Suggestions for a replacement for an obsolete optoisolator

[Mark Hammer]

Obsessive lurkers here may remember that I purchased a Heath TA-16 guitar amp about 6 weeks ago.  Everything on the amp works fine except for the tremolo.  The optoisolator module for the tremolo is a big black box containing a small incandescent bulb (just like the scematic shows) and a photocell, held in place by a wad of silicon goop.  The module is toasted, so I need to find a suitable replacement, either in a commercial product or via a DIY solution.  It was made by Silonex, but they appear to have stopped making such a unit an awful long time ago.

A look at the LFO portion of the schematic shows it tobe VERY similar to the LFO used in the EA Tremolo (and actually a great many others).  Unfortunately, I'm not well-versed enough in such things to know whether or how an LED might sub for the bulb in the circuit (or what small changes to the circuit might allow for that).

Anbody have any ideas?
SCHEMATIC:   http://harmony.demont.net/documents/schematics/amps/heathkit_TA_16_01.gif

[vendettav]

dotn want to seem a prick but i believe you'd get more help if the schematic wasnt tooo big... my two cents.

i coulnt even find the bulb in there 

[Govmnt_Lacky]

i coulnt even find the bulb in there 

Its the part labeled "LDR."   

Half of the LDR is the bulb. The other half is the Light Dependent Resistor

[vendettav]

lol yeah I know, i meant the schematic was huge to find that part. lol i zoomed out with my browser still couldnt get it to fit my screen

anyways, i guess you could put a DIY LDR/LED unit???

[R.G.]

The bulb drive setup looks very similar to the bulb setup in the Univibe. Since the unit is so physically small, the filament has to be small and hence pretty low power too. I'd get myself a few 12V or less rated grain-of-wheat bulbs and experiment.

If you have to go with an LED, make a DIY opto and tinker with the amount of current the LED gets in the driver til the sensitivity and swing get right.

[Mark Hammer]

dotn want to seem a prick but i believe you'd get more help if the schematic wasnt tooo big... my two cents.

i coulnt even find the bulb in there 

It was actually a lot bigger than I wanted too.  

The bulb drive setup looks very similar to the bulb setup in the Univibe. Since the unit is so physically small, the filament has to be small and hence pretty low power too. I'd get myself a few 12V or less rated grain-of-wheat bulbs and experiment.

If you have to go with an LED, make a DIY opto and tinker with the amount of current the LED gets in the driver til the sensitivity and swing get right.

And I would do that by varying the resistance between V+ and the LED?

[artifus]

have used grain of wheat (gow) bulbs with ldr's before - nice response, softer than leds.

*edit* r.g. beat me to it!

[JKowalski]

A DIY version should work fine... If you are using a LED then remember it's going to respond a little different - you won't have the natural time lag of an incandescent light.

Make sure to increase the current limit for the LED. And finally, you may have to tweak with the bias of the LED and range to get the LDR working within useful parameters - some trial and error.

EDIT: RG won this round...

Whats the deal with the shared 22 ohm resistor in the oscillator section?

[Mark Hammer]

Then I guess it's off to the Rat Shack (or rather, its Canadian replacement) I go.  

[petemoore]

  Yup RG pretty well outlined it.
   Find out about the bulb, try bulbs that fit what the LFO-driver is putting out/use resistors ?
  The old bulb contributed to the ramp rates, LED or new bulb might not match [definitely in that order].
   RS has some bulbs worth trying, automotive might also be a good source [if not physically too large].
  Incandescent bulb and 'bulb-life'...finding a suitable bulb and getting it to light is fairly likely what would put the tremolo right.
   Beyond that a decision about priority of 'vintage Vs. Function'...
  Use of the audio portion of the tremolo would be nice, especially if it works, if not worth replacing the capacitors or getting it to function...an audio injector and tweeking of the R-photo position [vary resistance here with large pot] to validate that signal attenuation is evident when R value changes.
  An LED driver and LFO circuit lifted from chosen tremolo...the tricky part would be if it fits and satisfying the power requirements, cobbling this in there might be the quick way to non-bulb tremolo with ramp-rates geared for LED.

[Hides-His-Eyes]

musikding.de make ready-rolled "bulb" ones for fender amps, I'm sure one of those would work to some extent.

[R.G.]

musikding.de make ready-rolled "bulb" ones for fender amps, I'm sure one of those would work to some extent.

Some of those Fender amps used an LDR and a neon bulb in the opto. A neon bulb is current sensitive for how bright it is, but it won't ignite and star emitting light until you get 75-90V across it in the first place. I haven't seen normal incandescent/LDR optos for Fender amps, but that doesn't mean they don't exist.

[Johan]

EDIT: RG won this round...

...he usually does...

I don't see a reason a LED with appropriate current limit resistor wouldnt work, but the responce might be a lot harder and more abrupt that a bulb. perhaps TWO resistors in series with the LED and a BIG cap striding the LED and one of the resistors as a way of mimic the lag (heat up/cool down) of the bulb?...
like this

>----resistor-----resistor-----LED-------->
                     I                              I
                     I----big cap ----------I

thoughts?
J

[Nasse]

I did a lightbulb tremolo some 30...40 years ago. The circuit I used suggested putting small dc current trough lightbulb (there was something like 1 k potetiometer wired such that it was parallel with transistor circuit driving lightbulb. Instructions said that you shpuld adjust the current such that the bulb does just not light but warms a bit and it had audible difference, it was faster or smoother... I used bicycle front light bulb but nowadays you can not find such bulbs, they are halogen bulbs, dunno if they make difference

Quess drivin the bulb under rated current/voltage gives longer life

[R.G.]

Quess drivin the bulb under rated current/voltage gives longer life

I once heard that the life of an incandescent bulb is inversely proportional to the twelfth power of the applied voltage. So at 100% of voltage, you get 100% of estimated life. At 105% of voltage, you get 1/ (1.05^12) = 0.557, or about half (!) the life for only a 5% increase over nominal voltage. Reducing the voltage gives a similar extension. That's why "long life" bulbs are rated at 130Vac, not 120. They typically have better filament supports as well to reduce mechanical stress.

[petemoore]

  Can you get a reading of the high voltage portion of the LFO swing ?
  careful, we're supposed to be having the other hand in the back pocket when we're in an amp...
 Then find a heavier than normal duty bulb as best you can...hopefully made for 'dim voltages' that has max rated V of a bit more than the high-LFO position reading of it's supply.