Fender Vibrato Circuit

[roadkingbluzs]

Replaced w/ metal film same values, it meters good, switch-on then out it goes. I will look again tomorrow Bob Thanks Again!!

[roadkingbluzs]

Replaced w/ metal film same values, it meters good, switch-on then out it goes. I will look again tomorrow Bob Thanks Again!!

[selloutrr]

I'm assuming by switching on and then it goes off. you mean the effect and not the amplifier powering off.

have you tested the opto Bug? I have a bug in a bandmaster go on the fritz. Before you order a new one if you confirm all the other parts are good, the voltage is within spec, and the soldering and continuity are correct I'd lean toward the opto.

[roadkingbluzs]

It seems only the 2.2M resistor fails. Every thing seems ok. The optobug seems to be working ,I can see light flashing and at various speeds per the speed control, other than that not sure of other portion/half of the optobug? Isn't it made up of two separate components. How would I test off circuit? Could this be causing the 2.2M R to fail?

[selloutrr]

how is the resistor failing? is it burning up?
have you tried sucking out all the solder from the eyelets it solders to cleaning the eyelet and resoldering?

if you really feel it's a voltage issue use a 1watt resistor but if you are burning resistors. I'd want to know why!

if you have another 1/2w resistor clip on the probes and meter ohms as you heat it up with a hair dryer and make sure you don't have a faulty batch.

is anything shorting under theboard?

Do you have continuity on the board itself? - to much moisture? Dust trapted under the board and sandwiched?

[martin manning]

It seems only the 2.2M resistor fails. Every thing seems ok. The optobug seems to be working ,I can see light flashing and at various speeds per the speed control, other than that not sure of other portion/half of the optobug? Isn't it made up of two separate components. How would I test off circuit? Could this be causing the 2.2M R to fail?

The 2.2M failing is hard to figure- even if you applied the full 410V B+ across it, it would only have to dissipate 0.08W. Has it failed again after you replaced it? Measure the voltage across its connections to see if its something like bias voltage (-42V) with the trem on. Then measure the resistance of a replacement to make sure it is what you think it is. This resistor is there to apply the negative bias voltage to the grids of the oscillator and the driver, putting them into cut-off when the trem is not in use. It also helps the oscillator start quickly when it's switched on (the grids jump to ground potential when the foot switch closes).

If you see the neon bulb flashing then the oscillator is working. The opto-bug thing is just a neon bulb shrink-wrapped to a light-dependent resistor (LDR). You can lift the un-grounded end of the LDR (the lead that is connected to the intensity pot's wiper) and measure the resistance from there to ground with the trem running. The resistance should swing with the lamp's on/off cycle.

[rp]

This might be a good post to ask this: Before I learned to leave good things alone, I would 'upgrade' the vibrato & reverb discs in Fenders with films. I always figured fender used discs here not just for frugality but for space and size limits on the boards. Every time I've replaced the old discs with metal polys it significantly changed the sound for the worse - colder, brighter, thinner, more metallic, lost the holistic quality, and I thought it even changed the speed and intensity. I disliked the speed/ intensity changes too so that implies it's not just drift and differing values, i.e. the new films would have made it dead on spec. I wasn't using hifi caps just average films. I noticed this in a few other amps too, Ampegs and Silvertones. Never once did i think the upgrade successful.

I don't understand this, I revere old parts but discs really are crappy. Anyone else experience this? Any explanation. Anyone else building their vibrato and reverb circuits with films and still getting the traditional big warm full sound?

[roadkingbluzs]

Thank you guys for added trouble shooting info. I am learning quite abit here. I won't be able to spend much time on this today. I did think of something possibly shorting under the board and will try to lift and view with mirror. May end up de-soldering / re-soldering. It possibly may be a bad solder joint somewhere. Amp was previously worked on by others so I will make sure of the circuitry again. The mystery will be solved!!

[roadkingbluzs]

A little help please. I am at replacing the optobug and have purchased one from AES. I have found some info but still a bit confusing. AES bug has a silver dot side with longer leads, is this the tube side ?Thanks Bob

[martin manning]

From AES web page:

[selloutrr]

Silver dot goes toward the tubes

[selloutrr]

lol you beat me lol

[roadkingbluzs]

OKay, LDR to pots , neon (siver dot ) to tubes! Thanks Gents . Hope this does it . Bob

[roadkingbluzs]

OKay, LDR to pots , neon (siver dot ) to tubes! Thanks Gents . Hope this does it . Bob

[roadkingbluzs]

It's a go! Nice and strong. In the replacement process I desoldered and resoldered various points of vibrato circuit. Thanks for all the input. Bob