Substituting 74xx logic - some advice please ......

[picbits]

Called out to look at £15k worth of machine today - its older than I am but mechanically sound.

I've traced the problem down to a misbehaving SN7490N decade counter. Obviously these are no longer stock items having been long discontinued.

The 7490 is part of a 0-999 counter circuit - the three BCD switches are commoned towards +5v with a 220R resistor with the 1 / 2 / 4 / 8 outputs going directly to the Q outputs of the 7490 via diodes. When the BCD switches = 7490 count, the common side of the BCD switches is +5v, in all other cases its zero volts (or near as).

With the 220R resistor, thats around 22ma sink with a single input enabled (worst case scenario) - the 74LS90 will only sink 8ma according to the datasheet while the 7490 is happy with 16ma.

This *could* be why the original 7490 has packed up - being asked to sink 22ma may have been too much for it but the machine has worked without a problem for the past *cough* years.

I could increase the pullup resistor to 680R and try a 74LS but not sure how that will affect other parts in the circuit - its fairly complex.

Any suggestions ? I've heard back from the manufacturer who has said the board has long since been discontinued and they don't have scematics for them any more .....

Ta

 

[ronv]

Are you sure the pull ups are on the output and not the input after the diode? I say this because the part has an active pull up built in.
If not I would try the LS part with a larger pull up.

 

[picbits]

It effectively goes +5v - 220R - switch - diode -|> 74 device output.

When the 74 output is high, there is +5v between the 220R and switch
When the 74 output is low, there is 0.7v between the 220R and switch

Effectively there are 12 switches all fed from the single 220R resistor. When all three 7490s match the value on the switches, the switch side of the 220R is +5v. In all other cases its <0.7v

Clever as it reduces parts counts but causing me grief. If this wasn't holding up a large order I'd be tempted to whack a PIC in there and do it the easy way.

 

[ronv]

Ttl

I think I get the circuit - see below. If it is like that and the output of the switches drives more TTL, I not sure why they used such a low value for the pull up, as an open TTL input is recognized as a high. Anyway it will depend on what the output drives (How man loads), but if it is four or less I would try the LS part with a 3.3K pull up to +5 in place of the 220 ohm.

 

[picbits]

Cheers ronv - thats exactly how its wired up. I found last night a place with a load of obsolete 7490s within an hour drive from here so I'll give them a ring and see how much they are asking for some,

 

[picbits]

Rang the chap with the obsolete stock and he wasn't keen to sell me any - aparrently they only deal with large amounts of sales.

Substituted the 220R with a 680R to see if I can get away with the LS TTL - no go. Obviously whoever has designed this has had his reasons for using a 220R resistor.

Was about to give up when I gave the obsolete parts chap another call, bribed him with cash and told him I could be over asap. He told me I had half an hour to get the 22 miles cross country before he went out and I'd miss him lol.

Made it just in time, got back and tried to desolder the old 7490 - the pads immediately lifted and came off the substrate - over the years the board has obviously got a bit more fragile !

Managed to get a socket soldered in its place, patched up the damage and inserted the new (old) 7490 and off we went - worked perfectly again (with the 220R resistor back in).

Stuff it - if it lasted 30+ years on the first 7490 then hopefully it will go on for another 30 !

Have advised the customer that there won't be many more repair opportunities due to the fragility of the board so might end up working on a microcontroller based fix for next time it dies !

 

[ericgibbs]

Cheers ronv - thats exactly how its wired up. I found last night a place with a load of obsolete 7490s within an hour drive from here so I'll give them a ring and see how much they are asking for some,

hi,
If you get stuck, I am sure I have few 7490 and 74LS90's on the shelf..

 

[ronv]

Score one for the good old days!

 

[picbits]

hi,
If you get stuck, I am sure I have few 7490 and 74LS90's on the shelf..

Thanks for the offer Eric - I now have a whole tube of them (or rather my customer does as they will be paying for them !).

I was very tempted to whip out the old 7490, program a PIC18F1320 (with intosc) up to emulate one and jerry rig it into place - the maximum count would only be 5 or 6 a minute so no great task.

I all worked out in the end - not going to be a cheap job for the customer though as there was the best part of two days of work and half a tank of fuel gone into sorting this out. On the plus side, the company they were going to get in would have charged ten times what I'm charging

 

[JimB]

Good to hear that you had some success!
Lets just consider this from a comercial point of view.

Called out to look at £15k worth of machine today - its older than I am but mechanically sound.

The machine is old and uses obsolete parts, did it cost £15k in 1975 or is it worth that today?
Whichever it is, the machine is obviously doing useful work or they would not want it repaired.
How much would a replacement machine cost?

not going to be a cheap job for the customer though

But what are you comparing it against?
This machine is obviously worth more than the average TV, it presumably does something profitable for its owner, and they can offset the repair costs against their tax bill. (Assuming this is not some shady outfit!).
So, your bill should represent money well spent for them.

On the plus side, the company they were going to get in would have charged ten times what I'm charging

If you know that for a fact, you should maybe be charging twice as much.
They should still be happy and your business will be more profitable.

Well done!

JimB

 

[picbits]

The machine to replace would cost them around £15k (as the customer informs me) - its a pretty simple bit of kit but very effective. The company is a reasonably sized one - around 12 employees.

I was actually their first employee many many years ago when they first started up - I used to study at the college across the field from them so spent my time between the two sites earning and learning what I could. Ironically I used to babysit the MDs sons, the eldest of who is now the MD of the current company.

As for charging more ? I have a fixed price per hour and a fixed callout fee (which I should really review as it hasn't changed for the past few years) which is what I consider a reasonable amount to ask for my services. Technically I'm their IT consultant but because of my background in electronics (and the fact I run an electronics business), if it goes wrong I get called.

They are the only company I've kept on from closing my IT business 12 years ago to concentrate on the electronics side of things. Its handy as although they take a fair amount of my time, the income pays for development and upkeep of the rest of my stuff. Generally because I'm able to think outside of the box, I can come up with a solution to a problem they have pretty quickly and more cost effectively than they can.

In fact the bit of equipment I designed and installed for them for a programmable filling station is the only bit of kit they've got that hasn't broken down yet

I've got some plans on some devices which should almost double their production facilities output but need to do some very careful prototyping and budgeting so when its presented to them they can't afford to refuse the offer.