WG6100 R901 and 907 smoke

gmontag

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I'm having a lot of trouble with the HV section of this monitor. R901 and R907 burnt up. I installed a cap kit and HV kit from Ian Kellog which replaced:

-caps
-all transistors
-diodes
-zener diodes
-most resistors
-555 IC

Still when I power up, R901 and R907 start to smoke and I get no HV. Seems like a short, but what should I check next? I didn't measure any shorts between the flyback terminals in circuit. I've triple checked Q900 and Q906 against frame shorts. The originals tested fine, but I replaced them anyway.

P.S. R928 measures 33K ohms instead of 56K ohms. Could that be it? My reading of old threads says no.

EDIT: Image added
 

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Should have mentioned

It's a P316 HV board
Deflection board has LV2000 and is running fine (no spot killer, lots of chatter)
 
A few things to check:

- C901, C902, C905, and C910 to ensure they are installed correctly and the correct ratings?
- D901 to ensure it is installed correctly.
- Do R902 and R903 test as good?
- Did you replace R918? If so, what position is it in (turned all the way or centered)?
- As a good measure, did you replace the solder on the flyback (T900)?
- Remove R928 from circuit and measure it, as it should be much closer to 56K Ohms than that. Remember there is a 5% or sometimes 10% variance.

Scott C.
 
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- C901, C902, C905, and C910 to ensure they are installed correctly and the correct ratings?
installed correctly. I can't measure capacitance with my DMM.

- D901 to ensure it is installed correctly.
yes

- Do R902 and R903 test as good?
yes

- Did you replace R918? If so, what position is it in (turned all the way or centered)?
no. It's measuring 17.5K in circuit.

- As a good measure, did you replace the solder on the flyback (T900)?
yes

- Remove R928 from circuit and measure it, as it should be much closer to 56K Ohms than that. Remember there is a 5% or sometimes 10% variance.
It's 33K out of circuit. I'll replace it.


Thanks.
 
- C901, C902, C905, and C910 to ensure they are installed correctly and the correct ratings?
installed correctly. I can't measure capacitance with my DMM.

- D901 to ensure it is installed correctly.
yes

- Do R902 and R903 test as good?
yes

- Did you replace R918? If so, what position is it in (turned all the way or centered)?
no. It's measuring 17.5K in circuit.

- As a good measure, did you replace the solder on the flyback (T900)?
yes

- Remove R928 from circuit and measure it, as it should be much closer to 56K Ohms than that. Remember there is a 5% or sometimes 10% variance.
It's 33K out of circuit. I'll replace it.


Thanks.
Do you have the means of testing D901? I have my doubts that's the problem, but good to test regardless. My gut tells me the problem is elsewhere.

Did you triple check your work for extra solder, bridged pads, etc.? Also look at Q905 (test it in and out of circuit).

Scott C.
 
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Do you have the means of testing D901? I have my doubts that's the problem, but good to test regardless. My gut tells me the problem is elsewhere.

Did you triple check your work for extra solder, bridged pads, etc.? Also look at Q905 (test it in and out of circuit).

Scott C.

D901 is new and tests good. Q905 is new, though I haven't tested it other than for a short. I went over every inch of the board with a magnifying glass, and I think it's ok.

I replaced R928 with a resistor network of the same value (56K).

The thing is, the symptoms are the same as before I replaced all the caps/transistors/resistors. All that work seems to have changed nothing. The monitor did work for a long time until recently. I'm starting to suspect the HVT since it's about the only thing that hasn't been changed. No cracks or heat damage are evident.

It still gets crazy hot when power is applied. R901 desolders itself. The side of the cage where Q906 is mounted is too hot to touch.

Picture attached.
 

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You've confirmed Q906 is good, correct? If nothing has changed, then start looking at what you have not changed, including the HVT as you suggested.

In looking at R918, that visually looks high, so you may consider marking it's current location, turning to mid-line and retesting. Then adjusting it back up until the proper voltage is achieved. I had a funky HV unit that wouldn't fail until at normal voltages, but turning this down helped me identify the culprit. May not be applicable in your case, but a very quick and easy test.

Scott C.
 
Probably the flyback or q901. Pull the hot, see if it's still overheating. Then check the waveform at the base of the hot or on the small transformer. If it's a nice 20 kHz signal, your fly back is toast. The schematic has nice DC levels marked so you can troubleshoot using voltage drops.
 
Probably the flyback or q901. Pull the hot, see if it's still overheating. Then check the waveform at the base of the hot or on the small transformer. If it's a nice 20 kHz signal, your fly back is toast. The schematic has nice DC levels marked so you can troubleshoot using voltage drops.

Thank you. What do you mean by "pull the hot" and "base of the hot"?
 
Thank you. What do you mean by "pull the hot" and "base of the hot"?

The HOT is the horizontal output transistor, which for this monitor is the BU207 mounted on the outside of the cage (Q906 - the bottlecap). Pull means remove it from the circuit and retest. The base of the transistor is the straight bar inside the circle on the schematic, which for Q906 connects to the small side of T901 and is normally around -27v.

I just repaired a unit that had a nice 20kHz square wave coming out of the 555 chip, but no HV. The clue was that turning the HV adjust pot at R918 had no effect on the square wave (it should change its frequency). The answer in my case was a bad Q902.

Also make sure the plug going into the BU409 (Q900 on the focus block side of the cage) is not flipped: the slot in the cage is made for the tab of the connector (so the flat side of the connector should be facing out).

And of course D901 ≠ ZD901.
 
I just want to close the loop on this. I replaced the flyback with one from Twisted Quarter and it fired right up. I dialed in the focus and all of the X,Y settings. It looks beautiful. Black Widow baby.

There's no obvious signs of damage on the old transformer. It looks identical to the new one. What would I check to prove it was bad?
 
Need a LOPT tester. Like this or newer model(s).

fbt.jpg
 
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