Rca 8-pt-7031

[dtvmcdonald]

I've been looking for one of these as I had one in college (except this is one is gray and my old one was red.)

One locally found me for a good price. It had been seriously serviced many times before.

A quick check on a dim bulb and variac at no more than 10 volts AC showed why it stopped being used: somebody had replaced the selenium rectifiers with 1N4007s ... backwards. It came up with negative B+.

Lots of shorted caps.

[old_coot88]

They musta thought the '+' mark on the seleniums meant the anode side. Wasn't there a fuse or fusible resistor that woulda popped?

[dtvmcdonald]

Nope, no fuse or any protection at all except a fuse between
B+ and the damper. I will add one.

[ZackN920]

Just curious, but when did you get this? Was it listed on Ebay? I've been watching a gray one on ebay for a few day's now. I noticed today the add was listed as unavailable now.
The same seller has a white one that I'm watching as well, that I'm real interested in, because that one has a stand. Don't know why it interests me so much but I really do like the look of them with the stands.
Good luck with the restoration. There good little watchers. I actually use mine quite a bit, that sits on my desk.

[dtvmcdonald]

I got it Friday. Its a local very very private deal. I've known about the set since October, but had hoped the seller could get a better deal, but he really was uninterested in the price. And I needed a project, so I got it.

The reason I hesitated is that I wanted a red one. What color is the inside of'
the red ones? I'm in no rush ... the cabinet is in fair shape and I can't refinish metal until summer anyway.

[dtvmcdonald]

I got it working. The picture is awful, as you would expect from the word description and numbers on
the IF and overall curves, which is 2.1 MHz wide at the -6 dB points. The pictures of the alignment
results seem to indicate that the sound carrier bump should be adjacent to the video one ... not so.

I had trouble with the HV and boost disappearing. This is probably due to a
bad connection at the damper or Horiz osc tube. I cleaned them with DeOxit
and it goes away.

This set does not have an insulated CRT anode connection (!) making
measurement easy. Its good to have an HV probe, I have an old 2000x one
that has quite a bit of AC response up to 100 kHz, so I can scope the Horiz
output tube plate.

The case looked terrible .. but in fact was almost pristine after rinsing with alcohol.
They used really tough paint.

[compucat]

I have one in a grey cabnet that I did a total recap on years ago. It has been very reliable. Performance is good but not exceptional. The tube is not aluminized for one thing. They are very well built with a power transformer and everything. I like the fact that they are so compact that they can be used and displayed almost anywhere. The all metal construction and real 1950s look is what I like so much about them.

[dtvmcdonald]

Performance is GOOD? Really! On mine the picture ... tuned correctly,
with the video carrier at the correct place ... is SOFT.

Other than that, its OK though the AGC is as expected a bit wimpy.

Doug McDonald

[dtvmcdonald]

With encouragement (meaning giving courage!) from the above, I decided
to try alignment again, this time ignoring the instructions.

I kept getting it better and better by playing with the adjustments.
Little by little, playing with all four (includes sound booster coil), I
got a quite OK picture, with response out a bit beyond 3 MHz,
though with a rather bad droop. But there is not sharp peak like
the instructions generate.

At that point I measured the carrier IF frequency ... it had moved up
3.6 MHz! (As had, of course, the oscillator frequencies).

I tried carefully lowering it back, but that simply sis not work, period.

I'm now much happier with the set.

As a bonus, the sound is much better.

[Sandy G]

I've heard these can be absolute Pills to work on. I have 3, along w/one of those "Arvin"-I THINK- combo 8" TV/AM radio sets that uses the same CRT.

[dtvmcdonald]

Actually they are quite easy to service for the most part. Its only a nightmare if
you decide to carefully unwrap every terminal that has a capacitor connected
to it, so you can do a politically correct lead re-wrap job. I only did that
to the vertical timing cluster, which had several way-off resistors so I replaced everything.
The two-part chassis comes apart in less than a minute and the set
works fine with them apart so access is easy.

I had lots of experience with one, of course, in the 60's. When I was in college
I had one. My suitemate had, envy envy, an old CTC-7. We also had
installed a proper rooftop antenna to the dorm and snaked coax all the way to our location.

[dtvmcdonald]

Circuit simulations (as well as general circuit knowledge) show why this set is so
difficult to align: L47, L48, C23 and stray capacitance across L47 and to ground
require a very deep dip somewhere between the sound carrier and the video
carrier. There are insufficient adjustments to adjust both the position of the
average of the two peaks and their separation. I had to adjust the average
to get the separation right.

[Eric H]

Originally Posted by dtvmcdonald

The case looked terrible .. but in fact was almost pristine after rinsing with alcohol. They used really tough paint.

I have an ad for these somewhere that says it's not paint but a coating related to Bakelite, possibly an early example of electrostatic coating?

[dtvmcdonald]

Here's the result. Note the screen burn in the center. How does this happen with an ion trap?



http://videokarma.org/attachment.php...1&d=1453778624

[jr_tech]

Ions generated past the trap, by the beam?

jr