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-   -   VHF tuner problem (http://www.videokarma.org/showthread.php?t=270150)

Jon A. 02-16-2018 07:23 PM

VHF tuner problem
 
2 Attachment(s)
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.

http://videokarma.org/attachment.php...1&d=1518829948

And here's the schematic for the stock tuner:

http://videokarma.org/attachment.php...1&d=1518844715

zeno 02-17-2018 09:15 AM

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:smoke:
LFOD!

Jon A. 02-17-2018 06:15 PM

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.

old_tv_nut 02-17-2018 07:02 PM

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.

zeno 02-18-2018 08:51 AM

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:smoke:
LFOD !

Jon A. 02-18-2018 04:47 PM

Quote:

Originally Posted by old_tv_nut (Post 3196355)
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 (Post 3196377)
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:smoke:
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.

Jon A. 02-22-2018 07:27 PM

1 Attachment(s)
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.

http://videokarma.org/attachment.php...1&d=1519349078


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