Work still in progress.
Yesterday I put a normal metal film resistor in parallel with the other two (R65/R68). Measured total resistance now is 1,33 ohm. OK.
Behavior is almost unchanged. With +15 V on the positive supply the main pulse appears only with R90 fully CW.
A strange thing I noticed: with the unit just powered up the pulse width initially is approx 500 ns or so.
While the unit warms up, the pulse width gradually decreases until it disappears completely after 1 or two minutes.
To make it reappear I have to decrease the supply to 14.5 V.
Suspecting some thermal drift, I tried to blow (with my breath) over the zone around R109 ( the resistor in series with the 5.1 V zener): the pulse reappears except to shrink in duration and then disappear again within 30 sec.
Oh yeah, in that pcb zone there are the resistors R106,R107 and R108 which generates the -3 V and - 2.5 V reference voltages. Measured, they are 3.1 and -2.6 respectively.
I don't think Tek made such a critical temperature circuit. I suppose that the optimal operating conditions are reached after a few minutes of warm-up and that the engineers have taken into account the heating produced by the 85 ohm resistor for the zener polarization.
Resistor values in the circuit area around Q86, Q88, Q80 are all in the range.
I will patiently check all the waveforms by comparing them with those shown on the diagram and I will check the voltage values on the basis of Q86 and Q88 also.
Varying the voltage between 13 and 15.5 V I noticed another phenomenon: the DURATION of the main pulse first decreases, then abruptly increases and then decreases again until it disappears completely at 15.1 / 15.2 V. Seems to be a timing problem, just like Albert said.
To better describe this phenomenon, a video would be needed. If I can do it, I'll upload it on the TekScopes forum.
Max
