HP 3850B Sweep Oscillator power supply fluctuation

[tonycox01]

Hi

I am bringing an HP 8350B sweep oscillator (with a 83952C RF plugin) back to life. It would not start up (kept blowing fuses) - which was down to a semi-seized cooling fan and an intermittent plugin/chassis connection.

Having got it running stably I'm beginning to check it out. The first thing I looked at was a main power rails. I've let the 8350 (and HP 3456 I'm measuring it with) warm up for an hour and checked the -40V, +20V and -10V volt rails.

All are close - but definitely out of spec because they are all "wandering" above and below the spec value:

+20V: 20.007V - 19.990V
-40v: -39.993V - -40.010V
-10V: -9.9979V - 9.9984V

They all go through their fluctuation range at about the same rate (~1-2 seconds).

I was hoping the kind souls on this forum may be able to give me some advice on where best to start. My thought was that there is some ripple in the PSU that may be causing this - but any pointers before I dive it would be very welcome.

thanks

Tony

(apologies  - I accidentally posted this to the "Test Equipment" forum first)

[wn1fju]

Am I missing something here?  According to Table 8-23 of the service manual for the 8350B, your voltages are within the limits

The fact that they are fluctuating slightly would not bother me at all.  But just to be certain, check them with an oscilloscope.  Just measuring power supply voltages with a DC voltmeter can overlook a lot of AC ripple, because the voltmeter will simply give you an average value.  Ripple would point to problems with a capacitor or diode bridge.  But the fact that you see movement in all (separate) power supplies leads me to think everything is actually fine.

The power supplies are loaded by whatever the rest of the circuitry is doing.  See if the 1-2 second fluctuation rate changes as a function of the various sweep rates.  Could all be perfectly normal behavior.

[tonycox01]

Hi wn1fju,

Thanks for your help. I just found Table 8-23 and as you say they look fine. I was working from my service manual (Section 5-1: Adjustments ) where the specs are as follows:

+20.000V +/- 0.001V
-40.000V +/- 0.002V
-10.000V +/- 0.001V

As you suggest I'll get the scope on the supplies and check them - and I hope I was worrying about nothing :-)

thanks agan

Tony