Two Faulty 5160 boards

[SpidersWeb]

So I grabbed another pair of genuine XT's, the 256Kb models. Unknown condition, and hard drive info I estimate they've been turned off for 17 years and left in a garage. Both hard drives and all cards work fine.

Anyway board #1:

- after removing a cap on the 12V line (which I'd had to remove off my existing 5160 too) the resistance on the 12V line went back in to the kOhm range
- applied power and another 12V cap exploded but the machine booted.
- machine was running fine for hours, I'd fully screwed the machine back together and was just using it casually and went to reboot, after the reboot the RAM test stopped at 64K and the machine locked up, reboot and now it's 'dead'.
- there was no odour or hot components or weird noises
- installing the Lansoft 27256 ROM in U18 made no change
- on my diags card I get clock and all the voltage signals.
- PSU tested, all values within 5% including -5V
- replaced intel 8088 with an AMD 8088 - no change

Board #2
- had to remove the same cap on the 12V line so the PSU could start
- no sign of life
- removed EPROM and tested, verified OK
- installed LanSoft ROM in U18, continuous beep
- reinstalled original EPROM, no beep or life.
- reinstalled LanSoft ROM in U18, continuous beep
- diags card LEDs show clock and voltages
- using the same PSU as for board 1

Have tried with and without expansion cards, even bare, I get no beeps (except board 2 + LanSoft continuous tone)

I also have a 5150 board behaving the exact same way, and another 5150 board I ended up giving away that was the same.
So I really want to know what is going on here.

I just realised my UV eraser has blown it's bulb, so making more EPROMs may be tricky unless I find some more 27256 chips, but I plan to try out the test1/2/3 bin files.

Beyond that, any ideas??

 

[Crypticalcode0]

for one if a cap blows I would call the machine defect and shut it down replace the cap and try again.

 

[SpidersWeb]

I have those caps removed on my working 5160, and Board #1 was working reliably for hours before the fault with the caps removed. So I'm not confident that's the problem. I will be replacing them anyway, but my previous experience was that these specific capacitors are not crucial to operation.

 

[Crypticalcode0]

A good engineer makes a design with a good margin 10% of operational deviation.
Those Capacitors were probably there to keep the Voltage stable on the board and that makes them none critical when the PSU doesn't act funny.
However some of the older chips are more sensitive to power fluctuations and that can and will eventually cause failure in one of these components.(Honestly though the more components in the first place the more that can break)

Which is terrible when you start to think about it, since most of the critical components have been filed as EoL product well over 20 years ago.

 

[modem7]

In this particular case (IBM 5160 motherboard), missing/open 12V caps do not affect motherboard start/operation, and that is where SpidersWeb's concern is at the moment.

On this motherboard, +/- 12V (and -5V) is not used by the motherboard and is passed through to the expansion connectors. +5V caps are a different story, particularly the ones near RAM.

 

[SpidersWeb]

In this particular case (IBM 5160 motherboard), missing/open 12V caps do not affect motherboard start/operation, and that is where SpidersWeb's concern is at the moment.

On this motherboard, +/- 12V (and -5V) is not used by the motherboard and is passed through to the expansion connectors. +5V caps are a different story, particularly the ones near RAM.

Unfortunatley I did make a mistake there, I just checked and one of the removed caps is on the 5V line but it's near the power connector - C58. It popped after initial power on.

Working 5160 (1986) has C58 C55 and C56 removed - C58 was gone before I even aquired the machine
Board #1 (1985) has C58 and C56 removed
Board #2 (1984) has C56 removed

Going to order more caps and a replacement UV bulb this week to help with ROM erasing. I'm guessing, beyond replacing the caps and trying test ROMs, I'm a little screwed without a scope or logic analyser :/ so any ideas or theories welcomed, happy to do any test even if it's a long shot.

I have verified the original BIOS chips and my 27256 LanSoft ROM though.

So far from these two complete machines, it's only the motherboards that are faulty, even the ST412+Xebec booted up PC DOS 3.2 (command.com dated 1985). They've also been handled carefully and with me attached to the case by strap.

 

[Crypticalcode0]

pull out your multi meter and measure resistance of a living system and measure resistance of those systems.
They should not be too far apart.

 

[SpidersWeb]

pull out your multi meter and measure resistance of a living system and measure resistance of those systems.
They should not be too far apart.

From memory both boards are within working range on all lines, but I can't remember the actual values, I will retest and post actual values tonight. For a proper restoration, I'd think every cap on all three boards would need replacement.

I had to check the values before power up to look for shorts, I think (from memory) 5V was in the 200-215 ohm range, which is really low but (from memory again) higher than my working board @ 160 ohms. Also inspected C58 on Board #2 - it needs replacing - I can see a big black mark coming up - so I will replace that before applying power again.

Interesting that I have 3 boards, made in different years, and the exact same capacitors are failing or have failed (C56 and C58 ).

I just had my desoldering iron with electric pump arrive, so replacing the caps will be a good exercise.

 

[modem7]

Unfortunatley I did make a mistake there, I just checked and one of the removed caps is on the 5V line but it's near the power connector - C58.

But C58 on a 5160 motherboard (both types) is on the -12V line.

Interesting that I have 3 boards, made in different years, and the exact same capacitors are failing or have failed (C56 and C58 ).

Not when compared to previous capacitor failures: http://www.minuszerodegrees.net/vcf_motherboard_failure_history.htm

beyond replacing the caps and trying test ROMs, I'm a little screwed without a scope or logic analyser :/ so any ideas or theories welcomed, happy to do any test even if it's a long shot.

Since the Supersoft/Landmark ROM doesn't run, and considering other things you've done, I see the next logical step being the use of the TEST1 ROM. It's very simple; output 33 to the POST card. It's going to tell you a lot about the state of each board, and therefore in which direction to head.

So I think that you should simply wait now until you're in a position to run the TEST1 ROM.

By the way. If things don't work out, note that I do chip level repair on these boards, however maybe two way travel over the Tasman Sea is uneconomical.

 

[modem7]

I had to check the values before power up to look for shorts, I think (from memory) 5V was in the 200-215 ohm range, which is really low but (from memory again) higher than my working board @ 160 ohms.

My boards vary considerably. Measuring passive resistance in a largely active circuit is really only useful when the power supply is not starting, and you need to determine if a short is responsible (and if so, on which voltage line/rail).

 

[SpidersWeb]

EPROM:
Well I have found spare 27C256 chips, just need to adjust the voltage on my EPROM writer. They're brand new, so are blank. I did a COPY /B test1.bin + test1.bin +test1.bin +test1.bin test_32K.bin to create the 32KB file but haven't copied it to an EPROM yet.

If that is a wrong move let me know, the test1.bin I grabbed was 8KB for a 5150. I figured popping it in 4 times will improve my chances of hitting the right start point. I have lots of cleaning and crud to do when I get home, so might not be for another day or two.

CAPS:
I've ordered 40 Tantalum caps, 10uF 16V, so I can replace that model on the boards. Should be doing that this weekend.

MACHINES:
I've ordered two replacment 5160 motherboards from a member off here, so I can get the units themselves running and enjoy them while I sort out their original motherboards.

modem7 - if I'm unable to repair these, I'd be very keen to send them to someone like yourself, because even if I have replaced them I want to know what is causing these dead-board failures. If it was just one I'd ignore it, but 4/6!

My history of dead PC / XT boards is:
- 5150 256KB - 1986 - left in storage for a few years - when removed dead board - LanSoft ROM (with adaptor) and CPU change - no luck
- 5150 256KB - 1983ish - purchased off ebay in working/tested condition, arrived dead, did the same ROM + CPU tests, no change
- 5160 256K - 1983 - a.k.a. Board #2 - 17ish years in garage arrived with shorted C56, dead (as per this thread)
- 5160 256K - 1984 - a.k.a. Board #1 - 17ish years in garage arrived with shorted C56, works 4 hours then dead (as per this thread).

- 5160 640K - 1987 - worked fine, C56 started shorting at 4am one night waking up the house, removed and runs fine now
- 5150 256K - 1983 - purchased off a VCF user, worked perfectly since I received it, no missing or damaged caps.

I have run the LanSoft 256 chip in XT machines before with success, so I know I'm doing that right, and after CPU swaps the processor goes back to the working machine, so I know I'm doing that right too.

 

[SpidersWeb]

So I think that you should simply wait now until you're in a position to run the TEST1 ROM.

Yep will run the TEST1.ROM first and see what happens and report back.

My impatience already ordered the boards, I told it 'no' but it just would not listen to me.

 

[modem7]

Well I have found spare 27C256 chips, just need to adjust the voltage on my EPROM writer. They're brand new, so are blank. I did a COPY /B test1.bin + test1.bin +test1.bin +test1.bin test_32K.bin to create the 32KB file but haven't copied it to an EPROM yet. If that is a wrong move let me know, the test1.bin I grabbed was 8KB for a 5150. I figured popping it in 4 times will improve my chances of hitting the right start point.

That is the correct thing to do.

 

[SpidersWeb]

Neither are responding to test1. No output.

Board #2 I found a damaged cap on the 5V line, and replaced the row with new caps, but no change. It just beeps continuously. On my diags board at power up, Reset lights up, then goes off and IRDY comes up - it's when IRDY comes up the beep begins.

Board #1 I checked the speaker output, 5V DC measured between pins 1 and 4.
Any idea what components to look at first that could be causing that? I can't see any visible shorts.

I have about 3 10uF and 10 4.7uF caps left - 16V two pin titanium

 

[modem7]

Board #1 I checked the speaker output, 5V DC measured between pins 1 and 4.
Any idea what components to look at first that could be causing that? I can't see any visible shorts.

The failure of TEST#1 to display 33 on a POST card means that the cause is low level.
That also means that the POST and initialisation code in the IBM BIOS ROM is not being executed at power on.
With no initialisation code running, certain chips will not be initialised, and that can result in certain lines not sitting at their normal TTL level.
The speaker driver signal can be traced back to two chips, the 8255 and 8253, that we now know are not being initialised.