A Tale of 3 Oscillators – Fixing Old Electronics

The oscillators I have are:

• HP 200CD This is a classic old oscillator based on the circuit that Bill Hewlett first figured out as his thesis project ( Stanford University in 1938 ) and which probably is responsible for the creation of the Hewlett Packard Company. The first big sale was reputedly to the Walt Disney company. It is a vacuum tube unit. I first used one in about 1960. Sine waves only.

• Eico This company sold many items both assembled and as kits. My first oscilloscope was an Eico scope. Not sure if this is a kit or production job. Like the HP it is tube based. Square waves have been added as an option to sine waves. Not as high quality a unit as the HP.

• General Radio General Radio is a very high class company up there with HP and Tektronix. This unit is probably a bit newer than the HP, it has a calibration sticker from 1974 so it is at least that old. It is largely a transistorized unit but still has one tube.

The circuits of all these units is a variable capacitor tuned oscillator with a incandescent lamp ( operated pretty cold ) to stabilize the circuit. This lamp trick is at the heart of Hewlett invention in these units. It is a trick that is still in use.

I went on-line and was able to find a copy of the manual for each of the units which I then filed away for future use.

The guitar amplifier guys would probably rip out all the electrolytic caps and replace them. I am both cheap and want to respect the vintage integrity of the units so I use an approach called reforming.  I will have more notes on this in an upcoming instructable, but the basic approach is to bring the B+ voltage up very slowly. I used a variac and brought the voltage from about 80 volts to 120 volts over about half an hour.  All the units powered up just fine after this procedure.  Before working inside discharge the power supply caps.

I used a compressor with a blow gun to blow out as much dust as I could. This was often a lot. Beware you can do damage with 100 pound per square inch air. Not everything came off. A selection of brushes down to a tooth brush loosened up the dust some more and then blew it out again. Repeated until no more progress, about 95 percent of the dust.

A cloth with just a bit of detergent was used to clean up some more including the face of the units. A cotton swap was used in the tight places. Really starting to look good now.


Do not go to anything damp to early, it removes some dirt, but what it does not get off it makes stick harder. You can use some solvents if you know what you are doing


I once knew a repair guy who would wash circuit boards first with alcohol and then distilled water. ( pure water by the way is not a conductor ). He claimed that the procedure fixed a significant proportion of the boards.

I used contact cleaner on the range switches and the scratchy potentiometers. Spray it on and rotate the contorl a good deal. In my case I later found ( when testing ) that I had to repeat the process on one of the controls.


Sometimes erasers can also be used to clean the contacts.

Binding post were tightened where loose. Look at the alignment of the hole for wire insertion and align as desired. Make the sucker tight. In some cases I cleaned up the post with a bit of steel wool ( do this away from your electronics bench and do not get the fibers into anything electronic ). I put just a bit ( wipe with a swab ) of oil on the binding post.


The dial that scraped was cleaned an positioned jut a bit farther from the case. Then it was reassembled. No scraping now.

Being careful because the case was open I powered up and tested. Mostly things were fine.


In the case of the HP which had its dial removed I adjusted the dial until it read correctly at 600 Hz, about mid scale.


From the Eico manual I followed the adjustment procedure. This did improve the flatness of the signal from end to end.


The GR unit still was flaky on two ranges. Poking the range switch with a chop stick and working with more contact cleaner got this cleared up in a few minutes.


Finally reassemble back in the cases and had 3 nice usable oscillators.


Worth noting: This was old gear but repair was simple, no new tubes, capacitors... No fancy diagnostic procedures. 100 percent success, some of which may have been good luck. I was going to do an instructable on fixing 3 Heathkit resistor substitution boxes, but basically the same steps, the same success.