Fender 68 Custom Deluxe Reverb trem circuit mod

For those of you in the EU area affected by RoHS (reduction of hazardous substances) regulations, you'll find that as well as lead-free solder being mandatory, there are a few other substances that manufacturers haven't been allowed to include since RoHS came into force in 2006. One of these is Cadmium. Not a substance you'd normally expect to find in a guitar amp, but some amp circuits use a LDR (light dependent resistor) in conjunction with a tiny light bulb, within some tremolo circuits (and trust Fender to label a tremolo circuit as 'vibrato' and a vibrato bridge as 'tremolo'). The more light that falls onto the LDR, the freer its electrons become t move and the lower its resistance becomes.

This LDR is almost always a CdS (Cadmium Sulphide) cell. (CdS was also used in most film camera light-meters). The LDR and lamp are enclosed in a back plastic sleeve to block out external light affecting the LDR cell, and has four thin legs poking out, which give it its nickname of 'roach' (through looking rather like a cockroach, and not a jazz cigarette).

So since 2006, Europe-bound Fender amps that use an LDR-based trem circuit have had the 'roach' removed and this has been replaced by a fairly complicated add-on circuit board that is screwed to the side of the amp that attempts to duplicate the simple action of the bulb's light varying in intensity and affecting the resistance of the LDR, which in turn drives the trem circuit.

This seems to be easier said than done, as the add-on board introduces a very audible tick/thump for each loudness cycle. There is a trim pot on the board which you use to minimise the tick/thump, but even when minimised, when you aren't playing, the tick becomes very audible, disappearing in the guitar sound when you play. I have a Fender 68 Custom Deluxe Reverb amp and the tick had put me off using the trem feature.

But as the rest of the circuit is unchanged, it is possible to remove the add-on board, and fit a 'roach' in its place. These are readily available as replacement parts, and in the UK you can pick one up for between £8-£10.

The add-on board has four wires running to the main circuit board, to where the four legs of the 'roach' would go, and another couple off to another extra PSU circuit board. All you need to do is cut these leads back (the extra PSU board remains as it feeds some necessary voltages to the main circuit board), and de-solder them from the main circuit board, and fit the 'roach' in their place. The extra board can then be removed and thrown away (you really won't ever think about replacing it in the future).

The board schematics and layouts are readily available, so its easy to see where the 'roach' fits.

The tricky part is de-soldering and re-soldering without damaging the circuit board tracks. De-soldering the lead-free solder they've used isn't that easy as it cools so quickly, so you've got to be quick with your solder sucker. You can use a hotter iron, but then you risk lifting the PCB tracks (in my amp, it's only a single-sided board with no through-hole plating that would help keep the tracks intact around the holes). Modern lead-free is more forgiving but this older stuff needs a high temperature and isn't nice to work with.

I did damage one of the track holes, which is why my first attempt didn't work, but I eventually realised this and with the 2nd roach I fitted, I bent the long length of leg over and soldered that to another joint on the same track about 1cm away.

Put it all back together and it worked, with a much nicer trem wave than before, and almost no background trem noise. It's not 100% silent, I don't think these things ever are when the trem circuit is engaged, but it is almost inaudible.

The amount of noise present is supposed to depend on the quality of the valve driving the trem circuit, so I'll replace the current one with a supposedly lower noise valve and see if I can get it any better. But it must also be related to the amount of pre-amp noise there is as well (the circuit varies the amp volume level so you'll hear the background noise level rising and falling, which will be more noticeable than a constant noise level), so good quality valves throughout will always help. But definitely 'roach' over add-on board any time!