Obsolete Components

Good idea! I'm not exactly in the same boat, but occasionally need the odd obsolete part, and I have a few old bits stuffed in various boxes.

You might as well post details of the network you're after, since you've mentioned it. Someone may have some (maybe even me!).

The resistor network I need is a Bourns 4416P-001-822 (16 pin 8K2). It is a wide body (10mm) surface mount and it draws a complete blank on all google searches. This component tends to 'leak' across the individual resistors which causes errors on a particularly sensitive circuit. I can't find this component anywhere! and there are no alternatives with this wide pin spacing! Very frustrating when you know what to replace but can't find any.

This problem of obsolescence is affecting so many corners of industry and I hate telling a customer in the back-of-beyond that their old treasured machine can't be repaired for the want of a 5p component!

Time for 'WRINKLY'S-ARE-US' to come to the rescue.

I recall curling the pins toward the underneath side of a standard DIP package to replace a surface mounted device. We used solder paste and a pencil type propane torch to solder it, as doing so with an electric soldering iron would have been nearly impossible.

Buy a bunch of 8K2 Discrete surface mount resistors and mount them on edge. If they don't reach the pads, a tiny L of copper wire or foil on one or both ends (both if symmetry is important) can make them reach.

I'd use a very fine soldering pencil.

This component tends to 'leak' across the individual resistors.

What exactly do you mean by leak? Can you measure different low impedances between the individual resistors? Is it possible that there is a problem with dendritic growth? Get a good magnifying glass or a good cheap modern camera with 1 cm macro facility and look at the edge of the body of the component where the legs enter. If you can see potential for water to enter the body during the water cleaning process when the card was made then it's possible that low impedance paths have "grown" inside the body of the component.

Why is all this relevant? Because if that is the problem then all the suggestions of mounting discrete components on the original pads are valid, and, should solve the problem.

One other possibility. Do you have the circuit diagrams? What are the resistors used for? If they are just pull-ups then replacing them with a 10K (more readily available)version in a similar package will almost certainly be OK.

By the way I agree with the main thread suggestion of a 'graveyard search' and 'equipment query' section.

Nice topic, as John says, you'll probably have to solder in some discretes, bit of a pain, but there ya go, should get zero crosstalk that way, but the matching maynot be so good.

Del

This is a fantastic idea, but I think about the issues that will arise. My back shop is full of "retired" equipment, some functioning, some not, that I use for sources for components. The primary issue is that I do not maintain an inventory of what I have, I just know. When I can't find a part at the local shop (buying from the Internet is not practical for me, due to logistics), then I check out my stock. If I find the part, I then have to extract it and test it to see if it is any good (assuming I have the appropriate test equipment). Or I try to find a substitute that is available locally. Surface mount? I don't even try to repair, because even with a microscope, I can not always identify the part (possible exception- a complete assembly, like a motor driver that appears functional). Things like stepper motors, etc., I have a lot of items that I will never use that I would be willing to share with others, but again, logistics raises its ugly head. I'm not sure how practical this would be, in the long run.

From what I remember of my years in the US, most major metropolitan areas have "surplus" stores where antiquated components can be had, but we do not enjoy such luxuries in this part of the world...

could you not make it with normal resistors or similair resistor network?

It is just a simple resistor network surely you make a work-around

Thanks for the interest guys - I guessed it might strike a chord. Probably the only way to make the data base work is for us to list any spares we might have and tag them onto our personnal web sites - Google should then pick up any matched searches. Unfortunately this would probably bring in a load of unwanted enquiries hence my idea to restrict it to within our CR4 comunity.

With the Bourns resistor pack I'm looking for it has 1.27mm pin spacing which is much smaller than standard DIL and problematic for discrete resistors. The board is used in a petrol pump so I'm very wary about using 'get-a-round fixes' in case it infringes Trading Standards, gives petrol away free or affects safety! Normally I can reverse engineer most things, even mask-programmed micros but this item has beaten me.

Equipment manufacturers today don't want us to keep their gear working - it stops them selling us new junk but for an old Navy Artificer who was trained to "fix everything or else", it's a hard philosophy to accept.

I think the idea of a "Public Boneyard/Hellbox" has much merit! Most people who have a well-stocked hellbox (from; oh, hell, look in that box) don't have much other than a mental inventory. Seems to me the problem would be getting the request in front of the guy that has the part stashed. But those who would make routine use of such a service would probably make it a practice to check in every once in a while.

Hello Tom,

Found this on the Bourns site. They show part number as 4416P-1-822. Link is to datasheet, hope this helps.

Bourns resistor network

Very interesting answer - I was told that this particular R network was a common cause of faults on this computer board so did a comparison check on 10 identical boards. Sure enough I found 3 boards with reduced resistance between the centre 4 or 5 resistors on pairs of networks. On swapping a single 'good' network from another board a previously defective board started to work normally which is pretty conclusive. The networks are part of an intrinsic safety/zener barrier for the shaft encoders within a petrol dispenser hence my reluctance to try any fixes using individual resistors or alternatives.

On checking the removed/defective network the breakdown between legs had vanished so I 'm not sure if I've zapped any dendites within the package during the hot desolder process or the fault was something else? There were no signs of tracking on the board and the pads were nice and clean and electrically clear of breakdown. The repaired board is working away happily. My next step will be to refit the suspect network in a good board and see if it works - if so, the problem with this particular component may be solved -thanks, I'll pass on the results to my network once I've done a few more tests.

I use a Pace rework station to do any solder work so the tools should be good for this level of repair.

Dendritic growth nearly always disappears when you remove the component whether it is under the component or inside. Bourns had a problem with bodies of resistor networks several years back where the dendrites were growing inside the component. If the contamination is inside the problem will probably return. You can easily see the "cracks" in the side of the component if you look with a very good magnifying vision system, or, as I said before a cheap modern camera with 1 cm macro will do.

Assuming that the networks are from the period when Bourns suffered the problem described by Randall, I would (strongly) advise not to refit the de-soldered parts. The reason is that, although you may have broken the connection to the dendrites, the majority of the dendrite track will be intact, so the reliable lifetime of the replaced parts is likely to be very short. (In this particular case, it is more than possible that a similar constraint would apply if you could acquire parts from someone else's reuse bin, so that route too would not be a cure-all)

However, given the difference between your application and what I had assumed, I would slightly change my recipe. I'm confident you are already aware that there is no special merit to the Bourns part. The sole requirements for the resistor network are adequate dissipation capability*, spacing between tracks, and known temperature-cycling performance when soldered on board (large ceramic components need carefully designed board pads and/or strain relief of the package pins). So, if you have the height available, I would use the largest practical piggy-back board, and wire it to use a pair of 4k2 and 3k9 resistor networks in series (so that they would be capable of providing the original dissipation). Be sure to make the pads exactly as recommended by the manufacturer of the network.

Personally, in a similar situation, I have used hmp solder to join two half-width networks, and soldered the ends onto the existing pads using standard solder. However, I can't recommend this without knowledge of three constraints - the layout and condition of the board pads, the temperature cycling that the unit is likely to experience in its expected locations^, and the soldering equipment you are using. (The advantage of the board route is that you can test it for connectivity and breakdown before mounting - and the networks should remain self-aligned to the piggy-back board during subsequent attachment)

Cleaning, drying, and passivation as previously suggested should ensure durability

*N.B. If the dissipation was significant in normal use, the self-heating would likely have kept the ceramic surface sufficiently dry to discourage dendritic growth.
^Naively, I would expect the failures to originate from warm humid locations.

Two great answers Guest: why not join up?

Hi Randall

Thanks for the compliment; I already did. But sometimes I have trouble logging on.

Fyz

That's what I'm saying. Why waste those good answers?

Great idea! I'm all for it!

The specific part (4416p-1-882) is listed by Anglia (the UK distributors). I know it doesn't guarantee anything, but worth a try (01945 474747)

If the exact same part code is available: I would guess that the part was introduced after the problem: so, removing the part cleaning the board and replacing the old part should be OK.