GT420 mk1
Here is GT420 MK1 schematic (current MK2 version is completely different circuit-wise). It's a LM386 based distortion with a fat silicon Rangemaster in front. What was neat in this particular pedal was hitting 386 not with a boosted signal, but feeding it with already dirty and compressed signal. This caused a rich, powerful distortion with reduced typical farting-out sizzly sound of simply smashed 386. Dual gain pot is crucial for this circuit and provides a wide range of available distortion. I included Rangemaster-based booster instead any other type, because it brings a favourable colour instead of sheer volume, what works great for the whole design (especially in 'dark' mode). EQ section was carefully tuned for a variety of interesting tones with only muff-style tone knob and a shape switch, but of course there is much room for experiments.
Type of 386 chip doesn't matter, nor exact transistor model. Hfe for rangemaster was circa 180 (bias around 6.6-7.1V at collector is fine - lower voltage will yield in more symetrical clipping and a bit more headroom) and for distortion input stage 330-350 (collector voltage around 5.0V). I also recommend to add 1k trimmer between 386 pin 1 and 10uf cap to possibly reduce high gain range a bit.
Satronik Fuzz Sustainer
Satronik Fuzz Sustainer has been manufactured in Poland around early/mid 80s. I examined two different units (one had factory sticker with 1985 date and seemed to be a bit more 'modern' than another) which showed many differences within parts values. Also, units I could find online had even further differences. The overall circuit design was the same and I believe general sound 'vibe' would be kept constant between these variations.
Circuit is designed with three 'blocks': input BJT amplifier with sustain control (which is just volume of this stage), 741 operational amplifier stage and output BJT stage with active tone control. Bootstrapped common emitter amp at the input provides decent input impedance, but is very tough to maintain consistent bias between many units even with bunch of well-selected transistors. Both examples I examined were biased for a heavily asymetric clipping. Op-amp stage works with gain set to around 10 000 (!) which provides distortion to even smallest input signals. It's 'null' control is set with trimmer and can be adjusted for nice gating effect or 'open' sustain. Tone control is an active baxandall variant with only treble potentiometer, while bass response is 'fixed' to neutral setting with two identical resistors.
In this schematic are reflected values of one pedal, while values of second pedal appear in round brackets - at notes below schem there's additional info for the second pedal and some others I've seen online. My replica was kind of mix between both, executed with formula I liked the most. There's demo at my youtube.