PCjr Monitor Issue

Hardware questions and modifications

PCjr Monitor Issue

Postby codavec » Sun Jan 28, 2018 7:09 pm

Hi guys. I just cleaned up my new-to-me PCjr monitor tonight, and connected it to the PCjr to test operation after replacing all the motherboard and power supply capacitors. Unfortunately, when I power on the monitor, the speaker emits a low frequency hum. The pitch changes when the computer is powered on. The monitor works otherwise, although I did have to adjust the V.SYNC knob to keep the image from vertically scrolling. Could the hum be attributed to aged electrolytic capacitors. I removed the rear cover and didn't see any bulging caps, but perhaps they're bad anyway?

Has anyone recapped the analog board in one of these monitors before? Disassembly seems a bit intimidating, so I'm reluctant strip it down to where I can remove the analog board. It's not that I'm afraid of the CRT, I just fear damaging the CRT neck, or other parts that hold everything together.
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Re: PCjr Monitor Issue

Postby codavec » Mon Jan 29, 2018 10:27 pm

I went ahead and disassembled the monitor to the point that I was able to remove the analog board. I'll be recapping both the analog board and neck board. I'm no electronics guru, but I speculate that there are capacitors responsible for filtering low frequency noise that could works its way into the audio circuitry. Anyway, I will update this thread again once the recap job is complete.
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Re: PCjr Monitor Issue

Postby Hargle » Tue Jan 30, 2018 6:51 am

I have at least 3 monitors that have varying symptoms that I am blaming bad/blown caps to, so whatever process you go through, the more documentation you can provide to us the better. A replacement caps list from Mouser or digikey or where ever you end up getting replacement pieces from would be absolutely welcome. thank you and good luck!
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Re: PCjr Monitor Issue

Postby codavec » Tue Jan 30, 2018 3:01 pm

I’ll compile a list of replacement capacitors and post it here. There are quite a few!
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Re: PCjr Monitor Issue

Postby codavec » Tue Jan 30, 2018 7:04 pm

So... As I was removing a few caps from the board, it didn't take long to find signs of leakage. If you look closely at the image, you can see a darker area in the middle of where the capacitor sits. That is cap goo.

For reference, the location on the board is labeled C953. The capacitor value is 330uF 180V. Don't mind the one missing at C304... I snipped it out so I could read the values.

Image
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Re: PCjr Monitor Issue

Postby geoffdaddy » Wed Jan 31, 2018 10:25 am

Just a note of caution... I know it's popular to replace electrolytic capacitors with tantalum caps on motherboards, (and in many cases that is definitely appropriate and OK), however in the case of your monitor I wouldn't do any such substitution. Tantalum caps can fail in dramatic ways, and they are less tolerant of ripple than their electrolytic brethren.
PCjr: 10MB RACore 01 hard drive expansion, Hotshot 640kB RAM expander, optical mouse, Xircom PE3, speech sidecar.
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Re: PCjr Monitor Issue

Postby codavec » Wed Jan 31, 2018 1:42 pm

Thanks for the tip. I had no intentions of swapping tantalum caps in place of the electrolytics. Replacement electrolytics are en route. I did specify that all replacements be rated for 105C instead of 85C.
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Re: PCjr Monitor Issue

Postby codavec » Fri Feb 02, 2018 6:42 am

The recap is 99% complete. I just overlooked a cap that’s hidden around the flyback transformer. Digging through my capacitor stash I found a cap with the correct uF rating, and one step up in voltage level. While it will work, it’s a cheapo cap that I purchased from RadioShack sometime ago. So, I’ve got a name brand cap on the way.

More updates to come.
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Re: PCjr Monitor Issue

Postby codavec » Sun Feb 04, 2018 5:48 pm

I finally had some time this weekend to compile a list of capacitors needed for the PCjr Display. A fully completed recap will include both the analog board (power board, whatever you want to call it), and the neck board. The neck board has a protective shield in place that must be desoldered before the capacitors can be removed. The analog board also has a cover that is held in place with plastic standoff clips.You'll need to desolder the ground strap from this cover before removing it to replace the analog board capacitors. On initial disassembly, there were no obvious signs of cap leakage. However, as I began removing caps, I found at least three that were leaking from the bottom. As always, exercise caution when working within a CRT monitor. High voltages may be present, yadda, yadda, yadda. Having powered up the monitor the day before disassembly, I made sure to bleed off the flyback transformer. If any of you aren't sure what this means, an Internet search will provide you with the information you need. In my case, there was no popping sound, or crazy sparks. There may be a bleed-off resistor in place to discharge the flyback, but I wouldn't count on it. Safety first, folks!

Anyway, on to the stuff you're really here for.

**UPDATE (March 4, 2018) - Here is a link to a layout diagram I created. It's available in Google Docs in PDF format.

The neck board has seven capacitors (in no particular order -- it was just the order in which I wrote them down):

1. C201 - 330uF 10v
2. C205 - 100uF 25v
3. C204 - 470uF 6.3v
4. C203 - 200uF 10v
5. C607 - 100uF 25v
6. C601 - 4.7uF 250v
7. C209 - 0.1uF 50v

The analog board has 25 capacitors (again, in no particular order):

1. C405 - 3.3uF 50v
2. C303 - 100uF 10v
3. C302 - 10uF 50v
4. C301 - 330uF 16v
5. C955 - 330uF 16v
6. C304 - 4.7uF 50v
7. C515 - 1uF 100v
8. C529 - 10uF 50v
9. C953 - 330uF 180v (LEAKING)
10. C530 - 10uF 160v (LEAKING)
11. C502 - 1uF 50v
12. C501 - 4.7uF 50v
13. C909 - 22uF 50v
14. C913 - 47uF 25v
15. C908 - 2.2uF 50v
16. C906 - 150uF 250v
17. C524 - 10uF 50v
18. C519 - 22uF 25v
19. C412 - 470uF 16v
20. C409 - 100uF 100v
21. C408 - 1000uF 6.3v
22. C407 - 3.3uF 350v (LEAKING)
23. C512 - 22uF 200v
24. C406 - 100uF 10v
25. C514 - 470uF 25v
Last edited by codavec on Sun Mar 04, 2018 2:33 pm, edited 2 times in total.
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Re: PCjr Monitor Issue

Postby codavec » Fri Feb 09, 2018 10:00 pm

SOLVED!

The replacement capacitors took care of the low frequency hum. For anyone else experiencing odd issues with their IBM 4863 displays, it may be a good idea to replace all the electrolytic capacitors before you send it to the scrap yard.
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