First post, by SSTV2
- Rank
- Oldbie
I've bought two EGA monitors and one of them is exhibiting an unusual fault - the raster is stretched horizontally to the point, that it wraps on itself, but screen geometry is normal. This happens in CGA mode (16KHz H-SYNC), in EGA mode (22KHz H-SYNC) it loses horizontal synchronization completely. I can adjust horizontal phase (horizontal position) and horizontal oscillator frequency (horizontal hold) on the monitor and it responds to the adjustments accordingly, but neither are affecting raster's width.
So far I've checked:
* all of the adjustment pots and their surrounding circuitry - no problems there;
* H/V sync. signal input IC (74LS86 quad XOR) and its surrounding circuitry - no problems there;
* every electrolytic cap around horizontal oscillator IC and adjacent area - everything's OK;
* film cap next to the horizontal width adjustment coil - perfect condition.
* Incoming power for the horizontal oscillator IC (11.85V) and H/V sync. input IC (5V).
I just can't explain such bizzare CRTs operation, I lean towards the faulty horizontal oscillator IC (TDA2595 based), but it's functional enough so that it would react to the adjustments and drive a horizontal output transistor.
What else could I check before substituting horizontal oscillator IC with a newly bought one?
Screen geometry on the first power on:
Fixed geometry, but raster is stretched and wrapped on itself (AMI BIOS diagnostics ROM screen, 286 MB):
H/V sync. and H/V control area: