Repairing a VU-Data Model 941B Oscilloscope
I picked this up at a Hamfest really cheap because it has a problem with its power supply. I don't need another scope but I couldn't help myself. It was only 10 bucks.
|
It is very similar in size and function to my Tektronix 326. It's dual channel and can run off of batteries, 12-volts DC power, or AC just like the 326. The one thing I like better about this scope than the 326 is the vertical channel inputs are on the front instead of the side.
|
|
The front panel is in nice shape. There doesn't seem to be much information available for this scope. I've not found a manual or schematic for it yet.
I did find several pages and a schematic on the 900 series scopes that is similar, but not identical to the 941. Parts of the power supply circuit are similar enough for the schematic to be a help with it. It looks like the 900 has a lot more discrete transistors and fewer opamps than the 941. |
|
I checked out the power supply and I see the pass transistor for the 15-volt DC power has been replaced and is not the correct part number. And the original 1979 filter caps are still installed. So I ordered replacements for all of them from Mouser.
The AC fuse is supposed to be 1/4-Amp, but someone replaced it with a 5-Amp fuse. Not very smart of whoever did it and I hope they haven't destroyed something expensive or that's unobtanium. Replacing a blown fuse with a much higher value is not going to fix the problem, but it's a real good way to do just the opposite. |
After installing the new filter caps and regulator pass transistor there was only a couple of volts output from it. I removed the output lead from the emitter to the rest of the power circuitry but still no good. Next the transformer secondary leads were unsoldered to check the AC out and it measures 38 volts end to end or 19 volts to center tap.
After reconnecting the transformer leads the collector was lifted to remove the pass transistor from the rectified DC and now there is 25VDC across the filter cap. After reconnecting the pass transistor the 25VDC is still OK and there is 15VDC on the transistor base from the 1N965B zener diode CR5, and 15VDC output. All I can figure is there must have been a bad solder joint on the PCB.
Now that there is regulated DC, the oscillator powering the toroid primary needs troubleshot because there is one burned resistor in the circuit.
After reconnecting the transformer leads the collector was lifted to remove the pass transistor from the rectified DC and now there is 25VDC across the filter cap. After reconnecting the pass transistor the 25VDC is still OK and there is 15VDC on the transistor base from the 1N965B zener diode CR5, and 15VDC output. All I can figure is there must have been a bad solder joint on the PCB.
Now that there is regulated DC, the oscillator powering the toroid primary needs troubleshot because there is one burned resistor in the circuit.
Page created 8/11/2022
Last edited 8/24/2022
Last edited 8/24/2022
