relatively OT: anyone ever replace a Vibrato bug?

ian87 · March 25, 2004, 06:25:04 PM

i have an awesome '60s Bandmaster, but the vibrato doesn't work. some dudes on the Fender Forum said it's most likely the "bug" -- the neon/LDR thing. it flashes OK and reponds to the Speed knob, but no vibe, just ticking.

tube is OK, footswitch is OK, so i'm thinking it's this bug thing.

anyone got any tips, tricks, or experiences to share?

thanks!

ian

The Tone God · March 25, 2004, 06:32:43 PM

If your sure its the "bug" or what it really is called, an optocoupler then you can replace it. Its just a neon bulb on a photocell not unlike the optocouplers people build for phasers and tremolos. It kinda hard to burn those out since the photocell isn't doing much and neon bulb can take teh voltage but it has happen a few time.

Have you look at it through one of the cracks while the amp on to see if the bulb is flashing ? You could cut it open slightly to see if the bulb is pulsing.

You can buy replacements fairly cheaply.

Andrew

ian87 · March 25, 2004, 06:40:32 PM

hey andrew, thanks! the bulb is indeed flashing, and flashes faster and slower according to the Speed pot.

does that mean that it's NOT the bug?

if it's not, i'm at a loss as to what could be the issue.

Paul Marossy · March 25, 2004, 06:40:40 PM

I replaced the one in my '74 Twin Reverb. It was really simple to do. I would do the flash test first, though. Are you sure that your preamp tubes are all good? I would start there before I did anyhing else.

ian87 · March 25, 2004, 06:41:41 PM

Quote from: Paul MarossyI replaced the one in my '74 Twin Reverb. It was really simple to do. I would do the flash test first, though. Are you sure that your preamp tubes are all good? I would start there before I did anyhing else.

yeah, i swapped the Reverb and Vibe tubes (the 'Verb works perfectly), with no success.

Paul Marossy · March 25, 2004, 06:45:39 PM

Probably "the bug" then. Sometimes those stripboards warp and can literally break a component in half. Maybe you have a resistor or something that broke in half... Or maybe that little neon bulb broke.

Another thought is that possibly the coupling cap in that section of the amp is bad.

The Tone God · March 25, 2004, 07:30:41 PM

Quote from: ian87hey andrew, thanks! the bulb is indeed flashing, and flashes faster and slower according to the Speed pot.

does that mean that it's NOT the bug?

if it's not, i'm at a loss as to what could be the issue.

Well the bug has two parts. The neon part which you now report is working. The other portion is the photocell. Potentially you could disconnect that from the circuit, stick a meter on the ends and messure the resistance to see if its changing. If it isn't then the photocell needs to be replace. If you get a changing reading then you have a circuit problem else where.

Another test could be replace the photocell with a pot in the circuit and play with that to hear if there is a change. If it works then the circuit is fine and the photocell is dead.

The oscilator seems to be working so check out the interface from the photocell to the rest of the circuit. You might a break in there or a bad intensity pot, etc.

Andrew

rhinson · March 26, 2004, 01:03:31 AM

hello there, it could be any number of things and all are easy to replace. if you need a new bug, get one from the hoffman amplifiers page---he sells them and they're much cheaper than you can get them elsewhere. also you might need to go ahead and replace both the cathode resistor and bypass cap (22mf/50v) on pin 3 of your vibe tube. if that doesn't work, then suspect the any one of the 3 caps that are connected together (.02, .01, and .01)---usually the .02 is the culprit but could be the others as well. i would do these things first before ordering the roach. if they don't get it, then it's the roach for sure, the photocell gets old and goes bad quite frequently in these older fender amps---i've replaced a bunch of them over the past few yrs. whilst working on amps. hope this helps. rh

zachary vex · March 26, 2004, 01:24:11 AM

you might not have to desolder it to test it. you could turn the amp off, bleed the HV caps if necessary, and clip your multimeter leads (set to high ohms) across the photocell leads (but don't bend or twist them... you might break the unit since they are relatively fragile and brittle.)

take a very bright flashlight or one of those little laser pointers and send some light into the slot on the end of the heatshrink on the bug... see if the resistance goes down. you might be able to purchase a new bug from http://www.tubesandmore.com/ or possibly even newsensor.com.

ian87 · March 26, 2004, 11:25:04 AM

kick ass. thanks to all for your excellent advice. looks like i'll be busy in the lab this weekend! :twisted:

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relatively OT: anyone ever replace a Vibrato bug?