k7203 Has HV but no Pic

[fings1]

Hi all,

I have a recently acquired K7203 (WGE-1972 Hots49A) chassis that I'm having an issue with.

The good news is I get HV and neck glow, however there is no picture. In fussing around with this, I've noticed that the voltages on the crt socket of the neck board get 8V on pins that I think should be ground. I get ~102V and 315V on the pins you expect to get higher voltages on but on all the others they are 8V. Not sure if this helps to identify the problem but thought it worth mentioning.

I have checked all the pots to make sure they work and no shorts. Same with the transistors.

Any ideas where to look when you have HV and no pic?

Thanks for any help.

 

[paul400]

the G2 voltage should be a few hundred volts to get any raster on the screen, if you are only getting 8v your flyback might be bad. post a pic where you measured.

 

[mecha]

is the heater working? if the neck doesn't glow there's no power to the electron guns. I would think you could turn the Screen way up and see something. if not then I would lean toward bad flyback. do make sure the solder to the flyback pins is good; I don't know the scientific reasoning for why it happens but the solder can break down there. I've seen it happen on the heater pins on K7000s in particular.

 

[M K L]

A flyback that doesn't work doesn't mean it's bad. It could just not be supplied (B+) and driven correctly (HOT collector pulse) on the primary side.

The only CRT socket pin (exposed on the PCB) where you can have 315V is the G2 (pin 7) which would mean the flyback is working. HV is said to be present but how was that established? Actually measured? Inferred from typical tube noise/static on screen?

And who knows what CRT socket pin he thinks one should expect to find 102V on? In fact one should expect to find about 100V on all three cathodes (pin 6, 8, 11). Then you have the heater voltage which on this monitor is a 6.1 DC voltage from the power supply and lastly we have the G1 which is tied to ground so 0V. Nothing else really.

If it's proven that the flyback works (i.e. it provides HV, focus and screen voltages) and there is a visible raster when the G2 is increased, the lack of picture can only be due to some problem in the RGB processing chain from input header to neckboard amps. Possibly a bad LM1203.

 

[Ohmerone]

My Tip: You Must replace the EL-Cap's

on these models Not check them If Original

From My Experience

with a few!

If the Neck board is not damaged

You should be able to turn up the screen

pot on the flyback and get a bright raster.

Even without the 12 or video B+ to the neck board

You will need the caps.
Flyback are a issue on this model as well
may not need it ATM.

Need to turn up the screen Pot First!
LUK

Check it out.

 

[fings1]

Thanks all for your great input.

I measured the pins again today and now I'm getting different readings. I do recall there was some variability when i checked them previously so something is amiss.

Here's the summary of what I have:

1) Heater not glowing now
2) Voltages on the neck pins (should have been clearer at first):
Pins 1,2,4,5,9,10 : 8V
Pins 3,5,8 73V (this has changed from 102V from the weekend readings)
Pins 7: 202V (this has changed from
3) Voltage coming from the flyback:
Pins 1,2,4,5,6,7,8,11: ground
Pin 3: 31 V
Pin 9: 127V
Pin 10: 181
4) I turned up the screen pot and there is no raster at all - no change in brightness or any lines. This occurred with the old readings and the new ones.

I've ordered a cap kit to replace all caps per God Emperor Ohmerone and see where that takes me.

Any thoughts, much appreciated.

 

[mecha]

caps aren't going to fix it. if you have no raster turning the Screen up, the flyback is bad.

 

[M K L]

Your pin numbering is not correct. I can't find a pic of the solder side of the neckboard so post one yourself.

The flyback pins:

1 = HOT collector pulse
2 = B+ (121V)
3 = NC
4 = NC
5 = NC
6 = NC
7 = GND
8 = ABL
9 = AFC/PROT
10 = Flyback pulse
11 = GND
12 = GND

Possibly your pin 9 = 127V is actually the B+ on pin 2. Don't measure it at the flyback though, there must be a more convenient point (it's also on the neckboard). Now post a pic of the neckboard so we can put some order there too and measure voltage on the G2 pin so we'll know if the flyback is working or not. It's possible that it's in shut down because the B+ is a little too high so you should first get it to the correct level (121V) and see. If it's still in shut down it could be the X-ray protection: flyback pin 9 sends a feedback voltage (set to the correct level by VR502) to the emitter of Q501 and if it's high enough Q501 becomes conductive and the voltage is sent to pin 13 of U501 which stops the horizontal oscillator. You should also make sure that U501 has 12V on pin 10 coming from the regulator U102 which is in turn supplied by the switching power supply (this 12V supply also goes to the neckboard).

 

[jlweatherly]

Appreciating all the info here, I have a K7201 that isn't getting HV that I will be digging into soon.

 

[fings1]

MKL...I'm not shocked I screwed up the numbering...so here are pics and my numbering logic. Basically backwards and missing a few pins to keep everyone on their toes...doh!

Main Board:



Neck Board:


Mecha...yes, a flyback is definitely on order.

I'll take another look at it all tonight with your observations.

Thanks all for your guidance!

 

[M K L]

That's a K7000 neckboard. The K7203 one is like this (note the six transistors vs the three of the K7000):



Anyway the CRT pinout is:



5 = G1 (GND)
6 = Green Cathode
7 = G2
8 = Red Cathode
9 = Heater (GND)
10 = Heater (6.1VDC)
11 = Blue Cathode

Not sure if the K7203 and K7000 neckboards can be swapped. The two 5-way headers have the same pinout but the wiring would need to be checked.

Pin 7 (G2) is where you should have a DC voltage of a few hundred volts adjustable with the "screen" trimmer on the flyback. I think you said it's 202V in a previous post.
Pin 10 (heater) is where you should measure 6.1V (DC not AC!). If there's no voltage try on point A.

Red dots = B+ (121VDC)
Blue dots = 12VDC

 

[Ohmerone]

Be sure to start your own thread
and not be confused by this thread!
Yours's is a different model.

Appreciating all the info here, I have a K7201 that isn't getting HV that I will be digging into soon.

Just replace the caps first!
then Post if not working.
Check the Plug connector
there should be Dc test points
printed on the PCB
near the plug.
B+
And listen for any ticking noise.
if ticking we can check some other test points
LUK