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

mark verbos markverbos at gmail.com
Thu Dec 1 14:57:11 CET 2022

Unfortunately the scraping off of IC labels was foreshadowing of the service manual only having the analog parts.
I do have a very nice scope. 
I hope that tomorrow I will be celebrating the whole saga finished, but we’ll see…..


> On Dec 1, 2022, at 2:06 PM, Mike Bryant <mbryant at futurehorizons.com> wrote:
> 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 <mailto:synth-diy-bounces at synth-diy.org>> on behalf of mark verbos via Synth-diy <synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>>
> Sent: 01 December 2022 12:47
> To: synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org> <synth-diy at synth-diy.org <mailto: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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