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Philips GR2.4 TV Chassis

Hello Newsgroup

Can anyone make comment to the following fault on a Philips GR2.4 chassis a 29 inch tube model now starting to drive me mad.

Fault symptoms are as follows: HT line low at only 8 volts instead of nominal 140 volts as per service manual. Chopper transistor tested and found to be ok on static diode test as did the Line Output Transistor. Scan coil plug M71 disconnected but power supply voltage still remains at 8 volts on the main HT line. Then disconnected power supply main HT line from the Line Output stage and substituted a 100 watt mains Light bulb as load but still the main HT line stuck at 8 volts. Disconnected resistor at pin 8 of TDA8385 SOPS chip monitoring over voltage conditions but still no HT or any set start up. I have the full service manual but does not contain a circuit description which isn't helping.

Could anyone make comment as to any further test to be made or basically explain the power supply start up sequence of this chassis which uses a TDA8385 S.M.P.S. IC and CNR50 opto-isolator to ultimately drive the chopper transistor. Incidentally, both latter IC's renewed together with new chopper and Line Output transistor but still no results. Alternatively, could anyone direct me to a circuit description on the Internet as this is absent within the service manual.

I would be most grateful for any help or comments offered.

Many thanks

Daz

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Dani.

snipped-for-privacy@antipspam.net wrote:

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Dani

On Fri, 09 Jun 2006 02:50:39 GMT, snipped-for-privacy@antipspam.net put finger to keyboard and composed:

I'm not familiar with your chassis, but I once saw a similar problem in a Sony set. The culprit was a dried out B+ filter capacitor. Other generic possibilities are the small electros on the primary side, or open resistors at the feedback input to the SMPS chip. AFAICS, when the TDA8385 is regulating properly, the voltage on pin 9 should be equal to its internal reference voltage, namely 2.5V. If the input at pin 9 is low, then I'd check that the IC's output at pin 2 is low. This would at least tell you that the IC is trying to do its job.

- Franc Zabkar

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Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar

Thank's both for your comments much apreciated, a little bit of information goes a long way.

Firstly, the fault does not appear to match exactly any of those commonly found on chassis in databases in the database links posted.

Hower, looking at this circuit again with reference to the cited comments on pin 9 being low this now becomes quite logical and will now do some testing based on this. The capacitor faults also cited will also be investigated but it should be said that some previous testing with capacitance meter and DVM resister checks have yielded no out of tolerance electrolytics or o/c ressitors thus far in S.M.P.S.. However, not all electrolytics were thoughly checked so will re-do more comprehensive tests in the PSU.

Will post up further findings based on pin 9 testing of the TDA8385 IC for general interest.

Once again, many thanks for helpful comments.

Daz

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