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XT clone not booting (it prevents the PSU from starting).

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flaviosr

Experienced Member
Joined
Aug 27, 2016
Messages
266
Location
Turin, Italy
Dear Group,
I have a NoName clone with a strange behavior.
If the PSU is not connected to the mobo and I turn it on, it starts and, for example, the HDD and the FDD turn on.
If the PSU is connected to the mobo and I turn it on, nothing happens... the PSU remains turned off.
Something in the mobo or in the PSU (or both)?
Thank you
 

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flaviosr said:
I have a NoName clone with a strange behavior.
Click to expand...
That board looks VERY close to a motherboard that I have.
See the 'X'GOLDEN TURBO-BOARD' entry at [here].

flaviosr said:
If the PSU is not connected to the mobo and I turn it on, it starts and, for example, the HDD and the FDD turn on.
If the PSU is connected to the mobo and I turn it on, nothing happens... the PSU remains turned off.
Something in the mobo or in the PSU (or both)?
Click to expand...
Most experienced readers will be saying to themselves, "These are classic symptoms of a tantalum capacitor that has gone short circuit. And it is probably one on the +12V line or the -12V line."

We can see if that is the case by getting you to do some resistance checks.

Take a look at the web page at [here]. That page is for an IBM 5150 or 5160, but your motherboard uses the same pin numbering. Do the for resistance checks and report the four results back here. We expect that you will find zero (or a few ohms) on either the +12V line, or the -12v line. ( Do not click on the links on that web page. Simply make the measurements then report them back here. )
 
Assumption: Short-circuit tantalum capacitor on either the +12V line or the -12V line.

• On my motherboard, I traced the +12V line to tantalum capacitor C3.
• On my motherboard, I traced the -12V line to tantalum capacitor C4.

1708328150454.png
 
modem7 said:
That board looks VERY close to a motherboard that I have.
See the 'X'GOLDEN TURBO-BOARD' entry at [here].


Most experienced readers will be saying to themselves, "These are classic symptoms of a tantalum capacitor that has gone short circuit. And it is probably one on the +12V line or the -12V line."

We can see if that is the case by getting you to do some resistance checks.

Take a look at the web page at [here]. That page is for an IBM 5150 or 5160, but your motherboard uses the same pin numbering. Do the for resistance checks and report the four results back here. We expect that you will find zero (or a few ohms) on either the +12V line, or the -12v line. ( Do not click on the links on that web page. Simply make the measurements then report them back here. )
Click to expand...
(Obviusly) you are right. Numbering the tests from 1 (top) to 4 (bottom) I have the following measurements:
1 // 384 Ω
2 // OL (Out of Limit)
3 // OL
4 // 0.3 Ω
The -12V line has a short circuit. :)
 
flaviosr said:
The -12V line has a short circuit. :)
Click to expand...
The XT-class motherboard does not use -12V itself. It simply passes the -12V from the PSU through to the ISA slots. C4 is present on the -12V line to do some noise filtering.

Remove C4, and then see if the short-circuit has disappeared.
 
That cap is not required for the machine to boot and/or run.

Hook up the rest of whatever you disconnected and see if it powers up.
 
The computer started but C3 exploded.
I have replaced C3 and C4 with electrolytic capacitors 10 uF @ 25V (the nearest I have) and the computer starts but it makes intermittent beeps continuously, not booting. Nothing on the screen. I have noticed that C3- is connected to C4+ and both to one pin of JP1 that I suppose if is the reset button. Any ideas?
 

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flaviosr said:
I have replaced C3 and C4 with electrolytic capacitors 10 uF @ 25V (the nearest I have) and the computer starts but it makes intermittent beeps continuously, not booting.
Click to expand...
There are many socketed chips. Have you re-seated them all?

In the re-seating process, be sure not to accidentally bend a pin up under the chip. Even now, there could be a chip where a pin is bent up under the chip, with the pin only just making contact with the mating contact in the IC socket.
 
Thank you for the suggestion. I am leaving for a business trip and I will do this weekend when I am back.
I keep you info.
 
I see "WTF-860221-A" printed on your motherboard. That is possibly a model/part number. I am recording it in this post in case someone with the same motherboard does an online search using that number.
 
I resocketed all chips (RAM chips apart, assuming that a bad RAM chip would not prevent the computer to start) and the computer behaves in the same way: it boots and after some seconds it start a continuous intermittend beeping.
I have seen that one BIOS is different than the other and "I hope" it could be with some issues... this ROM is in reality a EPROM.
Now I would like to compare the BIOS with the original one... I have only to find them! :-o
If any other idea, it is welcome!
 
flaviosr said:
I resocketed all chips (RAM chips apart, assuming that a bad RAM chip would not prevent the computer to start) and the computer behaves in the same way: it boots and after some seconds it start a continuous intermittent beeping.
Click to expand...
Different people have different definitions of 'start' and 'boot'. By your "boots", does that mean that you are seeing something on the screen ?

flaviosr said:
I have seen that one BIOS is different than the other and "I hope" it could be with some issues... this ROM is in reality a EPROM.
Click to expand...
The EPROM (a type of ROM) that has white sticker ("V3.3") is the motherboard BIOS.

The four EPROM's that have a green sticker are not BIOS chips. They are sure to be BASIC, possibly copies of the four Cassette BASIC ROM's found on the IBM 5150 motherboard. They are not required by your BIOS . For the purpose of diagnosis, you should remove them from the motherboard, just in case one is causing the problem. You can put them back after the motherboard is fixed.

And the motherboard BIOS only requires RAM bank 0 to be fitted, and so I suggest that you remove banks 1,2, and 3.

flaviosr said:
Now I would like to compare the BIOS with the original one... I have only to find them! :-o
Click to expand...
You could use the 'T U R B O - XT 1986 Speed 4.77/8.00MHz Version 3.10' BIOS, the BIOS from [here].

And you could create a 'Ruuds Diagnostic ROM', seeing what that shows. See [here].
 
Thank you for your help. I erased the EPROM and burnt the v3.1 and now the computer starts!
I have to reconnect everything and today I cannot but I am happy! :)
 

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