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Time to give up? (Video Issues Now)


Mtlatc
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I don't think that component is the 2-4.5uH inductor - that's the other large component next to the crystal with the adjustable core. I think what you have pictured is L107 (labelled as L207 in the TI console schematic) which is an 'SR inductor'. I'm not sure what that actually is. Can't find a picture of anything similar on Google.

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1 hour ago, Stuart said:

I don't think that component is the 2-4.5uH inductor - that's the other large component next to the crystal with the adjustable core. I think what you have pictured is L107 (labelled as L207 in the TI console schematic) which is an 'SR inductor'. I'm not sure what that actually is. Can't find a picture of anything similar on Google.

I’m sure you are right. It all seems to be coming together now. Thank you very much for your response. 

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31 minutes ago, Duewester said:

I think that is C101 a 12pf capacitor.

 

 

Screenshot_20230121-141837.png

Screenshot_20230121-142048.png

Screenshot_20230121-142246.png

Thank you very much for posting this drawing.  I have three manuals a don’t believe I’ve seen this one. Thanks again

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12 minutes ago, ti99iuc said:

Xad Nightfall replied you with the link of the exact component you will need to purchase :)

Awesome! Just checked.  Even figured out how to translate the site to English.😁 

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@Duewester The drawings you have clearly show a 12pF Cap.  I am being told this is the replacement part I need. What do you think?

 

https://www.pchub.com/uph-inductor-inductor-color-loop-axial-lead-14w-33uh-al0307-3r3k-p77788

 

Do you think this link is for the coil?

 

Thanks.

Edited by Mtlatc
pF not uF
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I've checked on a physical circuit board, and the component you originally pictured is L107 (or L207 depending on which document you look at). Referring to the photo of the circuit board above, +5V is coming in on the blue wire, it comes down and through R102 which is located just below L107, through L107 and across to one of the pins of the crystal and some other components.

 

The markings on L107 appear to be brown-black-brown which I think is 100uH. Not sure what the "S.R." means on the circuit diagram. You might want to find a similar item to the one you linked to that is closer to this value. But as it is only used for filtering the +5V supply, you might find that pretty much anything works.

 

Anyone any idea about why the circuit has that +5V feed into one side of the crystal? Such a requirement is not shown in the data manual, and none of the other computers that use the TMS9918 whose circuit I've seen have the same arrangement. Or is anything we don't understand always down to "to reduce RF emissions"?  ;-)

 

 

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Replying to myself: "Anyone any idea about why the circuit has that +5V feed into one side of the crystal? Such a requirement is not shown in the data manual, and none of the other computers that use the TMS9918 whose circuit I've seen have the same arrangement. Or is anything we don't understand always down to "to reduce RF emissions"?"

 

Looks like the crystal circuit is configured as an external oscillator source to the VDP, rather than the internal oscillator 'driving' the crystal.

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Just guessing... that this circuit is using Inductive Reactance, to achieve Series Resonance(S.R.).

R102, C100, L207, form an RCL, Tuned Circuit.

 

Series Resonance Circuit

 

Looks like T.I., switched to Capacitive Reactance, for the QI, model.

 

image.jpeg.47accfd9d2d2585878e08ce92076022c.jpeg

 

Might be an easy-do!

Replace R102, L207, with a 22pf capacitor.

 

--more--

 

Frerking - Crystal oscillator Design and Temperature Compensation 1978.pdf

 

EDIT: The downward pointed triangles in the QI's diagram, are GND. not VCC!

Edited by HOME AUTOMATION
linked pic. to PDF.
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On 1/20/2023 at 5:27 PM, Mtlatc said:

470441B2-DB4B-42CC-9D1A-B59553B7E79C.jpeg

I had a huge bag of these, when I was a kid. I think they came from R/S.

AIR the coil wraps around the connection lead at the top end of the device ...so this might be ok, it should have a low resistance.

Wonder if you can get any picture w/o it.:ponder:

Edited by HOME AUTOMATION
oops, pressed save too soon, still commenting... Done.
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I was/sm a whole column off in my assessment. On one page that is l207 on the component location page it is l107. Either way I'm all wrong with my assessment. 

Looks like it is indeed an inductor, but I've not seen one like it before. If the pros have a viable alternative they recommend, I'd follow their advice. My moral is "don't try to use your little phone screen to examine big docs and expect good results."

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