Power Designs Inc. Model 605 restoration

With eleven pow­er sup­plies already, I real­ly did­n’t need anoth­er one for the bench. Especially one from the ear­ly 60’s that was rat­ed at a max­i­mum of 6 Volts and 500 mA of out­put pow­er. But this one is a some­what rare Power Designs Inc. mod­el 605 pre­ci­sion pow­er source.

It ticked off most of my require­ments for a project.
Somewhat of an odd­ball or rare device — check
Little to no doc­u­men­ta­tion — check
Dirty, dam­aged, or obso­lete — check
Priced to sell because no one knows what it is — CHECK

The PD mod­el 605 was the pre­de­ces­sor to PD 2005 pre­ci­sion DC pow­er source. It has an out­put of 0 to 6 Volts con­trol­lable by two rotary switch­es and a 10-turn dial. The coax­i­al rotary switch­es have incre­ments of one Volt for the cen­ter dial and 100 mV for the out­er dial. The 10-turn dial poten­tiome­ter adjusts the out­put with a res­o­lu­tion of 100 µV per divi­sion. It uses a Bulova Electronics tem­per­a­ture con­trolled oven ref­er­ence pre­am­pli­fi­er for it’s pre­ci­sion reference.

All com­po­nent and wiring con­nec­tions are point to point using a hybrid style of eye­let and tur­ret con­nec­tion points. the con­nec­tion points are press fit into drilled holes in the cir­cuit board and sev­er­al top-hat style diodes are mount­ed thru drilled holes in the board also. The cable har­ness­es are laced neat­ly using a waxed linen cord with indi­vid­ual wires exit­ing the bun­dle at their con­nec­tion points.
The lat­est dat­ed com­po­nents installed in this unit includ­ing the ref­er­ence are from ear­ly 1964, mak­ing this unit 62 years old as of my writing.

I haven’t been able to find any doc­u­men­ta­tion or schemat­ics for this unit, but many of it’s cir­cuits car­ried over to the PD 2005 pow­er sup­ply which I did have doc­u­men­ta­tion for. When I received the unit it was miss­ing an impor­tant rear pan­el jumper across DC+, +Sense, and RV which sets the volt­age select­ed on the front pan­el.
After check­ing all the fus­es, trans­former wind­ings, rec­ti­fiers, and capac­i­tors it was time for it’s first pow­er up in pos­si­bly decades.

Well, it most­ly worked but the volt­age selec­tion con­trols were inter­mit­tent and non-repeat­able espe­cial­ly the sealed 10-turn poten­tiome­ter.
Luckily once out of the unit, the poten­tiome­ter was easy to dis­as­sem­ble by remov­ing it’s lock­ing ring which gave me easy access to the inter­nals. I end­ed up using Caig DeoxIT D5 clean­er and F5 which has lubri­cants to restore the prop­er oper­a­tion, and let it dry overnight before reseal­ing it. The rotary switch­es were cleaned with DeoxIT D5 which ful­ly restored their operation.

While wait­ing on an axi­al capac­i­tor order to arrive I did some more clean­ing and explo­ration of this unit. One item of inter­est was the sin­gle ST 6969 pass tran­sis­tor mount­ed on the side pan­el heatsink. I have nev­er seen this ST logo before.

The ref­er­ence network/ pre­am­pli­fi­er oven was man­u­fac­tured by Bulova elec­tron­ics div. and has an inter­nal cir­cuit board in the oven which includes a dif­fer­en­tial tran­sis­tor pair, two addi­tion­al tran­sis­tors, three Zener diodes, capac­i­tor, and assort­ed resis­tors. The oven heater oper­ates on 115 Volts AC and is con­trolled by an inter­nal ther­mal switch.

This mod­el used the same cir­cuit­ry as the PD 2005 for the reg­u­lat­ed ± 20 Volt inter­nal sup­plies. There are sev­er­al mod­i­fi­ca­tions rec­om­mend­ed for the PD 2005, so I also imple­ment­ed them on this unit which includ­ed plac­ing a 10 uF, 35 VDC tan­ta­lum capac­i­tor across CR9 the 20 volt Zener diode on the neg­a­tive sup­ply rail. Also rec­om­mend­ed in the PD 2005 ser­vice man­u­al is plac­ing a 0.01 uF, 200 VDC plas­tic film capac­i­tor across CR14 on the main board. The CR14 diode is the clos­est one to the ref­er­ence oven on the bot­tom side of the board. While I was at it I also includ­ed a tan­ta­lum capac­i­tor across the pos­i­tive 20 volt Zener reg­u­la­tor for good measure.

I was able to find a replace­ment chas­sis mount capac­i­tor for the main pow­er out­put cir­cuit. I end­ed up with a Vishay MAL205019222E3 2200 uF 100V sol­der lug capac­i­tor that fit the exist­ing mount­ing brack­et well but was quite a bit short­er than the orig­i­nal.
The out­put and rear ter­mi­nal capac­i­tors were also replaced along with the 8 uF capac­i­tors on the main board which were increased to 10 uF val­ues.
Heat-shrink was added to insu­late the exposed front pan­el AC ter­mi­nals to keep me from zap­ping myself when cal­i­brat­ing the near­by potentiometers.

The volt­age cal­i­bra­tion board is locat­ed just behind the front pan­el out­put ter­mi­nals. The upper poten­tiome­ter adjusts the 0 Volt set­point and the low­er poten­tiome­ter adjusts the 6 Volt out­put set­point. There are also sev­er­al fixed resis­tors on this board that can be jumpered or un-jumpered if the Zero poten­tiome­ter is insuf­fi­cient to bring the unit into calibration.

The Power Designs mod­el 605 cleaned up fair­ly well and is now ful­ly oper­a­tional and cal­i­brat­ed. It is an inter­est­ing bit of Power Designs Inc. his­to­ry that has been some­what lost to time but gave me some insight into lat­er mod­els, and was a fun project to work on.