Grundig TV CUC7880 or 7890 intermittent failure to power-up

[Bram Stolk]

This (Large screen) TV keeps coming back on my workbench with the label
""DEAD""
Actually it is NOT dead, the LED turns RED when mainpower is applied, but it
refuses to accept any REMCO or Front Panel commands.
Just letting the set sitting idle for 3 minutes brings back all LIFE to its
circuits and it plays as advertised with vivid sound and colours.....

Switching it OFF via the REMCO or Front Panel and trying it again to powerup
works as one would expect. I.e. perfect within seconds.

Pulling the main powercord and leaving the set unpowered for a week or so,
brings back the initial failure to switch on properly. Again, just sit and
wait for 3 or 4 minutes will restore normal operations.

As the Manual for this model does not warn for prolongued warm-up periods I
feel that some capacitor(s) has lost it's Farads resulting into this
extremely long power-up period required.

I checked numerous Caps for ESR but really could not pinpoint the defective
circuit. As normal ops always returns in a few minutes I have not much
chance to do any scoping and/or probing.

Is there any most likely cause of problem or any suggestion to disable one
or more protection circuits ?

TIA

Bram Stolk

 

[kip]

Sounds like a bad filter cap in the SMPS section..

kip

 

[Bram Stolk]

Kip,
You mean caps at secondary outputs of mainsupply ? There are about 10 high
capacity electrolytics in that area which I measured for ESR while in the
circuit. Probably a straightforward Remove and Replace with fresh ones might
get this set from my bench again (for a while)

I am also still suspecting RESET circuits at IC's 810 and 850 in the Control
unit, Think i'll hook up some storage/recording eqt at essential I/O's to
see whether a certain voltage will transit not nicely from Lo/Hi or vv...

When defective component has been isolated and identified I'll let U all
know.....

TIA
Bram Stolk

 

[kip]

Around the SMPS trany...
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