Well... a TV IS sort of a radio.
Posts: 1,106
Threads: 88
Joined: Jun 2011
City: Tacoma
State, Province, Country: Washington
I got this TV in the other day, after a 2 month plus sojourn through the postal system by Parcel Select (HAH!). It made it mostly intact, but the cabinet has some minor damage that I'll have to deal with.
The electronics were a bit more fun. Several caps to replace, along with several out of tolerance resistors (two were actually broken in half.)
For the longest time, I was getting good voltages pretty much everywhere, except for the HV. Eventually I found that there was a problem with the LV power supply (amazing they could make the thing work on 120V at all.. not even a voltage doubler.)
Anyway, I added a 300mH choke into the power supply filter chain (I had replaced the selenium rectifier with a silicon, so needed a resistor in there anyway. I used the choke because I had a lot of them around, and it did such a nice job of both bringing up the B+(the few volts made all the difference!) and cleaning up every last bit of hum that I decided to leave it instead of a resistor.
Well, the HV came right up after that... but still no visible signs in the CRT. I had left the yoke and ion trap off the CRT so I would have a better chance of seeing when I had HV up enough for the CRT. I had nothing. I held the ion trap up near the neck, and a spot appeared. Moving it around got a brighter spot... then it got TOO bright. 
Before that, the CRT face was perfect. This was the first case of INSTANT ion burn I've ever seen 
So now, I have this set with a nice bright picture, and two nice dark ion burns right in the middle.
Posts: 2,118
Threads: 112
Joined: Jun 2010
City: Medford OR (OR what?)
With a bunch of radios, I got a roundie tube, about 6", Motorola the other day. I haven't worked on a tube TV for 30 years. So, Brenda, you can continue to tell me what to do (or what not to do).
By the way, the picture tube HAD a light colored gasket where it met the front of the chassis. It took a while to figure out what this even was since it was a large pile of goop now. Anybody know where to get some sort of replacement. The picture tube is just hanging by a bracket near the yoke.
"I just might turn into smoke, but I feel fine"
http://www.russoldradios.com/
(This post was last modified: 11-09-2013, 10:05 AM by Phlogiston.)
Posts: 1,113
Threads: 14
Joined: Feb 2013
City: Irvington, NY
Renovatedradios used to sell the Motorola foam rubber surrounds, but the site now says they are no longer available.
Perhaps we can persuade him to make more, as I need one also.
Posts: 2,118
Threads: 112
Joined: Jun 2010
City: Medford OR (OR what?)
That would mean that I would have to fix it. Well, OK, I'm in. How do we persuade him?
"I just might turn into smoke, but I feel fine"
http://www.russoldradios.com/
Posts: 2,005
Threads: 361
Joined: Jun 2010
City: Dover, OH
I have a few old round tube TV sets myself along with a few square tube units. Old TV's can be fun, but can also be headaches to keep going. Sometimes you just get one going and something else fails inside and then something else and something else. Best way is to replace all capacitors and resistors to ensure at least some reliable service. It's not hard to see why there were once armies of service men out there to keep these going!
No matter where you go, there you are.
Posts: 1,106
Threads: 88
Joined: Jun 2011
City: Tacoma
State, Province, Country: Washington
Sounds like you have a VT-71 or it's ilk there. Those are PITA sets, but you feel like you've really accomplished something when you get them working right.
First thing's first, you'll have to replace those seleniums, and of course the 'lytics that go with them. You'll need to replace EVERY SINGLE PAPER CAP in the set before you even begin to get anything like television on that thing. The critical ones will be the huge paper caps (6KV!) that feed the sweep signals to the plates of the CRT. They're expensive little buggers, and the only place I've found for them is justradios.com. You may not find exact values there, but those are not the critical part: those spikes will turn any other type cap into Swiss cheese in a matter of seconds.
Pray your 7JP4 is still usable. Even used ones of unknown quality are going for upwards of $100 on ebay.
As for the gasket, I have one here someplace that I bought for a set that I restored that later had a fatal failure, and has been parted out to retrieve the expensive parts I had put in it. Since the gasket is used, if only lightly, I can let it go for $30. I paid $40 for it at renovatedradios.
Don't get me wrong, I like these sets, and they are a nice way to hone your skills, tracking down H-osc, V-osc and sync problems, and properly aligning those 27 MHz IF's (you'll need a service pack to do this, but they are plentiful and cheap).
Here's a nice workup on restoring these sets:
http://antiqueradio.org/motvt73.htm
Posts: 2,569
Threads: 336
Joined: Jun 2013
City: lawrenceville nj
WOW Brenda
I am truly amazed at how much you know and do you are like electronic guru  
Posts: 4,612
Threads: 51
Joined: Sep 2008
City: Sandwick, BC, CA
I would ask Bob Anderson about Motorola VT71 sets, he must have restored a 1/2 dozen sets by now using some version of that chassis, there were at least six or seven changes made to it during the VT71's production run. It's a 7'' electrostatic tube, a 7JP4, the odds are if the filament has continuity it can be made to work and produce a picture. The repro gaskets are not currently available new, but I'm sure that someone could find a way to cast one if they had a good one to make a mold from. The high voltage caps are also available from WJOE radio, I don't know what values they have but the prices are reasonable, those have to be replaced first, then the selenium rectifiers and electrolytics in the voltage doubler, then the usual paper caps and resistors.
Regards
Arran
(This post was last modified: 11-09-2013, 09:26 PM by Arran.)
Posts: 1,106
Threads: 88
Joined: Jun 2011
City: Tacoma
State, Province, Country: Washington
RATS!
I finally got everything working just right, and installed the chassis and CRT back into the metal cabinet. Then I went through the trouble to get the positioning rings and ion trap perfect.. put the escutcheon and handle back on. Then, and only then, did I realize that I had NOT put the speaker back in! Now I have to take it all apart again (at least pull the CRT back out) just so I can put the speaker back in.
That's what I get for being so clever! 
Posts: 1,106
Threads: 88
Joined: Jun 2011
City: Tacoma
State, Province, Country: Washington
OK.. TV is all back together and working great. I wish I hadn't burned that CRT.. it WAS in very good shape. Still, all in all, I'm pretty happy with it. It's a 14" Sylvania.
Posts: 7,215
Threads: 266
Joined: Dec 2009
City: Roslyn Pa
Something else that happens to the 7JP4's is the deflection plates can come loose or break loose inside the tube. The tube it self is fairly fragile. Got a few VT-71's myself.
Terry
Posts: 4,612
Threads: 51
Joined: Sep 2008
City: Sandwick, BC, CA
I'm just guessing here but I think one way to prevent an ion burn while setting a ion trap magnet is to set the brightness down as low as possible. I've never heard of this happening while setting the ion trap magnet but it obviously did in this case.
Regards
Arran
Posts: 795
Threads: 42
Joined: Dec 2008
City: Chicago, IL
That's actually not an ion burn. The electron gun is bent so both ions and electrons shot out the side of the neck. The external magnet bends the electrons back towards the screen while the ions being much more massive are barely affected.
What happened is the CRT was operated with the yoke removed so the intense electron beam was focused on one small spot and it destroyed the phosphor coating. The same thing can happen over time when a set is shut off many times and the image reduces to a bright dot.
You'll only see an ion burn on a CRT with a straight gun and no aluminized coating. That's pretty much only pre-war TVs and a few very early post-war sets. It looks like a dim spot in the center of the picture. (That stuff running down is the old CRT gasket)
[Image: http://farm9.staticflickr.com/8495/83723...c067_z.jpg]
(This post was last modified: 11-12-2013, 01:58 AM by Bob Andersen.)
Posts: 1,106
Threads: 88
Joined: Jun 2011
City: Tacoma
State, Province, Country: Washington
Oooh... I thought it was all the picture ink seeping out. 
Posts: 2,005
Threads: 361
Joined: Jun 2010
City: Dover, OH
Almost looks like a scene from a horror movie there. The TV is bleeding!
No matter where you go, there you are.
Users browsing this thread: 1 Guest(s)
|
