Trouble shooting and performing modifications on a HP 3437A System Voltmeter from 1984.
The HP 3437A is a microprocessor controlled 3 and 1/2 digit, successive approximation voltmeter, capable of sampling voltages at rates up to 5,700 samples per second.
It includes chassis isolated input terminals, a wideband input amplifier, auto-zero, auto-polarity, sample and hold, and 100% overange on each of the input voltage ranges (.1 volt, 1 volt, and 10 volts) over a frequency range of DC to 1.0 MHz.
The HP 3437A had a catalog cost of $3,165 in 1986 which is equivalent to $7,275 in 2019 with inflation.
This HP 3437A was an Ebay in state auction purchase that cost 8 times more to ship than the final auction price. It was well packed though and arrived in great shape.
The complete auction description was ” HP 3437A SYSTEM VOLTMETER. Condition is Used. “, and included one picture of the front panel.
The front panel LED’s that were on in the picture included “Data Ready”, Invalid Pgm”, “Listen”, “Talk”, “Remote”, “Binary Prgm”, “Enab RQS”, “10V”, “Int”, “Hold/Man”, and “ASCII”. None of the 7 segment displays were illuminated.
The price was right, so I put in a low bid and figured that it would be a fun winter project. I was the only bidder and picked it up at the starting bid price of $3.99. My guess was that the blank display, and the strange combination of status LED’s scarred most other bidders away.
Before turning it on a thorough inspection was in order. I always like to check for loose parts, and cables from shipping, along with physical signs of burnt parts and damage before turning it on for the first time. I also check the transformers, rectifiers, and capacitors in the power supply section for shorts, and general operational specifications. All checked well except for the transformer primary, which seemed to be a bit higher in resistance than I expected. Maybe some dirty voltage selection switches? Nope.….
The selected voltage on the switches was 240 volts.
After selecting 120 volts and replacing the fuse with a 500 mA rated part, I was ready to power the unit up.
The system came up with a functional display and working push buttons. Power supply voltages were checked and were all within specifications. The only thing that wasn’t working was the system fan.
HP for some strange reason decided to use a 3‑phase fan, and special drive circuitry with 35 discrete components to power it at a fixed speed.
Some of the HP 3437A units had failing fans, and HP issued a service note 3437A-10 to address replacement of the 3‑phase AC fan with a DC fan.
I ended up using a Sunon axial 60x20mm 12VDC fan PN# MF60201V3-1000U-A99
The original screws were too short for the new fan, so I used two stainless steel 4–40 by 1″ phillips machine screws which fit perfectly. Only two screws are needed as the other two holes are used for attaching the external air filter.
For my unit the replacement required a 22 ohm 2 watt resistor to be fitted across the collector and emitter pads of the removed Q202 transistor, and removal of R204.
Instructions for the fan replacement and circuit board modifications start on PDF page 174 of the “Operating and Service manual” linked at the end of the blog post.
The 3437A comes standard with 3 lug Triaxial input connectors on the front and rear panels. The price for triaxial connectors and cables is currently out of my hobby budget so I opted to replace the front connector with a standard 2 lug isolated BNC connector.
I know the benefits of using triaxial connections, but for my general use I am willing to trade those off to be able to use my existing cables.
Replacing the connector is not easy and took me around 1 hour to complete, but is completely reversible if I do ever decide to replace the triaxial connector.
With the new fan and connector installed, functional, and calibration checks went smoothly with everything well within specifications.
After a bit more cleaning and picture taking I placed the covers back on the unit and added it to my test equipment shelve.
Additional pictures of internal components:
Click on thumbnails for full sized images.
Hi-Resolution photos on Flickr — Album HP3437A