Image Map Image Map
Page 5 of 6 FirstFirst 123456 LastLast
Results 41 to 50 of 59

Thread: Apple IIc Monitor Tube displaying strange behavior

  1. #41
    Join Date
    Jan 2012
    Location
    Connecticut, USA
    Posts
    2,195

    Default

    Quote Originally Posted by Hugo Holden View Post
    You can replace these glass passivated diodes with the 4A rated BY448. If you strike one with particularly thick lead wires, use a BY228 that has a 5A rating. Both of these are over 1600V piv rated and have fast switching times, they are probably far superior to the diode that was in there in the first place and they are easy to get.
    Thanks Hugo, I will make an order for a set of both when I get home.

  2. #42
    Join Date
    Jan 2012
    Location
    Connecticut, USA
    Posts
    2,195

    Default

    Ok, I know its been along time but I have an update. I spent the majority of the day working on the two faulty IIc monitors. First off Thanks again Hugo for recommending those diodes.

    So the board with the missing and crumbling diodes; I replaced them all with the ones mentioned above. Voila! one Board, Psu-board and CRT now work perfectly! I decided to just replace the same diodes on the other board just because it may be a failure point ( I have never really seen diodes be a failure point but after dealing with these IIC monitors its possible).

    After doing so I tried the known good CRT monitor with the secondboard and second psu-board. Got display but much like mentioned early in the post. I decided to just try the second board but use the psu-board from the first working machine. Then I realized the second board was working great as well! The problem is with the psu-board. The caps were replaced a couple months back. That only leaves the transformer (which I cannot find any part numbers for in the case it may be bad) and two transistors (2SC1213C and 2SA673C) which I already put an order for replacements. If its not either transistor Could it be the transformer? The display comes in an out of vertical hold and there are waves just rippling through the image.

    Now lets talk the second CRT. I know we were talking about the internal grid being a possible problem and maybe turning the yoke. Honestly I never got to that. I did try the CRT on the known good board and PSU and it gets a steady display although quite small. Even with adjusting all the pots the image squished. I guess I will measure resistance in the yoke next as well.

    looking in Sams computer facts I see the power transformer is labeled 2869-239-0104 by PELCO but I cant seem to find anything online about it.
    Last edited by VERAULT; September 18th, 2019 at 12:03 PM.

  3. #43

    Default

    Compare the resistance of the joke windings between the bad and the good tube. The "squashed image" tube has probably shorted turns in one deflection coil. The good news is that probably you can rewind the bad yoke if it's the vertical windings that went shorted.
    If the other fault is in the power transformer, just measure the output(s) of the good one and find a similar replacement. You will find the same transformer probably only on another IIc monitor.
    Frank

  4. #44
    Join Date
    Jan 2012
    Location
    Connecticut, USA
    Posts
    2,195

    Default

    Ok so the horizontal winding on my good CRT reads 0.49 Ohms, Horizontal on the Bad CRT is 0.57 Ohms

    Vertical winding on the Good CRT is 2.19 Ohms, and on the Bad CRT is 4.24 Ohms. That is a pretty big difference on the Vertical winding on the bad CRT right?

  5. #45

    Default

    Quote Originally Posted by VERAULT View Post
    Ok so the horizontal winding on my good CRT reads 0.49 Ohms, Horizontal on the Bad CRT is 0.57 Ohms

    Vertical winding on the Good CRT is 2.19 Ohms, and on the Bad CRT is 4.24 Ohms. That is a pretty big difference on the Vertical winding on the bad CRT right?
    That is unusual. Often (for many monitors) the two vertical deflection coils on the yoke are wired in series so as to gain as much overall inductance as possible (I would have to look at the monitor schematic again) therefore, in a fault condition like shorted turns, the resistance of the faulty vertical coil pair should be lower than normal. I will have to check the schematic which I do not have on hand. If it was a scenario in this particular monitor design where the two vertical coils were originally wired in parallel, and one went open circuit, it could explain why you are seeing about double the resistance on the faulty coil compared to a good yoke. Check the solder connections, from the vertical yoke windings, of the enameled copper wire to each terminal on the yoke.

  6. #46
    Join Date
    Jan 2012
    Location
    Connecticut, USA
    Posts
    2,195

    Default

    I dont know. There was some black gook on the coil. I wasnt sure if there was corrosion. I sprayed some deoxit on it. (after I took the readings; made no difference of course)

    Solder connections look fine. I guess I can try reflowing them when I get the replacement transistors in.

  7. #47

    Default

    Quote Originally Posted by VERAULT View Post
    I dont know. There was some black gook on the coil. I wasnt sure if there was corrosion. I sprayed some deoxit on it. (after I took the readings; made no difference of course)

    Solder connections look fine. I guess I can try reflowing them when I get the replacement transistors in.
    check very closely on all the enameled copper wiring that is visible on the yoke, for a broken wire, perhaps one under tension, or a physical impact area on the yoke that might have damaged a wire.

  8. #48
    Join Date
    Jan 2012
    Location
    Connecticut, USA
    Posts
    2,195

    Default

    I gave it a look over and examined the copper wires from the solder connections back. Nothing obvious jumped out at me. I suppose I can post some pictures tomorrow.

  9. #49

    Default

    Quote Originally Posted by VERAULT View Post
    Ok so the horizontal winding on my good CRT reads 0.49 Ohms, Horizontal on the Bad CRT is 0.57 Ohms

    Vertical winding on the Good CRT is 2.19 Ohms, and on the Bad CRT is 4.24 Ohms. That is a pretty big difference on the Vertical winding on the bad CRT right?
    unusual... looks like the vertical coils are wired in parallel and you're missing one coil on the bad CRT's joke.
    I would unsolder both coils from the wiring posts on the bad joke and measure the resistance separately on both windings, one should be open or not making good contact in the current solder joints.
    Frank IZ8DWF

  10. #50

    Default

    Something is very wrong here. I found the Apple IIc circuit.

    As is usual, the V coils are wired in series and the H coils are in parallel.

    So it is not possible via any scenario I can see that your faulty yoke measures twice the resistance of the good one. You must have got them mixed up and the faulty one is half the resistance of the good one (as we first suspected).

    So likely the bad one is 2.19 Ohms, and on the good is 4.24 Ohms.

    Please double check this.

    The schematic states that the resistance (across the yoke terminals) for the vertical coils is 6.5 Ohms and for the H coils is 0.3 Ohms and the V coils are in series, the H coils in parallel.

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •