GE13PF6504 no picture but green splotches

[technoman9]

Greetings fellow CRT lovers,

Ten years ago, I was using a 13" GE TV model 13PF6504 made in 1985 as my bedroom TV. One night while I had it on, I heard a "blip" inside the TV and suddenly the picture went all black with a few green splotches. The audio went quiet except for a 60 Hz hum. I promptly shelved the TV until I could find time to look at it.

That day has finally arrived. Last night I pulled it out and turned it on to confirm the issue. Strange thing was, as the tube was warming up I could see normal no-channel TV static on the screen, but as soon as the raster finished filling up the screen it went right back to displaying the black screen with two green splotches on it (see attached image below). The audio also is still quiet except for a soft but noticeable 60 Hz hum.

So far I checked the large power capacitor and found it's not shorted or open. I did see, with my Flir camera, the Degaussing thermistor hitting 200 F in just a few seconds so I shut it down and have not fired it up since. I checked the horizontal output transistor D1453 and found complete continuity (just one ohm) between the base and emitter, but I need to pull it out to see if that's the circuit doing that or not.

I've worked on stereos and cassettes decks before but never a CRT TV, so am appealing to you all in the interest of learning how to troubleshoot this. Any ideas on how I should proceed? Are there known issues with these GE TV sets I should be aware of? Should a Degaussing coil thermistor be reaching 200 F or higher in short order?

[zeno]

DGS resistor will get very hot.
Odds are its going into HV shutdown.
Unplug the DGS, run on variac at abt 95 VAC.
Put an AC amp meter in the line & dont let it go above 1 amp.
Try set & see if it runs. More after you do that.
BTW the set was built by Gold Star or Samsung, dont remember which.

73 Zeno
LFOD !

[technoman9]

It is indeed a Goldstar set, that is who is on the CRT's label.

I suspect your high voltage (HV) shutdown hunch is correct. Plugged TV into a Kil A Watt meter, Genrad variac, and an isolation transformer in that order. Here are the results:

TV+meter+variac+isolation trans, 97 VAC: Got normal no-signal picture, normal static in the audio, and 0.7 A draw.
TV+meter+variac+isolation trans, 120 VAC: same results as above except current draw was now 0.83 A. Picture and audio still normal.
TV+meter directly into power strip: TV shows black screen with green splotches, no audio, and draws 0.16 A.

The results above were the same regardless if the Degaussing coil was plugged in or not. It appears that my Variac and isolation transformer together are enough of a current limiter to allow it to fire up. I know these sets have an "excessive X Ray" shutdown feature, is that the same thing as HV shutdown?

[zeno]

OK usually its too much main B+ ( typ 130vdc ). Usually its a shorted
regulator IC or a little diode off it. Hang a meter on that voltage
& turn up the AC. It should go up to a point & stop at the proper voltage.

HVSD goes by many names. Yours is a simpler design. Reason for unplugging
DGS is on a cold start the set will draw a lot of current for a second
& confuse your amp readings. Keeping the draw under an amp protects
the set if there is an overload in the HV. A normal set will draw apx
.5 to 1 A depending on size.

enuf fer now
73 Zeno
LFOD !

[JohnCT]

Originally Posted by technoman9

I checked the horizontal output transistor D1453 and found complete continuity (just one ohm) between the base and emitter, but I need to pull it out to see if that's the circuit doing that or not.

It's normal. The emitter and base are across the secondary of the horizontal driver transformer. The secondary is very low ohms. Besides, horiz outputs are pretty much binary go/no go, and since it produces a picture however briefly, you can assume the horizontal output transistor is good.

The green blotches are what RCA used to call "afterglow". This is just stray emissions from the gun that light up the screen because the TV has no HV bleeder to bleed off the HV. That plus some residual voltages at the gun show this.

As others have said, it sounds a lot like a HV shutdown. I used to be a LG ASC and repaired a lot of these. The two most common problems are a bad LV regulator and the B+ bypass capacitor. That cap is connected to the B+ input at the flyback, typically a lower value and higher voltage - 33uf@250 give or take. The regulator might be an "STR" device, either in a TO-3 case or an inline IC although some versions used discrete transistors for regulation. If you can post the pic of the power supply schematic someone will show you what to check.

A way we used to troubleshoot these is to *remove* the horizontal output and put in a 60W incandescent bulb from the collector to the emitter. The regulated B+ should hold steady at the schematic value as the AC is adjusted up and down 10 volts.

If the voltage drifts more than a volt, the regulator is probably shorted. If nothing else, I would change the B+ bridge filter cap and the B+ flyback bypass caps.

John

[technoman9]

Tonight I did as you suggested and measured the B+ voltage at pin 4 of the LV regulator, which I presume is the same thing as the B+ regulator, aka THE regulator. You can see it in the attached image; it's an STR3115.

With the set plugged into my variac and isolation transformer I measured 135V at the regulator's output. It varied +/- 4V as I dipped the variac down to 110VAC and back up to 120VAC. I then plugged the set directly into line voltage and got 165V!

With power off, I checked the two electrolytic caps in the power supply using my Tenma LCR bridge. The 470 uF measured high at 576 but otherwise they were OK. I then measured continuity of the regulator and found 2 Ohms between pins 1 and 4. I'd expect it to be something closer to the 390 Ohm resistor that's across the same point.

I think the above proves the regulator is shorted and fubar'd. Before I order the NTE replacement, is there anything else I should check? I will check the flyback bypass capacitors tomorrow, I believe those are C437 and C443 in the second image, correct?

[zeno]

For the IC buy from a trusted dist. There are many fake semi's out there.
Only other thing I would change is that little diode at the IC. Sry cant
read the C ##.
The 4.7mfd on the 177V is good to change on ANY brand TV. Probably
in the top 10 failures. It can give all sorts of video problems often
subtle.

BTW nice working on it with you. You have a good grasp on things
& you listen ! Wish they all did.

73 Zeno
LFOD !

[JohnCT]

Originally Posted by technoman9

Tonight I did as you suggested and measured the B+ voltage at pin 4 of the LV regulator, which I presume is the same thing as the B+ regulator, aka THE regulator. You can see it in the attached image; it's an STR3115.

Yep, that's a 115V regulator. These do require a load so you can't measure the output with the TV in shutdown to get an accurate reading. If the sweep is running, you can get an artificially high B+ reading because a weak or open bypass cap will feed back to the regulator.

Still, you should not be getting a shorted reading across the big pass resistor, so the regulator is bad.

As Zeno says, careful where you get them. I just checked and don't have any of these left, but you might be able to find old stock somewhere including the bay.

Originally Posted by technoman9

I will check the flyback bypass capacitors tomorrow, I believe those are C437 and C443 in the second image, correct?

C443 is the bypass filter for the 177 volt line and won't cause the shutdown but will cause pic issues.

C437 is the cap that snubs voltage spikes from the sweep when it's running, and this cap will cause excessive B+ even if the regulator was good. I'd change them both anyway.

John

[technoman9]

Zeno, John: Thank you for the kind words, it's a pleasure learning from you! I studied electrical engineering a decade ago but since have gone into a software/systems engineering career and thus rarely handle circuit design or repair. However, I often troubleshoot these software systems and so am good with following instruction and using tools to figure things out, though with CRT TVs I'm still learning the "system" if you will. It can be stressful learning electronic repair by one's self when one doesn't have any mentors to teach by example, so I'm grateful to have your patient guidance here.

Zeno: do you mean diode D803 in the power supply image, or the diode seen in the middle of the flyback transformer image, D441? Do you suggest replacement because of common failure or preemptive concern?

I checked those capacitors as you two suggested and indeed found C443 4.7 uF to be open. C437 checked out good but I will get new ones of both anyhow. I will be inspecting the rest of the electrolytics on this board to see if any others have gone bad. Thanks for the explanation about the capacitors' use. I absolutely love learning the "why" behind circuit design choices and possible failure modes.

I'm going to order the NTE replacement for the regulator unless you suggest otherwise: https://www.ntepartsdirect.com/ENG/PRODUCT/NTE15018 NTE seems to be trustworthy and I can order directly from them. The specs match the STR3115 perfectly.

[zeno]

I think its D805 the one under the reg IC on schematic. . Common to
short & cheap insurance.
Hint for today. Note how all the PS parts are 800 numbers.
The whole set is blocked off by stages that way. SO Vert may be 400 #'s hoz
500 #'s audio 100 #'s etc. Makes it easy to find things without manuals.
Most sets are made that way.

73 Zeno

[technoman9]

Ahhhh now I see, I missed that one originally. OK I will inspect and order a replacement. Will probably be a few days before I get the parts from Digikey.

And thanks for the tip, I see that numbering scheme now. Nifty!

[vortalexfan]

Originally Posted by zeno

I think its D805 the one under the reg IC on schematic. . Common to
short & cheap insurance.
Hint for today. Note how all the PS parts are 800 numbers.
The whole set is blocked off by stages that way. SO Vert may be 400 #'s hoz
500 #'s audio 100 #'s etc. Makes it easy to find things without manuals.
Most sets are made that way.

73 Zeno

Panasonic used a similar numbering scheme for their electronics (TVs, VCRs, etc.) where certain parts of the TV or VCR circuit had a "x00,x000 or xx00 series" number that corresponded to certain circuits in the VCR or TV (and also made for easy cross reference for service data and parts replacement.)

[technoman9]

Alrighty, all of the parts I ordered finally arrived so I'm starting to put them in.
As I was getting ready to install the new regulator, I noticed the old one had a clear sticky goo on the back of it. I presume this to be some sort of heat sink compound to help transfer heat. I happen to have some Silicone Cooler Grease from Startech so am thinking of using that.

I've replaced cooler paste on a couple of CPUs in my lifetime but never a voltage regulator. Is there any particular way it needs to be applied? Is Silicone Cooler Grease OK to use on voltage regulators? My searches on the topic have not found much.

[technoman9]

Scratch that, I am quite the idiot. There is plenty of information online about heat sinking transistors. That's what I get for trying to research this as I'm trying to fall asleep.

Will report out once I get the new regulator installed.

[damen]

Did the parts supplier include a new mica insulator with the new regulator? If not, make sure the one that was attached to the old part isn't damaged.