VU Data PS975 Scope Supply Troubleshooting DC-to-DC Inverter

[kalinrs]

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  .

I am sorry for the long post, but I was trying to give as much info as possible.

[mij59]

There could be a short in one or more of the windings of T2, check the secondary output voltages of the T2, e.g. +8V, -8V, etc.

[kalinrs]

Thanks,

I checked all output voltages on T2 and I get between 5-6mV AC and 250mv AC depending on the rail. The larger normal output voltage would get higher mV output.

I also checked the resistance of the secondary windings and I think they make sense - between 1 and 8 ohms, again rising with higher output voltage. Strangely, though the high voltage one (M15 and 16) only has 1.6 ohm (while disconnected from T3). However it also gets the highest output I was able to measure (250 mV).

[mij59]

The secondary  AC voltage are very low,  what  AC voltage do you measure on the primary winding 1,2 and 2,3 ?
Did you check C95 ?

[kalinrs]

Thank you, mij59

I get about 2.2V on 1 to 2 and 2 to 3.

However, I just realized that my Fluke 45 gives me different reading if I set it on a specific range (v.s. auto range). I am not sure if the switching is so noisy that it tricks the meter.

So I went ahead and I measured everything one more time and although I get a few hundred mV on most of the secondaries, I get 0 (jumps to a few mv as soon as I plug it in and goes away) on the +-12V winding. I wonder if that could be the reason? It still has 1.6 ohms across it. I went ahead and measured 8 to 9 and it was 1.8 ohms, while 9 to 10 was 0.9 ohms. I would think they should be the same...

C95 seemed OK, but I changed it just in case and no change.

Thanks again.

[mij59]

Just to make sure, disconnect pin 13 and 14 of T2 and measure again.

[kalinrs]

Sorry, I didn't mention that the measurements above were with the winding (13 and 14) disconnected from the rectifier.

I also continued after the bridge and found C139 (-12V filter) to be shorted. Great, but replacing it did not correct the issue... I hope that the cap failure did not cause a short in that secondary (judging by the different resistances of 13 and 14 to ground).

I will continue checking all of the other rails for any other bad filter caps. I will try to also check the diodes in all of the bridges for shorts.