Hewlett Packard 8012B Pulse Generator

Cleanup and repair of a Hewlett Packard (HP) 8012B pulse gen­er­a­tor pur­chased on Ebay as “Untested does not pow­er on, sold as is”.
The 8012B was orig­i­nal­ly pro­duced by the HP Gmbh Boeblingen Divison, Germany, but this lat­er mod­el was made in USA. It pro­vides vari­able tran­si­tion times down to 5 ns and the rep­e­ti­tion rate can be adjust­ed from 1 Hz to 50 MHz.

Hewlett Packard (HP) 8012B pulse generator

I was able to pick up this sup­pos­ed­ly bro­ken 8012B on Ebay for under $15 USD, plus about the same amount for ship­ping. As usu­al the front pan­el of the unit was cov­ered with label residue and a ID label, plus plen­ty of pro­tec­tive grime. There was only one bro­ken knob, but oth­er than that the unit looked in over­all good con­di­tion for it’s age.
After receiv­ing the unit I gave it a quick inspec­tion for items such as the cor­rect fuse, back pan­el volt­age set­ting, trans­former pri­ma­ry and sec­ondary resis­tance read­ings, pow­er sup­ply fil­ter capac­i­tor in-cir­cuit checks, pow­er sup­ply diode checks, and an over­all visu­al inspec­tion of all boards and con­nec­tions.
Everything checked out great, so I pow­ered it up…
It was very unevent­ful. There was no “Line” pow­er indi­ca­tor lit up on the front pan­el or any oth­er signs of life, but my Watts up? Pro pow­er line meter showed a steady 390 mA being used at 120 VAC. I quick­ly turned on my TDS 340A oscil­lo­scope, and after adjust­ing some front pan­el set­tings I had a pulse wave­form on the dis­play. All of the front pan­el switch­es were very inter­mit­tent and noisy, but the unit seemed to be working.

HP 8012B A6 board with new capacitors

As soon as I pur­chased the unit I ordered new elec­trolyt­ic capac­i­tors for the pow­er sup­ply, since I knew that this was a mid-80’s unit. Date codes on the inter­nal com­po­nents indi­cate that this unit was pro­duced in ear­ly 1986, and sold for $1,500 in 1982 which is equiv­a­lent to $4,450 in 2022 dol­lars. I used a pair of Nichicon 1,500 uF 63 volt axi­al capac­i­tors rat­ed for 105 degrees C for C29 & C30, and had a new-old-stock Sprague 100 uF 25 volt capac­i­tor for C45.

Crusty film capac­i­tors from A5 and A6 boards

I also end­ed up replac­ing all the film capac­i­tors on the A5 and A6 boards, because they looked a bit crusty and sus­pi­cious. After removal, all of the film capac­i­tors checked out good and were well with­in tol­er­ance, but I had some high­er qual­i­ty spares and still was­n’t impressed with the epoxy dete­ri­o­ra­tion of the orig­i­nal components.

HP 8012B LED replace­ment of front pan­el “Line” indicator

Up next was the non-func­tion­ing front pan­el “Line” indi­ca­tor. The orig­i­nal lamp was a T1 28 volt incan­des­cent bulb. The lamp is pow­ered by the plus and minus 17 volt pow­er sup­plies for a total of 34 volts DC. There is a 270 ohm drop­ping resis­tor R33 in series with the 28 volt bulb. I replaced R33 with a 4K ohm resis­tor, and used a green T‑1 LED as a replace­ment. The cath­ode was sol­dered to the plat­ed thru-hole ring and anode was attached to the new resistor.

HP 8012B new LED “Line” indicator

The only adjust­ments that I made were to the trim-pots for the +/- 17 volt pow­er rails. As per the man­u­al instruc­tions I set the pow­er rails as close to 17 volts as pos­si­ble using the rec­om­mend­ed test points on A6.

HP 8012B Power sup­ply tran­sis­tors, adjust­ments, and test points

The volt­ages in oth­er areas of the A6 board were very close to the set volt­ages, but when check­ing the A5 board I noticed a drop in volt­age on the pow­er rails. Some of this drop is to be expect­ed as the A5 board is fur­ther away from the pow­er source. Upon clos­er inspec­tion of the A5 board, I noticed the use of some very small cir­cuit traces from the edge card con­nec­tions.
My fix was to use some sil­ver coat­ed PTFE wire to jumper the lim­it­ing cir­cuit traces to pro­vide an addi­tion­al and bet­ter cur­rent path for cir­cuits deep­er in the A5 board. I did this for both plus and minus pow­er rails.

HP 8012B A5 board with pow­er rail jumpers

The next hur­dle was to gain access to the front pan­el slide switch­es.
To prop­er­ly clean the switch­es the front pan­el needs to be removed, which is not an easy task. The out­put BNC needs to be unsol­dered, and all poten­tiome­ter knobs and mount­ing nuts should be removed along with numer­ous Pozidriv screws on the back of the pan­el. I then care­ful­ly removed the slid­ing con­tacts and car­ri­er knobs where pos­si­ble. The grease had become extreme­ly vis­cous over time and took some time to remove with iso­propyl alco­hol. After a thor­ough clean­ing I used used DeoxIT D5 for the enclosed switch­es and for a final clean­ing of the dis­sem­bled slide switch­es as it also pro­vides some lubri­ca­tion after it dries.
I was so frus­trat­ed while remov­ing the front pan­el I for­got to take pic­tures. I don’t want to do that again any­time soon.

HP 8012B Amplitude vernier con­trol dual potentiometer

The remain­ing issue was a very hard to turn Amplitude vernier con­trol. This is where the bro­ken knob resided, and was prob­a­bly the rea­son for it’s demise. I used some low vis­cos­i­ty sewing machine oil where the shaft enters the poten­tiome­ter, and some DeoxIT D5 inside the dual 50 ohm poten­tiome­ter, then rotat­ed the shaft. This was done sev­er­al days in a row, until the shaft turned freely..

HP 8012B A6 board heatsinks and trim potentiometers

I ran through all the per­for­mance checks in the ser­vice man­u­al, and did­n’t need to make any adjust­ments. The HP 8012B is not an easy unit to work on, but it per­forms very well.

HP8012B front pan­el dur­ing test­ing with tem­po­rary knob

While doing a final inspec­tion of the pulse gen­er­a­tor, I decid­ed to replace the 36 year old Corcom pow­er line input fil­ter with a new Schurter fil­ter #5110.0333.1.
While I had the pow­er leads dis­con­nect­ed, I also added some heat­shrink tub­ing around the exposed con­duc­tors on the fuse holder.

HP 8012B trans­former and pow­er input section
HP 8012B replace­ment pow­er line fil­ter and insulation
HP 8012B back­plane board A7
HP 8012B back pan­el with added extra heatsink

Here are some saved images from the TDS 340A oscilloscope:

HP 8012B set to 10 MHz 10 ns pulse width at 3.8V
HP 8012B set to 100 kHz 1 us pulse width at 5 volts
HP 8012B dou­ble pulse mode at 5 volts

I am cur­rent­ly sav­ing up my cof­fee mon­ey to hope­ful­ly acquire a HP 8082A which fea­tures tran­si­tion times over a range of 1 ns to 5 ms, and pulse rep­e­ti­tion rates as high as 250 MHz.

Higher-res­o­lu­tion images avail­able in my Flicker HP 8012B album

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