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Thread: Europlus - Strange Timing Issue

  1. #1

    Default Europlus - Strange Timing Issue

    Hello all.

    I have been working for some weeks on an Apple II Europlus I picked up from eBay.

    The machine has never booted to the Apple ][ screen correctly - it has only ever shown variations of ??? - i.e. all question marks on the screen or ??@@??@@ occasionally.

    I have tested most of the ICs on the board with my TL866II, and have replaced the ICs that I have been unable to test.

    I have removed all of the RAM except for the 1st 16k, and I know this is good, tested working RAM.

    I have tested the ROMs, and have left only ROM F8 in the board.

    The CPU is a tested good 6502A (note A, not 6502 - the 6502 in the machine I replaced, as it tested faulty)

    None of this has worked - I always have just ??? on the screen.

    I have spent some time diagnosing with my oscilloscope, and I have noticed that both the 1MHz signals are quite "noisy" - It's not a pure wave. Removing the 74LS161 at D13 cleans up the signal massively, but replacing this IC with another 161 did not fix the fault, nor did fitting a new IC socket, or replacing the 100nF caps around row D.

    I have replaced Q1 and Q2 (and all the caps in the timing circuit) to no avail.

    Can one of you point me to where I should look next? I am pulling my hair out a little, having pored over the Apple II circuit description, the red book, Sams Facts over and over trying to pin this down.

    Here is a video of the clock signals:
    https://photos.app.goo.gl/9KGgfJkZAHcPPSM96 -- at the CPU
    https://photos.app.goo.gl/Gt4VLY5BHrPncHiK8 -- B13
    https://photos.app.goo.gl/YQ9pVQ8wVXP7z1uH6 -- 7MHz
    https://photos.app.goo.gl/j9eFjDRVJVUFoBEc8 -- 14MHz

    To my eye, the 14 and 7 Mhz clocks look OK, but the 1MHz is noisy....

    Help!!!

  2. #2

    Default

    Can nobody assist? Is the 1MHz signal meant to have that "echo" I am seeing?

    I am using the composite connector as the ground if this makes a difference?

  3. #3
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    Default

    On my Europlus, the clock signal is as noisy as hell but at the moment at work I cant see your pics.

    Will have another look when at home.

  4. #4
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    Default

    Quote Originally Posted by Gary C View Post
    On my Europlus, the clock signal is as noisy as hell but at the moment at work I cant see your pics.

    Will have another look when at home.
    Sorry, will be weekend before I could get a look at it

  5. #5

    Default

    No worries at all - thankyou ever so much for your time in any case, I will follow the steps outlined here - the symptoms are fairly similar: http://www.vcfed.org/forum/showthrea...h-much-testing

  6. #6

    Default

    Quote Originally Posted by nrg13 View Post
    Hello all.

    I have been working for some weeks on an Apple II Europlus I picked up from eBay.

    The machine has never booted to the Apple ][ screen correctly - it has only ever shown variations of ??? - i.e. all question marks on the screen or ??@@??@@ occasionally.

    I have tested most of the ICs on the board with my TL866II, and have replaced the ICs that I have been unable to test.

    I have removed all of the RAM except for the 1st 16k, and I know this is good, tested working RAM.

    I have tested the ROMs, and have left only ROM F8 in the board.

    The CPU is a tested good 6502A (note A, not 6502 - the 6502 in the machine I replaced, as it tested faulty)

    None of this has worked - I always have just ??? on the screen.

    I have spent some time diagnosing with my oscilloscope, and I have noticed that both the 1MHz signals are quite "noisy" - It's not a pure wave. Removing the 74LS161 at D13 cleans up the signal massively, but replacing this IC with another 161 did not fix the fault, nor did fitting a new IC socket, or replacing the 100nF caps around row D.

    I have replaced Q1 and Q2 (and all the caps in the timing circuit) to no avail.

    Can one of you point me to where I should look next? I am pulling my hair out a little, having pored over the Apple II circuit description, the red book, Sams Facts over and over trying to pin this down.

    Here is a video of the clock signals:
    https://photos.app.goo.gl/9KGgfJkZAHcPPSM96 -- at the CPU
    https://photos.app.goo.gl/Gt4VLY5BHrPncHiK8 -- B13
    https://photos.app.goo.gl/YQ9pVQ8wVXP7z1uH6 -- 7MHz
    https://photos.app.goo.gl/j9eFjDRVJVUFoBEc8 -- 14MHz

    To my eye, the 14 and 7 Mhz clocks look OK, but the 1MHz is noisy....

    Help!!!
    Digital oscilloscopes can play tricks on this kind of signals, but I think it isn't a trigger issue since 14 MHz and 7 MHz look fine.
    Do the sync output on the 6502 outputs some pulsing pattern continuously or it stops soon after reset is lifted?
    Is the reset circuit working? Some early apple II boards didn't have power on reset, but I think all plus/europlus should have the newer revisions.
    You didn't mention if you checked IRQ/NMI lines too?

    HTH
    Frank IZ8DWF

  7. #7

    Default

    Powered on with good ROM F8 installed, checked the following:

    Pin 2 /RDY = High
    Pin 4 /IRQ = High
    Pin 6 /NMI = High
    Pin 7 /SYNC = Pulsing
    Pin 40 /RESET = High

  8. #8

    Default

    https://photos.app.goo.gl/nyMWijwVXAM49RLc8

    Here is the Pin 7 signal - I think I see the same echo/shadowing as I do on the 1MHz clock.

    As an aside, I checked the 2MHz clock at Pin 4 of D2 - same noise.

    https://photos.app.goo.gl/tb2ANBzUT2DQEh3dA <<< Video of the 2MHz

  9. #9

    Default

    So... Playing around with the trigger levels on my scope made the issue go away with the "noise" on the clock signals.

    I can now see a clean 1MHz at the CPU, and also a good sync signal. However, still seeing all ??? on the screen.

    https://photos.app.goo.gl/J6PRMGycoqMPqEvf6

  10. #10
    Join Date
    Dec 2010
    Location
    Seattle, WA
    Posts
    2,015

    Default

    Quote Originally Posted by nrg13 View Post
    https://photos.app.goo.gl/nyMWijwVXAM49RLc8

    Here is the Pin 7 signal - I think I see the same echo/shadowing as I do on the 1MHz clock.

    As an aside, I checked the 2MHz clock at Pin 4 of D2 - same noise.

    https://photos.app.goo.gl/tb2ANBzUT2DQEh3dA <<< Video of the 2MHz
    When I look at your scope traces I would call that "jitter", not "noise" or "echo/shadowing".

    It looks like the clock signal is a reasonably well shaped square wave, but something is causing the cycle period to jump around instead of being a constant 1 microsecond cycle period for a nominal 1 MHz clock frequency.

    It might be interesting to take a single shot waveform capture and then scroll left or right from the trigger point and see if it captures any cycles where the period is significantly different, and see what sort of glitch the signal might have at that point. From the scroll window shown on the top of the scope display it appears that it might not have much capture depth to scroll the display to the left or right so capturing a glitch might take a bit of work.

    I don't know the Apple II hardware to guess what might be causing clock jitter, or if that might actually even be normal for some reason.

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