# Quick Question: Source of Weak Trem in Fender Amp...



## StevieMac (Mar 4, 2006)

Just wondering, if the trem effect works but is not as strong as I'm used to hearing, would you normally suspect the roach itself or more likely any caps/resistors associated with it? All preamp tubes are new & have been checked so it's not a tube....


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## jb welder (Sep 14, 2010)

Since you have already eliminated the tube as the problem, my first suspect would be caps, then resistors, then the roach. In my experience the roach is usually either dead or alive. Could be they can go weak, but that would be low on the list of common failures in the trem circuit.
Electrolytic caps are really the only parts in the circuit (aside from tube) that have a finite life span.


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## StevieMac (Mar 4, 2006)

Ok so, check both the .01 caps and the single .02 cap I guess? I was going to slow the trem speed down anyway so perhaps I'll put all new .02 caps in there.


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## dcole (Oct 8, 2008)

Check the bypass caps on the cathodes. They help the oscillator and bug driver operate at maximum gain so if you have oscillation all ready you may just need to replace the bypass caps.


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## nonreverb (Sep 19, 2006)

Is this an LDR type?


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## StevieMac (Mar 4, 2006)

Yes, it's one with the opto-isolator (roach) trem.


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## nonreverb (Sep 19, 2006)

If it's an older amp, change it....I keep a bunch of the assemblies for this very reason.


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## mhammer (Nov 30, 2007)

I'm guessing that the optoisolator uses an incandescent bulb and not an LED or other light source. In which case it would be the bulb that is going, not the LDR.

Richard, you'd be more familiar with these things than I would. Is the light-source/LDR a single physical unit or is there something like what you see in Morley pedals, with physically separate devices? Because if they ARE physically separate, one of the things that can happen with Morley pedals (and UNi-Vibes as well) is that one or both of the components gets nudged somehow, such that the light source is misaligned, relative to the LDR, and produces less fluctuation in resistance as a result.


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## nonreverb (Sep 19, 2006)

The LDR is the culprit Mark. The 'Roach' assembly as previously mentioned here is merely a neon bulb and LDR pressed together with shrink tubing. A few years ago I did some experiments to find out what was going on. The results were that the LDR actually degrades to the point that it's resistive reaction literally fades. Replacing the assembly cures the problem instantly. You can check the other components for problems as well but I haven't found much else that fails apart from the LDR and the tube.



mhammer said:


> I'm guessing that the optoisolator uses an incandescent bulb and not an LED or other light source. In which case it would be the bulb that is going, not the LDR.
> 
> Richard, you'd be more familiar with these things than I would. Is the light-source/LDR a single physical unit or is there something like what you see in Morley pedals, with physically separate devices? Because if they ARE physically separate, one of the things that can happen with Morley pedals (and UNi-Vibes as well) is that one or both of the components gets nudged somehow, such that the light source is misaligned, relative to the LDR, and produces less fluctuation in resistance as a result.


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## jb welder (Sep 14, 2010)

Thanks nonreverb, I stand corrected. The LDR in the opto assembly is a part that will age/degrade. 
Further digging implies that weak trem. issues are usually solved by replacing the opto or the electrolytic cathode bypass caps in the trem circuit.
I guess when I was seeing more vintage Fender stuff it wasn't quite so vintage yet .


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