HP 1740A scope no trace fix

[ddelectrical]

Hi all, great news! My HP1740A scope had a missing trace. I finally found that Q3 on the Vertical amp board, A5, was open circuit. I replaced it with a 2N3906, (e and b legs crossed and insulated as the pin config is different to original), and it works fine. The original transistors were marked with HP in house code and was, I think, a 20V pnp. The 2N3906 is a 40V type and works well. Trace is crisp. hfe original = 180, hfe new part = 300.

The hybrid unobtainium alien technology ic's are both fine!

There are quite a few threads here on this problem so I hope this points to a cure for those with the same fault.

All 4 transistors in the main vert amp are now the same type. It is possible that the original ones had a slightly higher frequency rating but these new ones are 200MHz rated and this is good for most of the work I will be doing with this scope.

BEWARE: the picture in the service manual shows A5Q3 in the wrong orientation!!!! If you look closely at the actual pcb you will see a little symbol of the correct position for this component. A5Q1 is correct.

If one or both of these transistors has shorted the section in the manual about using external trigger view to ascertain if pre or main amp has failed does not apply. In my case one was open cct so no visible trace could be seen anyway.

Note: replace both transistors if one has failed to ensure trace balance.

Good luck!

[stellarpower]

Hi there,

I'm new here and think I have the same problem with my scope, I was wondering whether I should start a new thread or post here, please could you tell me which would be preferable?

By no trace I assume that you have a scanline but no vertical deflection, is that right? Mine starts up okay, (there's a metallic click on first power on though, not sure about that), but after about a minute there's no trace and neither of the vertical position pots will budge the trace. My electronics knowledge is a few years old and really basic, I actually bought the scope as I thought it'd help me learn better but it had this error intermittently a few years ago, has been sat in a cupboard since, and now I have a project for which I need it, it's non-functioning!

Thanks for your help!