Hewlett Packard HP1740A Repair

4 minute read
This post is part of the series 'Test Equipment':
  1. Test Equipment for Test Equipment
  2. Two Adret Signal Generators
  3. Hewlett Packard HP1740A Repair

February 2023

I finally got around to repairing the HP1740A that has been languishing in my loft for many years. I was actually digging out a nice little Trio-Kenwood CS1830 oscilloscope to pass on to a deserving friend, when I decided on the spur of the moment to re-visit the 1740A and see if I could get it working.

I remembered something about 'noisy' traces, and someone suggesting that might be symptomatic of a faulty hybrid driving the Y-Plates. If that turned out to be the case I would probably scrap it.

However ... The fault turned out to be nothing more than a minor adjustment issue. Last time I looked at the 1740A, I was working blind ... no service manual. Some years ago, I accidentally bought a 1740A service manual, thinking I was buying a manual for my 1745A. Last year my 1745A developed a fault after not being switched on for several months, which was nothing more that a 'popped' capacitor on one of the vertically mounted boards. So, Since the 1740A had been in the loft for many years, there was no way I was switching it on from 'cold'. I ran it up slowly via a variac, and as expected, the EHT power supply fizzed a bit. But, there was a trace, two actually ... and the Y-amplifiers were working. However the Focus was ineffective, as was the Intensity control ... I couldn't turn it all the way down.

There was a rather sorry looking bulging 1500uF capacitor on the Low Voltage PSU board which I replaced. This didn't fix the problem though, but thanks to my Ebay error many years ago, I now had a service manual! ... And in the Adjustments section of the manual, para 5.41 is titled INTENSITY LIMIT ADJUSTMENT, which sounded promising.

Bingo! A small tweak of A15R2 resolved the brightness and focus issues ... and a slight tweak of the Astigmatism control on the rear panel cleaned up the traces. The EHT unit was still fizzing slightly but I put that down to coming down from a cold loft into a warm environment. So I switched it off and left it overnight with the top and bottom covers removed and the lid off the EHT unit.

The next morning I powered it up via the variac again ... no fizzing. That's always a good sign. So I removed the variac and powered it up direct from the mains ... silence ... nice clean traces, and a quick run through of the controls and a few tests appeared to show that the 1740A was working ...

HP1740A - 100MHz 2-CH Analogue Oscilloscope

... that was until I noticed that Channel-A was more sensitive than Channel-B. Investigation revealed that the on-board 50-ohm termination for channel-A was open-circuit. This turned out to be just a dirty contact, quickly fixed with a well aimed squirt of De-Oxit. But the channel was still somewhat over-sensitive.

Further investigation revealed that this fault was curiously restricted to ranges involving a '1' or a '5', yet worked perfectly on ranges involving a '2'. This revelation narrowed the fault down to two switches; Switch-I and switch-J which along with Switch-H, control the gain of the A3 Vertical Preamplifier Assembly.

Sounds simple to fix, but sadly it isn't. HP employed hybrid amplifier modules in these oscilloscopes and the one on the Vertical Preamplifier Assembly is faulty. I temporarily replaced it with the hybrid out of my 1745A and it worked perfectly. So now I need to find a replacement hybrid ... HP Part Number 5081-3030.

Coincidentally, as well as the bulging 1500uF capacitor on the PSU board, C11 (100uF) on the A8 Main Sweep Assembly was also showing signs of distress, and thus replaced. This is the same capacitor that failed on my 1745A last year.
LeCroy 9400 - 125MHz 2-CH Digital Oscilloscope

As far as I can discern, the HP1740A and HP1745A are functionally identical, with the only difference being the 1745A's slightly larger display. I also have a LeCroy 9400 digital oscilloscope (right) which is a beast of a machine, and although it works very well for its age, and is far superior to the Tektronix 7D20 (above), I still prefer analogue oscilloscopes when it comes to examining fast edges on waveforms. There's something reassuring about real-time displays rather than viewing something that has been sampled, digitized and spent time in a buffer before being sent to the display as a representation of what is being examined.

Hewlett-Packard, HP1740A