WG 25K7401 white screen

debaser138

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I've got a WG 25K7401 that I'm working on. Before powering it up, I found an open resistor at R120 so I replaced that. HOT and fuse were good and everything else looked ok. Fired it up and all I'm getting is a white screen, with or without a video signal connected to it. The B+ as measured at TP202 is 118v. Haven't installed a cap kit yet (the caps seem to be original) but one is on the way.

How reliable is the U500 RGB amp chip on the neckboard? Any chance that is bad? What else should I be looking at?
 
Digging into this a little more things are pointing to an issue with the blanking circuit. The flowchart says to check Q501 and Q502 on the neckboard. Q502 seems fine but on this neckboard, there is no transistor at Q501, only a jumper wire between the collector and emitter points where the transistor would be. The schematic shows OPT next to Q501. Not really sure what that means.
 
Digging into this a little more things are pointing to an issue with the blanking circuit. The flowchart says to check Q501 and Q502 on the neckboard. Q502 seems fine but on this neckboard, there is no transistor at Q501, only a jumper wire between the collector and emitter points where the transistor would be. The schematic shows OPT next to Q501. Not really sure what that means.

Will need to replace the caps..
things to check for no video.. R510 30 ohm and z500 for 12vs neck bd.
i would confirm i have the voltages to ic u500 first..
Check D310,for short, R310 a 1.2 resistor for open the video B+ voltage
you can have a raster but no video.
 
Will need to replace the caps..
things to check for no video.. R510 30 ohm and z500 for 12vs neck bd.
i would confirm i have the voltages to ic u500 first..
Check D310,for short, R310 a 1.2 resistor for open the video B+ voltage
you can have a raster but no video.

Cap kit is on the way.

R510 had the wrong value (around 48 ohms) and it was obviously replaced at one time. I installed the correct 30 ohm resistor.

Z500 was showing 022mV in circuit. I pulled it and it tested 707mV so it looks to be ok.

I checked the voltage at pin 28 on U500 which should be +12v (if I'm reading the schematic right). That was measuring 4.58v. What would be causing a low +12v on the neckboard?

R310 measures fine but I can't find D310 which should be right next to R310. I see it on the schematics but can't find it on the chassis. It's possible it's one of the two diodes right by R310 on the chassis but it's hard to make out the #'s on the board as the diodes there are covering them up. If it is one of those two, both of them test fine. Can someone identify the exact location D310?

Also, I only had the monitor on for a minute and R510 got pretty darn hot. I thought I smelled something burning and it might have been that resistor. I tested it after powering down and it looks to be ok.
 
Cap kit is on the way.

R510 had the wrong value (around 48 ohms) and it was obviously replaced at one time. I installed the correct 30 ohm resistor.

Z500 was showing 022mV in circuit. I pulled it and it tested 707mV so it looks to be ok.

I checked the voltage at pin 28 on U500 which should be +12v (if I'm reading the schematic right). That was measuring 4.58v. What would be causing a low +12v on the neckboard?

R310 measures fine but I can't find D310 which should be right next to R310. I see it on the schematics but can't find it on the chassis. It's possible it's one of the two diodes right by R310 on the chassis but it's hard to make out the #'s on the board as the diodes there are covering them up. If it is one of those two, both of them test fine. Can someone identify the exact location D310?

Also, I only had the monitor on for a minute and R510 got pretty darn hot. I thought I smelled something burning and it might have been that resistor. I tested it after powering down and it looks to be ok.

Video b+ goes to the crt cathodes
And to the 3 Watt resistors on the
Neck nd then to the collectors
Of the video amp transistors
Is there any voltage on these?
If you ha e video b+ then
Don't worry about d310

If the ics shorting it to will run hot
Like r510 and pull down the DC
to the ic if u desolder the pin at ic
And check z500 voltage

it should jump to 12vif it is good
And r510 will run cool again.
Let is know
 
Video b+ goes to the crt cathodes
And to the 3 Watt resistors on the
Neck nd then to the collectors
Of the video amp transistors
Is there any voltage on these?
If you ha e video b+ then
Don't worry about d310

I measured the voltages at the collectors of the video amp transistors and this is what I got:

Q503 - 96v
Q504 - 110v
Q505 - 150v

If the ics shorting it to will run hot
Like r510 and pull down the DC
to the ic if u desolder the pin at ic
And check z500 voltage

Could I just lift the non-banded leg of Z500 out of circuit and measure the voltage at that end? It should read +12v correct?

If I can't do that, which leg of U500 should I pull out of circuit (pin 28)?

I do have another U500 tho I don't know if it works. Perhaps I'll just remove the U500, put in a socket and try this other one.
 
Last edited:
Lift 1,23,28
So none of them are touching
The trace.
Does the zener hold the DC
Around 12vs or same?
Replace c507 next. That's if the ic
Not pulling down the DC
You can swap the neck board
Or use its parts.
U could remove the zener
To test DC level should go to around
15vs don't want to over voltage
The ic. If its good
 
Got this working tonight and wanted to post a follow-up. I installed a cap kit and that didn't change anything. I didn't think it would but most of the caps were original so they needed to be replaced anyways. I swapped in another Zener diode on the neckboard at Z500 and no change.

At that point I figured the LM1203 was bad. Removed the old one, put in a socket and installed a new one. Fired it up and the voltage was good at 12v but still just a white screen. If I turned the brightness down I could see a very faint picture on the screen.

So I pulled the chassis again and started at the RGB signals at the neckboard. That's when I discovered that all three of the 2N3906's were bad. Should have seen this earlier. I pulled all three of these and also discovered that two of the solder pads for one of the 2N3906's had a very small splash of solder that was connecting them. Removed this, installed 3 new 2N3906's and fired it up. Bingo - we've got a picture! Dialed it in and it looks great.

This was the first 7400 series monitor I have ever worked on so it was a good learning experience. Thanks again for the help everyone, especially Ohmerone.
 
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