Missile Command DRAM issue

[RGP]

Hi everyone, need some advice and some myth debunking.

I've got a Missile Command PCB that is giving me some beeps in the self test, garbage on screen, 4 low beeps, 1 high, 3 low - which points towards the ram at K4.

This is listed in the schematic as a 100017-001 - looking at the board its a TMS4116-20.

I know this is a 4116 DRAM which requires +5, +12 and -5v. I know these commonly appear and can be the source of problems on Williams boards too.

I heard that Missile Command boards use precise timings of certain chips so I was wondering, what can I replace it with - do I need to find an exact TMS4116-20 replacement or would an MK4116N-3 be ok? Do I need to replace all 8 DRAMS with the alternative if I can't find the exact replacement?

20 refers to the speed of the RAM right?

Need a bit of debunking before I do anything with this one.

Thanks

 

[Mitchell Gant]

4116 RAM's are prime for failure, they were 'large' memories with many transistors for their time, and the triple power rails don't help reliability either.

Should not be a problem replacing a single 4116 chip as long a it's same or faster access time.

The -20 most likely means 200nS access time, a -3 will be 300nS and may be too slow. Different manufacturers use different numbering so often it's an educated guess that -20 is 200nS and -3 is 300nS. Should really find the manufacturers datasheet to be sure.

There's no harm in trying a 300nS access time if it's easy to plug in a socket, it just won't work if it's timing critical.

 

[cmonkey]

If the -5V rail has gone then potentially all the 4116's will have gone with it. The single biggest cause of failure of ZX Spectrum's is the losing the -5V rail by failure of TR5 which usually takes out all the lower ram 4116's. Dunno how it is in a Missile Command but I speak from experience with Spectrum's.

 

[RGP]

Thanks, i'll try and find a spec sheet for the TMS and the MK ones to see if the -3 does mean 300ns. So if I replaced with something say 150ns it would happily run slower.

I checked the -5v rail and its present, wondering if its possible to mod these like you can a Williams to run on 4164's which are cooler and only need +5v.

 

[cmonkey]

Yeah, you can replace the ram with a faster one no problems (usually!), it's replacing them with slower ones that can cause issues.

I can't see any reason why you shouldn't be able to mod the board for 4164's or 4132's.

 

[Equites]

Lovely, far too nice for general public consumption.

I would be tempted to leave the PCB running 4116 RAMs. As long as your power rails are good then they should prove as reliable as 4164. I am a purist though.