tremolo "roaches"

[johnhoef]

Hello,everyone,
I am currently working on a Randall commander II. The tremolo does not work due to a faulty "roach". As some of you know, and some may not, the "roach" is a four legged component that contains a light bulb, and a photo resistor. The bulb flashes per the tremolo settings, and the photo resistor controls the volume of the amp. So, to my question....I am having difficulty finding a replacement, and wondering if a Fender roach would work? For that matter, are these roaches all essentially the same for different amps? Wondering if I could make one, but would have to know the specs of the photo resistor and light bulb. Any thoughts on how to fix this amp? Any help would be appreciated..thanks.
John

[Enzo]

Schematic for the amp? I have no idea what sort of lamp the amp circuit expects in there. Fender bugs want a neon lamp that runs in a high voltage circuit (meaning B+) Others might have LEDs, hard to say.

[johnhoef]

Enzo, Thanks for your interest, and I appreciate your help. So far I have been unable to find a schematic for this amp. I did open up the roach, and found an incandescent lamp, and photo resistor. I have tried a few different lamps, and none work. I think they are loading down the lamp driver circuit too much. The original lamp lost it's leads, but I managed to temporarily connect it, and it works normally (the bulb). So, it's a matter of finding the correct bulb. The photo resistor is changing in resistance value as I shine light upon it using my ohmmeter, however, in the circuit it has no effect, so I suspect this is the reason why the tremolo did not work.

[Mick Bailey]

From the description I expect it's the same device fitted to a number of 70s Randall amps - see post #33 here
https://music-electronics-forum.com/...ad.php?t=16112

[johnhoef]

Mick,
Thanks for that link...I'm afraid it doesn't help me much. Interesting reading, though. I understand the Randall amps are similar to Fender in design, so perhaps a fender device will work. Another thought is to experiment with a pot in place of the photo resistor to find the optimal resistance range for good tremolo, then find a photo resistor that works in that range. Next, through experiment, find a good light range that works...find a bulb...etc, etc. A lot of work! Sure would be nice to find the correct device and simply solder it in - done!
Any further help is greatly appreciated!

[olddawg]

I’ve seen techs make them with a lamp and a photo resistor inside a piece of heat shrink. I’ve also seen an led and a resistor substituted for the lamp. It usually just takes finding the right value resistor. You should be able to measure the lamp power supply voltage? Also.. rule one in fixing Tremelo circuits is replacing all of the electrolytic caps. I’m not a big fan of shotgunning, but it’s usually necessary imho...

[Randall]

Is it a RG-120, is so here is the schematic. Not much help though, no values listed.

https://music-electronics-forum.com/...7&d=1258694524

[johnhoef]

Randall,
I finally did find the schematic, and it's the very same one you just sent me...thanks anyway for that. This amp, contrary to the Fenders, uses an incandescent bulb instead of a neon bulb. I did find a replacement "roach", but it has the neon bulb in it. So, still no solution other than making one myself which I will probably end up doing.

Olddawg, I agree with you in replacing the electrolytics in the tremolo circuit....may as well as long as I'm in there.

Stay tuned.........

[Randall]

I've made them with white LEDs and a photocell under shrink wrap when dealing with low voltage supplies.

[olddawg]

Why not take a white, bright LED in series a pot wired like reostat and slowly turn the pot from max resistance until the LED flashes. Measure the pot and replace it with a resistor that close. Then use it with the photocell half of the bug in some heat shrink.??? Quick, easy, and dirty.. lol.

[johnhoef]

Olddawg,
Ha! I had to laugh with your suggestion. This is exactly what I did last night. Indeed, a white LED with a photo resistor. First, I put a pot across where the photo resistor was connected, to see what resistance range is necessary. I found one in my collection that was just right. Then, I found the blinker output needs to see a certain load, so put the LED across those two connections, then put a pot in parallel, and adjusted until the blinker worked just right, coinciding with the duration control. Then, I had to put the pot in series with this, so the intensity was just right. And indeed, it works nicely again. If any of you are interested, I can provide a circuit diagram of my home made "roach" if any of you are confronted with this. Keep in mind, this design is likely only going to work with Randall amps. Maybe other amps that use incandescent bulbs in the roach.

[patlaw]

Yes, please post the details.

[johnhoef]

Patlaw,
Glad to post my design. Keep in mind, you may have to make adjustments with these resistor values, considering what type of photoresistor you use, and the type of LED. I simply looked through my photoresistor collection and picked one that was in the correct resistance range. I'm afraid I do not recall the value, but I think it was 0 - 2k. R1 is the resistance the amp wanted to see, and R2 adjusts the brightness of the LED.

[Randall]

What voltage is feeding the LED?

[johnhoef]

Randall,
Sorry, I never did measure that voltage. I simply put a pot in series with the LED, and adjusted it for best performance. Then just measured the pot, and replaced it with a resistor of that value. It was quite dim to my surprise, due to the characteristics of the photoresistor along with the close proximity between the photoresistor and LED. I suspect if anyone tries to do this, the photoresistor they choose will likely have different characteristics than the one I used, so they will have to experiment like I did. The LED I used is of the standard super bright variety, but now in hindsight, I could have used a normal brightness one.