Makvision/Wei-ya 3129D PCB rev 6 repair help

[Pitou]

Hello all,
I've recapped this monitor and I had C32 backward. Since then, I only hear a click or tick from the T3 transformer and relay.

I have found C59A which is a Diode P6KE30A that was short and replaced it.

I've checked almost all parts in or out of circuit and I'm now lost. If anyone can help.

I tried using Claude and you can see all the steps I did so far. (Measuring transformers, FBT using DMM, LCR meter, etc)
You can see the whole conversation here:

https://claude.ai/share/feba45e4-26cd-40d8-8473-d394db1afcef

Any help will be appreciated.
Thank you.
Pitou!

 

[Big_P Big_p]

First thing first, I have noticed that the pads on these things are relatively easy to damage. So have you checked that all your cap replacements are actually connected on both sides.

Second, was it working before you recapped it?

 

[Pitou]

Yes it was working before recap. I agree pads are very flaky.I'll double check them

 

[Big_P Big_p]

So the clicking in these things is tylically associated with HV shutdown/protection. Next point of call if all the caps check out for continuity is checking the HOT, check the middle pin is not shorted to gnd.

 

[Pitou]

I did check all the pads for continuity and all is good. Middle Pin of Q36 (2SC5144) to GND gives 1.1ohm.

Thanks for the help!
Pitou!

 

[Big_P Big_p]

Ok. 1.1ohms between the middle pin and GND sounds like a short. I dont have a 3129d infront of me to check though, so maybe just double check you get a beep in continuity mode. I am guessing that the cap backwards blew the HOT, from memory its common for bad caps to short and kill the HOT.
Now you are going to have the fun time of finding an original HOT.

I have some if you need one but I am not sure they are original. I am not even sure how to be sure they are original these days as they are only available on the secondary market. They are typically either original pulls or copy chips, some people claim NOS supply but I tend to doubt them (especially if they are in China).

 

[Pitou]

ok. It's very possible, but I'm still not sure it's the HOT itself.
I removed it. Center pin to gnd gives around 20kohm at start and increasing to around 39kohm (probably going through a cap)

Here is what the HOT gives as readings with a DCA55. See pictures

 

[Big_P Big_p]

A 1.1 ohm reading in circuit is most likely a shord to GND. If your not certain its the HOT you need to find what it is.

Ill have a closer look at this when i get home from work.

 

[Pitou]

ok good. I'll peek around in the meantime.
Thanks again!

 

[Pitou]

I search some more looking at the schmeatic but couldn't find anything wrong so far.

 

[Big_P Big_p]

If the HOT is still out of circuit test if you are still getting 1 ohm between the middle pad and GND.

 

[Pitou]

Center pin to gnd gives around 20kohm at start and increasing to around 39kohm (probably going through a cap)

 

[Big_P Big_p]

That sounds normal. Did you buzz out the middle pin to GND in continuity mode before you removed the HOT? The ohms reading by it self is not all that useful. If not dont worry get me 2 ohms readings with the black multimeter probe on the centre leg and the red probe on one side, then the other.

 

[Pitou]

It did beep at 1.1ohm with the HOT in place. Anyway my Fluke 18B+ beeps at lower than 70 ohm.
On the board itself without the HOT in place, I get 18.38kohm on each side. Black is on the middle pin C

On the transistor itself, black on C:
Red on pin 1 = 71.1kohm
Red on pin 3 = Open, infinite 0.L

Thanks!

Pitou!

 

[Pitou]

I went back to primary side and notice that pin 7 of KA3842B reads about 71vdc. Is something more around 5vdc expected?
I removed the KA3842 chip to take measurement

Also, I read 0v from pin 12 of T3

 

[Big_P Big_p]

71vdc on vcc of the KA3842B is took high it should be abour 20vdc. Check for a short between vcc and gnd to see if its still ok. Then check the resistors, diodes and cap connected to pin 7.

 

[Pitou]

no short between vcc and gnd. All parts around it are fine out of circuit. Also Q1 (FS14SM16A) measure ok with the DCA55
Update: with the chip in place, voltage varies from 10 to 15v, but is unstable. In sync with the tick

 

[Pitou]

I captured pin 3 and 6 of the KA3842. Pin 6 on CH0 and Pin 3 on CH1.
If that can help.