Hi all,
I was given a VU-Data portable scope in unknown condition (all the guy knew is that it doesn't power on).
I have been trying to troubleshoot the power supply but I am running out of ideas. I have attached the scanned schematic of the supply section from the manual.
Unfortunately, the manual does no troubleshooting on the supply section other than saying that the inverter frequency is 16kHz (could change with abnormal loads) and normal current is about 1 amp.
What I have found out so far:
1. The -10V rail drops to about -3V when switched on and starts climbing to about -3.5V (it is current limited at around 1.1 amp by the voltage regulator). The panel LED lights up, but it is dim.
2. I receive no output whatsoever on any of the secondaries of T2 (or just a few mV at power up).
3. The resistor R215 dissipates some of the power (-3V to -3.5V across it).
What I have done so far:
1. Since I can only measure the supply with the heatsink removed from Q73 and Q74 (it covers the board) I am only running it on for about 10-15 seconds at a time.
2. Disconnecting the voltage regulator and applying a 10 ohm resistor on the output lets it hold -10V at 1amp without issues (not the voltage regulator).
3. Measured the frequency of the switching transistors (Q73 and Q74). One starts at around 13 kHz and drops to about 7. The other starts at 7 and increases to about 10.
4. I checked all the caps (tantalum) and they are not shorted.
5. Checked CR50 and CR51 and they are good.
6. Checked all of the resistors in the inverter and they are the correct values (not burnt or shot).
So I figured I would start chasing a short on the output.
1. Checked all of the outputs of T2 to ground and none are showing low resistance (after the rectifiers).
2. Disconnected all of the T2 secondaries (one at a time) and checked the inverter operation (no change whatsoever).
Sooo, I am running out of ideas. I don't know what could possible be causing the inverter to load up or fail. Any ideas? Any help is very appreciated! I have had it for a while and just sitting there and not functioning is driving me nuts
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I am sorry for the long post, but I was trying to give as much info as possible.