racal 9525 computing counter - replacing obsolete(?) sn7490an

[mab2]

Hello all,
I've got this old racal counter with nixie tube display, but when i came to use it recently it seems to have gone faulty:-

Basically, the gate lamp flashes as expected but the display doesn't change - it either comes up with the 'blurred' digits as shown, or 04000 or 04800 when turned on and doesn't change or reset.

poking around with a 'scope and I think one of the sn7490an's (decade counter) on the display board may be dud (when the gate is flashing and it should be counting I can see pulse trains at 1/10 frequency going from one counter ic to the next except between the last 2 - although i don't think this by itself could explain the static display), and possibly the next row of ic's has an issue (sn7475n - Quad Bistable Latches).

But my current supplier of components no longer stocks these; Is there a newer equivalent that will do a straight swap that i should use, or do i have to find a supplier of these exact devices?

I do have a newer and better counter TBH, but I like to keep these oldies going. I don't know why it's failed - the power supply rails seem OK

thanks

 

[mab2]

They do stock an sn74ls75n - would the LS version work OK?

Assuming the LS variant is compatible: they only do a sn74ls90d (SMD soic 14) alas, so I still need to find something for that one.

 

[rjenkinsgb]

The 74LS parts should work fine in just about any circuit that originally used 74 series.
The only exception I can think of offhand is if an output is driving something that needs a relatively large current, rather than just IC to IC signals.

You can easily get the ICs you need from such as Mouser, Digikey, Newark, RS or whichever.

eg. 74LS 90

https://www.mouser.co.uk/Search/Refine?Keyword=74ls90

Re. the fault, I'd look very carefully at the gating sequence signals and verify they are being applied to the appropriate pats in order?

During each count cycle the input to the counter chain should be enabled, then at the end of that the 7475 latch control input should be pulsed so they store the result from the counters.
Then the counters should be given a reset pulse and after that, the whole cycle repeats.

 

[mab2]

thanks - it sounds like you know a lot more about how counters work than I do.

re components: I was looking at farnell - if they listed the ones I was looking for they were 'no longer stocked ' which was why I was thinking they'd gone obsolete. I usually build up a basket with them until it's big enough to get free postage. I'm glad the LS are ok - I think the only ones that are not driving logic are the display drivers - which are high voltage of course.

 

[KeepItSimpleStupid]

My favorite obsolete parts supplier has them: https://www.utsource.net/sch/7490.html

 

[rjenkinsgb]

I think the only ones that are not driving logic are the display drivers - which are high voltage of course.

That should not be a problem, it's the intended application for the part.

I was thinking of where designers use such as R-C delays or transistor / relay drive on the outputs of ICs.
Standard TTL could sink a minimum of 16mA and has an input load of 1.6mA for a "low" input level, while LS and later have different drive and load capabilities.

 

[mab2]

Hmm. OK.

well before i start changing parts I ought to do some more testing - I'll have to look for those enable & latch control pulses you mentioned - i just have to find some time...