Search found 26 matches

by Tiger
Sun Nov 22, 2020 12:02 am
Forum: ZX Spectrum Lounge
Topic: 128k +2 Mysterious Videos issue.
Replies: 36
Views: 9816

Re: 128k +2 Mysterious Videos issue.

:( - random pattern - looks like (Upper-)RAM damage :shock:
by Tiger
Sat Nov 21, 2020 11:29 pm
Forum: ZX Spectrum Lounge
Topic: 128k +2 Mysterious Videos issue.
Replies: 36
Views: 9816

Re: 128k +2 Mysterious Videos issue.

In my Speccy's I have mostly switching power regulators instead of the 7805, so this will do no change. The interference fault in my case was definitely the (faulty) external power supply. 8-)
by Tiger
Fri Nov 20, 2020 8:43 pm
Forum: ZX Spectrum Lounge
Topic: 128k +2 Mysterious Videos issue.
Replies: 36
Views: 9816

Re: 128k +2 Mysterious Videos issue.

In my case it was a faulty capacitator in the (switch) power supply. I repaired it today and now everything is ok. It seems that 105° caps are not a guaranty for a long lifetime... :o
by Tiger
Thu Nov 19, 2020 9:54 pm
Forum: ZX Spectrum Lounge
Topic: 128k +2 Mysterious Videos issue.
Replies: 36
Views: 9816

Re: 128k +2 Mysterious Videos issue.

Today I had nearly the same problem with "crawling dots scrolling over the screen". Tested an ZX Spectrum Issue 6a PCB. But an other Speccy, wich worked fine before also shows these dots. I used an adjustable bench top power supply and after I took an other power supply every thing works fine. Seems...
by Tiger
Wed Sep 30, 2020 7:41 pm
Forum: ZX Spectrum Lounge
Topic: 128k +2 Mysterious Videos issue.
Replies: 36
Views: 9816

Re: 128k +2 Mysterious Videos issue.

Is it possible that it's a variation of the "snow effect"? http://www.zxdesign.info/dynamicRam.shtml :roll: This effect occurs if the CPU produces uncontrolled /RAS-Signals. To prevent this you can pull /MREQ (Pin19) of the CPU high. I think a resistor <1Kohm or a wire from pin 28 (RFSHL) to +5V (pi...
by Tiger
Fri Sep 25, 2020 6:18 pm
Forum: ZX Spectrum Lounge
Topic: 128k +2 Mysterious Videos issue.
Replies: 36
Views: 9816

Re: 128k +2 Mysterious Videos issue.

Hallo Chris, mit meinen Englischkenntnissen stoße ich nun leider an meine Grenzen - da Du ja in Deutschland lebst, verstehst Du es ja evtl., wenn ich in deutsch schreibe: bei dem Fehlerbild tippe ich auf eine Schwingungsstörung die sich da am Videoausgang zeigt, also eine überlagerte Wechselspannung...
by Tiger
Mon Sep 21, 2020 12:14 pm
Forum: Hardware
Topic: Spectrum 48 Iss 6A
Replies: 5
Views: 4937

Re: Spectrum 48 Iss 6A

I repaired some Speccy's and often found that there where faults with sockets. Often PCB wires where shortened under the sockets or broken. Best is to test all connections of the sockets ...
by Tiger
Sun Sep 20, 2020 10:37 pm
Forum: ZX Spectrum Lounge
Topic: 128k +2 Mysterious Videos issue.
Replies: 36
Views: 9816

Re: 128k +2 Mysterious Videos issue.

Also possible is interference in the 12V power line - check C41 (100µF) an C23 (22µF). But 'cause I never owned an +2 I'm not sure...
by Tiger
Sun Sep 20, 2020 12:33 am
Forum: ZX Spectrum Lounge
Topic: 128k +2 Mysterious Videos issue.
Replies: 36
Views: 9816

Re: 128k +2 Mysterious Videos issue.

I'm not sure but it seems there is a faulty capacitator at the IC 2 (TEA 2000). Maybe C45 (1nF), C47 (1nF), C32 (330pF) or C3 (10nF).
by Tiger
Sun Sep 06, 2020 10:23 am
Forum: Hardware
Topic: ZX81 generates high pitched sound on CRT
Replies: 28
Views: 11624

Re: ZX81 generates high pitched sound on CRT

Sorry that I tried to help you -