Electron: no output from RGB but video and UHF out OK

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emeu1
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Electron: no output from RGB but video and UHF out OK

Postby emeu1 » Wed Apr 18, 2018 5:16 pm

Hello,

as the topic says, I have a perfectly functioning Electron.
Only problem is, I get no picture on my television when using the RGB socket whereas the Electron generates a perfect picture on the television when I use video out or UHF out.
Here is a description of what I already tried:

1. Electron connected via video out to the television: works, perfect picture.
2. Electron connected via UHF out to the television: works, perfect picture.
3. Electron connected via RGB to SCART to the television: no picture at all. No snow, no lines, nothing, just a black screen.
4. Tested the RGB to SCART cable and television with my BBC model B: works, perfect picture. So I think it's safe to say that the television and RGB to SCART cable are not the cause (not faulty).
5. In the Electron: switched the LK3 link to the other position: still no picture (from RGB socket).
6. In the Electron: made link LK4, connected the Electron with a video cable to test if all three colours are there. Result: all three colours are there. Thus the ULA does generate the red, green and blue signals.
7. Checked the Electron Service Manual for possible further testing. Unfortunately it doesn't describe the situation that there is no signal from the RGB but signal from video and UHF.

My questions:

1. Has anyone ever had this situation before and knows what the cause is and how to fix it?
2. If not, any suggestions what the next thing is I could/should test?

Best regards,

Erik

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Re: Electron: no output from RGB but video and UHF out OK

Postby Bat » Wed Apr 18, 2018 5:40 pm

Hi,
I've no experience with Acorn video outputs but from my AV knowledge I would suggest that the sync signal is your problem. I know on the model B it comes from the composite output, don't know about the electron.
Cheers,
Gavin :)

emeu1
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Re: Electron: no output from RGB but video and UHF out OK

Postby emeu1 » Wed Apr 18, 2018 8:02 pm

@Gavin,

thanks for your reply.
I've checked the signal lines for CSYNC from ULA till IC19, from IC19 to IC14 and from IC14 to the RGB socket.
The lines are all OK (measured 0 ohm).
I doubt that IC19 is defect. It plays a role in the CSYNC, red, blue and green signals to form the video out signal.
And the video out signal is OK.
IC14 plays a role in UHF out signal. So I think this IC is also not defect.
The connections on the RGB socket itself also seem (visually) OK.
So, I'm still nothing further. :(

Best regards,

Erik

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1024MAK
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Re: Electron: no output from RGB but video and UHF out OK

Postby 1024MAK » Wed Apr 18, 2018 8:45 pm

The later PCB issues have a buffer chip between the RGB outputs from the ULA and the DIN socket.

It could be this chip.

Sorry, I'm away from my Elks, so can't check to give you any further details.

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
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emeu1
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Re: Electron: no output from RGB but video and UHF out OK

Postby emeu1 » Thu Apr 19, 2018 6:41 pm

Hello Mark,

thanks for your reply.
I've done some further testing by measuring the CSYNC level (in volts) starting from the ULA and then step by step until the socket itself.
Long story short, as soon as I pressed on the socket, I measured 3.7V on the CSYNC pin.
Without pressure, 0V.
It looks like a bad solder joint or cracked copper line near the socket and not a bad ULA or defect IC.

Best regards,

Erik

emeu1
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Re: Electron: no output from RGB but video and UHF out OK

Postby emeu1 » Sun Apr 22, 2018 9:35 am

Hello,

for all those interested, yesterday I found the cause.
The RGB socket turned out to be fine, however, the jumper on LK3 not.
It was internally broken and as such, didn't reliably make a connection between the central and 0V pin of LK3.
Earlier, when I measured the voltage on the CSYNC pin on the RGB socket, I used the 0V (ground) pin of LK3 as my 0V reference point.
While pressing with one of the measuring pins of the multimeter on the RGB socket, I also "counter pressed" on LK3 with the other measuring pin.
This way, the jumper temporarily connected and thus allowing the CSYNC signal pass thru to the pin on the RGB socket.
As soon as I stopped measuring (released pressure), I assumed the signal was still there on the RGB socket but, of course it disappeared!
Something you don't expect if you have just measured it.
So, all that was needed was a new jumper. :)

Best regards,

Erik

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1024MAK
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Re: Electron: no output from RGB but video and UHF out OK

Postby 1024MAK » Sun Apr 22, 2018 11:49 am

Good that you have finally found the problem. And thanks for reporting back.

Well done :D

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
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