Nice piece of test equipment with the optional ovened crystal reference.
This was an intermittent issue, where the gate and external standard LEDs would blink wildly and the counter would lock up. Tapping the case would bring it back to life and it tended to settle down after being powered on for a while. However it would still fail at random intervals.
Gerry Sweeney did a video on a similar counter (model 1999) where he concluded that the reference oscillator connector was at fault. Things weren’t so simple with mine!
Taking to the oscilloscope and circuit diagram soon relieved some strange goings on.
The internal reference is used as a source to drive the counters electronics. Even when applying an external reference signal the fault was still occurring. A nice clean sine wave could be seen on the scope from on pin 4 of PL14. IC17 provides signal conditioning and converts the sine wave to a square wave clock. If the fault was present, probing any point on IC17 would reveal a 4 kHz signal superimposed on the negative portion of the wanted 10MHz reference.
Checking the -5.2 V supply rail confirmed that this 4kHz parasitic oscillation was coming from the regulator formed of IC8b Q15 & Q16. A bit of carefully aimed tapping and prodding pinpointed C59, the main reservoir capacitor for this stage, starting or stopping the parasitic almost on demand.
On the bench, as you can see, the ESR on this capacitor is very variable. Tapping or shaking it caused the value to change dramatically.
A new electrolytic has been fitted and the counter is now working perfectly. 🙂