Hewlett Packard 3551A Transmission Test Set

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.

HP 3551A front pan­el after repair, quick cleanup, and knob swap

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.

HP 3551A pow­er sup­ply assem­bly attached to case

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.

HP 3551A case with bat­ter­ies reinstalled

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.

HP 3551A replace­ment 5 volt 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.

HP 3551A send fre­quen­cy adjust­ment hybrid potentiometer

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.

HP 3551A A1 top board

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.

HP 3551A A3 bot­tom board
HP 3551A front pan­el inter­nal jacks and switches
HP 3551A A3 board fre­quen­cy gen­er­a­tor section

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

HP 3551A A2 sev­en seg­ment LED dis­play board

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.

HP 3551A noise fil­ter board A4

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.

Leave a Reply

Your email address will not be published. Required fields are marked *