jhp@sactoh0.sac.ca.us (Jim H. Puga) (03/24/91)
Has anyone had any success with the RE Desrambler?
I ordered the PC board from D&D Electronics and culled
all the parts I needed from around the country. I put everything
together and the thing won't work. I've put it on a scope
and the waveform looks like it should. All the voltages
measure out fine. Basically, I don't get a clean signal
out when I put one in. I'd appreciate it if any one could
give me some help on this thing.
Thanks alot.
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"There's a lesson here somewhere." --Space Ghost whit@milton.u.washington.edu (John Whitmore) (03/28/91)
In article <1991Mar24.040723.15957@sactoh0.sac.ca.us> jhp@sactoh0.sac.ca.us (Jim H. Puga) writes: >Has anyone had any success with the RE Desrambler? > >I ordered the PC board from D&D Electronics and culled >all the parts I needed from around the country. I put everything >together and the thing won't work. I've put it on a scope >and the waveform looks like it should. All the voltages >measure out fine. Basically, I don't get a clean signal >out when I put one in. I haven't built one (yet), but have looked over the circuit a bit; there are two MAJOR FLAWS in the scheme as shown. R37 is a 2.2k ohm pullup resistor, connected to CMOS at 12V; that means a tad under 6 mA. Unfortunately, normal operation DOES deliver 6 mA, but the guaranteed current is only 1.3 to 3.4 mA (those are the figures for 10V and 15V power supplies; 12V is somewhere in that range). To get the circuit to work reliably, R37 should be more in the range of 22 kOhm. Likewise, R26 is the same value that RCA's app note ICAN-6247 recommends, BUT that app note feeds the signal from a 1V pulse (not 12V); change R26 to 47kOhm. Actually, I think it'd be a tad more reliable if the transistor Q4 were omitted, and the base/emitter pads connected by an 8 uH inductor, and R36 omitted (R36= infinite ohms). The only purpose for Q4/R36/C34 is to stretch the RESET pulse a tad, and the transistor isn't a good way to do that (because the base/emitter junction will get 12V transient reverse bias, which can be harmful). There have been some useful things posted in the past regarding this decoder; I'll e-mail 'em. "Don't get a clean signal out" can mean a lot of things. Start looking at the regulated power supplies, and go from there. It might be useful to unplug IC2 and IC3, and see if a jumper from IC2 pin 2 to IC3 pin 15 (i.e. remove the 'decoding' functions) results in a duplicate of the input signal on the output (this will show if the uA733 and buffer amp are the noise source). John Whitmore