sci.electronics.repair@googlegroups.com

Google Groups

John Robertson <spam@flippers.com>: Jul 07 12:19AM -0700 >> John :-#)#   > i soldered a header into the reverse position on the chassis, and gave it a try. it did the same thing as before, just inverted. now the bottom half of the screen has an image that compresses toward the center, a total collapse on the center, and no image in the upper half of the screen. does that rule out a yoke problem?   > thanks   Based on Rick's points above I think we can rule out the yoke. Nice work Rick and yourself for that test!   If you have a 'scope then now is a good time to check the outputs of the LA7851's sawtooth wave generator. Also 'scope the output of the vertical drive IC to is if it too is still a saw tooth or not. Should swing pretty much the entire source voltage (24VDC?) if I am not mistaken...   John :-#)# -- (Please post followups or tech inquiries to the USENET newsgroup) John's Jukes Ltd. MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3 (604)872-5757 (Pinballs, Jukes, Video Games) www.flippers.com "Old pinballers never die, they just flip out."

John-Del <ohger1s@gmail.com>: Jul 07 06:52AM -0700 > battery voltage is good at 123, 12v and 24 v are good. 123 volts at the flyback. > the vertical output ic is a LA7833. pin 6 has good 24 volt supply, and pin 2 ( output) is a nice 24 v square wave. i recapped the monitor, and the image is exactly the same. now im stuck.   > thanks   I didn't read back through all the posts, but if you haven't, check the pump up diode. The anode will be connected to the Vcc and the cathode to the pump up pin of the vert IC. A weak diode here will cause all sorts of havoc (don't check it with a dmm - they often sag under load). Also, I know you said you recapped the monitor, but make sure the yoke coupling capacitor was included. This is typically a high value capacitor (around 1000uf or more).

mhooker32@gmail.com: Jul 07 07:55AM -0700 On Friday, July 7, 2017 at 9:52:26 AM UTC-4, John-Del wrote: > > the vertical output ic is a LA7833. pin 6 has good 24 volt supply, and pin 2 ( output) is a nice 24 v square wave. i recapped the monitor, and the image is exactly the same. now im stuck.   > > thanks   > I didn't read back through all the posts, but if you haven't, check the pump up diode. The anode will be connected to the Vcc and the cathode to the pump up pin of the vert IC. A weak diode here will cause all sorts of havoc (don't check it with a dmm - they often sag under load). Also, I know you said you recapped the monitor, but make sure the yoke coupling capacitor was included. This is typically a high value capacitor (around 1000uf or more).   i may have hit the wall here. pin 14 of the LA7851 is a nice 5v sawtooth. pin 15 is a 12v volts square wave. pin 16 is kind of an ugly bowl shape waveform. pin 17 has nothing. on the la7833 pin 4, which i believe is the input, has the same ugly bowl shape wave, where i think it ought to have a saw tooth? pin two, the output has a very nice 24V square, which i dont think it should have. should be a 24v sawtooth? odd that it puts out a very defined square, instead of just some ugly shape, if something is wrong. i think the pump up is ok, as the output is 24v, but maybe im wrong. im thinking of just replacing the 7851 as a hail mary, and then bailing if that doesnt work.

rickman <gnuarm@gmail.com>: Jul 07 12:09PM -0400 >>> thanks   >> I didn't read back through all the posts, but if you haven't, check the pump up diode. The anode will be connected to the Vcc and the cathode to the pump up pin of the vert IC. A weak diode here will cause all sorts of havoc (don't check it with a dmm - they often sag under load). Also, I know you said you recapped the monitor, but make sure the yoke coupling capacitor was included. This is typically a high value capacitor (around 1000uf or more).   > i may have hit the wall here. pin 14 of the LA7851 is a nice 5v sawtooth. pin 15 is a 12v volts square wave. pin 16 is kind of an ugly bowl shape waveform. pin 17 has nothing. on the la7833 pin 4, which i believe is the input, has the same ugly bowl shape wave, where i think it ought to have a saw tooth? pin two, the output has a very nice 24V square, which i dont think it should have. should be a 24v sawtooth? odd that it puts out a very defined square, instead of just some ugly shape, if something is wrong. i think the pump up is ok, as the output is 24v, but maybe im wrong. im thinking of just replacing the 7851 as a hail mary, and then bailing if that doesnt work.   This would be a lot easier if you had a working unit to compare waveforms. It doesn't make sense to me that pin 15 if the LA7851 would be a square wave. I think I'd try to find something wrong with the parts between pins 15 and 16. Or maybe the components from the LA7833 pin 2 to pin 17 on the LA7851. I'm guessing this latter circuit is for linearization of the ramp and might cause the problem you see if a part is bad. It's a bit hard to figure out what all this does. Looks like the whole thing is pretty highly optimized.   Are the signals on pin 2 and pin 15 of the two parts the same polarity or opposite? I can't see how you would get a bowl shape signal on pin 16 if they are the same. Have you played with the vertical size and linearity controls to see how they affect the problem?   The charge pump circuit was bothering me as the diode seemed to be backwards. Then I realized it isn't making 24 volts from 12 volts, it's pumping up the 24 volts to something higher! What is the voltage on pin 3 of the LA7833? The cap is only rated for 35 volts, so it isn't making 48 volts. But clearly pin 3 should have something on it higher than 24 volts. What does the pump drive signal on pin 7 look like?   --   Rick C

jeff.panasuk@cfl.rr.com: Jul 07 07:24AM -0700 > Back in February or March of 2010 the TV began changing from any channel to channel 2 intermittently. Sometimes very frequent. Other times more intermittent but it would not go more than a few hours without the problem popping up. Replaced IC0102 (MN14834HH) Main IC for the Channel Selection Ckt but no help. Remote control IR circuit not the problem as I disabled it. Over time have replaced some bad unrelated hw (burnt resistor, Caps). Have schematics and service manual but can't isolate the problem. Nice picture and everything else works fine. Unit in storage but every 4 months or so, when I see it and think about it, I fire it up. Hate to trash this old tv. Kind of part of the family now. Has anyone ever heard of this before?   Thanks for the info. I'll add it to my troubleshooting short list. I cleaned the switch S0125 on 7/3 and the TV has been running solid through today 7/7. This has been the longest period without the problem since it first popped up in 2010. The TV is now back in action on its shelf in the kitchen area.

John-Del <ohger1s@gmail.com>: Jul 07 06:01AM -0700 On Thursday, July 6, 2017 at 10:53:10 AM UTC-4, amdx wrote: > is supposed to ship it. No notice of shipping, I provided notice to > return on 6-28-17, he has had 6 business days to return. > Mikek   A lot of TV "repair" facilities are guys we call "trunk monkeys"; repair shops operated out of the back of a car. These guys have no training and no state license, but just swap boards around hoping to run into a repair.   Yes, it was indeed poor troubleshooting on the buyer's end and now it's aggravation for you. Samsung LED failures are epidemic, and the repair shop should have known this. Most shops built their own LED testing tools years ago, but now anyone can buy a tester for LED strips that will light even give a voltage readout of the run voltage of any given strip at the tester's preset current limit, and do so without disassembling the display.   It's possible that the buyer made the return request in the hopes you would credit back the money and tell them to keep the board. It wouldn't surprise me if you don't hear from them again.   Too bad you also didn't remove the LED strips from the carcass before scrapping it. These bring good money and are in high demand.

frank <frank@invalid.net>: Jul 07 07:10AM > using the LOOP control to reduce the effect or lower the power supply > voltage. You've probably already tried this, but I thought I would > mention it anyway.   Yes, I've tried the loop control but it doesn't change much of anything on that display.   > to about 15v across the tube, and stayed there. Grab a DVM and > measure the voltage across the device. My guess(tm) is that it's > about 15v and holding.   the test voltage is a half waveform of the 50 Hz (in this part of the world) coming from a secondary winding and some transistor buffers. The voltage waveform is sent to the test probe and the X axis of the oscilloscope. So if the trace on the scope goes all the way to 240V, I think it's not firing. With 100K series resistor I can't see any vertical (current axis) movement even at 1mA division.     > For fun, put a capacitor (300v or high) across the tube with a high > series resistance and see if you can get it to oscillate as a > relaxation oscillator.   another thing to test, ok.     Frank

tabbypurr@gmail.com: Jul 07 02:40AM -0700 On Friday, 7 July 2017 05:51:38 UTC+1, Jeff Liebermann wrote: > so easily. So, if I don't need a vacuum, and want plenty of ionized > neon atoms to provide conduction, I would pressurize the tube with as > much neon gas as I could cram into the package.   People do use old bulbs. A place I stayed in the 80s was still using 200w carbon filament bulbs in the bathrooms to stop them freezing. Back in 1900ish nothing would have been thrown away. Even here I recently used a lamp that look like it was from the 60s or earlier.   ISTR being told 15w lamps stayed vacuum a very long time after 60 & 100w went gas filled, I don't know if that's correct or not.     NT

You received this digest because you're subscribed to updates for this group. You can change your settings on the group membership page. To unsubscribe from this group and stop receiving emails from it send an email to sci.electronics.repair+unsubscribe@googlegroups.com.