Checking out a HP 3551A sold as “For parts or not working, will not power on”, that I actually purchased for parts.
My original intent on purchasing this unit was to salvage the knobs and connectors for repairing some of my other HP gear. I did manage to swap out four of the knobs before my curiosity got the better of me.

The unit was well packed when it arrived, but from the outside view of the case I could tell it had a rough life. There were several large dents in the aluminum case, which happened long before its most recent travel to my house. All the knobs and jacks were dirty, but in excellent condition, so the knobs were immediately removed, cleaned, and swapped out.

So why did the seller decide that this was a parts unit, and not functioning?
I started with my usual checks such as:
- proper line voltage selection — Correct
- power line fuse correct amperage — Correct but BLOWN
- transformer main and secondary resistance checks — OK
- power rectification diodes — OK
- in-circuit capacitor checks — OK
- voltage regulation pass transistors — OK
- on-board fuses — OK
- aluminum case dent causing short — YES
It seems that the case damage had caused a short to the collector on one of the voltage regulator pass transistors. That in turn caused the 250 mA line fuse to perform it’s duty to save the power supply circuitry.
After a bit of banging with a hammer, and some twisting metal using some smooth flat jawed pliers, the case was restored close to it’s original shape and the short removed, Several layers of Kapton tape were applied just for good measure, because you never know if the dent may reappear.

Before reconnecting the batteries to the power supply I thought I would check to see if they had any charge left in them. I was somewhat surprised that all three sets had a few volts still in them. The two 12 volt packs immediately began charging when connected to my Power Designs 6050C power supplies. I had no such luck with the 5 volt 4‑cell 2 AH battery pack.

I was able to replace the original pack with a new 4.2 AH pack and attach it to the original mounting plate. After the packs had charged for many hours I reconnected the pack connectors to the power supply, reconnected the front panel and main board assembly, and replaced the blown fuse.
The unit was powered up using the battery power selection button first and worked perfectly, I then selected the ~AC power button and plugged in the line power cord. Again everything worked as it was designed.

While photographing the boards I came across this unusual potentiometer.
The send frequency adjustment knob on the front panel has an unique adjustment operation. Over a range of 270 degrees rotation in either direction it operates in a fine adjustment mode, and when it comes to the end of 270 degrees, it continues in a coarse adjustment mode and moves the original 270 degree fine range to the new coarse position. There is approximately a 10:1 ratio between the coarse and fine movement vs resistance change.

Looking at the diagram on the back of the 50K ohm potentiometer, it shows three wipers and two separate rotary resistors. I found this very interesting and found the patent #3,380,010 by M. W. De Young and Elmer T Johnson dated 1968, assignor to John Fluke Mfg.
It looks like the same potentiometer was also manufactured for HP by Precision Control Mfg. Co. of Seattle, WA.
It is a very different potentiometer in that it combines a coarse adjust and fine adjust potentiometer using a single non-coaxial shaft, and solves the issue of a fine adjust Vernier being at the end of it’s travel in a dual or coaxial fine/coarse potentiometer system.

Unfortunately, I could not find a schematic for this model, but it seems that the top main board contains most of the digital logic circuits, along with the speaker driver and controls.



The bottom board consists of analog circuits and the interface for the telecommunications send and receive circuits.

The A2 display board uses red seven segment LED displays and supports 3.1 digits and +/- polarity indicators. It is used for displaying signal frequency in kHz, noise, and signal levels in dBm & dBrn.

Perpendicular to the A3 analog board is the A4 noise filter board.
Now that I have the unit working, I am not quite sure what I want to do with it. I don’t really have a need for a transmission test set, but it does have an audio range frequency counter, and a very nice sine wave generator good for 40 Hz to 60 kHz, along with an AC level meter all included in a portable battery operated package.