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Incompatability of AST Sixpackplus and NEC V20 Experienced with Clone XT

S

SailorEd

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Feb 17, 2015
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Recently reloacted to AZ from Thailand
As noted in previous posts, I have also experienced the incompatibility of the SIxPackPlus board with the NEC V20 CPU chip.
Booted OK until installed board. When installed, NOTHING; only a few rotations of fan in switching power supply. Remove board and boots OK.
Wanted to install card to expand memory from main board of 256K up to 640K. I have now ordered Intel 8088 on e-bay to see if the SixPackPlus will boot on it.
It is a clone system with two floppy, HDD and no math co-processor.
 
SailorEd said:
When installed, NOTHING; only a few rotations of fan in switching power supply. Remove board and boots OK.
Click to expand...
Classic symptoms of a short circuit in the card (which overloads the power supply, stopping it from working).
The short circuit is possibly in one of the tantalum capacitors.
 
SailorEd said:
Thanks for the suggestion. I have the ohn meter but not the knowledge to do the test. Would appreciate your advise on how to do it or where to go for the test procedure. Thanks Ed
Click to expand...
1. Remove card from computer.
2. Turn card over so that the solder side of the card is showing.
3. Put mutimeter into resistance mode.
4. Refer to the diagram at [here].
5. Measure the resistance of the +12V line by placing one of the meter probes onto the +12V pin, and the other probe onto either of the two ground lines. Note the resistance.
6. Repeat step 5, except use the -12V pin. Note the resistance.
7. Repeat step 5, except use the first +5V pin. Note the resistance.
8. Repeat step 5, except use the second +5V pin. Note the resistance.
9. Repeat step 5, except use the -5V pin. Note the resistance.

If any of the resistance measurements are zero or a few ohms, then there is definitely a short citcuit (on the measured line).
Perhaps report back your measurements.


Note: Both +5V lines are measured, just in case a card maker decided to use only one (i.e. because of very low +5V consumption).

Note: There is an assumption that the card maker has used both grounds on the solder side of the card, which is what I would expect. That can be proven by verifying that there is zero ohms between the two grounds.
 
I prefer to use a slightly simpler method. This involves testing for shorts where the PSU connects to the motherboard with the power off.

First, remove all the cards from their slots. Then check the +5V, -5V, +12V and -12V connections for shorts to ground. If all is good there, insert the questionable card into a slot and repeat the test for shorts at the same four locations as just done. If there is now a short on one (or more) of the power rails it is due to the card you just inserted.
 
I have checked the resisance and got these results 20K scale Pin to Grd: 3-.29 5-open 7-0 9-0 29-.29
When using 20M scale appears that capacitors where being charged as readings on on 9 and seemed to start about 1.9 and go toward 0
Does this tells us anything about the condition of the board?
thanks
Ed
 
SailorEd said:
I have checked the resisance and got these results 20K scale Pin to Grd: 3-.29 5-open 7-0 9-0 29-.29
When using 20M scale appears that capacitors where being charged as readings on on 9 and seemed to start about 1.9 and go toward 0
Does this tells us anything about the condition of the board?
Click to expand...
Well, the +5V and -5V appear to be okay.

The +12V and -12V show "0", suggesting a short circuit on both lines, however, it is highly unlikely that that is the case (both short).
I see that you are using a high scale. What do you read when you use the 20 ohm scale ?
 
SailorEd said:
Checked using 200 ohm scale (no 20 scale on meter) with following results: 3-Open; 5-Open; 7-7.4; 9-Open; 29-Open.
Thanks for you input on what to do next?
Click to expand...
Partial short on the -12V line. Could be capacitor; could be something else. But forum history weighs heavily towards capacitor.

You can prove the 'short circuit on -12V line' hypothesis for yourself. Put a piece of sticky tape over the B7 contact, and then carefully (so as not to affect the tape) insert the card back into your computer. If the computer starts, it proves the hypothesis.

Provide us with a photo of your SixPakPlus card, or, point us to a photo of same on the Internet. If your SixPakPlus matches mine, I am in a better position to advise further.

Also, if you have not already, do a detailed visual inspection of the card, because the faulty component may be visually showing as damaged. With the capacitors, you are looking for signs like:
http://www.minuszerodegrees.net/images/bad_tantulum_1.jpg
http://www.minuszerodegrees.net/failure/faulty_decoupling_cap.jpg
 
I physically examined both sides of card and see no physical deterioration of any components or leads touching.
I placed tape over the B7 contact and installed installed card. Computer started OK!! Of course did not recognize card had been installed.
I have attached pic of board.
Thanks again for all the help.
image1.jpgimage1(1).jpg
 
SailorEd said:
I have attached pic of board.
Click to expand...
Your SixPakPlus card matches mine. Down near the edge connector are three blue tantalum capacitors, C44, C45, and C43. The middle one, C45, sits on the -12V line. Remove it (unsolder, or clip off) to see if it is the source of the partial short.

By the way, when you remove the sticky tape, it is a good idea to clean the contact in order to remove any 'sticky' residue.

SailorEd said:
Computer started OK!! Of course did not recognize card had been installed.
Click to expand...
I expect that the -12V is only being used to support the serial port. I covered up the B7 contact on my card, and as I expected, my card's RAM was still seen at power-on. That suggests that your card has additional issues. Switch/jumper configuration? Bad RAM in bank 0.
 
Thank you for checking with your card. I will continue forward with looking into ram and switches/jumpers. Seller of card told me it was removed from machine with 256K on the main board the same as my machine so I expected switches to be set correctly.

I noticed that the sixth row of ram chips (counting from the end of the card inward) is different from the other five rows. The sixth row of chips are MCM6665BP20 all of the other chips are TMM4164AP-15. This is correct?

Before getting your comments, today I checked the resistance of the three capacitors and the C45 is the only one that shows resistance. Tomorrow I will try to remove it from the card and see what happens. Will let you know what happens.

Thanks again,
 
SailorEd said:
Before getting your comments, today I checked the resistance of the three capacitors and the C45 is the only one that shows resistance.
Click to expand...
Stone is correct. Making a measurement of C45 with it in-circuit, results in a measurement of the combined resistance of all devices on the -12V line, not just C45.

SailorEd said:
I noticed that the sixth row of ram chips (counting from the end of the card inward) is different from the other five rows. The sixth row of chips are MCM6665BP20 all of the other chips are TMM4164AP-15. This is correct?
Click to expand...
Not a problem. My card uses a mixture of both MCM6665BP20 and TMS4164-15NL.

SailorEd said:
Thank you for checking with your card. I will continue forward with looking into ram and switches/jumpers. Seller of card told me it was removed from machine with 256K on the main board the same as my machine so I expected switches to be set correctly.
Click to expand...
On my card, switch 7 on the switch bank is off. Looking at the documentation for the early versions of the card, switch 7 does not do anything, and there is nothing to indicate if switch 7 needs to be left off, or left on. You could try switch 7 off and see if it makes a difference, but I don't think it will.
 
Making Progress. Removed C45, checked with Ohm meter and short was gone. Installed in machine and it booted through the ram check and reported error as follows:
512K OK
08 RAM Bad 80004

Does this indicate location of bad chip? How are the banks of ram and chips identified on the AST card?

Thanks again for all the help getting to this point. I have some additional ram chips that I expect are OK; but not 100% sure.
 
It looks like the error is in the 5th bank. You can verify this by switching the 5th bank chips with the sixth bank chips. If the error moves accordingly you will know that there is a bad chip in the (now) 6th bank. Replacing those chips one by one with a known good chip will locate the bad one.
 
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