Image Map Image Map
Page 5 of 5 FirstFirst 12345
Results 41 to 48 of 48

Thread: 32k PET 2001-N - Video and running issues

  1. #41

    Default

    Well, I don't have an 80 column PET anymore and that 2001N that dhoelzer is working on is also only 40 columns and non CRTC.

    I have the Tynemouth board but the diags on that only test the first 1K of ram.

  2. #42
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    2,723

    Default

    40 column non-CRTC is fine - it works OK with that.

    The table that programs the CRTC is right at the beginning of the EPROM (apart from an initial 'JMP START', an ASCII 'V' and the version number). There then follows an 18 byte table as follows (these numbers are in decimal):

    db 49 ; Register 0.
    db 40 ; Register 1.
    db 41 ; Register 2.
    db 15 ; Register 3.
    db 32 ; Register 4.
    db 3 ; Register 5.
    db 25 ; Register 6.
    db 29 ; Register 7.
    db 0 ; Register 8.
    db 9 ; Register 9.
    db 0 ; Register 10.
    db 0 ; Register 11.
    db 16 ; Register 12.
    db 0 ; Register 13.
    db 0 ; Register 14.
    db 0 ; Register 15.
    db 0 ; Register 16.
    db 0 ; Register 17.

    If you can find this table, we can change it to initialise the CRTC however you want. There are some of the other values I have found in the source code itself (the .a65 file).

    Dave

  3. #43

    Default

    Can anyone make out what the specs are of the mini transformer on the 9" CRT board? Mine is pretty chewed up and I'd like to replace it. I think it reads as T701, but I'm not certain. Just to the upper right of the of the middle of the lower left quadrant (if that makes sense) of this schematic:

    http://www.zimmers.net/anonftp/pub/c...01N/321445.gif

    Thanks!

  4. #44
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    2,723

  5. #45

    Default

    Quote Originally Posted by daver2 View Post
    Super.... but any idea what its specs are? I can't seem to find anything anywhere that describes the component.

  6. #46

    Default

    Quote Originally Posted by dhoelzer View Post
    Can anyone make out what the specs are of the mini transformer on the 9" CRT board? Mine is pretty chewed up and I'd like to replace it. I think it reads as T701, but I'm not certain. Just to the upper right of the of the middle of the lower left quadrant (if that makes sense) of this schematic:

    http://www.zimmers.net/anonftp/pub/c...01N/321445.gif

    Thanks!
    T701 is the driver transformer for the horizontal output transistor. You will likely not see any specs for this anywhere in most VDU manufacturer's manuals.

    Its job is to couple the signal from the driver stage to the base-emitter circuit of the HOT (Horizontal output transistor). However, it is a little more complicated than meets the eye, in that when the driver transistor conducts, it builds up a magnetic field in the core of the driver transformer T701. At that time the HOT's base-emitter junction is reverse biased (the HOT turned off for flyback).

    When the driver transistor turns off (its collector current is cut off), the magnetic field in T701's core attempts to collapse, as it does it forward biases the the HOT's base-emitter junction, damping the field collapse, turning the HOT on and scanning about tht 2/3 of the H line scan toward the R hand side of the VDU's raster. You can see from the dots marked on the diagram, the polarity of the transformer windings of T701.

    It is highly unlikely T701 would fail. Generally, a small transformer like this would have about a 10:1 to 15:1 turns ratio and the secondary winding will have a low DC resistance of a few ohms or less. The primary may have a resistance of tens of ohms.

  7. #47

    Default

    Quote Originally Posted by Hugo Holden View Post
    T701 is the driver transformer for the horizontal output transistor. You will likely not see any specs for this anywhere in most VDU manufacturer's manuals.

    Its job is to couple the signal from the driver stage to the base-emitter circuit of the HOT (Horizontal output transistor). However, it is a little more complicated than meets the eye, in that when the driver transistor conducts, it builds up a magnetic field in the core of the driver transformer T701. At that time the HOT's base-emitter junction is reverse biased (the HOT turned off for flyback).

    When the driver transistor turns off (its collector current is cut off), the magnetic field in T701's core attempts to collapse, as it does it forward biases the the HOT's base-emitter junction, damping the field collapse, turning the HOT on and scanning about tht 2/3 of the H line scan toward the R hand side of the VDU's raster. You can see from the dots marked on the diagram, the polarity of the transformer windings of T701.

    It is highly unlikely T701 would fail. Generally, a small transformer like this would have about a 10:1 to 15:1 turns ratio and the secondary winding will have a low DC resistance of a few ohms or less. The primary may have a resistance of tens of ohms.
    Well, it's not so much that it "failed"... it's that it's corroded from mouse urine.

    Is there a simple way to measure the resistance of the primary and secondary and figure it out from that? it seems feasible...

  8. #48

    Default

    I've finally got some time again. Board is all clean (or as clean as it's going to be) and I've started replacing components. I'm finding it super difficult to work off of the low-ish quality CRT diagram with the hard to read scrawl for the components. I've taken some time to try to transcribe the schematic into a bill of materials spreadsheet.

    Is there anyone willing to take a gander and see if you see any obvious errors? Specifically, look at the rows where the Comments column has question marks; those are the things that I can't read and/or understand.

    Schematic: http://www.zimmers.net/anonftp/pub/c...01N/321445.gif

    Link to BoM spreadsheet: https://drive.google.com/file/d/1qPI...ew?usp=sharing

    Once this is verified/corrected, I'll send it to Zimmers to make someone else's life easier.

    Thanks!
    Attached Images Attached Images

Tags for this Thread

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
  •