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ZX Spectrum border and sound problem

skydiver021skydiver021
in Hardware
After recap stange problem with ZX Spectrum (issue 3b) with sound and border. See video at: https://drive.google.com/file/d/0BwhYHKW6xjI-aVdMcEY4ajVkcVk/view?usp=sharing (pay attention to the sound) I was replaced again with old caps and same thing. Upper 32KB is removed, when install upper 32KB same thing except the sound is little slow. I was replaced ULA, CPU, ROM, RAM, (same thing with original and replacement module from zx.zigg.net) LM1889N, all 74LSxxx IC, two quartz crystals all transistor except TR 1,2,8 and 9. My friend take osciloscope screenshoot from ULA pin 28 and speaker here it is:
https://drive.google.com/file/d/0BwhYHKW6xjI-V1YwZ2Zzd2NzLVU/view?usp=sharing
I was try with different power supply, same thing like original. Does anyone have an idea?
Important notes: When type LOAD "" ENTER border flicking and sounds stops. When breaks loading flicking border and sound continue

Comments

  • 1024MAK1024MAK
    @skydiver021 This is a digital section fault, so changing capacitors and transistors will make no difference. From what you say, it does not look like a failure of any chip either. It could be that there is a solder splash or similar short circuit between PCB tracks.

    Without more information, maybe between the Z80 CPU /INT circuit and another line. Very carefully visually examine the area around each and every solder joint / pad that you have worked on. Both on the top and bottom of the PCB. If you don't find anything, you will need to use a digital multimeter (set on resistance, Ohms range) to test for short circuits.

    Mark
    Sinclair FAQ Wiki
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  • skydiver021skydiver021
    edited March 2017
    Right in the target Mark. Small allmost invisible solder splash across two lines mixed with white component label on a upper side of a board, very hard to find. Strong led light, magnify and patience :)

    Thanks Mark.
    Post edited by skydiver021 on
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