SOT: Panasonic 9" B&W CCTV monitor

debaser138

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Not exactly arcade related but close. At work we have an old Panasonic CCTV setup with 2 cameras and a 9" black & white monitor. This monitor connects directly to the two cameras and actually sends low voltage over the coax cable to power the cameras and get a video signal from them. Recently the monitor quick working so I opened it up. I discovered a dime sized hole burned thru the board right over where D4 & D5 are. I tested these diodes and they appeared fine but I replaced them anyways and ran some jumpers to accommodate the damage from the hole. I also replaced the 4 capacitors by those two diodes (C11, C12, C18 and C19).

I powered it up but it still does nothing. I'm getting no voltages at either of the three test points (TP2, TP3 or TP4). I checked all the resistors and diodes in the circuit and they test fine. I thought perhaps the diode at D1 was bad (even tho it seemed fine) so I replaced that. No difference. I'm getting around 160V as measured coming out of D1. I also replaced the 2SC3461-LMCA at Q1 but no change. At this point I'm wondering if the transformer at T1 is the issue, tho it looks fine. I'm also wondering if a failing cap at C7 would cause some issues. Gonna bring in my ESR meter to check that cap along with the rest of them in this circuit.

I've attached a simple and more detailed view of the power supply circuit for this monitor to see if any of you monitor gurus had any ideas on what the problem may be.

Any help would be appreciated. Thanks!
 

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If you're getting voltage out of bridge rectifier D1, I would think that resistor R2 might be open or increased considerably in value.
 
Thanks Ken. I had previously checked that resistor along with all the others and it was good. The thing is the schematics have it as 1 ohm resistor when in fact it's a 15M ohm resistor. The schematics messed up a few other values as well.

I brought the monitor home last night so I could thoroughly go over it. After comparing the traces on the board with the schematics, I found two bad traces in the section where the board had burned. Ran some jumper wires, turn it on and it powered up!

It was 1am and I was too excited to see if I fixed it so I drove into work and hooked it up to our security cameras and what do you know - it was working great!

We've had this system up and running for almost 20 years. Will be interesting to see how long it lasts.
 
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