Re: Philips GR2.4 TV Chassis

On Fri, 09 Jun 2006 02:50:39 GMT, antispam@xxxxxxxxxxxxx put finger to
keyboard and composed:

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

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
--
Please remove one 'i' from my address when replying by email.
.

Relevant Pages

  • Re: RCA Model P46733AT CTC178LA Side Pin Out
    ... In the 178 chassis those usually destroy the pin ... minimum the pin output transistor again. ...
    (sci.electronics.repair)
  • Re: unit hums loudly regardless of volume
    ... and I checked the voltage between ... >>> Tried the turining around the plug trick, ... >>> hand was on the record player chassis and got a fairly good shock. ...
    (sci.electronics.repair)
  • Re: Aerial Amp - feeling a tingle
    ... induce 'half mains' voltage into an unearthed co-ax courtesy of the ... coupling caps intended to 'isolate' a potentially live chassis in the ... modern SMPSU designs intended to 'RF bond' the chassis to the 'screening ... tolerate high voltage breakdown without going permanently short circuit. ...
    (uk.tech.digital-tv)
  • Re: Need help with vintage amp
    ... You don't say what the plate voltage is on the ... Hum on these Gibsons can be difficult to isolate. ... Remember, amp off when pulling. ... ground to the chassis. ...
    (alt.guitar.amps)
  • Re: plate voltage
    ... The other one is in your pocket, you sit on it, or it is ... but it *must not* touch the chassis. ... One hand, no completed circuit, regardless of which hand. ... voltage bay... ...
    (alt.guitar.amps)