Hi Mark,
Thanks a lot for that.
I guess for anyone just wanting to use the standard MTX to play games etc. from tape, then the limitation of not beng able to use expansion cards might not be a problem. Personally though, I'd want the flexibility to be able to plug in at least the RS232 card. Actually, I just remembered, my original "MTX512" has not 32k n the main board, Memotech soldered a 32k RAM card to the internal edge connector, so that one needs to have at least 1 expansion card powered, although I guess the RAM card probably does not take much power.
So, maybe your original design is better? - Just a bit bulkier?
Regards
Dave
Power supply
Re: Power supply
Actually old DRAM is fairly power hungry 
Still thinking about this.
The problem is:-
(1) the 16k DRAM chips used for the video RAM needs +12V, +5V and -5V and all three need to power up at the same time (or in the correct order),
(2) the rectifier system is a number of half-wave types back to back.
Add (1) and (2) and it is a bit limiting.
I did look at using a 20VA 6V transformer instead of a 20VA 9V type, but the output voltage is too low for the transistor boosted +5V circuit .
If you want a neater, more compact solution, there are companies that will produce a custom transformer (well custom windings on a frame from a selection of standard sizes). However, have never ordered any myself (yet!).
Mark
Still thinking about this.
The problem is:-
(1) the 16k DRAM chips used for the video RAM needs +12V, +5V and -5V and all three need to power up at the same time (or in the correct order),
(2) the rectifier system is a number of half-wave types back to back.
Add (1) and (2) and it is a bit limiting.
I did look at using a 20VA 6V transformer instead of a 20VA 9V type, but the output voltage is too low for the transistor boosted +5V circuit
.If you want a neater, more compact solution, there are companies that will produce a custom transformer (well custom windings on a frame from a selection of standard sizes). However, have never ordered any myself (yet!).
Mark
Standby alert “There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer in Somerset later in the year
Not as many MTXs as Dave!
Re: Power supply
Hi Mark,
I did wonder about the practicalities of getting a transformer specially made too, but as yet, have not done anything about it either. I was thinking that it might be possible to get a limited run of complete assemblies made, i.e., complete with case (though not matching the MTX) and power lead. I'm not sure what the minimum order quantity might be, but if it was not too large and the cost was not prohibitive, I am contemplating doing this, then either selling the extra units to other Memotech'ers that I know (not to make a profit, just to share the cost), and/or putting them on ebay - I know a few MTXs have sold without PSUs, so I think there may be a demand for at least a few there too.
mmm, I will definately investigate this further. What would you suggest that the voltage and VA ratings should be for a replacement unit? I think that the existing PSU is pretty close to being maxed out, so it may be better to go for 10-20% more power anyway?
regards
Dave
I did wonder about the practicalities of getting a transformer specially made too, but as yet, have not done anything about it either. I was thinking that it might be possible to get a limited run of complete assemblies made, i.e., complete with case (though not matching the MTX) and power lead. I'm not sure what the minimum order quantity might be, but if it was not too large and the cost was not prohibitive, I am contemplating doing this, then either selling the extra units to other Memotech'ers that I know (not to make a profit, just to share the cost), and/or putting them on ebay - I know a few MTXs have sold without PSUs, so I think there may be a demand for at least a few there too.
mmm, I will definately investigate this further. What would you suggest that the voltage and VA ratings should be for a replacement unit? I think that the existing PSU is pretty close to being maxed out, so it may be better to go for 10-20% more power anyway?
regards
Dave