Fuse Change in HP33120A Function Generator

Simple post. Most of the function generators that we have here are fitted with a fuse holder that allows us to quickly swap out busted fuses. This one in particular however is not.

Changing the fuse in this one is about just as simple except you have to take it apart before you can change it out. I just thought it would be neat to show the guts of the function generator.

The photo below and to the left is just the new fuse in the sockets. It looks like a goofy looking resistor.

The photo to the right is measuring the resistance of the old busted fuse. It's measuring 6-ish Megaohms which doesn't make for a very good fuse.

HP 3310A Disassembled and Potentiometer Repair

I originally wasn't going to make a post about fixing a potentiometer on the HP 3310A function generator because I thought it should have been relatively easy to replace/repair a simple potentiometer and wouldn't really have a lot more content to add to what I already did. Holy Packard was I so wrong. More like semi-wrong. Repairing the potentiometer was relatively easy as expected, but all the disassembling to get to it so I could even work on it was just incredible. I know in the last post I said the same thing, but friggin' kudos to HP for making a solid box of interlocking, military grade spec parts that is just an incredible pain to take apart. A majority of the photos that I took were not initially for a blog post, but rather documenting how to get this contraption back together once I realized that this was going to be a lot more extensive than unscrewing a few parts.

In the last post about the 3310A, I kind of went over a little of the first bit on how to start taking this thing apart so this is sort of going to pick up from there in the breakdown process.

The potentiometer that is used to vary the amplitude of the function generator was really erratic every time it was adjusted which suggested that the wiper contact was all gunked up with all sorts of gross just from use over time. So the initial idea here was to just replace it, but first I had get to it. Due to how the function generator was constructed, it was necessary to take it almost completely apart.

This is the side panel that screws down each of the printed circuit boards to the box itself so that it's nice and secure. Naturally, each set of screws are different sizes and have specific placements so this photo was a lot more useful to me once I had to put it back together.




But what's this? It only takes off the metal frame, but at least now we can see the board that has the potentiometer on it.

Getting the board out itself was not necessarily professional, but once the metal side frame is removed you can flex the box to wiggle out the circuit board.

Here are more photos of the board in question once it had been wiggled free. There are two other circuit boards that interface with it, one of which needs to be removed completely to get our potentiometer free.

Now that I can get a good look at the potentiometer, I figured that it might be worth a shot to see if cleaning the wiper track might solve the issue... but this only became an option after finding that the instrument room did not have potentiometers with the proper physical dimensions.

So prying back the clamps on the dust cover plate revealed this was kind of a unique potentiometer. Rather than having a single contact tooth, this potentiometer had contact teeth so to speak. The photo shows a comparison of these two kinds of potentiometer contacts.

Anyways, just from many years of use the contact teeth made tracks in the resistive strip sort of causing an eroded mess of shavings. I am not certain, but I am under the impression that the resistive strip shavings were preventing a good electrical connection. Cleaning up the tracks with ethanol worked out nicely.

Before reassembling everything, it was suggested to me by Dave Azpell that a tiny hole should be drilled in the back of the dust cover. If the potentiometer started misbehaving again ethanol could be easily applied without having to take the whole thing apart again.

 

  Here it is mostly put back together with a little video showing the output of the function generator on an oscilloscope while I adjust the amplitude with the newly cleaned potentiometer. 

EDIT (9/17/2015): Someone emailed me about getting some scans from the service manual. So here are the two sets of scans I did. I did a very basic one that included the board layouts. The second one focused on the A1 board that has the triangle amplifier and +/-5V detector.

Here's the link to my Google Drive

Repairing HP 3310A Function Generator

It is not uncommon that lab TA's report that their lab equipment is broken in some way or another. Usually it's some trivial thing like they forgot to check if it's properly powered or didn't know how to check if the fuse is blown, but from time to time the equipment is in need of some serious maintenance.

So this was brought into the back and the TA just mentioned that he couldn't get it to work. This older model function generator are usually pretty solid and don't have fuses so if they're broken then they are super broken. After some initial tests just to double check if the TA was not using it correctly, it was found that there was no output for any of the waves.

After taking the thing apart, and I would just like to give kudos to HP for their work in designing their function generator in such a way that it was a huge pain to take apart. I mean this this is a rock solid piece of equipment and my guess is they really didn't want it to be taken apart unless it really had to because it was a mystery box of sliding parts to get this jigsaw box apart. Anyways, it is pretty common for the countless detaching and reattaching of cables to the output by students just naturally wears them down and in this particular case, breaks the connection so that new leads need to be soldered. To the right, there is my handiwork with a new resistor in place. The original connector that was attached was just destroyed so I had to fashion a new connector to the board as well. This actually solved part of the problem because I was able to get the function generator to output a square wave, but the Sine and Triangle wave parts were still flat lining.

Well that was the easy task, now I had to look for something a little more malicious as to why this thing wasn't working and you can see it in this picture to the left. Some old military grade tantalum capacitors leaked and corroded away at the copper trace. On the right you can see that the trace was just eaten away.

So the service manual was found, but wasn't really necessary in this particular case because it was fairly obvious what was the culprit for the bunk outputs. I thought it was kind of interesting though and figured I would show the photo of the schematic, C2 and C3 should be the caps that I needed to replace if I remember correctly.

The only thing that needed to be done to repair this would be to fly wire the two separated traces together and drop in some new tantalum capacitors. Not too tricky.

The above photo is the new wire soldered to connect the two traces, and to the right are the new two orange caps that were soldered in.

And would you know it, everything started working again. Successful repair job to keep this HP function generator up and going for at least another year before someone drops it or something.

Big thanks to Dave Azpell for keeping the service manual and giving me guidance on what to check for to get this guy up and running again.