STE motherboard works ok but high res output is VERY dim

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Chandler
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STE motherboard works ok but high res output is VERY dim

Postby Chandler » Thu Feb 22, 2018 10:57 am

As the title says, i have an STE which does work fine in LOW/MED res from the monitor output and TV port

But high res the output is black, however if i turn up the monitor controls to max i can see a faint desktop and move the mouse around etc

It's not the monitor as tried the STE on 3 different screens, same prob

Any ideas as to what the problem could be? bit odd as clearly the high res has not totally failed

gilles504
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Re: STE motherboard works ok but high res output is VERY dim

Postby gilles504 » Thu Feb 22, 2018 12:37 pm

check output of the 74S257 (U405 from this schematic http://www.gossuin.be/images/schemasata ... 600dpi.pdf)
also check for continuity from pin 7(MONOMON) of this chip to the monitor DIN14
(check on the pin and not the pad to check for a bad soldering)

tzok
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Re: STE motherboard works ok but high res output is VERY dim

Postby tzok » Thu Feb 22, 2018 1:28 pm

The monochrome video signal in STE is digital (ON/OFF). If the signal levels are wrong this can be due to loose connection (cold joint) on PCB (probably on the socket to PCB connection) or the buffer (marked by red frame on a schematic below) circuit is broken.

Image

MONOMON signal is used for detecting the monochorome monitor, it is not a video output.

ranix
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Re: STE motherboard works ok but high res output is VERY dim

Postby ranix » Thu Feb 22, 2018 3:43 pm

the video socket gets pushed around a lot over its lifetime, I'd also try resoldering the connector

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Foxie
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Re: STE motherboard works ok but high res output is VERY dim

Postby Foxie » Thu Feb 22, 2018 4:14 pm

It could be a bad solder joint, but it could also be a bad transistor. Q403 is a 2N3904 that buffers the mono output. You could try replacing it and also reflowing the solder joints. If none of that helps, I suspect the shifter.

gilles504
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Re: STE motherboard works ok but high res output is VERY dim

Postby gilles504 » Thu Feb 22, 2018 8:06 pm

@tzoc: oops, yep I read the schematics too fast ;)
if it's not a bad joint Q403 is a candidate but also check the capacitor C414 that may be near to short and change output level.

Chandler
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Re: STE motherboard works ok but high res output is VERY dim

Postby Chandler » Fri Feb 23, 2018 1:45 pm

Repaired! it was a damaged solder joint on the underside of the mobo from one of the legs from Q403 to a small capacitor (C414) looks to me like it was damaged at factory as i don't see how it could have happened otherwise

Anyway thanks to all the replies as looking at Q403 lead me find the cause :0)

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troed
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Re: STE motherboard works ok but high res output is VERY dim

Postby troed » Fri Feb 23, 2018 1:51 pm

Nice!


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