Krohn-Hite 5300A Function Generator
Written by Bryce Ringwood   

The Krohn-Hite model 5300A is an electronic function generator. It is a piece of test or laboratory equipment that produce varying voltages with time. This instrument will produce sine, square and triangular waveforms from 0.002 Hz to 3 MHz in 9 ranges. In addition to this, it will produce a sweep output (output varying in frequency). The output can also be triggered.

I unashamedly scrounged this instrument from a colleague, because I wanted an audio oscillator to check out the Crosley 173 – I wanted to trace WHERE it was going bad.
Of course, this instrument is monumental overkill – but it would certainly do the job. If I could fix it, that is.

The problem was that it did not come with a service manual, or even operating manual, and when I switched it on – nothing happened. When I removed the cover, I was greeted with a circuit board of prodigious complexity. Fortunately the device had an overload protected power supply, and the sense resistors were hot. This is an almost sure bet that a capacitor has failed. I regret to say that I embarked on a ruthless hunt, isolating different parts of the board by cutting traces (neatly!) until the culprit was found.  This really isn't the best way to do things, but it is the quickest and I was impatient.

Now the device came to life – well most of it. The sweep section didn't work. I noticed that someone had previously been messing with a circuit involving a 2N2905A transistor and hadn't done too neat a job. I replaced the transistor neatly, removing all traces of flux and rubbish and tried my luck again. This time it worked.

Finally, I calibrated the instrument using a computer oscilloscope program. (Krohn-Hite will still recalibrate these instruments for you for a nominal $600-00, should you need real accuracy.)

A better (but slower) way to correct a faulty capacitor, is to unsolder and check each one in turn using an ohm-meter. This usually works, with the culprit showing a resistance in the 0 – 10 Ohms region. This is how I would check for trouble on an instrument that wasn't my own.

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