Hello folks,
You know what I miss? I miss the way HP and others used to include detailed service information, and even full schematics, in their product manuals. Ah, the good old days. Now they act like a simple power supply is a closely guarded secret.
I found this forum last year while searching for repair data for my 54845A scope, after its power supply stopped working. Mainly what I found out is that there are a lot of people in the same boat, who could repair the darn thing if only they had a schematic. If you're one of them, I have good news: I'm an engineer, I'm retired, I'm stubborn and I don't like secrets. So...
I reverse-engineered the entire power supply, drafted a complete schematic, and I'm placing it in the public domain here. It's my gift to all you folks who take the time to help each other out.
This isn't some sophisticated, digitally-managed modern marvel; it's a very conventional 25-year-old, 300-watt design with a wide-range PFC front end, an isolated flyback converter for standby and bias supplies, a two-transistor forward converter with post-regulated outputs, and some protection and control circuitry. All the semiconductors are generally non-special and still available.
First, the obligatory warnings: This information is provided WITHOUT WARRANTY OF ANY KIND and you use it entirely AT YOUR OWN RISK. There's 380 volts DC in that chassis, even in the standby state, so if you're not completely comfortable with troubleshooting and repairing high-voltage, line-powered assemblies, seek qualified help. Also, there may be differences between this schematic and your particular unit. Be sure to read the notes on sheet 1.
Other notes...
1. The PFC stage and standby/bias flyback supply are always operating whenever AC power is applied. To enable full operation, the scope connects about a 200 ohm resistance between J9 pins 3 and 11. The supply can also be enabled for bench-testing by applying 200 ohms between J4 pins 1 and 2. (The design of the control input is weird: If the resistance is too low, or zero, the supply will not turn on.)
2. The overcurrent threshold for the +5 V output is configurable according to the resistance the scope presents between J9 pins 3 and 10. In my 54845A, the resistance is 1210 ohms. It may be different in other models depending on the expected nominal current, or it may vary during operation. I did not investigate further.
3. There are separate overvoltage detector circuits for each output, and an additional detector monitoring J9 pin 7, with a nominal threshold of +2.64 V. I did not investigate the source of the voltage on that pin, but it measured only 220 mV during operation. If an overvoltage condition is detected on this pin or on any output, the supply shuts down until either AC power or the enable input is removed then reapplied.
4. There is no output current limiting. If an overcurrent condition is detected on any output or at the DC-DC converter input, the supply shuts down for about 1 second then attempts to restart.
Of course, I can't resist offering a few editorial comments about the design.
1. The enable input circuit seems unnecessarily complex, but maybe there was some reason for it that escapes me. I assume that the 15 mA constant current source is intended to provide wetting current for the mechanical relay contact that enables the supply, but a simple resistor to VCC would have done that.
2. If the intent of the R-C network in the reset circuit (R292-C239-CR216) was to provide a simple power-on reset function, it doesn't work, because the comparator is inverting, and since C239 is initially discharged, the low-active output (RESET_N) is deasserted, not asserted, at startup.
3. Connecting the summing node of an op amp (U203 pin 2, sheet 9/D3) directly to the external wiring harness at J9 makes it highly susceptible to noise and is a poor design practice.
On the other hand, the actual power-conversion circuitry seems well designed and robust.
Incidentally, the actual failure of my power supply was the result of some poor workmanship on the part of whoever repaired it before I acquired it. The daughterboard is mounted to the main board using right-angle pin headers, and about half of the plated-through holes in the main board had been damaged through negligence. If you need to remove the daughterboard, make sure you thoroughly unsolder each pin so it's loose in the hole.
Good luck! C.W.
*** THERE IS A NEWER SCHEMATIC *** See further down the thread.