[time-nuts] HP 58540A GPDSDO Repair

[afarson at telus.net]

I have an HP 58540A GPSDO (GPS Time & Frequency Receiver) in my lab; it supplies 10 MHz clock to various instruments. An LED on the unit's front panel shows green for normal operation, or red when an alarm condition arises.

Last week, the LED went dark. The +24V power feed was OK, so I suspected an internal fault. As no schematic or service information was to be had on-line, I started tracing out the circuitry on the 58540A-60002 power board (the lower board inside the case), The +24V feed is applied to 24/5V and 24/12V DC/DC converter modules via a 2SB1621 PNP Darlington switching transistor (Q1), which in turn is driven by a small SMT NPN transistor (Q2). An MB3761 voltage detector IC (U1) switches the Q2/Q1 chain off outside a predetermined voltage range.

In the course of tracing out th circuit, i found that R8, a 1kΩ SMT resistor, was open. R8 is in the path between Q2 collector and Q1 base. This removed the bias from Q21 base, thus turning off the +24V feed to the two DC/DC converters. R8 is an 0402 part, and was apparently undersized for the power it was required to dissipate (≈ 0.6W).  The 58540A was new when I acquired it, and had failed after 6 months' 24/7 service. 

I replaced the failed R8 (which had broken in two) with a 1K8, 0.5W leaded part This brought the unit back to life. The 58540A is now back in service.





 

73, Adam VA7OJ/AB4OJ

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