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Lear Siegler ADM-31 Repair

I think your power supply is fine. You should look for a short circuit (probably a bypass capacitor) on the main PCB.
What happens: the SMPS tries to start, but current limit kicks on, due to the short circuit on the 5V rail, then it shuts off (clicking sound) and
after a while it starts again, overcurrent kicks on again and the cycle repeats forever (or when you remove the power).
Frank IZ8DWF

If it's like the SuperBrain supply, it needs a load on one of the lines to generate the other voltages.

Short circuits: Look for pesky tantalum capacitors. They sometimes go bad and short out. I've had this on a SuperBrain and an Exidy Sorcerer. Hard to find which one, but I looked for smoothing caps connected across the 5v / 12v rails and ground; commonly around memory banks, then disconnected them one at a time until it started working. And indicentally, the tants on the SB look like plastic bodied diodes.

Have you got a circuit diagram?
 
I have a circuit diagram for the smps and monitor but not for the main pcb. There may be one somewhere but not on the internet. From the attached pic, could you point out what the tantalums look like? The ones beside most of the ICs are ceramic caps, correct?



Main PCB 02.jpg
 
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I'd say they all were. They're polarised, right? (Have a band or + at one side). Like I said, if they look like plastic packaged diodes and they are marked as capacitors, it's likely they are tants. Take C76 for example (centre of the board at the bottom) it is polarised (has + marked on the board, connected to the side of the tant that has the band painted round it). Definitely a tant.

So, do some continuity checks first. See which rail is short circuited. You will have 5v and 12v, possibly -12v. Put a meter across them with the power off. See which one is shorted. Then work out which tants are connected to it, again with the meter. Then, for each tant, desolder one leg and lift it off the board (so it's standing up a bit). Test the tant for shorts. If it is shorted, pull it out.

You may find one or more failed tants. On one or more power rail. Pull all failed ones, then try to power up - it may work as these caps are just for smoothing, generally. Plan on replacing them though!
 
I have a circuit diagram for the smps and monitor but not for the main pcb. There may be one somewhere but not on the internet. From the attached pic, could you point out what the tantalums look like? The ones beside most of the ICs are ceramic caps, correct?

tantalum may be the black axial ones on the sides of the board. I've seen them in the yellow color, never black, but look enough like tantalum ones.
The ones next each IC are ceramic.
You're not going to find a short with a simple continuity checker. Try to understand if you can use one of the methods I've previously described in this thread.
The more likely short is one or more tantalum capacitors, but really it can be any other component sitting across the supply rails, I've had many examples in my repair life.

Frank
 
JonB, maybe you're referring to the wrong cap because, to me, C76 is a non-polarized ceramic cap; not a tantalum.
C76.jpg
The ones along the sides (like C75) appear to be tantalum solid axial bullet caps, as Frank said.
C75.jpg
I tested each of them with my DMM in continuity mode on both the GND and +5 rail and did not find a short in any of them.
I placed the black probe to a known GND pin and touched the red probe to to the side marked (+) which is the +5V rail and heard a "chirp" but no "beep".

Frank, of the three methods you suggested for finding a short, I don't have the proper test equipment to perform any of them. I only have a DMM, an Oscilliscope and a Logic probe.
 
Yes, those are the ones. I can't see the board too well in the pictures - didn't come out very high resolution on my screen.

Try the 12v line. That is what failed on my SuperBrain, and while I don't wish to argue with a more experienced electronics repairman than me, the method I described earlier is exactly how I found it.

If you have no schematic or cannot determine which rail is which, a very quick test would be to go over all the tants with the continuity checker. Some will be used on the 5v rail, some on the 12v and so on. This will not tell you which one failed but it'll tell you which rail is shorted.

Here is my temporary repair.
IMG_4750.jpg

That's a shorted out tant on the 12v rail in the middle of the SuperBrain's DRAM memory. I just disconnected one end and left it standing up for now.

I've worked out how to get the high resolution picture from Photobucket and I can see it has AM9114 1024x4 static RAM; looks like a total of 4K. It'll still need more than just 5v though, if only to drive the serial port. It has MC1496 chips, which I think need 12v and -8v. Datasheet here : http://ee.sharif.edu/~comcirlab/mc1496.pdf

Edit: Just found a service manual here: http://bitsavers.trailing-edge.com/...A+_Dumb_Terminal_Maintenance_Manual_Jun83.pdf - the PSU supplies 3 +5v rails, +12v, -12v and 15v. Looks like there is a -5v too. See Appendix B.
 
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I have a circuit diagram for the smps and monitor but not for the main pcb. There may be one somewhere but not on the internet. From the attached pic, could you point out what the tantalums look like? The ones beside most of the ICs are ceramic caps, correct?



View attachment 1023439
Hey Lorne, sorry for necroreplying to this old thread, but I've just got an ADM31 with a blown PSU (that someone else previously tried to repair and removed bits from). Before starting my own repair or replacement of it I'm trying to collect all info I can get on the 31 - which seems to be strangely scarce. Did you actually have some schematics for the PSU or monitor boards for the 31 specifically or was it for the ADM-3A or ADM-5?
 
Hey Lorne, sorry for necroreplying to this old thread, but I've just got an ADM31 with a blown PSU (that someone else previously tried to repair and removed bits from). Before starting my own repair or replacement of it I'm trying to collect all info I can get on the 31 - which seems to be strangely scarce. Did you actually have some schematics for the PSU or monitor boards for the 31 specifically or was it for the ADM-3A or ADM-5?
I too am looking for the schematics for an ADM31 - the switching supply is a Boschert model 1001 date code 7943 Revision J. My supply has a burnt out resistor R2 and possibly other issues - low output voltage on the +5 was the first thing I noticed and powered down right away. I will recap this as well of course. Too old to trust them!

I did find this Boschert patent which somewhat covers the supply, but not enough to identify resistor values: https://www.freepatentsonline.com/4037271.html

John :-#)#
 
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Here's a picture of the original Filter cap and the replacement I bought from Digi-Key. The original is a 16uf 63V B.P. cap. I could not find an exact match so went with the closest which was a 10uf 63V B.P. but as you can see from the pic it's a LOT smaller. This has got me thinking that perhaps I did not buy the correct one. I read somewhere that a 10uf was a suitable replacement for this cap. Does anyone have any advice on this one?

View attachment 1020491

I'm sorry I missed this old thread, but there are still some points to make that might prevent others from digging a grave for the VDU:

The size disparity is there because the two capacitors are in no way interchangeable, if you put that small part in there, due to its much higher ESR it would overheat and burst its guts.

It is not a "filter cap" it is a coupling cap.

It is worth doing some research on how CRT VDU's work, their H and V scanning systems, before undertaking any repairs. Read up on energy recovery scanning.

The original 16uF bipolar capacitor is a very special part and is nothing at all like any bipolar electrolytic or any other electrolytic you can buy from any supplier. It is likely fine too, and doesn't suffer the general fate of old electrolytics,

Generally these are the horizontal yoke coupling capacitors. They form part of the S correction linearity function (look that up) to assure good H scan linearity and they block DC from the H yoke scanning coils. They have an extremely low ESR in the order of 0.1 Ohms. The peak currents in this circuit are as high as 3 amps.

They can be replaced with a film capacitor, generally a 100 to to 250V rated one of the same capacitance, as I say, likely the original is just fine. Like this for example:


The thing is that you get this generalized advice on the internet about replacing electrolytic caps in "everything". But, the people who are saying this make global non-specific recommendations. So a lot of folks following this (like sheep) apply that to all the capacitors that look like electrolytic capacitors that they see, in the hopes of fixing their broken machines. And as a result they can get themselves into a real pickle.

The better move is to ignore you tube videos, buy some books on CRT's and VDU technology, read those, understand how and why things work (when they do) and move on from there to figure out what to do when they don't work.
 
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I too am looking for the schematics for an ADM31 - the switching supply is a Boschert model 1001 date code 7943 Revision J. My supply has a burnt out resistor R2 and possibly other issues - low output voltage on the +5 was the first thing I noticed and powered down right away. I will recap this as well of course. Too old to trust them!

I did find this Boschert patent which somewhat covers the supply, but not enough to identify resistor values: https://www.freepatentsonline.com/4037271.html

John :-#)#
John, I was wondering if you could take some decent pictures of the power supply and terminal pcbs and dump the firmware for the archive
 
I also have an ADM 31 and just discovered that the ganged POT on the back end of the PC board is broken and cant get any OHM reading from it. any chance of someone with the same unit could please ohm out the two pots values? also the traces on the back side of the board are missing on one of the pots.
could do with a closeup picture of the pot area of the board front AND back please.
 
Probably its because I own it now. lol. and as soon as I can I will follow up on that dump and pictures. I'm working on the power supply and ohming it out trying to make a schematic. there's not that much to it.
 
I have the rom dumps from MY adm-31. my board is diffrent than lgallair above. attached is picture of what my board looks like and the roms from my board.
there in bin format and they are labled as marked. i woud like to find a 129867-3 image. anyone?
 

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