This is a call-out for some assistance in solving a noise issue that prevents a delay pedal from being usable.
Some of you may be aware that NOS MXR boards and other stock sitting in long-term storage were purchased by a number of individuals, and placed for re-sale, some of it here and some on E-bay. Among the various items were the old 118 model green analog delay pedal. In its various incarnations, this used a trio of SAD1024 chips and eventually a 4096-stage Reticon R5101. Given that the basic design had some nice features (a tracking lowpass filter that adjusted treble-cut with delay time), it was tempting to want to figure out some way to make the pedal live again, even if the requisite R5101 chips could not be sourced. The "solution" was to retrofit a small board with a PT2399 digital delay chip covering approximately the same delay range. You can see the way the board was implemented in these two pictures, sent to me by Erik in Italy (the person with the misbehaving pedal).http://img.photobucket.com/albums/v474/mhammer/MXR118.jpghttp://img.photobucket.com/albums/v474/mhammer/MXR118flipside.jpg
The noise being made is best-described as a kind of "thwup-thwup-thwup" helicopter-like noise whose pitch and frequency corresponds to the delay time setting. Erik sent me soundfiles for diagnostic purposes, and I think that verbal description pretty much captures it.
You will note that the original 118 ( http://img.photobucket.com/albums/v474/mhammer/MXRAnalogDelayschem1.png
) used a clever scheme whereby the master clock was divided down one way for the BBD chip, and another way for the switched-resistor lowpass filters. The filter sections are very much like those used in the MXR Envelope Filter, with duty-cycle determining how long a CMOS switch section stays open or closed on average.
While the original used this arrangement, the retrofitted version cannot...sort of. Instead, it uses a dual-ganged pot. One half adjusts the master clock on the board, as per usual, and the other half adjusts the delay time of the PT2399, in exactly the same way it is done in the Rebote, FAB Echo, and similar pedals. I wouldn't call it "authentic", but the goal is to have more treble cut at long delays and less cut at short ones. The sound files I heard demonstrate that it works.
Unfortunately there is also this helicopter noise that prevents it fro being a nice-sounding pedal. Erik did not mod it himself, but bought it as is, and would like to make it usable. Does anyone here have any experience with these refurbished/modded 118s? We have considered some of the usual culprits like wire leads to close to the on-board transformer, but to no avail. This leads me to wonder if the problem is endemic to the retrofit itself.