ZX Spectrum 48k - no CLK signal

edited November 2013 in Hardware
Hello

I'm trying to repair ZX Spectrum 48 mainboard issue 3B.
When i check signal with oscilloscope on X1 or on ULA pin 39. I get stable 2V no clock pulses. Can broken ULA chip be the cause of no clock signal?

regards
Maciek
Post edited by gottrek on

Comments

  • edited November 2013
    gottrek wrote: »
    Can broken ULA chip be the cause of no clock signal

    It could be or:

    - a broken X1
    - a broken C73
    - a broken PCB track
    - a shorted PCB track

    Make sure your probe doesn't load down the crystal or add too much capacitance. It doesn't take much to upset the oscillator.
  • edited November 2013
    Zorn wrote: »
    Make sure your probe doesn't load down the crystal or add too much capacitance. It doesn't take much to upset the oscillator.
    Use a correctly adjusted 10:1 probe :)

    Do you get a picture on the TV?
    What about the clock output on ULA pin 32?

    Mark
  • edited November 2013
    I'm using 1/10 oscilloscope probe.
    X1 is ok. I have tested it in DIY quartz tester. I get nice sinus signal of 14MHz.
    I have replaced C73 with 15pF ceramic capacitor. Someone has changed it to 33pF.
    I'am still getting no clock signal on ULA pin 39 or 32.
    I don't see any broken or shorted tracks on PCB.
    Any other idea? What else can i check?

    PS.
    Maybe this is important. According to service manual when there is no 3MHz clock i should check ULA pin 13 for 3V but i'am getting here 4V.
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