HP1740A Delay Calibration

[tombi]

Hello,

I have a very annoying problem with a 1740A. The unit had a couple of minor faults that I repaired but now I am trying to calibrate it and stuck on a step.

Adjustment step 5-51 has you set the delay to 0.2 and then line up the bright dot (delayed timebase dot) with the small graticule 2mm from the left by adjusting A7R169

Unfortunately I have A7R169 at the minimum and I can't get it there. It is only slightly off (maybe 2mm - one minor graticule line).

So should I be able to adjust the delay to zero or is there supposed to be a small gap? I would think I should be able to if a delay of 0.2 is supposed to set it to  2mm from the start. Anyway I can't and the offset seems relatively consistent across the screen.

I checked TP4 on A7 and the voltage is around 0.9 or 0.8 when the delay is turned to zero. The manual indicates it should be 1.3 to 11.3 so this seems already lower than they expect.

The sweep signal looks a little low in that it starts at about 800mV. I thought this might be why this is happening.

So I followed the setup on the A8 schematic and stopped the sweep to take voltage measurements. They are mostly ok except that collector of Q3/Q13 is -1.5V instead of 1V as indicated. I checked other places like Q4 and Q7 and these voltages look mostly ok. TP2 is right also.

C11 had been replaced on A8 before I got the unit. I replaced it again (with a 100uF capacitor as the previous person put a parallel combination of around 63uF in). Otherwise I've not changed anything here.

I checked the opamp and that manages the integration loop and it looks Ok (inverting/non-inverting voltages match).

Any idea where to look? Is that -1.5V likely to be the cause? If so what might be the cause of that?

So service manual is here

Thanks,

Tom

[David Hess]

If the delay is calibrated like Tektronix does it in most cases, then the delay's mechanical dial is suppose to be offset from the 10 turn potentiometer's "zero".  Not coincidentally the offset is suppose to be about 0.2

[tombi]

Ok are you thinking this could be a mechanical issue?

I have an old 475 actually and the 1740 dial is different - On the 1740 when you turn it to zero, the outer dial zero is half a value out. When you turn the inner dial to the 1 position the outer dial clocks to zero. Also when you turn the inner dial round a turn, the outer doesn't clock to 1 until the inner gets goes past 9 and gets to zero.

This makes me think the knob may have been removed and re-installed incorrectly.

Thanks - that sounds like a good line of inquiry to follow!

Tom

[tombi]

BTW - I gave up. The delay is basically 0.1 out (I adjusted it to be exactly 0.1 out).

I completed the rest of the calibration procedure with no issue and the scope is functioning pretty well.

So I am calling it done.

Tom