I have noticed a few of these chassis having the increased standy B+ and
have serviced the chopper, regulator circuit before returning then to
service. It is an interesting question tho, did the HV block fail because of
the increased B+ or is it just co-incidence? Due to the age of these sets
the caps are going bad anyway and need replacing. Good point David!
"Leonard Caillouet" wrote in message
news:BR6Ae.31654$up5.29048@lakeread02...
Interesting. I never noticed the standby B+ going that high. I'll have
to check more closely. We have several routine checks on the 169 chassis
on any set that comes in that includes the caps in the power supply so the
get changed if they are bad. I often don't check the standby B+ until
after the repair is done so I had not noticed that. We do, however,
change lots of HV dividers in sets where the PS caps are fine. In those
cases the B+ is checked and I have not seen it out of range. I guess the
small margin of safety on the divider is the problem. I have also changed
quite a few that were dne by other shops previously and the grommets were
missing or the lead was not inserted correctly. So the OP should be
careful to install it correctly or be prepared to change it again before
long.

You wold think that they would not start the horizontal osc. until B+ is
regulated. But then, you would think a critical part like a high voltage
splitter would be built with a greater safety factor.

Thanks, David.

Leonard

wrote in message
ups.com...
High B+ regulation in standby.
The margin for error is very small on the hv in the splitter.
As the capacitors fail in the power supply, the standby voltage will
drift over time up to as high as 165V, versus the 140~145V that is
suppose to be there.
Since that is a 15% increase in initial B+ at start up, which is beyond
the 10% margin for the insulation in the hv block, the hv is too high
initially at start up and this starts internal arcing at start up.
Over time these carbon tracks finally cascade and the hard carbon arc
short is started. The problem arises in that the regulator kicks into
run mode in about one second after turn on. The regulator is then
running off the horizontal feedback pulses for regulation. Since this
happens so fast and is only slightly above the x-ray trip voltage, the
x-ray circuit never triggers. Each turn on with standby B+ running
high will cause an arc over. The big problem we found was that if the
smps was not repaired and was the cause of the failure, it most times
would take 6 to 9 months before we would get a call back on the set
being dead. So shop policy had to be verify the smps secondary
voltages on ALL ctc169 sets, cr4118 and cr4116 must be in proper range
and clean looking on the o'scope. Extra 5 minutes of work to guarantee
no future problems.

The cr4118 failure can also cause the B+ to run high as it starts to go
leaky and fail. However I have never seen it fail to the point that
the B+ will run over 150V before the 15V line was too low for the set
to attempt to start up.

It is mainly the 2.2uF capacitor on the standby feedback line of the
smps that goes open that causes this problem. It is a simple matter to
measure the standby B+ voltage on the cathode of cr4116 with the tv
turned off to verify that there is not a problem.