Philips PM5544 electronic testcard

[ppppenguin]

This is an important item in TV history, something we very much want to show at the museum. A genuine first.

The LS74 is absolutely fine. I don't need to look for a 7474.

The remaining problem looks like a SN7525N dual sense amplifier. Even the data sheet wasn't that easy to find. The part looks like unobtanium.

I did a very nasty knife and fork job on the PCB to swap the feeds from two bits of core between different 7525 chips which seems to prove the point.

Might be possible to do some serious bodgery with a dual fast comparator and half a 74LS08 quad AND gate. I've got some LM319 around: https://www.ti.com/lit/ds/symlink/lm119....8114914269 and some LS08.

[Mike Watterson]

I'll see have I a 7525. Maybe some ancient HDDs or floppies used them.

[ppppenguin]

Thanks. Possible. Looking at the data sheet they were designed specifically for core stores.

[Mike Watterson]

Seems it was ONLY used for core, hence rarity.
The AND stage is just for ENABL£, so its feasible that some sort of dual op-amp or comparator might do that. But a quad nand will do the two channels, with the 2nd pair of NANDs as inverters.

Application
https://corememoryshield.com/ti-core-mem-1973.pdf

Data (signetics version)
SN7520
SN7521
SN7522
SN7523
SN7524
SN7525

http://www.elektronikjk.com/elementy_czy...SN7523.pdf

The sensitivity / threshold doesn't look onerous
They seem to be designed to work up to 10MHz, so a video op-amp suitably wired is likely viable. Need two.


A little plug in board so an SN7525 can be added later?

[Mike Watterson]

Oh, and unless they are in a box of really old unsorted TTL, I don't have any. The SN75xxx parts I have are later interfaces, no use for cores.

[Mike Watterson]

Jeffery, I rooted in the shelves at the back of the workshop, which oddly you never saw (maybe good!). I thought there was a box of general DIL and one of ancient RAM and CPUs. I found the general DIL one and it had the RF attenuators and dummy loads I misplaced 2 years ago! But no ICs, so they must be elsewhere.
THANKS!

I only checked the 16 pin DIL ICs in the box that still has ICs. CPUa that time forgot like RCA CPD1802 and stuff for 6502, 6800 & Z80s inc I/O. I think all DRAM. So there may be another box. However unlikely any SN7525 now I've seen datasheet. Likely most of the 16 pin DIL is 64K x1 DRAM, but there might be older. There is also longest 0.3″ you might see. Likely Cache RAM for 486 boards. I think the attic has a vast variety of SIL/SIM DRAM going back to 286 and some older DDR.

[ppppenguin]

One of my fellow trustees who's good at this sort of thing has found two from an ebay seller in Holland. He's ordered them at £3 each.

I've built a frig with a LM319 fast dual comparator and a 74LS08. So far it isn't working. Although it's sensing a differential signal I think it may need the threshold to be biased. There is a bias input on the 7525. For the LM319 I'll have to put a sniff of DC through one of the terminations.

[Mike Watterson]

Seems to need 18mv or 40 mV?

Great that a couple have been found. I only even saw a core store once. It was framed and hanging on the wall. One of the last using stupidly tiny cores. RAM and ROM came in rapidly after TTL, so the SN75 series core sense amps can't have been in production very long! The various SN75xxx parts for serial comms lasted much longer.. The 75176, 75188, 75189 etc..I

[ppppenguin]

Putting DC through the terminations won't work because the core is effectively shorting the comparator inputs together. I'll try a resistor in series with each input and then bias at the comparator terminals. The 319 has a low bias current for a comparator, under 1uA, so there shouldn't be any DC problems. The input offset voltage and current are also acceptably low.

[Mike Watterson]

Input capacitance vs series resistance?
Would a series capacitor work to allow DC offset?