Testing A Trem Optoisolator

[slorinczi]

Photocell Question for Sunn Transistor-driven Vibrato

Thanks for all the excellent info gents! I'm struggling to fix the vibrato on a Sunn Sceptre. The bulb fires just fine, but the vibrato effect is very weak. I measured the LDR resistance, but under both light and dark it was in the high Megohms, making me think it's gone bad.

Seems like the "standard" VTL5C3 should work, but I do want to make sure I'm in the ballpark as far as voltages go. Excuse my inexperience, but if I'm reading this schemo correctly, the bulb expects to see 4.5V:



According to Vactec, the unit specs are:

LED Reverse Breakdown Voltage: 3.0V
LED Forward Voltage Drop @ 20mA: 2.0V (1.65V typ)

So does this mean I have to "kill" 2.5V or so to avoid burning out the bulb prematurely?

Thanks,
Seth

[loudthud]

The problem with the Sunn circuit is that when the bulb goes bad, it kills the signal because the LDR is in a high impedance state. I have attached a schematic of the later model 190L. The changes that were made didn't change the basic operation of the circuit but the LDR was moved to the next stage between the tube and the tone stack. In this position it is after the Reverb pick-off so the modulated guitar signal isn't going thru the reverb. (That must be kind of strange.)

The driver transistor is probably close to being saturated so the current is set primarily by R114 in the 190L schematic. Adjust it as needed to make the new part work.

[slorinczi]

Hmm, got it, and thanks for the info! Now I'm thinking that I may try to insert a generic (i.e. Radio Shack) CdS part into the photocell circuit and see if that does the trick, given that the bulb fires fine.... Not sure of the specs I need, I'm guessing high MegOhms when dark and under 100K when light?

[Sweetfinger]

It should be possible to rebuild the Sunn photocell if you can save the bulb. Order a replacement photocell for a Morley volume pedal and stuff it inside the old package, or make a new package out of some tape and heat-shrink, or a plastic pen casing..whatever.
Part # R-VPC01
here:
Antique Electronic Supply

[slorinczi]

Thanks much. As it turned out, I did try to rebuild the unit with a new CdS cell, but that didn't improve performance at all. I then carefully rebuilt the old one, and for whatever reason it started working just fine. Now the amp is sold, so it's thankfully not my problem any more!

Thanks,
Seth