Electronics > Repair

HP 1740A Channel B not working and flickering on A

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I picked up an HP 1740A oscilloscope recently. I replaced the fuse and it now turns on just fine, and somewhat works. However, channel B doesn't work at all (only the flat line on the graticule shows, even when probing the 1V CAL signal). As for channel A, it works except for TIME/DIV larger than .2mSEC, it flickers rapidly even when not proving anything, and the RESET light flickers along with the displayed signal.

I'm pretty new with all this. Any advice on what to check?


Firstly go over to the thread about restoration of a HP 1741 by forum member 'tekfan' and read that for any tips etc.
Its in the test equipment section, and has been recently resuscitated !. Next download the manual from the link I posted in that thread. That site should have the 1740 but the 1741 is very close for the most part.
 The power supply needs verifying to be operational first. And do be careful working on the live innards as there is dangerous voltages present, know what you're doing.
Actually the very first thing to do is check for loose or oxidized interconnections which could easily be the case with gear this old. Also dry joints are highly likely in the power supply PCB so check for those then measure the supply rails  etc.
 Once the power supply has been cleared of any issues then move to the vertical input stage and check the resisters around the the hybrid for any that have drifted high, this Will cause bias shifts in the DC coupled stages and could easily cause the loss of the input signal getting to the CRT.

Thanks for the reply! I had already checked the power supply voltages across the capacitors, and all six are very close to the reference values in the book (the scope actually came with the service manual). I just checked them again, and sure enough, they're all good.

What do you mean by the hybrid? I've been checking resistors on the "vertical preamplifier assembly" right near the input, and those seem fine, but I'm not really doing so intelligently just randomly checking values and comparing to the book... Any help with what exactly I should be looking for?

EDIT: I also just noticed that when switching to display B, the vertical position adjustment does not change the flat line position at all (it's above the graticule).

 The hybrid refers to the custom HP IC module which is A3A1 on the schematic page 8-19 (note ; unobtainium be careful measuring around it when powered, hopefully not already faulty!).
Measure the volts on TP9 and TP10 are they the same with no input signal?.  What voltage is on R36 and R61 wipers (vertical pos. pots). Does each side of those pots volts change when varying the pot position?.
 R64 and R66 100 ohm resistors, measure the volts on the end  that connects to pins 36 and 34 (of A1) wrt 0V are they the same for each channel when selected ?.
Measure the volts applied to pin 20 and then pin 1 of the hybrid amp when each channel is selected. These are the channel enable for A and/or B.

So I'm having some trouble figuring out where R36 and R61 are on the circuit board. I did however measure the rest.
With no input, I get 0V on both TP9 and TP10. With the CAL 1V into channel A, I get 0.045V on TP9, and 0 on TP10; and with the CAL 1V into channel B, I get 0.045V on TP10 and 0 on TP9. Looking at the diagrams, this seems to make sense to me.

Measuring the voltage from R64 and R66, on the ends that connect to pins 36 and 34, I get 11.67V on channel A, and identical on channel B. Presence or lack of input does not change anything. I'm not sure if that makes sense from the diagram, but it's at least reassuring that I'm getting the same on both channels.

I then measured the voltage on pin 20 and pin 1, which appear to be TP5 and TP7 respectively. Presence or lack of input does not change anything. I get 4.863V on TP5 (pin 2) whether channel B or A is selected. However, on TP7, I get 2.801V when channel A is selected, and 4.867V when channel B is selected.

Judging from the diagrams, those are the A and B on/off inputs into the hybrid. So it looks to me like the hybrid is never getting the signal to turn on channel B. Which is good I guess because it means the problem doesn't look like it's in the hybrid itself. Looks like the problem is somewhere in the circuit shown on 8-23. So thanks a lot for already allowing me to get this far!

I'll try to do some more diagnosing on my own and will update if I manage to find anything. But I'll also keep checking the forum for further advice.


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