D9400 Repair......I think I can claim victory...

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Holy hell this was challenging. So I picked up a D9400 monitor and chassis that I plan to use for my dynamo HS-9 Marvel Vs Capcom 2 cabinet.

Seller mentioned that him and others tried to get this monitor working but was at a point that he just wanted to get rid of it. He also mentioned that the chassis would click but nothing else.

Decided to not power the unit and try to find as many issues as possible.

Repair Notes:
1. Replaced shorted out HOT (Q207)

2. Replaced shorted out Q206, previously read false open diode readings for D209, D210, D212, ZD201, D208 due to Q206 being shorted between gate, drain and source pins.

3. Placed transistor isolator plastic behind IC301, this was causing ic301 pin 4 to have connection to ground, causing D517 to be reversed biased and letting the magic smoke out. Of course I didn't know this until first turning on the unit and seeing smoke come out of D517. This lead me down a pretty big rabbit hole, story below if interested.

4. Replaced D517 😮‍💨

5. Tube is up and running now, but is in definite need of a cap kit.

Jumped Down A Rabbit Hole

After checking the power supply area I decided to power the unit up. After flipping the switch, the power supply clicked and the green LED came on but then immediately shut off and came back on. The second time this happened magic smoke started coming from D517 near T501. Looking into it, D517 in circuit read shorted with the cathode leading to T501 and the anode trace pathway leading to pin 4 of the vertical amp IC301.
  • Looking at the schematics the only way D517 is burning up is if it's being reversed biased
  • Checked for shorts from pin 4 to the other IC301 pins, no shorts.
  • Followed traces, not finding any other path that could be causing this issue.
  • Found that Pin 4 somehow has continuity to ground......wait what???🤔. How can this happen if pin4 is not shorted to any other of the IC301 pins. Checked D517 and it also showed continuity to ground
  • Even more confusing, with D517 removed, both solder pads still showed continuity to each other....WHAT???!! This means that both cathode and anode sides are connected to ground.
  • Spent an hour checking all of the related circuits and looking for shorted components to ground.
  • I decided to un solder jumper J114 that connects pin 4 of IC301 to D517 area. Bingo! D517 anode and cathode solder pads are no longer showing continuity.
  • This meant something in the IC301 pin 4 circuit was connected to ground.
  • I removed IC301 since I couldn't find anything out of the norm with the solder location.
  • Doing so pin 4 solder pad no longer showed continuity to ground. For the hell of it I replaced IC301 but it should continuity with ground again........
  • BINGo.....Sigh...the people before removed IC301 and removed the back insulator and reinstalled it without one. This caused the short to ground once a screw and bolt are used to hold it up against the heat sink.
  • Reinstalled IC301 with back isolator, though used a zip tie to hold it against the heatsink instead.
  • Reinstalled jumper J114 and replaced the burnt out D517 diode.
  • Fired up the unit and was pleased to see the X-Men vs Street Fighter intro

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Last edited:
Great you got lucky!
Need to replace the caps
Unless you want it back to repair later!

They are essential. To replace at least once since it came out
And need to clean off glue around the PWM ic

Btw Q901 is the Hot for high voltage only
Not q207 and q206 it's for deflection only
That's the letters HSOUT on schematic mean

As for D517 that's for vertical deflection
Won't cause horizontal scan deflection or high voltage shutdown
It use the plus in minus voltage from the power supply and those caps need to be replaced
Or will overheat the ic

Just hold down dc with ps secondary voltage
Anytime a short on vertical circuit the ic can be removed and still have hv just a horizontal line

Btw testing a mosfet should only tested with an ohms across the pins I would expect r 260 to be open if there's an issues with shorted q206.207
From my experience
I would replace the caps now even if it works.
I personally hate repeat work.
It's your call good luck!
 
Great you got lucky!
Need to replace the caps
Unless you want it back to repair later!

They are essential. To replace at least once since it came out
And need to clean off glue around the PWM ic

Btw Q901 is the Hot for high voltage only
Not q207 and q206 it's for deflection only
That's the letters HSOUT on schematic mean

As for D517 that's for vertical deflection
Won't cause horizontal scan deflection or high voltage shutdown
It use the plus in minus voltage from the power supply and those caps need to be replaced
Or will overheat the ic

Just hold down dc with ps secondary voltage
Anytime a short on vertical circuit the ic can be removed and still have hv just a horizontal line

Btw testing a mosfet should only tested with an ohms across the pins I would expect r 260 to be open if there's an issues with shorted q206.207
From my experience
I would replace the caps now even if it works.
I personally hate repeat work.
It's your call good luck!
I appreciate your concern and warnings about the caps, and I 100% agrees with you, I'm waiting on the cap kit to come in and I never planned to use it for primetime until that's done. That and because the cabinet it's going into is a tight fit which would require me to remove the tube again to get the chassis out to replace the caps, I rather wait and get the caps done👍 first.

I thought the same thing about R260 but it read normal, though I plan to swap it with a 3W resistor.

Thank you for the explanation of Q207 and Q206, I guess I saw "HOT" on Q207 and just assumed.

As for D517, not sure what to tell you there, once I solved that issue the tube did fire up were it was in shutdown prior. Though I did go through reflow the solder prior to finding the connection to ground on IC301 so maybe that mucked up my results.

Again I appreciate your knowledge as I've used your help before from other posts.
 
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