Electronics > Repair

HP/Agilent Infinium 548xx scope - Power Supply Schematic !

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Conventional Wisdom:
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.

Incredible first post!

And thanks, I have a working 54820A scope and a spare parts donor with a faulty power supply. Hopefully they have used the same PSU design in the entire product range. This might be very useful in the future.

Wallace Gasiewicz:
Thanks for the PS Schematic. 
Did you also put in a new Hard Drive? Lots of them fail. 
Mine has been upgraded to SSD.

Conventional Wisdom:
Mankan, thank you for the kind words.

Wallace, I haven't had to replace the drive yet, but the SSD is a good idea.  This thing takes a LONG time to boot.  I expect that the challenge will be to find one with the 2 mm IDE interface.

Like everyone else within the last few years, I did have to grind open the DS12887A to wire in a fresh lithium cell.  Amazing how many instrument manufacturers didn't imagine (or care) that their products would be used for more than 10 years.

Wow, great work thankyou!

I have a HP 54825A and it has the factory upgrades to make it run better. However, the PSU has been unreliable so this info is really useful!

The physically smallest electrolytic caps in my PSU had all gone high ESR and I've replaced them. There's also some sort of status flag on one of the headers that would trip every so often and you have identified this as a brownout pin. Thanks again for this!

When this pin tripped on mine it would halt the scope and a message about a failed fine interpolator would show on the screen in red text as below:

Fine interpolator gain cal failed: Service is required

This didn't make any sense because the fault was actually in the PSU. Something was upsetting it and thanks to your info I now know something in the PSU was falsely tripping the brownout detection in the PSU.


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