[sdiy] Hardware reverb questions/74LS vs 74S vs 74HCT

[Roman Sowa]

Nowadays you'd use 74LVT or 74ACT for high speed and current drive

Roman

W dniu 2022-12-01 o 14:06, Mike Bryant pisze:
> No idea on the design itself, but yes if you are driving lots of RAMs 
> address lines in parallel than you need 74S devices as they can drive a 
> higher load.  There are also some CMOS HCx devices with higher drive, 
> but remember you would need HCTx to go with the other devices.
> 
> Do you have a schematic and a scope ?
> 
> ------------------------------------------------------------------------
> *From:* Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of mark 
> verbos via Synth-diy <synth-diy at synth-diy.org>
> *Sent:* 01 December 2022 12:47
> *To:* synth-diy at synth-diy.org <synth-diy at synth-diy.org>
> *Subject:* [sdiy] Hardware reverb questions/74LS vs 74S vs 74HCT
> Hi there,
> 
> I have been trying to repair an EMT 244 reverb on and off for 10 years. 
> It is built from logic chips and 2901 bit slice processors, I guess, 
> since they scratched off all the labels on the ICs. When I bought it the 
> power supply was dead, which I fixed and it worked for one day then 
> locked up. I learned from forums that the ICs (which I could see very 
> clearly) and the IC sockets would corrode badly on these and I would 
> need to replace all 65 or so sockets. I have done that, it was a 
> nightmare (it’s a 4 layer PCB with +5 and ground internal signal 
> planes). I also got a chip identifier from a Chinese ebay seller to 
> identify the chips. Thing is, the identifier doesn’t tell me if the 
> 74_xx is S or LS or whatever so I have to guess based on the 1978 design 
> year. I now have the input meter successfully showing input signal. But, 
> no signal gets through the RAM and out to the DAC. Recently I have 
> learned that S series is actually faster than LS, which I had filled the 
> thing with LS chips. And HCT is even slower. But, maybe it’s not the 
> speed that’s the problem. Now, I have put back in all the original 
> chips, in the new sockets, still no go. The only chips that I cannot 
> bring back the originals are the 74174 flipflops on the address lines of 
> the RAM and the (what I assume to be) 74241 latches on the RAM outputs. 
> I have S series versions of those 2 coming tomorrow.
> 
> Last night I got to thinking that the chips addressing the RAM might be 
> the only ones that NEED to be S series, since there are 30 static RAMs 
> being driven.  The mention of bit slice processors the other day made me 
> think that somebody on this list might have knowledge on this subject. 
> This whole reverb thing is a black art. Anyone?
> 
> 
> Mark
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