Brightness control inop. Contrast ok. CRT is good and has no H-K short --Now what? - Videokarma.org TV - Video

jpdylon · #1 10-07-2007, 11:00 PM

Got this set as a freebee. The CRT has very low hours on it. Its been sitting in a garage for 15 years since it developed its problem.

The previous owner said the brightness control stopped working. It still doesn't work. The contrast, color, tint, and sharpness controls work fine. The sub-brightness control in the video section has no effect either. it just seems that the brightness is always at minimum.

The control itself is fine, as are the lines from the control panel to the chassis. The resistance from the high side of the pot to ground is 3k ohms, so i do not believe a short in a control is causing the problem. The sub brightness control is ok as well. I haven't been able to discover anything obvious that could be causing the problem. There is an IC on the video board that may be part of the luminance circuit, but I don't know for sure without any literature.

I would like to save the set just for the shop since its small and I can put it up on a shelf. --Any thoughts?

MRX37 · #2 10-07-2007, 11:31 PM

I wonder if the flyback has a brightness adjustment...

I don't know what would stop the brightness control from working, but a bad IC sounds pretty likely.

In the meantime, perhaps a suitable kludge would be to adjust the screen voltage on the flyback.

radiotvnut · #3 10-08-2007, 12:17 AM

If you will post the make & model; I'll see if I've got a Sams on it. That may tell us something. I have cranked the G2 (screen) control up on the flyback to improve brightness or for troubleshooting purposes.

jpdylon · #4 10-08-2007, 12:39 AM

Its a Montgomery Wards with what looks like a Sanyo chassis. the model # of the set is GNB12915a / Chassis M32-63. Cranking up the G2 only improves overall contrast and adds retrace lines.

I noticed that the sharpness control make the brightness erratic depending on what's being displayed on the screen. I highly suspect the video IC. It gets very warm and has a discoloration on the bottom (pictured)

MRX37 · #5 10-08-2007, 10:39 AM

Wow. That's the first time I've heard of the G2 voltage improving contrast instead of brightness.

Yeah that video IC looks like it could use replacing. No clue where you'd find one, unless you run across a similar TV that has a good one.

radiotvnut · #6 10-08-2007, 12:16 PM

The number on that IC crosses to an NTE822 (and ECG822 would be the same thing). That chassis looks more like a Philips set and the part number on the IC looks like a Philips / Magnavox part number. There was not a Sams listed for that model number. If you'll post a picture of the whole chassis; I'll see if I can identify it.

jpdylon · #7 10-09-2007, 11:31 AM

chassis shots. I guess it does kinda look philips-ish -especially with that NAP crt...

jpdylon · #8 10-09-2007, 11:33 AM

Quote:

Originally Posted by radiotvnut

The number on that IC crosses to an NTE822 (and ECG822 would be the same thing). That chassis looks more like a Philips set and the part number on the IC looks like a Philips / Magnavox part number. There was not a Sams listed for that model number. If you'll post a picture of the whole chassis; I'll see if I can identify it.

Thanks for that reference. Mouser has them in stock so I'll get one when I order from them next.

radiotvnut · #9 10-09-2007, 02:53 PM

That is a Philips chassis. The early versions of that chassis were introduced by GTE and were used in Sylvania and Philco sets from the late '70's. When Philips bought Sylvania in around '81; that chassis was kept in production under the Philips name. I think that chassis was discontinued in the mid '80's. I had a couple of those chassis. If I still have them; you are welcome to the IC.

zenith2134 · #10 10-09-2007, 09:51 PM

Cool tv. I had an '81 Sylvania GTMatic 19incher with a similar chassis. I trashpicked it a few years back, all I did was increase the vertical height pot a tad, and the thing kept on runnin'

Gave it away with the hope Itd be cared for. Decent sets, IMO. Also had a Maggie from about 1984 I found in the same house, C4 I think the chassis was (?) Anyway it was a NAP crt with a ton of hours on it, and it still looked great.

jpdylon · #11 10-09-2007, 11:11 PM

Quote:

Originally Posted by radiotvnut

That is a Philips chassis. The early versions of that chassis were introduced by GTE and were used in Sylvania and Philco sets from the late '70's. When Philips bought Sylvania in around '81; that chassis was kept in production under the Philips name. I think that chassis was discontinued in the mid '80's. I had a couple of those chassis. If I still have them; you are welcome to the IC.

Thanks. If you do happen to find the spare chassis I would definitely like the IC.

jpdylon · #12 10-25-2007, 10:56 PM

It was indeed the IC. The set is working beautifully now.

A big thanks to radiotvnut.

radiotvnut · #13 10-26-2007, 12:46 AM

Glad that IC solved the problem. There are two other common problems with that chassis that may show up down the road. The first is a very bright screen with retrace lines at turn on and then it shuts down. That's caused by a defective 47 uf, 250 volt (I think) cap at the rear of the chassis by the flyback. The other problem is a set that fires up and shuts down instantly. I've seen that two section filter can in the power supply cause this problem. I still have that old chassis if you need any more parts down the road.

Kiwick · #14 10-28-2007, 05:36 PM

Quote:

Originally Posted by radiotvnut

That is a Philips chassis. The early versions of that chassis were introduced by GTE and were used in Sylvania and Philco sets from the late '70's. When Philips bought Sylvania in around '81; that chassis was kept in production under the Philips name. I think that chassis was discontinued in the mid '80's. I had a couple of those chassis. If I still have them; you are welcome to the IC.

That's in no way an original dutch Philips chassis... they're entirely different and uses exclusively Philips-made components... in an 80s Philips chassis you should see their turquoise resistors all over the place...

that's more likely a japanese OEM chassis made for Philips' American market sets, i guess it would have been too complex to modify one of their PAL KT3, K30 or K40 chassises for NTSC reception...