"John Robertson" wrote in message
...
On 2017/07/08 7:38 PM, John-Del wrote:
On Saturday, July 8, 2017 at 9:58:10 PM UTC-4, wrote:
On Saturday, July 8, 2017 at 7:15:05 PM UTC-4, Dimitrij Klingbeil wrote:
On 07.07.2017 19:36, rickman wrote:
wrote on 7/7/2017 1:01 PM:
On Friday, July 7, 2017 at 12:09:33 PM UTC-4, rickman wrote:

The charge pump circuit was bothering me as the diode seemed to be
backwards. Then I realized it isn't making 24 volts from 12 volts,
it's
pumping up the 24 volts to something higher! What is the voltage on
pin 3
of the LA7833? The cap is only rated for 35 volts, so it isn't
making 48
volts. But clearly pin 3 should have something on it higher than 24
volts.
What does the pump drive signal on pin 7 look like?

--

Rick C

pin 3 of 7833 is 25.65 v. pins 2 and 15 are opposite polarity.
pump
drive signal is a .5 volts upside down sawtooth.

i appreciate the time taken here. i could just send this out, but i
think i am close, just not there.

If pin 7 pump drive signal is only half a volt, I would say your
problem
is there. Pin 3 should clearly be more than 25 volts and it is this
signal that drives that voltage. Is pin 3 at all steady or is it also
a
waveform? You should measure the voltage with the scope, not the
meter.
They don't use a smoothing cap so I guess the voltage will pulse with
the drive. During retrace you don't need a high voltage on pin 3.
Retrace is when the voltage goes below 24 volts and the diode is
forward
biased to charge up the cap again.

I think you said you already replaced the LA7833, so there are only
two
other parts. When you say pin 7 is "upside down", do you mean it is
normally high and drops low or it is going negative? I would expect
it
to be near 24 volts during the picture time and drops low during
retrace
as the cap charges. I guess the question is how does the waveform on
pin 3 look? I would expect it mostly near 48 volts dropping slightly
below 24 volts during retrace. Pin 7 should definitely have a
significant pulse voltage on it.

Did you check D360 (the "bootstrap" diode from 24V in the pump
circuit)?

It may be shorted and pulling the "pumped" supply "down" to 24V. The IC
will most likely need the pumped supply for one half of the screen
only..

i did pull up one leg of D360 and checked it, it checked good. pin 7
never goes anywhere near 24 volt. there is a .5 volts poorly defined
waveform, almost looks like ripple. im thinking it should be a 24v
sawtooth? i am thinking something on the board may be pulling down pin
7?

thanks

You can't check this diode with a dmm... I posted above:


"I didn't read back through all the posts, but if you haven't, check the
pump up diode. The anode will be connected to the Vcc and the cathode to
the pump up pin of the vert IC. A weak diode here will cause all sorts
of havoc (don't check it with a dmm - they often sag under load)."


That is a most interesting point, the 1N4937 diode is indeed a fast
switching diode and it certainly appears to be critical for building the
vertical deflection. Your point about sagging under load could indeed be
correct for this problem as described by the OP.

Its a fairly common type number - I've certainly encountered *MUCH* less
reliable diodes along the way.

Not that I've never had to replace one, but they're not prominent in the
stock faults list.