68 'twin Vibrato

[tommy d]

This amp has spent 20 ears as a well maintined bass amp in head form using pretty much the dry channel exclusively. I bought a Traynor YBA-1A and have decided to restore the Twin on the Vibrato channel and use it for guitar.

The reverb works but is a little noisy but the vibrato is dead, meaning that the nobs work (I hear a slight volume change) but no vibrato effect. Is this most likely the LDR? How do I test it?

The pedal is a reissue and when I hook it up to the amp, the reverb channel is very noisy unless the single lead labeled REV goes to the Vibrato RCA and the dual VIB lead to Reverb RCA. Is my pedal wired wrong? Backwards right? Sounds like a ground loop to me can I drop a cap or diode to separate. See pictures for a thousand words.

http://picasaweb.google.com/dunham.t...y=qkUSsgqwrZw#

Thanks!

[Dave Curtis, dB AudioTech]

The Tremolo won't work unless the RCA jack is shorted. The Reverb stops working when its RCA jack is shorted. That's how the switching works on these. Forget the footswitch for now and short the trem jack (short jumper, shorting plug, screwdriver, etc) to see if it'll work. Yes, it could be a dead roach, but also could be other things...

[MWJB]

"Is this most likely the LDR? How do I test it?" Could be. Fire up the amp, chassis out, with speaker load connected, with the trem engaged (RCA shorted), turn off the lights - is there flashing from the LDR? If not replace the assembly. Also check for broken/failed oscillator caps (.02, .01, .01) and cathode bypass caps (25v/25uf). In fact, you may as well replace the lot in one go (as well as power supply, bias supply & other cathode bypass electrolytics) rather than do 3 separate jobs.

"Is my pedal wired wrong?" sounds like it, sheilded lead goes to reverb, non shielded to trem.

[tommy d]

The pedal is out of the equation. I tested the amp shorted as specified and I have the light but it does not flash. What does this mean?

Thanks!

[MWJB]

It's not oscillating, replace the caps mentioned.

[Mars Amp Repair]

I haven't looked at the schemo, but if this amp has those white electrolytic caps, then most likely the 5uf @ 25V on in the tremolo ckt is shorted. I'd replace all the white ones as a preventative action. Also, replace the cap with the same 5uf value (or close like 4.7uf as it will change the tremolo frequency...betcha can't guess how I found that out ;-]

glen

[tommy d]

I went to Ace Electronics in Houston and picked up a 25mf 100v tant cap (it was all they had in 25mf) and the bias and rectifier e-caps too.

The tremolo was ticking with a Sovtek tube so I tried a fresh JJ and no ticking! Sounds great!

However, compared to Music Man 2-10 65 tremolo, the lowest Twin setting is kinda fast! The current config was tested and found to be stock. Can I run 1 meg resistor in series to make the pot a 4 meg to 1.1 meg pot?

thanks again,

Tom

[MWJB]

To slow down the vibrato replace one of the 0.01uf oscillator caps with another 0.02uf...you could feasibly do both, but I'd do them one at a time & listen in between.

[tommy d]

A single substitution of .02 for .01 did the trick. The Twin rides again as a sweet guitar amp. Thanks for the help everybody - total cost $8.32

[Dave Curtis, dB AudioTech]

I went to Ace Electronics in Houston and picked up a 25mf 100v tant cap (it was all they had in 25mf) and the bias and rectifier e-caps too.

Radio Shack should have a 22uF electrolytic (Nichicon).

The tremolo was ticking with a Sovtek tube so I tried a fresh JJ and no ticking! Sounds great!

The official Fender solution (Service Bulletin number 9):
******************************************************** *
"The ticking caused by the Vibrato is caused by improper lead dress.

It can almost be "cured" by connecting a .01 mfd 600 volt mylar capacitor
on the 10 meg ohm resistor in the vibrato circuit. this resistor is located
on the parts panel. Remove capacitor across the 10 meg ohm resistor (old
modification) if in place."

(IOW, Solder the cap from the junction of the 10 meg resistor and the
optoisolator to ground.)

"If this does not produce the desired results, then the leads should be
dressed as follows and excessive lengths shortened.

1. Dress the leads to the vibrato speed and intensity controls away from
the tone controls and filter leads.

2. "Bunch" the leads to the components on the parts panel which connect to
the tube socket of the 7025 (12AX7) vibrato tube."
******************************************************** **

However, compared to Music Man 2-10 65 tremolo, the lowest Twin setting is kinda fast! The current config was tested and found to be stock. Can I run 1 meg resistor in series to make the pot a 4 meg to 1.1 meg pot?

There is a trimpot on the ckt board that "fine tunes" the trem on the MM; at least my 212-65 had it. See attached pic. I was able to get dual-volume oscillations by adjustment; pretty cool. I miss that amp. If you have another trimpot on the left side of the board between those 2 resistors, leave it alone; that's the bias pot.

[tommy d]

Thanks for the info Dave and I'll the give Tremolo adjustment a shot on both amps. I am a sucker for slow, rich tremolo with surfy reverb and a strat on the neck p/u.

Last time I was under the hood on the MM, I just adjusted it per the service manual but didn't experiment. I really like the 210-65 (tube PI version). Truly a great amp that a surprising number of guys had, let go and now regret it.

I put the 9 series Accutronics in it and JJ KT77 did the caps, substituted some IC's and lastly threw in a pair Eminence Ram Rods for a real honey of an amp. If anybody is interested there is great thread on LM307h and quad opamp substitutions on the pacair musicman users group. http://www.pacair.com/discus/message...tml?1193000471

this brain candy can really bring your MM to life!

[Dave Curtis, dB AudioTech]

The '68 doesn't have a trimmer on the trem; just the MM. I didn't mean to imply that.

That pic is from one without the 12AX7 driver.

My (well-gigged) 212-65 had the driver tube also, and the stock alnicos sounded nice. I had a pair of G12M-70s in it for a long time. I sold it after I got my '76 Pro Reverb; shoulda kept it. It had a great clean sound and a lot more headroom than the PR; more like a Twin. Now, if I need headroom I bring the VT-22...

I still have a bunch of ICs, also. You just never know.

[tommy d]

Radio Shack should have a 22uF electrolytic (Nichicon).



The official Fender solution (Service Bulletin number 9):
******************************************************** *
"The ticking caused by the Vibrato is caused by improper lead dress.

It can almost be "cured" by connecting a .01 mfd 600 volt mylar capacitor
on the 10 meg ohm resistor in the vibrato circuit. this resistor is located
on the parts panel. Remove capacitor across the 10 meg ohm resistor (old
modification) if in place."

(IOW, Solder the cap from the junction of the 10 meg resistor and the
optoisolator to ground.)

"If this does not produce the desired results, then the leads should be
dressed as follows and excessive lengths shortened.

1. Dress the leads to the vibrato speed and intensity controls away from
the tone controls and filter leads.

2. "Bunch" the leads to the components on the parts panel which connect to
the tube socket of the 7025 (12AX7) vibrato tube."
******************************************************** **



There is a trimpot on the ckt board that "fine tunes" the trem on the MM; at least my 212-65 had it. See attached pic. I was able to get dual-volume oscillations by adjustment; pretty cool. I miss that amp. If you have another trimpot on the left side of the board between those 2 resistors, leave it alone; that's the bias pot.

Hi Dave,

I finally got around to the TR vibro mod you described and it works just great. A slight creeping tick remained after all my other fidling so when I was recapping the P/S, I threw in the filter cap. The result was a noticeable improvement and I don't think I will mess the the potentiometer dress because it looks so tidy as is and the tube dressing was bunched pretty well stock.

Thanks again!

PS the MM vibrato trick was a winner too! I sound positively "swampy"

[Dave Curtis, dB AudioTech]

You're very welcome! Glad things came out good.