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Looking for a replacement part.

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robotBuilder
(@robotbuilder)
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@mtlatc 

If you are sure you the component is faulty and you will not be using it again you can always dissect it and see what the bulge is.

 


   
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mtlatc
(@mtlatc)
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Posted by: @davee

Hi @mtlatc ,

That new picture does complicate the situation.

Sorry, but I think the only way to solve the situation is for you do some actual circuit tracing to identify the parts and compare with the schematics. Starting with the schematics, you probably only have to trace 4-5 parts.

Best wishes,

Dave

Unfortunately, I am not that confident in my ability to doing the circuit tracing. I have already (most likely) confused C101 with L100.

 


   
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mtlatc
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Posted by: @robotbuilder

@mtlatc 

If you are sure you the component is faulty and you will not be using it again you can always dissect it and see what the bulge is.

 

 

Not doing that. LOL. I'm not absolutely sure it is the fault. I have ordered both the parts suggested here, and the other post.  Additionally, it has been suggested that my issue is really the crystal, so for fun, I ordered one of those also.  

 


   
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(@davee)
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@Hi mtlatc 

Re I am not that confident in my ability to doing the circuit tracing. 

Can you be more specific? ... I should mention, tracing a circuit board is a confusing task, and it's really easy to make a mistake or two ... or lot more. I've tried to indicate the general method below, but not explained everything in excrutiating detail, so if you don't understand something, ask.

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The only trick I know, is to go round a small area, making notes, checking with the meter, and be on the look out for errors in your tracing.  When you find a mistake or something doesn't feel right, just check through and fix it. You will make mistakes as you go ... just accept it is a trial and error kind of process, but gradually the mistakes will become clearer, so you can fix them.

I assume you have a meter that measures resistance ... and maybe has a convenient buzz mode, as part of resistance, to indicate a low resistance path. Use this to check the connection between parts.

-------------------------

The hardest case is when you don't have a schematic to start with. In your case, schematics are available .. get yourself a copy, find the page with the TMS9918 chip, and print out 2-3 copies of that part of the page enlarged, so you have space to highlight and make notes. In this case, just the small snip below might be enough, but usually it involves a larger sheet of paper!

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You may not have a board layout ... so also be ready with pen and blank paper, and make a physical sketch as you go. Draw a simple outline of each device and label as e.g. L101, etc as you identify them. In your case, start with the TMS9918 ... it is one of the largest parts on the board!

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Then it is a case, take a guess at  two connected parts on the schematic, use your meter to check if the pins connecting the two match up. If they match, start your board layout sketch, and highlight that 'wire' as 'done' on your schematic (colour with highlighter) ... if the pins are not connected, try to figure out, why not. Is it the other pin of one (or both) components? Have you got the wrong component(s)? You may be able to physically see the tracks on your board, to eyeball check where they go for clues.

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Repeat above, until you have identified all of the parts in question.

As an example, using the snip from before:

image

The TMS9918 will be obvious to spot and hopefully have the number printed on it. Assuming you know how the pins are numbered (if not find the data sheet), find pin 40.

The schematic shows it should connect to one pin of three parts ... L100, C101 and R101.

Then for each of those parts, check the other pin, goes to where you expect, e.g C101 other pin goes to ground.

Repeat this process until you have all the connections in that corner of the schematic.

Note the R101 and the C101 are high/very high resistance components, so only one of each of their pins will show low resistance to Pin 40 of the TMS chip. But the L100 coil will have a very low resistance, so your meter may not be able to differentiate between the end connected to C101 from the end connected to the crystal Y100.

You will probably be able to see which pin is which, but for this purpose it doesn't matter. Just be aware, parts like this can be confusing, and typically cause some head scratching first time round.

L107 will also be very low resistance, so its to pins may be difficult to differentiate.

--------------------------

As I started, it is a confusing process, but with a little patience, some good lighting and magnifying glass, and maybe a few cups of your favourite beverage (preferably not too alcoholic!), you should be able to figure it out.

Good luck! Dave


   
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mtlatc
(@mtlatc)
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@Davee Everything got in the way today, but I did manage to check the continuity in that area and everything looked good. Hopefully I can do some real checking tomorrow.  Tho I'm still not confident I'm doing everything right.


   
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(@davee)
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Hi @mtlatc,

  re: Tho I'm still not confident I'm doing everything right.

Few, if any, of us are confident of doing everything right.

Personally, I just do my best, hope that I get at least some of it right, and try to find ways of checking what I have done.  Please note, when checking, try to find a check that is different and/or separate from the original way you did something ... humans are good at spotting patterns, etc. but terrible at spotting their own mistakes.

After trying to trace out the circuit as I described, I would ask myself a few questions, whilst I still had the unit on the table, and meter probes to hand. The questions partly depend on the circuit you find. Hence, these are only examples that will need modifying accordingly.

  • The photo showed a 'classic' adjustable inductor, with a ferrite core in a plastic tube, and a coil on the outside of the tube, which was the same general appearance as the product photo I showed. If, that isn't the adjustable inductor connected to the crystal, what is it connected to? (You may have download the full schematic to answer that question.)
  • What is the 'mystery' damaged part with coloured stripes connected to?
  • Did you find any differences between the board and the schematic? If so, what?
  • The schematic clip showed L107, an inductor used for filtering, which does not need to be adjustable. Have you identified it? In 'modern' components, I would often be a cylindrical part, like the part shown in that repair article, but it might look quite different. It could even be your mystery damaged part.

Best wishes and good luck, Dave


   
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