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CR11 - Now I'm curious

NeXT

NeXT

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Billdeg pushed a Unibus controller for the CR11 card reader my way, mentioning that it was "very dead".

He wasn't joking.

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Looks to be an 8881 that went into meltdown. I've never seen a chip blow up so violently like that. There's burns straight through the PCB.

The full engineering drawings are here, thank GOD so I will be attempting to repair the board. The blown IC in question is E09 but I can't immediately tell where the obvious spike in voltage came from. The reader? The backplane?. I'm sure that whatever happened any surrounding circuits are also toast. It's going to be an absolute joyride to test everything out. :rolleyes:
 
Hi All;
I Know the feeling, my 11/40 had the same kind of meltdown, on an Ic on the Timing Board.. a pin was attached to an 8881 from a DL-11 Board..
Just clip it out, put in a socket, and use some wire-wrap wire to connect anything that got burned up or away..
My best guess would be that sometime in its Past either it or something wired to it got plugged in Backwards or somewhere there was a short (cause unknown) to another voltage, which could have been +12 or --5, it could have been the +30, if one of the supplies had a meltdown, either way one could have wiped out alot, since they weren't using modern Switchers for Power Supplies..
You might want to Plug it into, a Home-made Backplane, that has the voltages normally needed for this Board, and Look with either a meter, or preferably a scope and see that all the other pins going to the Buss have the correct Voltage on them, and not like +12 on a +5 volt line..
I have been through this before..

THANK YOU Marty
 
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Well the initial repairs are complete. I would like to point out that those off-brand desoldering guns on ebay work AMAZINGLY well on double sided PCB's this old. I lost two traces and a pad (and a few other pads that were not critical) during the meltdown but that was all patched up and two sockets were installed. All capacitors and resistors were tested and everything seems to be within tolerances and not shorting or open. Because the CR11 interface is so simple it only needs +5v so a current limited bench supply can power it without even needing a backplane and I can immediately see if anything wants to immediately fail or if there's any nasty voltages on Unibus pins there shouldn't be. As of right now the board seems okay and only draws 750ma so excluding the probability of additionally fried logic from whatever caused the first meltdown (plus the currently missing 8881) the board is now safe to insert into a machine and proceed to do any additional debugging.

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A helpful hit, when I have a card removed from the system I will always connect the ground and +5 volts to one of the bypass capacitors on the bottom of the card. They already have the polarity identified and prevent shorting multiple pins with using clip leads. How’s your supply of N8881 chips? Think a SN7401 may be the same but not one hundred percent on that and you can still find the 8881 on eBay from time to time. As far as the damage to the original goes got to remember that you have a five volt source with all most limitless current feeding the device so once it starts to go it just keeps burning. It’s just pure speculation on my part but think the old epoxy cases are starting to deteriorate to the point of letting in moisture and that starts the burning, that along with the bus driver chips running hot under normal operation it won’t take much for them to run away and destroy themselves.
 
Hi All;
Can one of You Please tell me where would or who would be a good place to get an 8881, as I could use some, but Don't know who to trust on the internet ?? The 7401 is pin compatible, but not Amperage draw the Amperage is quite less.. The 7439 is also pin compatible, and also closer to the amperage..

THANK YOU Marty
 
I got four readers ATM. You'll probably see three of them for sale here soon enough. ;)

I gotta second that. $15 for a chip off ebay seems a bit much. I don't have any in inventory.

A helpful hit, when I have a card removed from the system I will always connect the ground and +5 volts to one of the bypass capacitors on the bottom of the card. They already have the polarity identified and prevent shorting multiple pins with using clip leads.
Click to expand...
AA2 and AB2 are not used on this board so I can get away with clipping the leads on like this. If the clips slip and it shorts out the bench supply immediately kills the power. I have more appropriate clips somewhere to attach lines directly to the IC's but they don't seem to be in their drawer....
 
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Marty said:
Hi All;
Can one of You Please tell me where would or who would be a good place to get an 8881, as I could use some, but Don't know who to trust on the internet ?? The 7401 is pin compatible, but not Amperage draw the Amperage is quite less.. The 7439 is also pin compatible, and also closer to the amperage..
THANK YOU Marty
Click to expand...

I have never seen a source for NOS (or even pulled) DEC 8881 devices. 8641 and 8640 are generally available. Not 8881's.

So I would keep some spare dead boards around and use them for 8881 pulls for replacement. It's work to get them out but not much other choice. And get some good desoldering equipment.

7401 is pin compatible, but lower drive. It may work however in many applications, depending on usage on the board.

Don
 
Sounds crazy but it works for me. Get dead cards like parallel port drivers or other weird peripheral stuff and because you don’t care about the board it take a butane touch like you would use for soldering copper pipes and heat the bottom of the old boards to the point where the chips will start to fall out. Sounds extreme but not that much more stressful to the components then when they were wave solder in and have had way better results doing this then trying to desolder the chip without destroying it in the process. Now days when I change a chip the first thing I do is destroy the suspect device by cutting all the legs at the device package and then desoldering each leg by itself and cleaning the hole afterword’s. Once the pads are all clean then clean the surface around it with alcohol and install a new socket and then the new chip. But when trying to salvage components from an old card then just destroy the old card itself to try to salvage the devices and remember that you then have to set up a test bed to check the removed device before you go ahead and use it.

http://www.ebay.com/itm/N8881N-14-DIP-SA-/290927208649?pt=LH_DefaultDomain_0&hash=item43bc9ce8c9

Not cheap but appear to be new with a 1980 date code.
 
Hi All;
Thank You QBus for the link, but it is would still be quite expensive for the amount that I need, but still is better than $15.00..
I have done like You said, and used the Torch, You need to watch out for the Fumes that come off of the Boards while doing this.. The fumes are quite nasty smelling, and most likely Bad for You..
Another way, that I like and use now is I use a Large Solder Pot to unsolder the Ic's (I use a torch to Help Heat up the pot, it takes a lot lesss time to do so..) And I set the Board on the solder pot in the area of the Ic's that I want, and just pluck them out, and Yes, sometimes they do fall out, it depends on whether the pins are Bent or still straight.. But, the Fume factor is alot lower.. And I can take out as well (most of the time) R-Packs and such small items..
Sorry, the second photo isn't focused, the camera indicated it was focused..

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I don't know how to get rid of the thumbnails photo's..

THANK YOU Marty
 

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I have not tried soldering pot technology yet but it looks like a whole new way of getting burns and making a mess all around the shop! Will have to look for one on EBay. The blow torch thing is definitely something you do outside; having heavy leather or welding gloves is real helpful too and don’t hold the board over your head while looking for things to pop out!
 
Get a used toaster oven and after removing the card handles/plastic tops you can heat the whole board up and pull the chips off like they are not even soldered in. Again though, do this outside or in a well ventilated area.

I quickly researched the 74LS01 as a substitute for the 8881 and it seems that the only difference is a lower input capacitance on the 8881. That shouldn't really be an issue in Flip Chip applications however the higher speed rated chip is the 74HC01 and would work equally as well.
 
NeXT said:
Get a used toaster oven and after removing the card handles/plastic tops you can heat the whole board up and pull the chips off like they are not even soldered in. Again though, do this outside or in a well ventilated area.

I quickly researched the 74LS01 as a substitute for the 8881 and it seems that the only difference is a lower input capacitance on the 8881. That shouldn't really be an issue in Flip Chip applications however the higher speed rated chip is the 74HC01 and would work equally as well.
Click to expand...


While I have used 7401's as 8111 replacements in my 8E's I think the 8111 has a higher drive ( sink) current and this may be significant when driving a well loaded or long bus. A better substitute may be the 7439, if you can get it, or a 7438 if you dont mind playing silly bu**ers with the pin out as its backwards, or I have heard of 2 7401's piggybacked together. If its not driving a well loaded bus then a 7401 should be fine though I would use a straight TTL not an LS, Iv not tried a 74HC
Dave H
 
I'm with Ray on the keeping of junk boards. I am surprised that even the most useless of boards (TQK50, for example) don't sell for higher prices just to be a source of qbus interface ICs.

I am a huge fan of the radioshack desoldering iron (64-2060). At $15 they are awesome. I recover 40 pin dips with them with no problem. The replacement tips are cheap enough and when the heating coil burns out, just buy another $15 iron (but I seem to have four of the bulb and tip assemblies around because I can't bring myself to throw them out.) The only other better value for the money is the radioshack wire wrap tool.

There are some tricks with the desoldering iron though. 1.) if at first you don't suck the hole clean, resolder it with new solder, then suck it again and 2.) the ground pins that may go to heavy traces or a ground plane may be hard to get hot enough to suck clean. For these I heat that last pin with a normal iron while pulling on the chip (I have one of those spring loaded dip pullers.)

Lou

PS. As a teenager I stunk the house up for three days when I took a torch to a board in the cellar to get ICs off. After that lesson, I just leave the ICs on the boards and desolder them as needed.
 
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Hi All;
Lou, I think I have the same De-soldering tool, and I Really like it..
"" I am a huge fan of the radioshack desoldering iron (64-2060). At $15 they are awesome. I recover 40 pin dips with them with no problem. The replacement tips are cheap enough and when the heating coil burns out, just buy another $15 iron (but I seem to have four of the bulb and tip assemblies around because I can't bring myself to throw them out.) The only other better value for the money is the radioshack wire wrap tool. ""
I have not had my heating coil burn out yet..
But, for really big jobs I use my Solder Pot..
"" There are some tricks with the desoldering iron though. 1.) if at first you don't suck the hole clean, resolder it with new solder, then suck it again and 2.) the ground pins that may go to heavy traces or a ground plane may be hard to get hot enough to suck clean. ""
Sometimes an Iron on one side and the De-soldering tool on the other will work as well.. Also, If you have a Large Pad that need to be unsoldered, I stick a pin in it and heat it up, pushing it through and then twirling it until a hole appears, then it will let You suck out the rest of the solder..

THANK YOU Marty
 
m_thompson said:
There are some on eBay now. Just search for "SIGNETICS N8881".
Click to expand...

Saw those. Many are from China. I am very suspicious about getting these parts from mainland China sources.

In any event I found this source: http://www.gerberelec.com/browse/?PartNo=n8881&-search=Start+Search

Good price. Ordered a few tubes of 25pcs. They are marked date code 1990 and are Signetics stamped. Tested a few in circuit and they work fine.

Gerber has ~24K pcs in stock at $1.28 each. They have lots of other standard TTL parts as well.

I last bought stuff from them some 30 odd years ago when I was back in the Boston area.

Don
 
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