Bringing one back to life.

Hardware questions and modifications

Re: Bringing one back to life.

Postby geoffdaddy » Tue Jul 07, 2015 2:28 pm

One suggestion I would make for you is remove the motherboard from the case, and do a thorough inspection of all solder joints first thing. I found a few that didn't look too good in my initial attempt to repair mine. Also, try running the motherboard out of the case with the power supply board attached so that you have a little better access to things. On mine I attempted to press firmly (but not too hard!) on various parts of the motherboard/chips while cycling power off and on. That's mainly how I isolated my problem when I found the fault with the CAS delay chip. It would work with a moderate amount of pressure on that chip (actually a fancy potted transformer that is in a DIP package) so there must have been an internal fault in that part. Using my oscilloscope after I isolated the problem area revealed the issue. The replacement I got works a champ and still works great to this day.
PCjr: 10MB RACore 01 hard drive expansion, Hotshot 640kB RAM expander, optical mouse, Xircom PE3, speech sidecar.
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Re: Bringing one back to life.

Postby Trixter » Tue Jul 07, 2015 2:36 pm

Stonent wrote:Again, symptoms are display lights up but just grey (as in the screen is blank but the brightness makes it look grey) and a continuous tone from the audio.


Mine did that when I was trying to use a 640KB upgrade daughterboard (sandwiches between the original CPU socket and a new CPU). I also found I got the exact same response if the PCjr had no CPU in it at all. Hope that helps...
You're all insane and trying to steal my magic bag!
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Re: Bringing one back to life.

Postby jharre » Tue Jul 07, 2015 9:05 pm

Have you tried a different display?

The composite video output on the back of the PCjr (marked "V") can feed a display or TV set (or an old VCR) with a composite input. You just need a simple cable with an RCA plug on both ends. Even an old audio cable will work.

It sure won't be the finest video quality, but it might be enough to see if your computer wakes up and spits out error codes.

Jim
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Re: Bringing one back to life.

Postby Stonent » Thu Jul 09, 2015 11:12 am

Thank you, that should be extremely helpful to me considering there's no ISA slot to plug a POST reader into (and I'd have to buy one first)

(This was in response to the post about reading codes from the serial port)
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Re: Bringing one back to life.

Postby Stonent » Thu Jul 09, 2015 1:16 pm

jharre wrote:Have you tried a different display?

The composite video output on the back of the PCjr (marked "V") can feed a display or TV set (or an old VCR) with a composite input. You just need a simple cable with an RCA plug on both ends. Even an old audio cable will work.

It sure won't be the finest video quality, but it might be enough to see if your computer wakes up and spits out error codes.

Jim


Yep zip on every TV I've connected it to. (RCA cable to the yellow video connector with composite selected)

And when I probe the Composite port on the PCJR with my oscilloscope, I don't see a NTSC wave form like I do in this video where I probed my Raspberry Pi.

http://www.youtube.com/watch?v=MhbC_YgKvNU (This is the video of an NTSC signal from a Pi, not my PCjr)
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Re: Bringing one back to life.

Postby Stonent » Thu Jul 09, 2015 1:20 pm

geoffdaddy wrote:One suggestion I would make for you is remove the motherboard from the case, and do a thorough inspection of all solder joints first thing. I found a few that didn't look too good in my initial attempt to repair mine. Also, try running the motherboard out of the case with the power supply board attached so that you have a little better access to things. On mine I attempted to press firmly (but not too hard!) on various parts of the motherboard/chips while cycling power off and on. That's mainly how I isolated my problem when I found the fault with the CAS delay chip. It would work with a moderate amount of pressure on that chip (actually a fancy potted transformer that is in a DIP package) so there must have been an internal fault in that part. Using my oscilloscope after I isolated the problem area revealed the issue. The replacement I got works a champ and still works great to this day.


It's been outside the case the whole time. I checked joints and the all look perfect. In my pressing around I did notice the 74LS04 chip that's next to the end of the 8088 processor is very hot.

I checked voltages and got this:

Image

Since that's just an inverter, I would think that you'd have pairs of high and low pins. So either that chip is bad, or something near the chip is misbehaving.

Pins one and two look like they may be ok. Pins 12 and 13 concern me a little bit. 11 and 10 look almost shorted, as well as 5 and 6. 9 and 8 I'm not sure if the voltage is too low to invert. And 3 and 4 I'm not sure either.

I can't tell on the schematic which LS04 that chip is because there seem to be at least 2 or 3 that interface to the CPU. The board layout, I can't read the lettering for it.

Oh regarding your CAS delay chip. Is that the odd looking chip near the RAM? I think I tried looking it up and just got some old documents that called it a digital delay line.
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Re: Bringing one back to life.

Postby Stonent » Thu Jul 09, 2015 6:21 pm

Here's a video I did.

http://www.youtube.com/watch?v=NxkZ3nRdNss

Can anyone find pin 12/13 on the schematic? I can't find them anywhere. ZM44. Most of ZM44 is on the first page but that's just 4 of the 6 inverters accounted for.
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