Fading Composite output

About original Memotech hardware.
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acsi
Posts: 36
Joined: 16 May 2019 08:49

Fading Composite output

Post by acsi »

Hi

So the CFXII is purring along nicely however after all the work changing caps and getting the machine back up and running I now find that after around 5 minutes the composite output has faded out and the TV has lost signal.

As part of recapping the motherboard I recapped the video board and everything seemed fine at the time. I thought maybe the one of the regulators was an issue as one (the 5v) had pins soldered onto it and then to the motherboard. I just replaced all 3 and have a nice stable voltage with no change when the video fades out.

The issue seems to be related to the video board as if I unplug it and plug it back in (without powering Down the MTX) it starts working again for awhile. Due to this I don't think its heat related but was wondering if it could be the LM1889 as I did see a voltage drop on the output pin?

I don't own a oscilloscope unfortunately.

Has anybody experienced anything similar?
acsi
Posts: 36
Joined: 16 May 2019 08:49

Re: Fading Composite output

Post by acsi »

OK while typing the previous message I remembered that I have a dead 48K Spectrum board (tracks lifted) which has a LM1889N on it.

Bit of desoldering and swapped them over and it's been running fine for the last 10 minutes. :D

Excellent finally I may get to put the lid on it and leave it there. :lol:
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Dave
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Re: Fading Composite output

Post by Dave »

Did you change the caps on the video board too?

If not, C51 could have been a contender too. It is possible to pin the video board for mono output which eliminates most of the other components. If the video still has issues, C51 is the most likely cause

Regards
Dave
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1024MAK
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Location: Looking forward to summer, in Somerset, UK

Re: Fading Composite output

Post by 1024MAK »

acsi wrote: 19 Dec 2021 09:46 … I recapped the video board and everything seemed fine at the time.
Assuming that the +12V to the LM1889 is correct, that you replaced the electrolytic capacitors on the video board, that the replacements are good, and that their polarity is correct, then a signal that fades over a time period of 5 minutes or more is likely a problem with a DC level in an amplifier circuit.

If after replacing the LM1889 it’s fine, then you have your answer ;)

Mark
acsi
Posts: 36
Joined: 16 May 2019 08:49

Re: Fading Composite output

Post by acsi »

Yep its all good now. Played Escape from Zarcos for around 30 minutes with no further issues so it must have been the LM1889 at fault. :D
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