Dealing with the gate inputs is easer said then done. On this one they tied the inputs together at the board level under the IC socket. I am starting to think getting this running will be a redesign of the clock. If I shoot out 1Mhz at pin 8 at a TTL level that should get the unit up and running again. So I might be in to making an adaptor board. That can use the socket to get 5 Vdc and output the 1Mhz. would most likely add to the stability as well with modern parts.
Zen
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[email protected]Subject: Re: [TekScopes] Sick TG501 Main 1Mhz ref not running.
I reasoned that this was a really poor way of building an oscillator and that, since TTL really isn't a good analog part, circuit operation could be dependent upon the individual part's characteristics. When I build oscillators from logic parts, I usually use Schmitt trigger parts; Tektronix didn't. 74HC132, 74hct132 or 74ls132 might work like a chram. Not wanting to reengineer the Tektronix circuit (and parts of it using the other 2 sections of the part), I took the path of least resistance and dug around in my junk bin of little used ancient parts and came up with another 74L00. The part I used was old - possibly older than Tektronix's part. The circuit sprang to life and that was good enough for me.
Crystal oscillators can be tricky since individual crystal characteristics can vary widely. You might be able to get it to run by playing with the capacitances; however, you might not be able to calibrate the frequency. Try lifting one leg of C105. A 74HC00 might be interesting to try. You can always try an LS00 or an HCT00. Note that you can reduce capacitive loading a bit by lifting one input of each gate and conecting it to +5V.
