HP 54616B PSU repair

[axemaster]

I've been using a HP 54616B oscilloscope for a few years, and recently the power supply seems to have died. I've opened up the scope and pulled out the power supply. Based on my examination of the PCB, the PSU architecture appears to be the following (in order):

• AC input, fuse, mechanical power switch
• powerline filter
• isolation transformer (1:1)
• bridge rectifier to mains voltage DC
• flyback transformer primary + big N-chan MOSFET for switching
• flyback transformer secondary, multi-tap
• bunch of low-voltage rectifiers and regulators
• Ribbon cables to carry the power throughout the scope

Testing the PSU, I found that everything seems to work fine up to #4 - the mains DC voltage is present. However, there is no switching of the MOSFET. As a result, no power is transmitted across the flyback transformer, and there is no power at all on the secondary side.

I probed the MOSFET gate and there is no logic signal. So I believe the circuit responsible for generating the switching signal has failed.

I am currently probing step by step through that control circuit.

Does anyone know the correct switching frequency for the flyback transformer? In the worst case, I could jury-rig my own controller.

Also, it would be very helpful to know the correct DC output voltages from the supply. Is this documented anywhere?

Thanks so much for your help!

[axemaster]

Here's some supporting info:

Switching MOSFET = IRFPC50
https://www.vishay.com/docs/91243/91243.pdf

Switching controller IC = UC3842
https://www.ti.com/lit/gpn/uc3842
----> I have measured the output pin of this IC and there is no logic signal

Switching controller optocouplers = TCDT1103
https://www.mouser.com/datasheet/2/427/TCDT1100-1767073.pdf

Some of the transistors near the controller = MPS6651
https://www.mouser.com/datasheet/2/308/1/MPS6601_D-2316415.pdf

"Precision shunt regulators" near the controller = AS431
https://www.diodes.com/assets/Datasheets/AS431.pdf


NOTE - I also have a HP 54600B. I opened it up with the idea of cannibalizing the power supply. But the PSU for that scope is clearly a different model (though similar).

NOTE - Found DC test point voltages on page 132 of service manual...

[xavier60]

Check for open circuited high value resistors.