Beckman 7360R Universal EPUT® and Timer Restoration

Several decades ago I owned two Beckman / Berkeley mod­el 7360–20 Universal EPUT® and Timer’s. Because of their size and weight and an upcom­ing move, I gave them away to a friend. In the past few years I have some­what regret­ted that deci­sion, even though it was prob­a­bly for the best. So when I saw this 7360 up for auc­tion, I decid­ed to make an offer on it. I was able to pur­chase it at a rea­son­able price, but as expect­ed the ship­ping was a bit high due to it’s size and weight.

Beckman 7360R Universal EPUT & Timer

So what is a “Universal EPUT® and Timer”?
It is an elec­tron­ic counter which per­forms a wide vari­ety of fre­quen­cy and time mea­sure­ments. The EPUT® part of the descrip­tion is an acronym for “Events Per Unit of Time“
EPUT® Meters auto­mat­i­cal­ly count and dis­play the num­ber of events that occur dur­ing a pre­cise time inter­val, also known as Frequency.
The 7360R will mea­sure fre­quen­cies up to 1 MHz, and time mea­sure­ments in one μ‑second units, along with time inter­val between events on sep­a­rate chan­nels, man­u­al­ly con­trolled event counter, time inter­val and phase dif­fer­ence on sep­a­rate chan­nels, extend­ed Scan inter­val, and self-test check.
In my opin­ion, pret­ty impres­sive for a 52 vac­u­um tube and 70 neon bulb piece of test equip­ment designed in the mid 1950’s. This unit was pro­duced in the lat­er part of 1962 going by the date codes on some of the components.

Beckman 7360R con­di­tion before cleaning

The unit arrived in good shape con­sid­er­ing it’s weight, with only it’s top cov­er com­ing loose dur­ing trans­port. All of the tubes were still in their sock­ets with no visu­al dam­age.
The orig­i­nal pow­er cord had been sev­ered, so I was fair­ly sure that it had not been pow­ered up in many years or pos­si­bly decades. The unit had been stored in non-opti­mal con­di­tions and was very dirty with rust spots and cor­ro­sion in sev­er­al areas. Time to start the long ardu­ous task of clean­ing inside and out.

Beckman 7360R after clean­ing top down view

The dirt­i­est area was the divider adjust­ment brack­et which is locat­ed on top of the unit and not pro­tect­ed by a cov­er. I end­ed up unsol­der­ing the test point jacks and remov­ing the poten­tiome­ters and wiring har­ness so that the rail could get a good scrub down. While I had the brack­et off I also removed the side pan­el to clean in some hard to reach places.

Beckman 7360R bad Hopkins MPA film capacitors

During my clean­ing I came across sev­er­al of these blue Hopkins type MPA film capac­i­tors that had devel­oped cracks in their out­er shells. Most of these were across the +120 volt pow­er cir­cuit feed­ing sev­er­al dif­fer­ent point of use cir­cuits on the unit.
After see­ing this I decid­ed to replace all the elec­trolyt­ic and paper dielec­tric film capac­i­tors in the unit.

Selenium full-wave bridge rec­ti­fi­er left side of image

Another area of con­cern was the Selenium full-wave bridge rec­ti­fi­er used in the -110 volt pow­er sup­ply. Selenium rec­ti­fiers do not age well and this one need­ed to be replaced. The issue when replac­ing Selenium rec­ti­fiers is their high­er volt­age drop when com­pared to mod­ern sil­i­con rec­ti­fiers. I end­ed up chang­ing the two par­al­lel 22 Ohm 2 watt resis­tors (R111 and R112) with two 43 Ohm 3 watt resis­tors in par­al­lel for a total of 21.5 Ohms vs the orig­i­nal 11 Ohms. It took me sev­er­al tries to find the right com­bi­na­tion as I start­ed off with a high­er resis­tance so as to not to end up with too high of a volt­age initially.

New Silicon full-wave bridge rec­ti­fi­er replace­ment for Selenium

I used a VS-KBPC608PBF Vishay Silicon rec­ti­fi­er rat­ed at 800 volts and 8 amps which allowed me to mount it in the same loca­tion and use the case as a heat sink.

Beckman 7360R AC input, trans­former, and car­tridge rectifiers

The two fuse hold­ers for the 117 volt AC line were miss­ing their caps and so were replaced with new Bussmann HKP-E-HH fuse hold­ers, along with a new pow­er cord.
The +120 pow­er sup­ply uses four 1N1084 car­tridge style Silicon rec­ti­fiers rat­ed at 500 mA and 400 volts each in a full-wave bridge configuration.

Beckman 7360R main trans­former con­nec­tion legend

I also moved the main trans­former pow­er input ter­mi­nal from #1 (117 volts) to #2 (126 volts) as my aver­age house volt­age is usu­al­ly 125 volts. The trans­former was man­u­fac­tured by Oeco Corp. for Berkeley Scientific which is a divi­sion of Beckman Instruments Inc.
There are two 6.3 volt sec­ondary AC wind­ings. The 15 amp 6.3 volt wind­ing is ground­ed on pin #8 and the 12 amp 6.3 volt wind­ing is con­nect­ed to the minus 110 volt DC sup­ply on pin #10.
As an ini­tial test I dis­con­nect­ed the 97 and 176 volt AC sec­ondary wind­ings from their respec­tive rec­ti­fiers so that I could check the fil­a­ments on all the tubes.

Beckman 7360R tube fil­a­ment ini­tial testing

All the tube fil­a­ments worked with no issues.
After a bit of a wait for my parts order it was time to get the select­ed capac­i­tors replaced.
I opt­ed for leav­ing the two orig­i­nal elec­trolyt­ic 125 uF at 350 volt can capac­i­tors (C101 & C106) in place and iso­lat­ing the pos­i­tive ter­mi­nal on both capac­i­tors. C106 on the neg­a­tive 110 volt pow­er rail has a com­plete­ly iso­lat­ed can from case ground.

Working on replac­ing the main fil­ter capac­i­tor on the minus supply
Capacitors replaced on input chan­nels and con­trol circuit

The 1 MHz fre­quen­cy divider cir­cuit had 5 crusty com­po­nents which at first I thought were leaky capac­i­tors, but they turned out to be 3.9 Meg Ohm 1 watt 1 % pre­ci­sion resis­tors used in the adjust­ment of each decade divider. Most like­ly they orig­i­nal­ly had a con­for­mal coat­ing that had dete­ri­o­rat­ed over time. They were replaced along with (C612, C615, C617) the 0.0047 uF, 0.047 uF, and 0.47 uF paper film capacitors.

Crusty 3.9 MOhm 1 watt 1% resis­tors in fre­quen­cy divider
Capacitors and 3.9 MOhm resis­tors replaced in fre­quen­cy divider

Now that all the nec­es­sary capac­i­tors had been replaced it was time to tack­le the DCU’s (Decimal Counting Units). The Beckman 7360R uses two types of DCU’s, the 785A which is capa­ble of count­ing at a 1.1 MHz rate and only used in the A207 posi­tion as it’s cas­cad­ed out­put is one-tenth of it’s input rate.

Beckman 785A Decimal Counting Unit (DCU) top view
Beckman 785A Decimal Counting Unit (DCU) side view

The 785A cas­cades into six 775 DCU’s in series. The 775 has a max­i­mum count­ing rate of 120,000 counts per sec­ond and like the 785A has a direct read­out from 0 to 9 using neon bulbs to back­light the num­ber being displayed.

Four Beckman 775 Decimal Counting Unit’s (DCU) installed in 7360R

Both types of DCU’s uti­lize four dual tri­ode vac­u­um tubes which make up four flip-flop cir­cuits per DCU. The 785A uses type 12AV7 tubes while the 775 DCU uses type 5963 tubes.
Other than a good clean­ing no oth­er main­te­nance was need­ed for the DCU’s. I did label each DCU so that I could get them back into their orig­i­nal sockets.

All tubes and DCU’s back in their sock­ets and ready to test

With all the tubes and DCU’s installed it was time to per­form a full sys­tem pow­er up test. I used a Variac to slow­ly bring up the AC line volt­age while mon­i­tor­ing the pow­er sup­ply DC volt­ages with three mul­ti­me­ters.
All of the volt­ages end­ed up well with­in spec­i­fi­ca­tions, and I fine tuned the +120 volt sup­ply using the adjust­ment on the back pan­el to exact­ly +120.0 volts.
I also had num­bers dis­played on the front pan­el, even though they were not quite cor­rect, which I some­what expect­ed with all the com­po­nents replaced and adjust­ment poten­tiome­ters moved while clean­ing them. I did try to put the poten­tiome­ter’s back in their orig­i­nal posi­tions after clean­ing, but they almost nev­er end up exact­ly where they started.

Beckman 7360R Oscillator Shaper cir­cuit with­out crys­tal oven

I was also mon­i­tor­ing the crys­tal time base fre­quen­cy using the back pan­el BNC fre­quen­cy out­put J401 dur­ing test­ing and noticed that the fre­quen­cy did not lev­el out at the spec­i­fied 1 MHz and con­tin­ued to drop. I also noticed that the crys­tal oven was not turn­ing off so the sys­tem was pow­ered down.

Monitor 1 MHz fre­quen­cy stan­dard used in the Beckman 7360R

This 7360R uses a heat­ed oven crys­tal fre­quen­cy stan­dard type SA 1078 made by Monitor Products Company of So. Pasadena, California. It uses a 115 volt AC heater ele­ment of around 900 Ohms and pro­duces approx­i­mate­ly 14.5 watts of heat. For tem­per­a­ture con­trol it uses a her­met­i­cal­ly sealed bi-metal­lic thermostat.

Backside view of the Monitor Prod. SA 1078 fre­quen­cy standard

I had orig­i­nal­ly thought that the con­tacts in the ther­mo­stat were stick­ing, but it end­ed up being the sup­pres­sion capac­i­tor placed across the ther­mo­stat con­tacts. The capac­i­tor was very short­ed at 0.06 Ohms and only rat­ed at 85 ℃ with a capac­i­tance of 0.01 uF at 400 volts DC. It was also wrapped inside the fiber­glass insu­la­tion against the heat­ing element.

85 ℃ capac­i­tor used inside the Monitor fre­quen­cy reference
Monitor SA 1078 fre­quen­cy stan­dard with new capac­i­tor installed

I was able to mount a new 105 ℃ rat­ed capac­i­tor so that the fiber­glass insu­la­tion could be wrapped between it and the heat­ing ele­ment pro­vid­ing a bit more ther­mal iso­la­tion.
After reassem­bling the crys­tal oven it was plugged in to the Y401 sock­et for anoth­er sys­tem test.

Crystal fre­quen­cy stan­dard after warm-up and adjustment

The crys­tal fre­quen­cy stan­dard was now switch­ing the oven on and off to reg­u­late it’s tem­per­a­ture. After 45 min­utes of warm-up time I was able to use the “XTAL ADJ.” screw to set the fre­quen­cy to with­in 0.1 Hz using my HP 5305B / 5300B exter­nal GPS dis­ci­plined fre­quen­cy counter.

Now that I had a good fre­quen­cy stan­dard I was able to fol­low the cal­i­bra­tion pro­ce­dures in the ser­vice man­u­al. After about an hour of adjust­ing the Shaper, Divider, and Attenuator cir­cuits I had a work­ing Beckman Universal EPUT® and Timer.

Beckman 7360R com­po­nents that were replaced

Well now that I have it run­ning, what do I do with it?
Due to it’s size it won’t end up on the work­bench equip­ment shelves, but I do have some dis­play / unfre­quent­ly used equip­ment shelves with pow­er where it will reside. Hopefully it will find some use when trou­bleshoot­ing some equal­ly vin­tage test equip­ment in the future. And if noth­ing else it will most like­ly bring a smile to my face on a cold rainy day when I pow­er it on to view the warm glow of the Neon dig­its on it’s display.

Beckman 7360R count­ing at a 1 KHz rate while work­ing on knobs
Beckman / Berkeley model 7360-20 Universal EPUT and Timer images
My orig­i­nal Beckman/Berkeley mod­el 7360–20 Universal Eput and Timer

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