Rca 8-pt-7031

[dtvmcdonald]

Last night, while I was examining the video amp response, the picture died.

Too-low HV (about 1700 volts) and a variable buzzing sound from what appears
to be the horizontal output box. I can't tell if it is in the transformer or at the
rectifier socket.

To make a long story short, the buzzing disappears if I remove the HV rectifier.
The waveform at the CRT anode connector shows aperiodic dips, a dozen of so
horizontal periods apart. This does not change if the CRT base is plugged in or not.
Similar glitches appear at the HOT plate cap.

I removed the HV box and found little visibly amiss. There was a little black crud
on the insulating material covering the tube socket, which I removed. No difference.
The audible noise and glitches disappear entirely if the HV rectifier is removed.
They are reduced but still there if I disconnect the CRT anode connector.

I did notice that one pin (a filament one) of the HV rectifier tube socket was filled with the waxy stuff, which I cleaned out with no change.

Tests show that with or without the HV rectifier tube the boost voltage is normal.
With HV rectifier removed the pulses at the HOT plate appear reasonably
normal, with the rectifier in, they are low. My HV probe has only a rough calibration
for frequencies above 1 kHz.

I then removed the wires to the yoke. The buzzing was much less prominent
but other symptoms appeared similar, with or without rectifier tube, except that the
glitches became somewhat more regular, varying with the vertical hold (not signal applied) but not at all in sync with it. (Due to variation is current draw from the boost?)

Removing the wires to the width coil removed the boost voltage and the large
single cycle HV pulse at the HOT plate cap.

The HOT gets warm, but no sign of distress and its not really "hot" as opposed to warm, and I don't think its warmer than when the HV was OK.

So the question! I've never had to service a problem like this before. Should
I first assume that the problem lies with arcing under the wax (?) covering the
HV rectifier socket, including the filament dropping resistor? Should I remove that
covering to examine what's under it? Is it really wax, its rather hard for wax.
Its a tan color. How do I remove it? Heat gun to melt it, solvents, chipping???

What are the experts recommendations?

[jr_tech]

Is the 1.5 ohm resistor in series with the 1V2 ok?

jr

[dtvmcdonald]

Is the 1.5 ohm resistor in series with the 1V2 ok?

Well, its not open!, at low test voltage. I can measure it in
series with the flyback winding. I can't get to it as its buried under the
blog of (presumably) wax. It could have problems in use. That might do it.

As I said ... should I remove the wax (?) blob?

******************

OH! I just realized something ... due to tehe way the circuit works, I could remove the
rectifier and install a semiconductor "stick" directly between the HOT plate cap
and the nealby CRT anode. Were do I get such a thing? Remember the HV is 5 to 7 kV,
but I presume one for 15 or 20 kV would work.

[jr_tech]

I would be tempted to test it at 300 ma (1V2 heater current), supplying current from a bench supply and looking for glitches in the current.

IIRC a hair dryer will melt the wax, but it has been a long time since I worked on one of these with a hv problem. I think that I ended up replacing the entire hv module with another one from a friends set.

jr

[dtvmcdonald]

I tried connecting 7 1N4007s in series to make a HV rectifier.

With it the TV works fine.
Thus, its a spark to ground at the 1V2 cathode.

I'll use the diode idea.
I found the NTE518 which is for this purpose. But would
7 1N4007s in series be the same thing?

[dtvmcdonald]

While waiting for the NTE diode I played with improving the
picture. The set uses a very very odd contrast control system
using a pot in the video amplifier cathod and two bypass caps,
one 150 and one 820 pF. The frequency response varies
tremendously with contrast setting.

I found an improvement: add from the cathode to ground
two things: one is a .0012 uF cap, the other is a series
combination of 47uF and 390 ohms. This does mean the
contrast can't be turned as far down, but the frequency
response becomes quite flat from 1/3 full gain (control
about 80% up) on down. At full gain there is a 6 dB
enhancement at low frequency, but this generally is too
high a setting.

[dtvmcdonald]

I tried aligning the IF again (The RF stages were OK to start with). Remember
earlier I had moved the IF up about 3 MHz.

I actually looked carefully at the slugs in the tuner coils and realized that
they were all set on the bottom side of the windings (i.e. partially out
of the windings on the side nearest the chassis.

I tried moving them, one at a time, to the same frequency on the outer side.
This resulted in a similar response. Next I tried aligning at the normal
frequencies. The picture appeared exactly as in the instructions for the
first time! That's an abysmally bad response, nevertheless. I again
tried playing by hand and lo and behold it was easy to get, instead of
the specified peaky response, the usual slope for the vestigial sideband,
then a flat top, followed by a steep drop to 3.5 MHz (from the compulsory
dip caused by the interaction of L47 and L48) and the specified small peak
at the audio carrier.

After reinstalling the CRT HV kludge, I got a perfect picture. Not super
high resolution, of course with both IF and video limited to maybe 2.5 MHz,
but no blurring or ringing. It stays that way from zero to about 85% rotation
of the contrast control.

I love playing with TVs like this! I hope this thread on a super UNexceptional
and UNinteresting (but cute) TV will be of help to somebody in the future.

RCA PT-8-7010 PT-8-7030 PT-8-7011 PT-8-7031 PT87010 PT87011
PT87030 PT87031 PT-8-7030T PT-8-7031T for searchers.

[maxhifi]

May I ask, what sort of sweep frequency generator did you use during the alignment? I have the same RCA TV, and it really has a bad picture after a full recap, reading the above goes a long way to explain why.

[dtvmcdonald]

Originally Posted by maxhifi

May I ask, what sort of sweep frequency generator did you use during the alignment? I have the same RCA TV, and it really has a bad picture after a full recap, reading the above goes a long way to explain why.

I used an old Eico 368, fully refurbished and converted to
post-modulation markers. This is utter garbage, especially linearity
and accuracy of the sweep. The marker dial is quite
accurate. However I also have a super accurate synthesizer and
use a HackRF with SDRSharp on my computer as a spectrum analyzer, also
very accurate. (You can buy one of those DVB dongles for like $7 from
China and while not 6 MHz wide they make a great spectrum analyzer to set
the markers.) I have not bought a 50s-60s 45 MHz only much
better one since I have some non 45 MHz IF TVs. I can also borrow
the best that money can buy ones if needed (DC to 150 GHZ).

I'll add the pictures I just took.

This one is a video sweep from a DVD player off Digital Video Essentials,
DVD version. The tic marks at top and bottom start at
1.0 MHZ and go up by one MHz steps. You can see the resolution
stopping at about 3 MHz. This is Channel 5 off an agile modulator.

http://videokarma.org/attachment.php...1&d=1454288347

This one is a scope trace of the above, off the CRT cathode. Note that
the 7 pF of the probe noticeable reduces response. You can see the
1 MHz tics which are set at exactly 2 major divisions.
This looks horrible, and is, but oh so much better than what the
service manual shows!

http://videokarma.org/attachment.php...1&d=1454288347

This is the same but channel 12 off a dedicated Ch 12 B-T BAVM-SAW modulator. Just looking at the screen its not different from the above.

http://videokarma.org/attachment.php...1&d=1454288347

This is a single frame from a modulator on a cable box.

http://videokarma.org/attachment.php...1&d=1454288347

This is a single field, not frame, from the modulator and an OTA STB.

http://videokarma.org/attachment.php...1&d=1454288347

Thumbnails appear in wrong order.

[dtvmcdonald]

The NTE518 HV diode arrived and works fine.

Its odd that they offer a 10 kV one rated 25 ma but nothing
higher voltage at that current rating. I suppose three in
series would work nicely on a color set.