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Waves of Joy

January 16th, 2017

Victory is Mine

People are probably dying to find out whether I succeeded in getting my old HP 3300A function generator working, so here I am to help. It is now running.

What is a function generator? It’s sort of the opposite of an oscilloscope. A scope displays electrical waveforms. A function generator creates them. I don’t know all the reasons why this is useful, but I can list one. If you have something like an amplifier, and you want to see whether a signal is finding its way through it, you can hook a function generator up to the input. This keeps a varying voltage going into the amp while you check the circuit at various points. For me, as a guitar amp builder, the alternative is to hook up a guitar and pick a string over and over. That’s work, and as I have often said, work is against policy.

I suppose you could also check an amp’s frequency response by sending different frequencies through it.

For some reason I no longer recall (probably the guitar amp thing), I bought a used Hewlett-Packard function generator a few years back. I don’t know when it was made, but I know they were selling them in 1965. It’s the size of a big briefcase, and it weighs maybe 25 pounds. It will produce several types of waves (sinusoidal, square, etc.) at frequencies up to 100 kHz. I’ll bet it was really expensive when it was new, because it contains approximately 4 million components which were installed by hand.

A few weeks back, I pulled it off the shelf to see if it worked, and I was disturbed to find that I had no cord for it. Did I ever have a cord for it? Search me. I could not find it.

The cords we have on computers now are called C14/C15 cords. C15 is the female end. I leave it to you go guess what the male end is. As you know, these cords have three conductors, and the connectors at the ends are in a line, with the ground conductor above the others (or below them, if you hold it that way). The function generator had a male receptacle for a cord, but it was a bizarre old setup known as the PH-163. I have written about it before, presumably to everyone’s delight. It’s sort of like a C14/C15, but it has oval pins.

You would think there would be ten billion PH-163 cords for sale on Ebay, but there aren’t. The best deal I found was about $25, for a cord for a machine which, for all I knew, would never work. My solution? I took one of the thousands of spare cords I’ve accumulated, and I bought a receptacle for it. I opened up the receptacle hole in the generator cabinet and installed the new receptacle, and that allowed me to use a new cord.

I was really proud of that job, because I had to use a whole lot of tools, and the stuff I was working on was very small. I had to use a Dremel, two files, a drill, a soldering iron, a solder sucker, a power screwdriver and some other stuff I forget.

Because I know you’re excited about this, I will post a photo, looking down at the receptacle.

The two red things are new capacitors I put in. I will tell you about that shortly.

I got it wired up, and I plugged it in. POOF. It failed to work, and the lights in the garage flickered. Seemed like there was a short.

I looked it over and asked questions on forums, and I learned something interesting. Sometimes old capacitors short out for no apparent reason. One day, you have a capacitor which blocks DC and many AC frequencies, and the next day, you have what might as well be a straight copper wire with zero resistance. The transformer on the generator had a dual capacitor across the main power wires, presumably to filter out crap, and when I checked the cap, I got 4 ohms of resistance (a short) on one side.

A dual capacitor (that name may be wrong) is a capacitor that looks like a single capacitor yet which contains two capacitors that share one lead. It will have one lead in the middle that goes to both caps, and it will have two leads on the sides, each of which goes to one cap. I do not know why HP chose to use a dual capacitor, but it seems like a bad idea, because when one side crapped out, I had to replace both sides.

I guess that’s not really true, but I felt like it was stupid to keep an old capacitor when its twin had just kicked the bucket.

Meanwhile, the generator’s fuse had melted. After I tried running it in the garage, I took it in my office and plugged it in, and the breaker that powers the outlet popped. I assume this is what killed the fuse. I should have used a current limiter, but I didn’t.

Here’s something that may be useful to doofuses like me who can’t read fuses. The fuse in the generator said “6/10A” on it. I couldn’t figure that out. Six amps? Ten amps? Six amps on even-numbered days and ten on odd-numbered days? It turns out it means 0.6 amps, which is something the manufacturer could have stamped on it instead of “6/10A”. I had to go on Ebay and order new 6/10A fuses.

I also had to order the caps. I have maybe 2000 caps on hand at a given time, and when the generator needed one, I didn’t have a single 0.01uF.

When everything arrived, I put the correct fuse in the machine and solder the caps in place. You can see them in the photo. I even saved the funky old insulation from the old cap and put it on the new cap leads.

The solder sucker I used is wonderful. If you to to an electronics store at random and ask for a solder remover, they will give you a clumsy foot-long plastic thing which is hard to use accurately. I found a little metal Japanese solder sucker about four inches long, and it’s much more precise. Solder tends to get stuck in the nozzle, so you you have to stop and get it out, but it’s worth it because it’s so easy to use.

I got everything put together and plugged the machine in. No joy. After going through all this, I found that the on switch was dead. No problem, right? Buy another one for three dollars on Ebay. Sadly, that option was not open to me. This thing had an expensive “Marco Oak Press-Lite” switch, which is apparently an aircraft-grade switch. It lights up when it’s on. New ones are selling for sums well into two figures. No way was I paying that.

I figured out which pins on the switch needed to be shorted to turn the machine on, and I soldered a wire between them. Now the machine turns on when you plug it in. I ordered a small toggle switch, and when it arrives, I’ll cut it into the wire I installed. I’ll put it in the front panel of the machine, and I won’t have to deal with the aircraft switch.

I got all this junk done, turned the machine on, put an oscilloscope probe on it, and got a signal! I was thrilled to the marrow. I got square waves, triangular waves, and sinusoidals. I got big ones, small ones, fast ones, and slow ones. The bizarre “sweep plugin” module that came with the generator worked, too. I don’t know what it is, but it changed the waveform. I still have one knob which is frozen, but at my age, that’s to be expected.

So now I have a huge signal generator which can probably be replaced with twenty dollars’ worth of modern parts that take up ten square inches. I don’t care. It was fun to get it running.

If you’re still reading, here is the payoff.

1. If you repair electronics, use a current limiter when you turn them on. Don’t be stupid like me. I have a variac and a light bulb limiter, and I still decided the best option was to hit the switch and pray.
2. You need a metal Japanese solder sucker. Don’t bother with the big plastic ones. It’s an “Engineer SS-02.”
3. A 6/10A fuse is really a 600 mA fuse.

I still have to fix my old Hitachi oscilloscope. I don’t really have to; I could throw it out. But I should fix it. It could come in useful some day.

My definition of “useful” is unusually broad.

I realize my life is full of excitement and adventure. Try not to envy me.

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3 Responses to “Waves of Joy”

  1. bmq215 Says:

    Dual capacitors are certainly a thing but in that situation the three-lead part you describe well be an EMI filter. Looks very similar to a ceramic capacitor (and has some capacitance, 0.01uF being a likely value), but there’s also a choke included in the package (so four parts in one instead of two). Will your unit work well without it? Sure, probably. EMI effects can often be pretty subtle and transient. But there’s probably also a reason why it was included in the first place…

  2. Steve H. Says:

    Thanks for the information. Sometimes I’m amazed to find out how many things my electronics courses left out.

    I’m looking at the schematic, and where the component used to be, I see two caps. I don’t see anything else. No inductors.

    The caps are labeled “0.005,” which is strange, since the one that came out was definitely twice that big.

  3. Steve H. Says:

    I just found a technical document from Vishay, titled “AC Film Capacitors in Connection with the Mains.” It says they’re used to kill high frequency interference.

    Interesting note: it says “safe end-of-life behavior” is a consideration in the design of these caps.

    I guess the guys at HP didn’t read this paper. I’m pretty sure shorting out isn’t safe behavior.