I'm looking for some repair hints for the mentioned tv model.
The Display stay black after power on without sound displaying the last selected channel number. I had precautionary changed all electrolytic capacitors in the power supply after this first occurance of the mentioned symptom but had not really found a bad cap with high ESR during this replacement. Thinking all should now be ok, I was able to watch tv for the rest of the day, but after turning on the tv on the next day I get the same behavior as previously mentioned.
It's look like a problem of the remote control receiving unit.
I will start today again my investigation but in the case not having success it would be helpful for me to read your opinion on this topic.
On Mon, 21 Jul 2008 17:19:16 +0200, markus wolfgart put finger to keyboard and composed:
Does the TV respond to the front panel controls? Some sets will turn on when the CH-UP/CH-DOWN buttons are pressed. Others will turn on when the main power switch is turned on. Pages 2 and 3 of the circuit diagrams (see below) show a "POWER SW" signal which appears to provide this automatic power-on function. Otherwise check whether the uP is sending a "POWER ON/OFF" signal to your PSU (pin #2 of 5-pin connector).
Here are the circuit diagrams:
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- Franc Zabkar
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Please remove one \'i\' from my address when replying by email.
The tv don't respond to the channel keys but display the last channel number from the previous run.
Today I had found some datasheet from parts located on the mainboard. (2SD1878, 2SC2621E, TDA4940, TDA4944, AN5836, AN5860, LA7210, LA7220 and LA4445) for my later on investigation.
Unfortunately the parts named uPC1488H, TA8659AN Toshiba, OEC8024B Orion, and ?K1006? Sony IC102 had no positive hit by my search query :-(.
I had already found and download the schematic, but its hard to read it as it is a very poor quality scan.
No chance to read the voltage levels and even the part id.
If you have a better scan or pdf doc, please sent it to me, if possible.
I had not yet measured the supply voltages as after the replacement of all power supply caps located nearby a heat causing part the tv was running the whole evening like usually. After power down and power on the next day I got a black screen again and only the channel LED was on. (no audio tone, no noise on the screen). No respond on pushing the channel keys at the front panel.
I had checked the remote control transmitter led with a digital camera, looks fine, ir led is flashing the RC5 patterns when pushing a key.
In addition you right my tv model has to be started by the remote control main button when powered on starting on the last channel selected.
I hope to have more luck today to find the reason for malfunction, but its more complicated when I had to check signals on a running tv bacause of missing service manual.
On Tue, 22 Jul 2008 10:05:11 +0200, markus wolfgart put finger to keyboard and composed:
That's a vertical output. I wouldn't be looking there.
CXK1006L Serial Electrically-Erasable PROM (EEPROM) - With Seq/Controller
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(specs)
These look similar:
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(5150)
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I'd be looking for an open "startup" resistor. As you say, the circuits are hard to read, but these resistors are usually around 100K or more, and are often connected to the base of the chopper transistor. In your case the chopper is inside a 5-pin STR regulator. I would check R503 and R504 (220K) at pin 2 of this IC.
Check that the OEC8024B microcontroller is attempting to switch on the PSU by monitoring the "POWER ON/OFF" signal (pin #2 of 5-pin connector). If that signal is toggling, then you can confine your troubleshooting to the PSU (and the loads on the secondary side).
- Franc Zabkar
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Please remove one \'i\' from my address when replying by email.
many thanks for your kind help and your repair hints. I have not so much experience on repairing tv's as it's not my daily business, but the few tv's I had already successfully repaired in my life had mostly blown semiconductors (hv psu switching transistor, some of them had broken diodes) or bad because overheated electrolyt caps. I had never make the experience of malfunctioned resistors. So your information is for my a new lesson in repairing tv equipment. On the first view I had not found any scorched resistor but I will try to measure them all located in the place you mentioned. As I'm tomorrow and on Friday on a business trip, I will continue my repair action on Saturday and will write my report on Sunday.
After my two day business travel I start my next repair attempt on Saturday morning taking your friendly hints into account.
First of all I was very lucky to find some bad solder pads (at pin 2 and
5 of the TLP580 opto coupler), hoping thats the reason of the malfunction. But after mounting all boards in the tv case, I get the same black screen as mentioned before.
I think thanks good its weekend and I have plenty time to find the buggy part(s).
I start again to solder all suspect looking pads in the belief that one more is cold ant try to bring the psu to running outside the tv case.
But as Murphy is always not far away it will become a longer story to tell.
As I verified that pin 2 of IC501 (STR58041) gets a high sig from the opto transistor (about 1.3V) as told me by Franc I create a 500Ohm/100W dummy load for the 110V line of the psu and connected it to CP508 gnd - main b+.
I connected pin 1 of CP504 to pin 1 CP505 and pin 3 of CP505 to pin 2 of CP504 together (bridge the POWER-ON switch) and connected pin 1 and 2 of CP502 and in addition supply 0-3V to pin 1 and 2 of CP503 to simulate a power on sig of the mcu.
After some few trays increasing the voltage over 1,7V at pin 2 of CP503 I hear the switching sound of the psu and measure 113V at my load.
All looks fine and I was lucky and reinstall the psu in the tv case being convinced that now all will work ok and only a bad solder connection was the reason for the drop out as so often be the case.
During the Saturday evening tw was running stable, wife was lucky and I too. Unfortunately after two hours the same black screen scenario occurred as the two previous times before.
Becoming a bit angry I decide to continue my repair session on Sunday morning after breakfast with a relaxed brain ;-).
I decide to change all diodes from the primary side except the from the power rectifier D501 to D504 as this diodes are general purpose diodes and looks ok as I measured them with a simple ohm meter. The other diodes looks good too, but are fast recovery diodes and probably makes trouble when voltage revers, as some of them are located nearby hot resistors and the pcb is already of a dark color at this place so that these diodes are exposed to intensive heat too.
I replaced them all (D505,D506,D507,D508,D510) by a 1N4937 (fast recovery 600V diode found in my boxes) as well the D515 (which looks different to the changed EU1A, EU1Z). The Marking code is a big B and T and a smaller 8F and two rings at the kathode side. I'm not sure if this is a zener diode, but don't looks so according the schematic which is not in a best quality as mentioned before.
Despite this replacement the psu don't start when applying a 2V voltage to the power/on pin of CP503 as in my first attempt to repair it.
After this replacement of the mentioned diodes I replaced some resistors (R509 by 33Ohm/3W), (R504,R507 by 470k || 470k + 390k || 390k all 1/4W as not having a suitable 2x 220k), (R521, R524 by a new 100k resistor) and measured R510 as 180k, R507 as 56k R511 as 33Ohm R501 as 1Ohm.
Furthermore I check all tree Transistors Q502/Q503 (2SD863) and Q504 (2SC945) and they looks good when measuring the NP and PN doides.
Now I belief that the IC 501 was blown by my last attempt watching tv despite the fuse is ok.
When measuring at all five pins I get 313V DC at pin 3, near GND at pin
4, 0V at pin 1 & 5 despite the schematic show -32 or -39 V at pin 1 & 5 and about 1.3V at pin 2 when injecting 2V to CP503 at pin 2.
When I look at pin 3 via the scope I don't see any oscillation attempt.
Today I will buy a new STR58041 (IC501) in the hope that this ic is defective otherwise I will dispose the tv to the local junk dealer.
tv is now running after I had replaced IC501 (STR58041). Pin 1 (-40V) and pin 5 (-33V) had now voltage like listed in the schematic. Hoping this was my last repair attempt for this box.
Many thanks for your help Franc.
Was a nice but longbreathed training in tv repair task.
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