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Faulty ABI Electronics DDS-40 Logic IC tester

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alan8086 View Drop Down
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    Posted: 17 Oct 2016 at 12:22am
Hi, I'm new here so Hi to all of you. I have an ABI Electronics DDS-40 Logic IC tester That has developed a fault as follows...

It used to work, then I left it in the cellar for ages which it didn't like. Turned it on - lots of static and rubbish on the screen but still readable. Crackly noises too. Turned it off quick before anything started burning. I've been trying to fix it for the last 5 days. I've got somewhere - now instead of a screen full of random characters, if I cycle the power on and off a few times I get a clear, readable screen - the ABI Electronics POST but it fails the self test and says 'self test fail: kybd scan'

This is what I have tried:

1: Had it all apart, solvent washed all boards, socketed chips, PSU board, keyboard, VDU board, cables - everything.

2: checked voltages on the PSU output. there are 2 +5V, a -12V, a +12V. The two 5V have there own adjustment pots and there is one that controls the +12 and -12V together. I discovered if I increase the right hand +5V pot until around 5.5V, the POST screen comes on more regularly.

3: I noticed before I started getting POST, the relay on the top left of the main board wasn't coming on at power on. There are 4 LEDs nearby. one green that is on always and a red that has been on below the relay. The others have never come on. The Relay is an overpower protection device I think. I've had the low power npn tansistor out that controls the relay plus related capacitors and a diode. I tried a replacement transistor for a while but no difference.

4: It gets stupid here: I temp soldered some leads from the power input plug on the main board to an old AT computer PSU that has the correct voltages, just to check it wasn't a PSU issue. The board powered up ok with the usual rubbish on the screen but one of the main big traces from the relay to a diode fused red and blew. Fortunately I managed to fix it by desoldering the relay and soldering a wire bridge across the break. I tested the relay whilst out and resoldered it in, put the original low power transistor back, solvent washed the board and from then on the relay worked and I started getting the POST on the screen as described above. The red LED is now off also. Just the green one on now.

5: I have tried re soldering any dull joints anywhere.

6: I've turned the screws on 4 trim pots that have obscure solder mask markings just to see any effect. None.

I then desocketed everything and solvent washed (meths) all boards (twice) - carefully drying with a heatgun from a distance.

The best I can get is as described which was some progress but it isn't there yet :(

I've done a video of what it is doing now. If I can get the damn thing to upload to utube, I'll post a link.

Thanks for any help and sorry for the length!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ace` Quote  Post ReplyReply Direct Link To This Post Posted: 17 Oct 2016 at 12:54am
Can you not continue past the self test failure by pressing execute (or enter I forget)? I'd start at the keypad pcb maybe a bad key that's stuck on or a bad connector so it thinks it's not plugged in.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote alan8086 Quote  Post ReplyReply Direct Link To This Post Posted: 17 Oct 2016 at 12:55am


[IMG]http://i1095.photobucket.com/albums/i470/alan6502/ABI%20Electronics%20DDS-40XP%20Logic%20Chip%20tester/WP_20161012_013_zpsw9ir5cvv.jpg[IMG]


[IMG]http://i1095.photobucket.com/albums/i470/alan6502/ABI%20Electronics%20DDS-40XP%20Logic%20Chip%20tester/WP_20161015_013_zpstft5zcbi.jpg[IMG]

Edited by alan8086 - 17 Oct 2016 at 1:29am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote alan8086 Quote  Post ReplyReply Direct Link To This Post Posted: 17 Oct 2016 at 12:57am
No - there is no response from any of the keypad keys. The main querty keyboard doesn't do much either - just the caps lock light goes on and off if you press the key - which suggests the keyboard logic insode is functional?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote alan8086 Quote  Post ReplyReply Direct Link To This Post Posted: 17 Oct 2016 at 12:59am
One Youtube Vid:

https://youtu.be/HR7hgRJOH2c

Can you embed videos here?

Lots of pics of the insides...

http://s1095.photobucket.com/user/alan6502/library/ABI%20Electronics%20DDS-40XP%20Logic%20Chip%20tester

Edited by alan8086 - 17 Oct 2016 at 1:03am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote alan8086 Quote  Post ReplyReply Direct Link To This Post Posted: 17 Oct 2016 at 1:00am


Edited by alan8086 - 17 Oct 2016 at 1:01am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ace` Quote  Post ReplyReply Direct Link To This Post Posted: 17 Oct 2016 at 1:06am
You get the same error with the qwerty unplugged? I'd focus on the blue keypad pcb, maybe a bad solder joint or you need to pop the keycaps off and clean the switches. I've had to clean some of the switches on mine, trm also covered it in his repair thread.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote alan8086 Quote  Post ReplyReply Direct Link To This Post Posted: 17 Oct 2016 at 1:12am
Yes, its the same without the querty - o wondered if it was the keypad board. I've re soldered all the key switch connections. I havnt tried removing the keys though. The whole thing has been solvent washed however. I'll give it a go and see. I do know Ive had the data cable on the wrong way round. Its orientation isn't clear. On the solder mask there is a white rectangle around the socket with a bit missing from one corner - I assume that is pin no1 so I oriented the plug so the red side of the ribbon cable was next to the notched rectangle. tried it both ways.
Has someone done a write up about this model? I searched before posting and couldn't find mention of it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote alan8086 Quote  Post ReplyReply Direct Link To This Post Posted: 17 Oct 2016 at 1:29am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Purity Quote  Post ReplyReply Direct Link To This Post Posted: 17 Oct 2016 at 8:29am
Can't read any of the chips on the PCB.  Does it have a keyboard controller chip on it?  Something like an 8279c?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ace` Quote  Post ReplyReply Direct Link To This Post Posted: 17 Oct 2016 at 8:56am
This is where the keypad plugs in (small connector). Hard to tell from the picture but the pins on the PLCC socket don't look too good.



and you can see in this picture the correct way to plug in the keypad cable alan.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote alan8086 Quote  Post ReplyReply Direct Link To This Post Posted: 17 Oct 2016 at 12:09pm
- thanks very much for your replies, much appreciated!

Regarding the keyboard connector - its the other end I wasn't sure about, the keypad end. I hadn't noticed the metal retaining clip that locks the plug into the socket and managed to pull the socket/plug off its pins. It goes back in ok but I was a little unsure as to its orientation. I put it back so the red strip on the ribbon cable lined up with the notch on the solder mask marking and so that there was no twist in the cable when its all back together.

Regarding the PLCC socket - that is the one chip I haven't had out. I don't have the de socketing tool for it and didn't want to risk damage to it with small screw drivers etc. You may well be onto something there, Defiantly worth getting it out and inspecting the contacts. Is your machine the previous model? You have a Z80A in yours, on mine that PLCC chip is a CMOS Z80 derived CPU, a Z180 or something? Nice to see another similar machine anyhow :)

Re - the keyboard controller chip. I haven't seen a specific keyboard controller on the main board. Inside the querty keyboard, there is some ttl logic and discrete stuff plus some kind of micro controller made by NEC. I looked it up but didn't take any pics or remember its number. It had the components of a CPU, RAM and UV erasable ROM in one ceramic 20 pin dil package. A bit more in there than your standard USB/PS2 keyboard.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote alan8086 Quote  Post ReplyReply Direct Link To This Post Posted: 17 Oct 2016 at 12:11pm
By the way - the reason for me wanting to use this device in the first place was to diagnose a fault on one of the static RAM boards on my IMS 8000 computer. I am chuffed to say that I've diagnosed it to a faulty 5V power regulator. There are 4 on each board. I noticed a 3Kohm resistance across the ground plane and the 5V supply plane on one board where the other 3 boards just read open circuit - de socketed all the chips, de soldered one end of the bigger decoupling caps but finally noticed a low resistance across the source/drain pins of one regulator, an LM340T5. I removed it and the low resistance reading disappeared - 3 more LM340T5's on their way from eBay! :)

I would still like the logic tester back in action though. As well as its functional use, they have retro technology repair/historical value too!

Edited by alan8086 - 17 Oct 2016 at 12:26pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote alan8086 Quote  Post ReplyReply Direct Link To This Post Posted: 17 Oct 2016 at 12:17pm
I read through a post from another member on here about hacking the timekeeper battery/ROM and soldering in a normal BIOS battery - I may give that a go if I can get this thing working again.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ace` Quote  Post ReplyReply Direct Link To This Post Posted: 17 Oct 2016 at 12:27pm
You can get a plcc extractor dirt cheap off ebay, I got one free with something. The picture above is from a boardmaster 4000 which is a later version but they're very similar, I think it was even possible to get ABI to upgrade DDS-40 to a 4000 back in the day.

I tried holding down a key while powering up my 4000 to see if I could get the same error but it passed self test, so it looks like it is something on the main pcb rather than the keypad itself.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote alan8086 Quote  Post ReplyReply Direct Link To This Post Posted: 17 Oct 2016 at 12:38pm
I've been reading in another post about ROM Burners - I was going to get a TL866CS Programmer off eBay, cant afford anything more but I noticed it comes with a chip extractor. I'll have a go at removing the PLCC chip and see what's what from there.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote alan8086 Quote  Post ReplyReply Direct Link To This Post Posted: 19 Oct 2016 at 9:00am
Ok, I've removed the PLCC cpu chip - not a standard removal tool method btw. The contacts on the chip where quite dirty/pitted in places. I cleaned it up, replaced - but no difference :(

I did have the keypad cable the wrong way round at the keypad end. Accidentally pulling the plug off with the socket attached has caused some confusion. I have traced the +5v all the way back from the power input on the main board to where it enters the keypad board so its now oriented correctly - again, no difference.

I am wondering if I've killed something by having the plug in the wrong way. I've noticed some weirdness too - On the mainboard end, I've probed around the different pins in the keypad socket and found what I thought was the +5v pin but noticed that with everything turned off, that pin also shows continuity straight to ground - the metal casing etc. I can also not find any other pin that is either +5v or 0V. How can one pin be both unless there is a short to ground and if that is the case, how can it be working as well as it is? You would expect the PSU to trip out?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote alan8086 Quote  Post ReplyReply Direct Link To This Post Posted: 19 Oct 2016 at 9:09am
...I was wondering too if anyone who has one of these units could take a picture of where all the power cables screw into the terminals on the output side of the PSU. I'm certain I have them in the correct way but I don't understand why there is a big black lead screwed in to the anode case tab of the Thyristor next to the relay that goes to a 0v ground on the PSU? I have measured 5V between the two and you would think that would create a short? The psu has separate 0V and +5V supplies - one side for the mainboard etc and the other to the power sockets at the front of the unit that allow you to power a board under test. At the moment I have these disconnected to 'simplify' things. The unit still works as usual with just the essentials connected.

BTW - if this thread is getting too old/cluttering up the place, feel free to end it. I'll try and post something useful in return, although I'm more into 1970's/80's business machines than anything arcade related.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ace` Quote  Post ReplyReply Direct Link To This Post Posted: 19 Oct 2016 at 9:50am
What's the PLCC socket itself look like? the pins don't look too good in the picture above. You need to trace out the keypad connector and check everything connected to it (at least that's what I'd do).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote alan8086 Quote  Post ReplyReply Direct Link To This Post Posted: 19 Oct 2016 at 10:12am
The pins in the socket look quite clean and shiny - its the pins on the PLCC chip that look dull/pitted etc although I've cleaned them up somewhat. Probably worth another gentle go with some 1200 grade wet n dry or a wire brush on a dremel.

I have dug out the HP logic probe, logic pulsar and current tracer I bought along with the ABI unit when I first got my Intertec Superbrain. Having never used them I've just youtubed their use/function and feel a little daft now I can see their potential!! I will get to work tracing those keypad pins etc.

Thanks again for your continued replies.
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