Saturday, June 8, 2013

EICO Model 377 Revisited

Recently, as I was working on an audio amplifier project, the time came to test it with a frequency-controlled input signal in order to determine if it handled tone control properly. This was just the job for my recently-restored EICO Model 377 Sine and Square Wave Generator. Unfortunately...
... when the unit was powered up and allowed to warm sufficiently, I was unable to get any signal out of it at all. Zero. Zilch. Nada. Flatline on my oscilloscope.

Once I got to a good stopping point with my amplifier, I took the time to examine the Signal Generator a little more closely.

After taking the case off and probing (carefully! remember the 'one hand rule'!) various points within with my meter and my scope, I was able to determine that there was no signal being sent to the output tube. (For reference, the schematic can be found here.) The input to V-4 had nothing but some very low-amplitude noise on it.


Well, I backed up a little and looked at the first tube in the oscillator circuit, V-1. Specifically, I looked at the cathode, pin 5. The voltage there was 14 VDC, about 11 Volts higher than the schematic indicated. Odd. This pin connects to a small light bulb which doesn't light up, but provides the negative resistance used to form the sine wave. I wondered if perhaps the filament in the light bulb had broken, so I turned off power to the unit and gave it a minute to discharge the caps. Then I started to remove the bulb to check its continuity out of the circuit. As soon as I touched it, it nearly fell out of the socket. Aha! I screwed the bulb back down in the socket firmly and reapplied power to the unit.

Everything works again. Success!


~ eW

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