Tremolo apparently on, but not in the speaker

[slidincharlie (Carlo P)]

G'day all
I have a '79 Fender Deluxe Reverb amp. The problem lies in the tremolo circuit (or so it seems).
The tremolo is on (optoisolator bulb flashing, faint put-put from the speaker) but there is no tremolo effect coming out when I play my guitar.
I really do not understand...
What should I check?
TIA,
Carlo

[g1]

Does the intensity control affect the level of the "faint put-put" ?
If not, the opto LDR may still be bad on the resistive side.

[BosAmps]

Is the 12AX7 for the oscillator-circuit still OK? It is the hardest working tube in the preamp and perhaps it has become to weak to keep the oscillator running at decent levels... Otherwise the LDR is a weak link in the chain..

[Jazz P Bass]

Yeah.
#1 the oscillator has to be running.

If it is, then the 'roach' is bad.

[slidincharlie (Carlo P)]

Fixed.
The tube was weak, a new tube did the trick.
Yet this very amp has a weak tremolo: even now with a new tube it sounds good (that is, usable at a gig) only with the intensity knob on 10.
I have a couple optoisolator replacements in my parts box - I will see if they are more efficient.
Thanks for the tips.
Carlo

[BosAmps]

In your first post you mentioned that you could see the lightbulb flashing, which also could indicate that you can see the LDR or better: that the LDR (= light depend resistor) is also collecting light when the bulb is off. If it is collecting light, it will have less resistance than 'max' and this might limit the max range of the oscillator circuit..
Perhaps first try to seal the packaging and prevent light from coming into it.. I'm thinking of electricians tape, heat-shrink-tubing, anything that doesn't conduct and keeps the inside DARK!!!

Good luck!!

[slidincharlie (Carlo P)]

In your first post you mentioned that you could see the lightbulb flashing, which also could indicate that you can see the LDR or better: that the LDR (= light depend resistor) is also collecting light when the bulb is off. If it is collecting light, it will have less resistance than 'max' and this might limit the max range of the oscillator circuit..
Perhaps first try to seal the packaging and prevent light from coming into it.. I'm thinking of electricians tape, heat-shrink-tubing, anything that doesn't conduct and keeps the inside DARK!!!
Good luck!!

Thanks for this tip. I was just thinking at this issue this morning, so I opened the tubing apart in all three optoisolators (is this the correct name for the bulb+LDR assembly?) that I have, including this one inside the amp to make better insulation from light.
Anyway original Fender optoisolators are simply enclosed in a piece of shrink tubing, they are not better insulated than these that I have.
Now I wonder... isn't a chassis dark enough? I don't think that the optoisolator can get light from anywhere inside a chassis, or am I worng on this?

[52 Bill]

Hey Carlo! Good to see you back here.

The light that hits the photocell causes the resistance to lower, so if any ambient light gets to it, the overall volume of the amp will be lowered any time the intensity control is turned up.

Try this test. With the vibrato switch turned off (no light flashing), cover the top of the chassis and play through the vibrato channel. While you play turn the vibrato intensity control up and down and listen to see if there is any change in volume. If there is, then the photo cell is low resistance and should be replaced. If there is no change in volume, try uncovering the top of the chassis. If there is any change of volume when you uncover the optoisolator, then there is light getting to the photocell from the ends of the heat shrink tube.

Another problem with the photocells is memory, or the lack of ability to change resistance quickly. A good photocell should change from over a megohm to less than a few k-ohms. And it should do it quickly. If the effect is weak, replacing the photocell or the entire optoisolator for a more modern one will usually improve the depth.

The open ends of the original optos served a purpose. When you look into the extension speaker jack, you can see if the light is flashing without removing the chassis from the cabinet.

[Tom Phillips]

...A good photocell should change from over a megohm to less than a few k-ohms. And it should do it quickly...

Carlo,
Since you have opened the roach packages that you have you can make the readings that 52 Bill described. Just hook your Ohm meter to the Light Dependent Resistor (LDR) module and make a reading with it in the dark and verify that it is a megOhm or higher then expose it to the light and verify that the resistance drops to a few k Ohms or less. Any light will do. You do not need to use the neon bulb. Note to readers: When I use the term LDR I am talking about the two lead Cadmium Sulfide variable resistance device inside the "roach" / "Light bug" module. Photo attached.


CAUTION: The wire lead connections to the LDR are fragile so be careful not to stress the connection points.

[slidincharlie (Carlo P)]

Well well... I've learned quite a few things about optoisolators from this and another thread:
1) how to tell which end of the roach is which (that is, neon bulb vs LDR);
2) light going inside the roach affects its functionality and even the amp's volume.
I did the tests as per 52 Bill's advice, and it seems that the roach is okay.
I have re-assembled with better positioning and tighter shrink tubing two home-made roaches that I had, and have installed one of them in the amp. Now the tremolo works nicely.
Thank you all for the support - you guys on this forum are helpful and nice as ever.
--Carlo