What is the common failure mode for the Fender opto-isolator?

[AZJimC]

I've got a 72 Super Reverb, and have been looking over the trem circuit. The oscillator is doing it's thing, but symptom is very weak trem. At full intensity there is barely a discernible change when active. I have peeked into the end of the opto-isolator and can see the neon flashing, responding to the speed control as it should. It seems as if the photo resistor has stopped responding to the light. I also plugged the end with a q-tip to make sure the light coming in was not an issue.

Is this common, and/or are there any tricks to wake these things up? Are there any other components that might be contributing to this?

It is a AB568 circuit I believe.

[P Batty]

Very common, the LDR in the opto-isolator loses its dark resistance over time, the same thing happens to old lightmeters. If you want to fool around with fixing your roach,  Radio Shack has a 5 pack for $4 http://www.radioshack.com/cds-photoresistors-5-pack/2761657.html#.VM15AnZVZZc otherwise Doug has the whole assembly for $11.

[AZJimC]

Very common, ...

As I suspected, I guess I could check a local RS for stock, and give it a shot, as long as I don't break the wires off the neon... or get together an order for Doug next week.

Another option would be to order an ab763 board from Doug, and wrangle the thing into total submission. my only hesitance is that the amp is relatively untouched, 72 inside. I've never even played a true blackface, so I don't really know how great that would be. Reckon the demand for a silver face will ever come up, once most of them are "blackfaced"? Would likely have more value as a Hoffman board handwire currently, but you never know the future.

[EL34]

The LDR is very specific and not easy to find one that works the same
It's not worth messing with them


I know, I built them 15 years ago from scratch and it took 20 different combos of bulbc and LDr's before I got one that sounded good


I sold the formula to New Sensor and now I buy them from them



I stock new ones on this page
http://hoffmanamps.com/MyStore/catalog/Lamps.htm

[alerich]

As an aside, another common failure/problem with the roach style trems on the old fiber boards of the blackface / silverface era... if you hear a "ticking" sound from the trem it is not unheard of for those boards to become conductive over time. I have seen people solve the ticking problem by lifting the roach leads up off the board and soldering the leads as flying leads.

[P Batty]

The beauty of buying stuff from Doug is that every part he sells is optimal for building guitar amps. But designing the roach is above and beyond the call of duty. I am in awe of your expertise, Doug!

[EL34]

Thanks, but at one time in the 90's the supply dried up everywhere and so I went on a quest to build my own.
No one had them for sale anywhere

I ordered 10+ different lamps and 10+ different LDR's
Then I assembled them all different ways and made notes on which combos had good qualities
Then I assembled them again and again until I found the combo that sounded good

I sold the design to New Sensor in the 90's and they started building them

Now a days they are easy to get, so there's no need to try and build them
It's a very hit and miss affair because the neon lamp and the LDR have to be just right to get a tremolo effect that sounds good

There's a bunch of ways it can work, but not all of them sound good

I still have the ones that did not pass the test in a drawer here