hanging on by a thread" at the solder joints, at various points on the board. This stuff is old and getting brittle I guess.

[Enzo]

I meant the yellow wire at the top of the board, that quickly disappears through the hole in th chassis.

[52 Bill]

Kinda makes sense that the 100K resistor, which is very close, if not touching the cathode by pass cap would be cooked. That cap can get pretty hot I believe.

Just the opposite, if anything might get hot it is the resistor, not the cap.

[Chuck H]

Already stated, the resistor will get hot, not the cap. You probably couldn't get a steady read on the resistor because the cap is holding a small charge. You can't effectively measure resistance when any voltage is present. This shouldn't happen in perfect theory since it's bridged across it's leads to a ground, but it does happen. You'll need to isolate the resistor from the cap to measure the resistance. That crumbly lead wire DOES need fixing but it probably isn't your problem UNLESS it's also a cold solder joint. Only fixing it will tell, but don't get your hopes up. Now that you're inside with a hot iron close by it should go well enough. Not a lot left to check.

[keithb7]

Chuck, you were spot on with the 100K resistor. Would not read until I isolated it. I am learning plenty here, thanks for your contributions. The 100K resistor was fine once I measured it, after I had replaced it.
I tested the amp again to find that so far no change after the few things I cleaned up. Vibrato slow to kick in at first start up, then immediate activation, and seems to work perfectly normal after initial start up. Will follow more of the suggestions listed tonight after work. I need to make some kind of a jig to run the chassis live outside of the cabinet, so I can take some live measurements. Will get on that next. Thanks again kind folks.

[keithb7]

I think I got it. I changed the middle .01 cap that I had skipped in the vibrato circuit, and now the vibrato kicks in instantly from stone cold. The cap tested fine on my DMM. Maybe it was a solder joint, and I
caught it when I put the new one in and re-soldered everything? Oh well. Whatever. It's done. Thanks for all your help and suggestions. I learned more again during this exercise. I also found other potential
problems with old wires. Here it is as it sits now, working. 3 new orange caps, a 100k resistor, a new by-pass cap and a new roach. Gonna re-flow some joints and wrap it all back up.



You guys who offered help, I respect what you do helping rookies like me. Much appreciated. - Keith

[Chuck H]

It's worth noting that you read elsewhere that replacing the caps fixes this problem. Well, it did. I can't say why three working caps in an oscillator should cause this problem or why three seemingly no more suitable caps should fix it. All I can say is that these things happen. Techs hate it and non tech guru types love it. Tech wins out most of the time but a certain amount of experience helps too in chasing the snarky stuff. Is there a tech explanation? Sure. We'll never know just what it is though. At least not for this specific case. I, for one, am logging this in my cranium for the future. I mentioned that I've read the shotgun repair is popular for old trems. This would support why.

[Justin Thomas]

I can say this: every time I try to slow down the trem in a Fender by increasing those 3 caps, if I go too big & slow, like three .022s, it makes the trem slow to come on. If I go way big, it barely comes on at all. I'd be curious to know the measured values of the three caps?

Justin

Admittedly, that's only three amps so far...

[Pedro Vecino]

I´m sorry, but no method here. Is not possible to deduce the origin. You could reinstall the old three capacitors again. If they are in good condition after extraction (I have never found a bad one at that circuit) and the problem is also solved the origin of problem seem clear.