A test you can do on the secondary is check it for shorts...

The "cold" (not mains connected, see photo) side of the power supply appears to have jumpers on all the outputs. It would be fairly easy to just snip or desolder one end of these, one by one, to see if opening a loop allows the other voltages to come up. A repair would then also be very easy. Does that plan make sense?

I guess I'd start with the 126v loop (jumper JP065, upper right) since the others are fused, and all those fuses are still good.

 

Jp065 is a good place to start. If it fires up I would suspect the Horz output transistor. An ohms test on these transistors is not always conclusive, as iv had transistors that are leaky & test ok with an ohms test & lock on when the hv is fed to them. Its easier to replace a suspect faulty transistor.

 

The "cold" (not mains connected, see photo) side of the power supply appears to have jumpers on all the outputs. It would be fairly easy to just snip or desolder one end of these, one by one, to see if opening a loop allows the other voltages to come up. A repair would then also be very easy. Does that plan make sense?
...

With newer sets they will shutdown the PSU on many types of fault, basically if you start disconnecting things there is no guarantee the PSU will come up ok as it might detect things are not working and shut down. The fault you describe after your repair is normally called "hiccupping" and is a symptom that the PSU is faulty itself or has detected some other issue that may include over/undercurrent, over/undervoltage on the outputs.

As debe said test or replace the horiz out transistor, (I mentioned it before) but before I did that I would just connect a 30v 1A (or 0.5A) limited DC PSU on that cap on the 126v rail. If it rises to 30v ok you can be fairly sure there is not a major short there, and the test takes seconds and won't hurt anything.

On new sets the amount of feedback can be troublesome, it's not uncommon for the flyback to generate a DC voltage signal that feeds back to the main PSU, so if the PSU and flyback are both not perfect then it will shut down the PSU. You can do a lot of tail-chasing on sets like that!

 

OK, I'll try that on all the secondary side loops, after the diodes, starting with the horizontal. I was thinking I didn't have a suitable power source but I actually built a variable voltage, current-limited power supply years ago that will work nicely for this.

Tail chasing is definitely a problem, but I believe the only feedback from the secondary to the primary is via the optoisolator OR an overcurrent condition seen on the current-sense resistor on the primary.

 

There is a simple approach to this.

There is a way to test if the MOSFET drive is correct or not with a DMM.

I repair this daily.

Care to listen to me.

If so tell me the status of the Voltage at Source of MOSFET after removing it. u NEED TO APPLY POWER AFTER REMOVING THE POWER MOSFET.

Measure the DC voltage and AC voltage at the course and the positive of the filter cap. The filter negative should be DMM common negative point.

If you get around 300V DC with No AC voltage what so ever U are good with only if u don't have a an Active PFC circuit. If u have an active PFC then u should get a DC close to 400VDC.

The if the above measurements are correct then switch of, dicscharge all the caps in the primary side. Connect DMM at around 10V range DC. Negative to filter negative, positive to gate pin of MOSFET, with MOSFET still removed.
Apply power as u keep an eye on the meter. The meter will show around 2 to 4V DC and a little more if u switch it to AC.

Post back. U will find the fault pretty quick

Plus post the complete schema so I can study it.

 

I've been testing and just now found your post. Here's the entire schematic.
View attachment Thomson_ITC008_MAIN BOARD.pdf
And the training guide
View attachment THOMSON ITC008 training.pdf

Here are my latest results, which I think are getting me to the answer!

• RP006 and 7, the startup resistors tested good.
• CP024 AND 25 had about 6V across them, after being unplugged a few days. So one could be open but neither is shorted.
• CP040 had no voltage but there is a path to ground, so no surprise. No measurable resistance, so not shorted.
• CP008 and 9 both have >120V on them after several days, so again one could be open but neither is shorted.
• Applying 20V to JP065 - an easy-access jumper on the supply to the horizontal transistor - took my power supply down to under 1 volt at over 1A, maybe 2A. !!!!! That's very suspicious, right?

Many posts I've found online suggested replacing the horizontal transistor to fix a set that is buzzing but otherwise dead. So I'm proceeding right now to remove that whopper transistor, TL035 in the schematic.

 

gimme a miniute to check

No need to remove the HOUT. Just check for dead short between CE. IF u don't get zero ohms then it is OK>

 

It's dead, and it's a S2055N. I pulled and tested it with an LED and a 9V battery. The LED lights even when the base is grounded!

 

H out can only be tested in one way that is with DMM in diode check mode.
Positive to collector. CE should be open, CB should be open. If u get this results than it is OK

 

CE is dead short. CB is 38Ω. Das ist kaput!

 

Ahh then it is indeed kaput, but u still can fix the PSU. if it is faulty u know.

Check to see the supply is working or not ?

 

With HOUT removed it's totally safe to run the set. U should get the stated voltage at collector if the supply is OK.
And around 5 to 10VAC at base if the Y/C is producing the H. Pulse

 

Hmmm.... I was able to get the set to briefly power on now. An AC voltage of 21V appeared on pins 16 and 18 of LP003, where the high voltage is produced. The set quickly powers down and I hear various clicking sounds, like a relay click. There is specified voltage 132V on the horizontal supply cap (CP080) though.

I'm not discouraged, as I think what I'm seeing now is the result of feedback regulation.

 

The set will go to standby indicating a horizontal fault since there is no Hout.
The μC will monitor the H.Fly back pulse.

U can measure the PSU stated output voltages at switch on. Just keep pressing the remote stby button. U can keep the PSU cycling.
If u get the stated voltages then PSU is OK.

Just replace the Hout. Any Hout with a damper will suffice if it is rated above the s2055 Ic

 

Having trouble finding a replacement. The schematic shows ST1803DHI (10A 50W 20Ω base-emitter resistor), while the one I pulled was a S2055N (8A 50W 50Ω resistor). Of course there are lots of other specs.

NTE shows NTE2636 (8A 50W) as a replacement but it's not readily available either. NTE 238 (8A 100W) sounds good but is maybe overkill and pricey.

Anyway, any suggestions?

 

What about BU508DF

Trust me, BU508DF is a good replacement and I think it's cheap.

Now. don't worry about the base resistor.

But if u are having doubts just wire a 50 ohm 1W R across BE.

 

Neither transistor is all that expensive here in Australia ST1803DHI is $7.05. BU508DF is $3.23.

 

BU508 are a good catch-all but will give problems in some sets especially newer ones. Any supplier that specialises in parts for TV repair should have stocks of the proper transistor.

Now after heaps of time repairing PSU and replacing Hout, you may find it's got a shorted flyback transformer. Testing those is whole new story.

 

Yes, I can see why people just throw out sets rather than have them repaired. A fancy new set (ie. not a back breaking, power sucking, giant CRT) begins to look pretty good.

I found a local place that will order a NTE2636 for me, for $4. I have to wait, but no shipping charge. I live in an area with maybe 200k people, and this is the ONLY store in the area that would sell me one. Sad. Fingers crossed that this is the magic bullet.

Any useful testing I can/should do before powering it back up?

 

This is what ive used for the last 35Yrs for testing LOPT transformers for shorted turns.