Another local Hoffman! COLOR! - Page 3 - Videokarma.org TV - Video

Yamamaya42 · #31 07-17-2022, 02:39 AM

Progress, and step back, sort of? :O

After seeing the broken pin on the 25AP22A ( or so the sticker on it says ) , and not knowing if my fix attempt will work or not. I tried to bring the set slow up on Variac, this fond that C3 was REALLY bad, as it got hot VERY fast.

So I replaced all the sections of it, and tried again, THIS time power came up, I got hiss in the speaker, I heard vert start, but NO HV....

looked inside the HV cage, the HV rect was lit, and very hot. Pulled the cap off and got a good strong AC arc all over the place, removed the HV shunt, no change,,, BAD 3A3A, of which I have no spares.

so now, before I can make sure the CRT is any good, I must wait for the new 3A3C to get here so I can get HV up.

Yamamaya42 · #32 07-17-2022, 10:38 AM

OK...
It was NOT a bad 3A3A :O

http://suzaku.live-evil.org/0717221006a.jpg

had a burn out in the heater wire FOR it,,,,

crispy carbon!

http://suzaku.live-evil.org/0717221025.jpg

after the repair

Electronic M · #33 07-17-2022, 10:47 AM

Yikes. Hopefully that arcing and high current didn't hurt the rect of worse the fly.

Yamamaya42 · #34 07-17-2022, 10:57 AM

Quote:

Originally Posted by Electronic M

Yikes. Hopefully that arcing and high current didn't hurt the rect of worse the fly.

The Fly is OK, the way i found it was, I used a handful of 20KV 5mA HV High Voltage Rectifiers i had, doubled up on them so they would not fry.
but quickly found that when NOT hooked in circuit i had 25k HV, but they would burn up as soon as I tried to wire them into the normal way.

that is how i found 1he 5m to ground short on the heater winding for the rect tube and went to look for the problem.

Tom9589 · #35 07-17-2022, 11:29 AM

When I looked at the two pictures of the chassis out of the cabinet, I noticed that there were two transformer cans with a piece of black tape over the top adjustment hole. Any idea why they were covered? Was it a way of the manufacturer to discourage "diddlers" from adjusting these cores?

old_tv_nut · #36 07-17-2022, 12:59 PM

Quote:

Originally Posted by Tom9589

When I looked at the two pictures of the chassis out of the cabinet, I noticed that there were two transformer cans with a piece of black tape over the top adjustment hole. Any idea why they were covered? Was it a way of the manufacturer to discourage "diddlers" from adjusting these cores?

Curious. Can't think of a reason offhand. Possible that someone who worked on it did that?

old_tv_nut · #37 07-17-2022, 01:18 PM

What circuit are the taped cans in?

For relatively low frequency circuits where the stray capacitance is not significant, it would be possible to include a capacitor inside the coil assembly and automatically pre-tune it during the assembly manufacture, before installing on the printed circuit.

Motorola's coil plant made some pre-tuned assemblies, but I always thought they still had to be adjusted after installation in the chassis. I used eight pretuned coils (IIRC) in our redesign of the CBS video film player (EVR), at frequencies around 900 KHz and 1.8 MHz, [linear-phase chroma pilot and chroma subcarrier filters] but they definitely had to be touched up in the final product. This was due to the differences in strays between the winding machine and the final product. It would have been possible to tune samples in the final product and then have the winding machine duplicate the tuning, but it was not worth it because the printed circuit might be changed at some time (material or layout) and change the strays.

Yamamaya42 · #38 07-17-2022, 01:19 PM

Quote:

Originally Posted by Tom9589

When I looked at the two pictures of the chassis out of the cabinet, I noticed that there were two transformer cans with a piece of black tape over the top adjustment hole. Any idea why they were covered? Was it a way of the manufacturer to discourage "diddlers" from adjusting these cores?

no idea why it's like that

Good news is, the HV rect is OK!
bad news is, my patchwork did not hold.
the HV blasted through / around the heat shrink tubing and found a path to grd and arced again, I guess i should not have clipped it down

now time for more drastic measures.

http://suzaku.live-evil.org/0717221306.jpg

huge piece of tubing filled with silicone
this will take many hours to cure
and If it's dry enough to put pack together late tonight, I will slop more on it.

hope to be ready to power it again mon night.

Yamamaya42 · #39 07-17-2022, 01:28 PM

using this stuff
https://webaps.ellsworth.com/edl/Act...05&language=en

Yamamaya42 · #40 07-17-2022, 06:35 PM

I am really hoping this CRT is going to work, otherwise things will really suck pretty fast T_T

Electronic M · #41 07-17-2022, 10:53 PM

Once you have a good CRT change the bad HV wire properly. When that insulation kludge fails on you again a few weeks or months down the road you'll wish you had replaced the wire.

Yamamaya42 · #42 07-18-2022, 12:08 AM

a very dopey looking FBT

http://suzaku.live-evil.org/0717222342.jpg

there were some cracks in the outer " doughnut " that was allowing arcing.
but unlike the ones in the roundies, this one seemed ti be made of tar or something and not rubber.

there was no carbon tracks that I saw, just cracks and so on.

as far as replacing the heater winding, i really don't have the type of wire to do it right now, or really know what to even get to use.

Yamamaya42 · #43 07-18-2022, 08:22 AM

Thinking conservatively, it would be rather foolhardy now NOT TO replace the burned wire now while I have the FBT out of the chassis with it easy to get access to.
Not really knowing what type of wire to get, I made an educated guess.
Copper Core Flexible Silicone Wire Cable Red 10M 32.8Ft (22AWG 40KV)

40KV, well above what this set should generate, 22AWG, should be thin enough for the clearly 2 turns that I see around the core of the FBT, and hearty enough to power the 3A3A/C

Yamamaya42 · #44 07-18-2022, 08:50 AM

Re-post of original FBT photo.

http://suzaku.live-evil.org/0716221404.jpg

You can clearly see 2 turns on the heater winding on the core, should not be too hard to replace with the new wire ordered.

You can also see the crack (cracks) in the FBT outer layer which looks to be made out of some kind of hi temp tar, which was easy to melt with an soldering iron, there was a tendency for corona arcing from these cracks but saw no real carbon, and thought it very risky to try to remove it, so I covered it with several layers of dope.

dieseljeep · #45 07-18-2022, 10:10 AM

Quote:

Originally Posted by Electronic M

Once you have a good CRT change the bad HV wire properly. When that insulation kludge fails on you again a few weeks or months down the road you'll wish you had replaced the wire.

When the high voltage rectifier winding arced through, I used to cut out the wire and used an EDI solid-tube which was a solid-state replacement for a 3A3/3DB3 or a 3AT2, depending on the set. It was especially important on a Zenith that used resistance wire for the winding.
RCA's seldom needed it because the flyback was replaced before.