Videokarma.org

Go Back   Videokarma.org TV - Video - Vintage Television & Radio Forums > Vintage TV & Radio Tech Forum

We appreciate your help

in keeping this site going.
Reply
 
Thread Tools Display Modes
  #1  
Old 02-16-2018, 07:23 PM
Jon A.'s Avatar
Jon A. Jon A. is online now
Don't mess with Esther.
 
Join Date: Jun 2013
Location: Canada
Posts: 4,075
VHF tuner problem

Hey all,

After installing new electrolytic capacitors on a VHF tuner I can no longer get a clear picture. When I turn off the DVD player the screen looks like the tuner isn't hooked up at all. I rigged the set with one from my junk pile and got it working again for now. Adjusting the AGC delay makes no difference with the stock tuner but works fine with the temporary replacement.

My usual soldering iron temperature wasn't high enough to solder the leads to the feed-through capacitors so I'm wondering if I cranked it too high and damaged one. If that is the problem, I would like to know if those from an old cable converter box would make suitable replacements; I have no way of checking their value. I didn't touch the AGC input, just B+ and AFT correction.

Here's what I get with the stock tuner hooked up; hopefully someone has seen this sort of thing before.



And here's the schematic for the stock tuner:

Attached Images
File Type: jpg VHF tuner problem.jpg (82.8 KB, 47 views)
File Type: jpg 23-320-04 VHF tuner schematic.jpg (73.1 KB, 47 views)

Last edited by Jon A.; 02-16-2018 at 11:27 PM.
Reply With Quote
  #2  
Old 02-17-2018, 09:15 AM
zeno zeno is offline
VideoKarma Member
 
Join Date: Mar 2013
Location: New Hampshire
Posts: 3,001
Either the RF amp FET got blown by static or the AGC is
shorted out / opened. Any tuner should work if it uses
the same B+ usually 12 or 24VDC on a SS set. Most differences
in tuners are the shafts, isolation, etc. There was one CC2 that
used 2 different mfrs & you never knew which one you would get.

73 Zeno
LFOD!
Reply With Quote
  #3  
Old 02-17-2018, 06:15 PM
Jon A.'s Avatar
Jon A. Jon A. is online now
Don't mess with Esther.
 
Join Date: Jun 2013
Location: Canada
Posts: 4,075
Finding a bolt-in swap could take a while, it's a General Instrument unit and is the only one I've seen without manual fine tuning. The one I'm using right now is from a JVC-built set, it's just sitting in the cabinet.

Actually I think I'll just try to find out what kind of FET it uses, hopefully without dropping the control cluster again, then grab one of those along with a 100K resistor and go at it. I can't imagine what else could be shot.

Last edited by Jon A.; 02-17-2018 at 06:24 PM.
Reply With Quote
  #4  
Old 02-17-2018, 07:02 PM
old_tv_nut's Avatar
old_tv_nut old_tv_nut is offline
See yourself on Color TV!
 
Join Date: Jul 2004
Location: Rancho Sahuarita
Posts: 4,555
Those feedthrough caps are just 1000pf; am I reading the schematic right?
Should be a very non-critical value and a sub from your cable box ought to work.
__________________
www.bretl.com
Old TV literature, New York World's Fair, and other miscellany
Reply With Quote
  #5  
Old 02-18-2018, 08:51 AM
zeno zeno is offline
VideoKarma Member
 
Join Date: Mar 2013
Location: New Hampshire
Posts: 3,001
The feed thrus are pretty tough. I have even used a gun on them.
To be sure you can unhook the AGC & sub a voltage you
can vary. look at manual for range.
If you can change that FET you are a better man than I !!!

73 Zeno
LFOD !
Reply With Quote
Audiokarma
  #6  
Old 02-18-2018, 04:47 PM
Jon A.'s Avatar
Jon A. Jon A. is online now
Don't mess with Esther.
 
Join Date: Jun 2013
Location: Canada
Posts: 4,075
Quote:
Originally Posted by old_tv_nut View Post
Those feedthrough caps are just 1000pf; am I reading the schematic right?
Should be a very non-critical value and a sub from your cable box ought to work.
Yup, and I've compared it to other schematics, seems to be a very common value for tuner/IF applications.
Quote:
Originally Posted by zeno View Post
The feed thrus are pretty tough. I have even used a gun on them.
To be sure you can unhook the AGC & sub a voltage you
can vary. look at manual for range.
If you can change that FET you are a better man than I !!!

73 Zeno
LFOD !
I disconnected the AGC on the substitute tuner last night just to see what would happen; looked the same as if I had disconnected the B+.

Man, I read those feed thrus are rather delicate, so much for that.
Reply With Quote
  #7  
Old 02-22-2018, 07:27 PM
Jon A.'s Avatar
Jon A. Jon A. is online now
Don't mess with Esther.
 
Join Date: Jun 2013
Location: Canada
Posts: 4,075
I hope most VHF tuner FETs are the same or at least interchangeable because this one was partially covered in flux and scraping that off took the part number with it.

In case it's any clue at all it has four leads. For all I know they could all be like that.

Edit: Been doing some research, the 3N211 seems like a safe bet. I'm sure I saw "3N" on the original at one point but that doesn't tell me much. That bulkhead is soldered in, once it's free the rest comes apart with a screwdriver. It's full of carbon composition resistors which have probably drifted all over the place, may as well replace those with metal film resistors while I'm in there. I doubt surge currents are an issue in these.

That transistor may have been on its way out for a while, the temporary tuner is giving me the clearest picture I can recall.

Attached Images
File Type: jpg Mystery FET.jpg (82.1 KB, 21 views)

Last edited by Jon A.; 02-24-2018 at 12:33 AM.
Reply With Quote
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 08:59 AM.



Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.
ęCopyright 2012 VideoKarma.org, All rights reserved.