View Full Version : Weird problem with Sony KV-20FV12


old_coot88
12-26-2015, 02:41 PM
I have a 15 year old Sony KV-20FV12 That's been working perfectly up to now. CRT nice and bright, just like brand new. Then it started a "cycling" act, appearing exactly as if the CRT heaters were being switched on and off regularly. The pic doesn't go all the way out, just fades to like very weak emission on all three guns. Then it comes back up to normal, gradually cycling in and out over about a half minute period. The change is not abrupt, but like a gradual 'breathing' effect.
But the CRT heater voltage does not change, holding a constant 5.2V AC

Anybody had a similar Sony problem?

zeno
12-26-2015, 03:12 PM
Sony AKB circuits will mute the pix if a bad CRT but
I will assume yours is OK. Usually the pix blinks
when cold & does it less with warm up.
It is also sensitive to the G-2 setting so turn it up a tad
& see what happens. Basicly if any gun cant draw enuf
current the jungle IC shuts off all Y & C. During blanking
each gun is fired & current measured. You can usually see
three color lines at the top if you shrink the vert a little.

The CRT should have about 125V on the cathodes with
normal pix & 200V no pix ( zero beam ), G-2 abt 500V any pix.
Dont remember what the G-1 did in these.

73 Zeno:smoke:

old_coot88
12-26-2015, 05:30 PM
Sony AKB circuits will mute the pix if a bad CRT but
I will assume yours is OK. Usually the pix blinks
when cold & does it less with warm up.
It is also sensitive to the G-2 setting so turn it up a tad
& see what happens. Basicly if any gun cant draw enuf
current the jungle IC shuts off all Y & C. During blanking
each gun is fired & current measured. You can usually see
three color lines at the top if you shrink the vert a little.

The CRT should have about 125V on the cathodes with
normal pix & 200V no pix ( zero beam ), G-2 abt 500V any pix.
Dont remember what the G-1 did in these.

73 Zeno:smoke:
There's no change in pic size or geometry, or in chroma or video content. The CRT socket readings are-

Heater 5.2 VAC constant, and no change in the orange glow of the cathodes.
All G1s 51V constant.
Cathodes constant 150V with no pic, with pic they fluctuate 130V-170V with scene content.
G2 700V constant (G2 control painted solid, can't turn it).

Seems like totally normal readings. And they do not change during the half-minute "cycle" between a normal, bright pic and the washed-out "extremely weak CRT" image.
There's no intermittency or abruptness, and no response to tapping the boards or CRT neck. Just the slow, gradual 'cycling' between the two states.

All I can figger it must somehow be the jug doing it. Prolly just junk the set.

Findm-Keepm
12-26-2015, 06:16 PM
Solder connections okay?

Solder connections at the CRT socket were bad on my daily watcher 20" Sony. My daughter thought I was messing with the remote - to prove her wrong, she did the soldering....

The solder connections on the CRT board were quite solder starved.

jr_tech
12-26-2015, 06:28 PM
Something has got to be changing... corroded CRT pins, perhaps ???

jr

Eric H
12-26-2015, 06:32 PM
I would resolder the CRT socket on the board.
It could be losing some heater voltage even if you can't see them dimming, this was a fairly common problem, it happened to a Sony monitor I had.

zeno
12-27-2015, 11:05 AM
The G-2 sounds too high. It will break loose.
May be the pix is still there cause the G-2 is high
& it is actually blacking out but you cant
see it.
If thats the case you may have a weak CRT.
If so there is a trick you can try.

73 Zeno:smoke:

old_coot88
12-27-2015, 11:53 AM
I was reading the wrong scale (senior moment):o It's 700V on G2.
Anyhow I reflowed all the pins as suggested, and cleaned the CRT pins, and no go.
It's now in the fault state permanently, the typical look of a very weak CRT.
What had me buffaloed was the slow 'cycling' between the fault state and a nice bright 'brand new' pic. It's gotta be the jug. Still dunno how or why it was doing that.
Oh well, 15 years was a pretty fair life for the set.
Thanks for the feedback.

Dude111
12-27-2015, 03:09 PM
Is there anyone in your area that could look @ that set for you buddy?

I feel terrible for you,dont want you to lose her.....

Electronic M
12-27-2015, 04:02 PM
Is there anyone in your area that could look @ that set for you buddy?

I feel terrible for you,dont want you to lose her.....

Dude...It's just a 15 year old BPC!...I can go out any day of the week and get 10 working ones just like it for free....Hell when I see a BPC on the curb I don't even test it (if I even stop), but instead just rip out everything except the cabinet and CRT to use as parts for mid 70's and older sets that actually have some value.

zeno
12-27-2015, 05:53 PM
No need to quit yet. Lets do some free stuff first.

Use a pattern no a changing source. Color bars etc.

Measure the 200 V source. To find it find 3 resistors that
go from the collectors of the 3 color outs. Typically they
are 18 K 2W +- where they join is the 200V.

Turn down the color, brite & contrast all the way.
Turn down the G-2 until you see NOTHING. The paint
will crack when you turn it hard.

Measure the collectors again. If the AKB is killing the
pix they will be 200V.

Trick for all Sonys with AKB for all listening.
Put in video mode no source.
Ground one end of a 15-18K resistor.
One at a time put the other end on each color out transistor
collector..
This makes a voltage divider that will turn that gun on heavy.
A good CRT will show just that color BRITE & usually with
retrace lines. A bad gun will give a dim & mucky pix.
Reds will look brown etc.

Trick 2 last resort.
since Sony CRT's wont rejuve find the low ohms resistor
in series with the filament & short it. Works on most
brands with low E. Risk is an open filament or H-K short but
whats to loose at that point ? We did quite a few & got a year
or two out of many sets.

73 Zeno:smoke:

old_coot88
12-27-2015, 08:41 PM
Thanks Zeno. But I'd already closed the thing up ready for the scrapper. I really have no interest or incentive to repair "new" stuff if the problem isn't obvious and quickly fixable.
Intransigent fuddyduddy-ism.:grumpy:

kf4rca
12-28-2015, 08:46 AM
I rejuvenated the tube and it quit cycling.

Dude111
12-28-2015, 11:10 AM
Dude...It's just a 15 year old BPCI hear ya buddy....

old_coot88
02-04-2016, 12:19 PM
Just a little update regarding this critter. I had it all buttoned up ready for the scrapper, and then got to thinking what the heck, just use it like it is for a while. So I ran it with that horrible grey scale, near-zero emission, crappy crappy picture for several days. Danged if it didn't start "coming back to life" on its own over a period of several more days. I never touched the drives or G2 at all. Never adjusted anything. It came back to displaying a fully 'new set' picture just like it always had. And it's been holding that way ever since. Never seen 'nuthin' like it before. Don't know how long it will last, though.
Screen shot attached.

dieseljeep
02-04-2016, 12:42 PM
Dude...It's just a 15 year old BPC!...I can go out any day of the week and get 10 working ones just like it for free....Hell when I see a BPC on the curb I don't even test it (if I even stop), but instead just rip out everything except the cabinet and CRT to use as parts for mid 70's and older sets that actually have some value.

You might be seeing more "curbside setters" as Best Buy quit taking in TV's and computer monitors for recycling, free. They now going to charge $25.00, just like others do.

jr_tech
02-04-2016, 01:13 PM
So after resoldering the board conenctions, cleaning the crt pins, the cathodes took a bit of running at normal voltages to recover? :scratch2:

jr

old_coot88
02-04-2016, 01:28 PM
That's as good a explanation as any.

Dude111
02-05-2016, 05:03 AM
.....Don't know how long it will last, though.
Screen shot attached.Excellent!!!

I hope she is happy now and will give you excellent quality for along time :)

old_coot88
02-05-2016, 10:39 AM
Yeah, hope so too. When it was doin' the original 'cycling' act, there was no perceptible change in the heaters' normal healthy orange glow. A "Lazarus" CRT is real puzzler for sure.

zeno
02-05-2016, 07:28 PM
Newer sets do some strange crap. I had a few of this vintage Sony
with a small ( H & V ) & messed up raster. Who would suspect a bad tuner ?
The data line to the tuner would short & none of the IC's such as
the jungle would get the preset data from the E-prom. So you had to
change the tuner / IF & it just cost too much. Odds are it was a
chip in the tuner shorted but probably a SMD & not available.
BTW to trouble shoot just unsolder the CLOCK, DATA & ENABLE
pins at the tuner. If it gives a full raster its the tuner.......

73 Zeno:smoke:


Yeah, hope so too. When it was doin' the original 'cycling' act, there was no perceptible change in the heaters' normal healthy orange glow. A "Lazarus" CRT is real puzzler for sure.

kf4rca
02-06-2016, 08:45 AM
Have to remember that one. Thanks.

old_coot88
01-22-2017, 08:16 PM
Zeno, you being the resident Sony guru, lemme pick your brain a bit. The jug in this Wega KV-20FV12 is starting to get a little soft again. Before I think about pulling the back again, is there a heater series resistor? I can't find a schematic anywhere showing the CRT area. Presumably the resistor would be on the CRT socket board.

I know some other brands had 'em. Having retired from the repair trade in 1986, I'm not overly knowlegable on newer stuff.

Findm-Keepm
01-22-2017, 10:41 PM
Zeno, you being the resident Sony guru, lemme pick your brain a bit. The jug in this Wega KV-20FV12 is starting to get a little soft again. Before I think about pulling the back again, is there a heater series resistor? I can't find a schematic anywhere showing the CRT area. Presumably the resistor would be on the CRT socket board.

I know some other brands had 'em. Having retired from the repair trade in 1986, I'm not overly knowlegable on newer stuff.

3.3 ohm 2W resistor, R718 or R71B, depending on how the CRT board is marked...off pin 7 of the CRT socket.

old_coot88
01-23-2017, 01:30 AM
Wow, thanks. May be able to milk the old jug a while longer now. :tresbon: