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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 13th Feb 2018, 1:14 pm   #1
trsomian
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Default Philips SG90AE hum

Firstly, I don't normally dabble with TVs very much, but I have one of the above which largely works, but has audio hum when the picture is bright (not when the picture is dark. Seems like an electrolytic drying out to me; do others agree? The picture doesn't misbehave though.

I have downloaded the service manual, and it seems the audio is all in a TDA8191, which runs from a 20V supply off the main SMPS Obviously the smoothing capacitor for that output is a likely villain. I assume that the main DC link capacitor after the mains rectifier is good because the picture is OK.
Why would the hum change with picture brightness though?

All guidance and ideas welcome
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Old 13th Feb 2018, 2:29 pm   #2
Maarten
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Default Re: Philips SG90AE hum

A slightly misaligned IF stage will do exactly this. It is unlikely to come from any power supply line.
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Old 13th Feb 2018, 2:36 pm   #3
Sideband
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Default Re: Philips SG90AE hum

Agreed. The power supplies are derived from the line stage and the main switch mode supply, neither will give an audible hum if there is a problem. The SMPS runs at around 60kHz so you won't hear a hum if any decoupling is faulty.
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Old 13th Feb 2018, 2:39 pm   #4
Welsh Anorak
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Default Re: Philips SG90AE hum

How are you feeding a signal into this? If it's RF then Maarten will be right; if via SCART it's likely to be a mismatch, possibly source or cable troubles.
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Old 13th Feb 2018, 2:54 pm   #5
trsomian
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Default Re: Philips SG90AE hum

Thanks for the ideas, folks. The set is analogue UHF, so all input to it these days is from a set top box or other SCART input, so presumably IF problems are irrelevant.

Any suggestions as to how a SCART problem causes this, where the picture brightness causes audible effects; I thought the two data streams were separate
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Old 13th Feb 2018, 11:45 pm   #6
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Default Re: Philips SG90AE hum

Try using a fully shielded SCART cable. These are usually much fatter than standard cables as all the cores carrying each video signal have internal shielding. It could also be a grounding issue, or perhaps a bad ground on the SCART socket (dry joints).

Have a go at switching between composite and RGB (if supported). If it's only noticeable with composite, open up the SCART lead and check to see if there's a 75 ohm resistor on the composite feed. Most decent SCART cables will have termination resistors in place, but some cheaper cables won't.

Also, I know it obvious but make sure the plug is pushed all the way home (at both ends). With some older sets, especially, it often took a few attempts to get the SCART plug to fit properly (and stay in), as the socket was recessed instead of being completely flush with the casing.

I'm guessing they'd rather you purchased their own cable that fitted perfectly, of course..but cost an arm and a leg!
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