| Products > Test Equipment |
| HP 973A & 974A Multimeter Service Manual anyone? |
| << < (6/8) > >> |
| AMR Labs:
Your HP970A is a completely different DMM, its more like a hand-held probe type of instrument which was made by HP in the US. No relation to the HP971A-974A series made in Japan by Yokogawa. HP should have given the later multimeters a different model designation. |
| artag:
It's also a little earlier .. 1973 :) |
| Compulsive Fixer:
Greetings All My first post on this forum. I came here seeking information on the HP 973A multimeter after mine recently stopped working. I have had it for 25 years after winning it for renewing my subscription to a well known Australian electronics mag and it has been an excellent piece of equipment which has worked faultlessly all that time. A couple of weeks ago it stopped working so I pulled the back off and found the batteries leaking and covered with a blue deposit on the ends. I was rather surprised at this as I wrongly assumed that Duracells were reasonably leak proof. Fortunately the battery holder contacts cleaned up well with no signs of pitting or corrosion and I optimistically fitted new cells, but the meter powered up and presented a normal display as soon as I pushed the second battery into place and could not be powered down again using the ‘Power’ Button. The first thing I did was clean the switch contacts on the board and re-coat the button contact with a thin coat of powdered graphite rubbed in well but still no luck! So out with the magnifier and low power microscope to examine all of the tracks and components in the general area of the battery holder, but all looked good and well protected by the conformal coating. Using my cheapie multimeter, which I carry in the car glovebox, I was able to tediously trace a path from one of the switch contacts through a couple of resistors and diodes to the general area of IC7. The other contact carried +3V. My next suspicion was that some of the corrosive liquid from the cells had seeped in under the chips on that end of the board so I soaked just that end in warm water and detergent with some brushing, washed and soaked again in several changes of fresh water then into denatured alcohol and thoroughly dried with warm air. I had previously un-clipped the screen and was careful not to wet the rest of the board as I did not want to get water into any of the trimmers. All of this still failed to get the ‘Power’ button working, although all of the other functions of the meter seemed to be working normally. With nothing to lose I decided to look at the most likely culprit – IC7, a 74HC132A (a quad 2 input NAND Schmidt trigger) surface mount IC on the back of the board under the LCD screen as this had a clear path to the batteries through a hole for the battery holder locating pins. With the hot air rework gun I lifted this chip off the board and found it to be wet underneath with a film of pale green liquid. Close examination showed that the area around the solder pads had been masked off from the conformal coating but a small section of copper track from the pad for pin 11 to a via under the middle of the chip body had been dissolved away by electrolysis. Full of hope, I carefully bridged the gap in the track with a thin strand of copper wire and re-assembled everything but, alas, still the same problem. Once again I lifted the IC and re-examined the area under it with the microscope. The only slightly suspicious thing I could see was a tiny tab on the end of the pad for pin 10. There was no signs of any track leading from it to any other pad or via. Still convinced that I was on the right track I started drawing out the circuit around this chip but eventually gave up due to the number of fine tracks and vias in this area – it was going to take too long! So, here I am hoping someone can help with a circuit diagram or a photo of the tracks under this chip from a scrapped meter. I have spent quite a lot of time looking online for a low-cost replacement for this meter but nothing seems to compare, so I an really keen to fix this one. Any help or suggestions from forum members would be much appreciated. |
| helius:
--- Quote from: Compulsive Fixer on September 16, 2020, 04:20:03 am ---A couple of weeks ago it stopped working so I pulled the back off and found the batteries leaking and covered with a blue deposit on the ends. I was rather surprised at this as I wrongly assumed that Duracells were reasonably leak proof. --- End quote --- Oh, brother. I don't think schematics for this series ever appeared. However, they do have an unusual and distinctive feature, the soft power button. Most DMMs use a rotary switch position to turn off and disconnect power. The 973A has a key that switches power on and off: this means that it must have a path from the battery to the key, and a latching mosfet circuit that is switched on and off by the key. To facilitate latching it probably uses a capacitor as well. Another DMM with this design feature is the Protek 506, for which schematics are available and should be similar in this area. |
| Compulsive Fixer:
Thanks for that info helius, I had a quick look at the Protek 506 circuit (Hung Chang) and it appears to use the same IC so I will sit down and study it in more detail as soon as I get the time. The link to the Duracells was also most interesting. Mine were date stamped for Mar 2017 so not terribly old. They don't mention any guarantee on covering equipment damage but they were made in China so could even be counterfeits. |
| Navigation |
| Message Index |
| Next page |
| Previous page |