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Thread: tremolo "roaches"

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    tremolo "roaches"

    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

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    Lifetime Member Enzo's Avatar
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    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.

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    Education is what you're left with after you have forgotten what you have learned.

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    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.

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    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

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    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!

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    Old Timer olddawg's Avatar
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    Iíve seen techs make them with a lamp and a photo resistor inside a piece of heat shrink. Iíve 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. Iím not a big fan of shotgunning, but itís usually necessary imho...

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    Supporting Member Randall's Avatar
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    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

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    It's weird, because it WAS working fine.....

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    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.........

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

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    It's weird, because it WAS working fine.....

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    Old Timer olddawg's Avatar
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    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.

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    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.

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    Member patlaw's Avatar
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    Yes, please post the details.

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    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.

    Click image for larger version. 

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    Supporting Member Randall's Avatar
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    What voltage is feeding the LED?

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    It's weird, because it WAS working fine.....

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    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.

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    Old Timer J M Fahey's Avatar
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    Glad you solved it.

    In this particular case, we need 2 resistors.

    * a lowish value one, to make oscillator think itīs loaded with a filament lamp.

    Maybe somebody finds and posts the specs for **real old** optocouplers, with a real filament lamp inside (Gibson used similar ones)

    In principle, and considering we want low filament currents to make them easier to drive, I guess they used the famous (in my youth) "grain of wheat" lamps or their smaller oriental cousin, grain of rice:

    but until you get one (search around, they are still availble), a 12V/30mA resistor will be a good impersonator.
    Meaning 12V/.03A=400 ohm resistor.
    You used 1200 ohm, clearly oscillator is happy with some 10mA ... so ... who am I to disagree?

    * a highish one, in series with LED

    You used a modern high efficiency Led, I am surprised at the very low current, but these are very very efficient.

    I bet a cheap "special mixed bag of the week" LED will require way higher current, but feel free to experiment.

    * that said, it would be nice to get a real grain of **** bulb someday and make vintage perfect roaches.

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    Juan Manuel Fahey

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    JM,
    I used an 1800 ohm resistor, not 1200....and I agree, this is awful high to simulate a light bulb, however, this is the resistance value that produced the nicest sounding tremolo. So, perhaps we are NOT simulating a light bulb, but rather, we are finding a "sweet spot" that the oscillator likes better. I have a bunch of these tiny bulbs in my bulb stock, but none of them worked properly, producing a desirable tremolo sound. The original bulb is super tiny, so perhaps your "grain of rice" variety. My local electronics distributor does stock what they like to call "grain of wheat" bulbs, but I have no idea what to ask for. Now, in hind sight, perhaps the ones with the lowest current rating might work. I do like the idea of using an LED for a more dependable and longer lasting design. I agree, it would be really nice if someone could come up with the specs for the original roach used in these amps. But in all my years of working on amps, this is the first time I have seen a non-neon roach. Making vintage roaches? Cool idea, but not much of a market, I fear....if you do this, keep your day job!

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    Quote Originally Posted by johnhoef View Post
    I finally did find the schematic.......This amp, contrary to the Fenders, uses an incandescent bulb instead of a neon bulb.
    That's why I posted the original link - it shows an incandescent bulb.

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