HP 33120a (Attempted) Repair

[jastervoid]

I've got a sick HP 33120a function gen that I could use some help repairing. I get a 2 beep error code on startup and nothing on the display. According to the service guide (linked in below) the beeps indicate the main processor cannot communicate with the front panel processor.

I went ahead and checked all the power rails and they all check out ok. There's also a -13v supply using a 7805 (U1102 on the schematic) from the -18v rail. I had to desolder the VFD from the front panel pcb to access all the smd goodness underneath including the regulator.

So of course the output of the 7805 measures 3.6v instead of the expected 5v. The input voltage is only 5.2v. There's a string of 4 resistors (R1103-R1106) that are used to limit current through the 7805. Curiously they are marked as 10 ohms in the schematic but are all 75 ohms on the board.

I lifted the output leg of the 7805 and the voltage jumps to 5v as expected; so the regulator seems ok. Something is drawing too much current off that -13v rail.

Simple Ohm's law math tells me that maximum current through the regulator in this circuit is about 36ma. Measuring current on the output of the regulator shows 44ma, almost 10ma extra. This is where I get stuck and could use some help. How do I go about isolating the component that is drawing the extra current?

33120a Service Guide

[free_electron]

check the tntalum caps on the frontpanel.
if that doesnt work the lm339 that does the level shifting

[dc101]

You might try connecting to the serial port and seeing if you can read the specific error code.  Hopefully that will help you at least determine if it's the display, the keyboard or both.  U1101 also uses the -13 v rail.  Have you checked around that at all?

[dc101]

I'm assuming of course that you have already checked the front panel ribbon cable and it is ok?