Hi all

Have a fault with this set which I can't get to the bottom of. It
sometimes refuses to start up out of standby. It tries, and the
microcontroller activates the degaussing relay but it seems the power
supply won't start up properly and it reverts back to standby.

I have found the SMPS secondary voltages to be wrong, as per the
service manual. I can supply the schematics to anyone interested.

The following are the SMPS secondary voltages:

B1 should be 151V in standby and 139V when set is on. Reads 131V in
standby and when set is on.
B2 should be 16.8V in standby and reads 15.4V at first but builds over
a few minutes to 16.3V.
B4+ should be 18.2V in standby and reads 15.5V.
B4- should be -17.4V in standby and reads -14.6V.
B5 should be 7.82V in standby and reads 8.32V.
B6 should be 5.8V in standby and reads 6.11V.
B8 should be 10.42V in standby and reads 11.26V.

These voltages have different ones marked for when the set is on, but
I haven't listed them as they deviate about the same as they do when
in standby mode. They may rise or lower slightly, but still remain
different to the schematic voltages.

All line-stage/flyback transformer derived voltages are correct when
the set is running.

Having found no other cause for this very
intermittent failure to start up, I'm wondering if the low HT (B1) and
perhaps other rails is causing it at times for whatever reason.

I have replaced every electrolytic in the primary and secondary even
though most were fine, just to be on the safe side. I have checked
every diode and resistor and any that seemed odd were taken out of
circuit for testing.

The set wouldn't power into standby with the line-output transistor
removed and a dummy load (60W bulb) in place, it just tripped. It
will power into standby with no transistor present, however. I even
disconnected everything on the B1 rail except the feed to the
optocoupler but none of these things changed the B1 voltage of 131V.

The mains smoothing capacitor (220uF, 385V) was showing 300V across it
in standby mode, dropping to 280V when the set was running. I was told
it shouldn't drop like that and to replace it as a precaution. It read
198uF with very good ESR, but put in a replacement one reading 219uF
with very good ESR and rated at 400V. No change.

There is a potentiometer on the secondary side for adjusting the B1
and this reads fine (470R) but full adjustment gives from 124V to
137V on the B1 - this is still well below the 147V given for standby
voltage.

Please has anyone got any suggestions? If you wish to have a look at
the schematic to help, just say so and I'll get it to you (PDF
format, ~4.5MBs) in whichever manner you suggest.

Many thanks in anticipation.

But have you looked very closely for dry joints ?
Very Very closely..

kip
"Insomniac" wrote in message
. ..
Hi all

Have a fault with this set which I can't get to the bottom of. It
sometimes refuses to start up out of standby. It tries, and the
microcontroller activates the degaussing relay but it seems the power
supply won't start up properly and it reverts back to standby.

I have found the SMPS secondary voltages to be wrong, as per the
service manual. I can supply the schematics to anyone interested.

The following are the SMPS secondary voltages:

B1 should be 151V in standby and 139V when set is on. Reads 131V in
standby and when set is on.
B2 should be 16.8V in standby and reads 15.4V at first but builds over
a few minutes to 16.3V.
B4+ should be 18.2V in standby and reads 15.5V.
B4- should be -17.4V in standby and reads -14.6V.
B5 should be 7.82V in standby and reads 8.32V.
B6 should be 5.8V in standby and reads 6.11V.
B8 should be 10.42V in standby and reads 11.26V.

These voltages have different ones marked for when the set is on, but
I haven't listed them as they deviate about the same as they do when
in standby mode. They may rise or lower slightly, but still remain
different to the schematic voltages.

All line-stage/flyback transformer derived voltages are correct when
the set is running.

Having found no other cause for this very
intermittent failure to start up, I'm wondering if the low HT (B1) and
perhaps other rails is causing it at times for whatever reason.

I have replaced every electrolytic in the primary and secondary even
though most were fine, just to be on the safe side. I have checked
every diode and resistor and any that seemed odd were taken out of
circuit for testing.

The set wouldn't power into standby with the line-output transistor
removed and a dummy load (60W bulb) in place, it just tripped. It
will power into standby with no transistor present, however. I even
disconnected everything on the B1 rail except the feed to the
optocoupler but none of these things changed the B1 voltage of 131V.

The mains smoothing capacitor (220uF, 385V) was showing 300V across it
in standby mode, dropping to 280V when the set was running. I was told
it shouldn't drop like that and to replace it as a precaution. It read
198uF with very good ESR, but put in a replacement one reading 219uF
with very good ESR and rated at 400V. No change.

There is a potentiometer on the secondary side for adjusting the B1
and this reads fine (470R) but full adjustment gives from 124V to
137V on the B1 - this is still well below the 147V given for standby
voltage.

Please has anyone got any suggestions? If you wish to have a look at
the schematic to help, just say so and I'll get it to you (PDF
format, ~4.5MBs) in whichever manner you suggest.

Many thanks in anticipation.

I'll say. I've redone all joints in an effort to cure this fault.

Here is an image of the power supply schematic:

http://img150.imageshack.us/img150/8518/psu8ku.gif

It's an 83KB GIF image, so doesn't take long to load.

Have since replaced IC800, IC801, and tried substituting IC850. No
difference whatsoever.

Anyone?