Picker Nuclear Compact Scaler

Something a bit dif­fer­ent than my usu­al test equip­ment and cal­cu­la­tor projects is this Picker Nuclear Compact Scaler built by Intertech.
Quite a few peo­ple may be won­der­ing. What is a “Scaler” and what is it used for?
A “Scaler” is a count­ing device that total­izes the events detect­ed over a peri­od of time. Typically used with a Geiger-Muller tube or scin­til­la­tion detec­tor for mea­sur­ing ion­iz­ing radi­a­tion. It is often referred to as a nuclear scaler.

I have owned sev­er­al Nixie tube cal­cu­la­tors, and sev­er­al Neon bulb type coun­ters over the years, but when this came up for sale I knew it would be a fun project and a great learn­ing expe­ri­ence. The sale was for this Picker Compact Scaler / Transistorized and was sold as-is with­out any of its count­ing tubes, but the price was extreme­ly rea­son­able.
For me what makes this piece of equip­ment spe­cial is it’s decade count­ing glow-trans­fer tubes, and as an added bonus I already have a DIY scin­til­la­tion detec­tor that I built many years ago that this scaler will oper­ate with.

Back before there were Nixie tubes, there were mechan­i­cal and neon bulb based coun­ters, along with “glow trans­fer count­ing tubes”, com­mon­ly known as Dekatrons.
The first glow trans­fer count­ing tubes were invent­ed by John Reginald Acton with the patent assigned to Ericsson Telephones Limited (ETL) in 1951, using the “Dekatron” brand name.
This scaler uses five Dekatron type tubes for it’s counter and dis­play which were man­u­fac­tured by Sylvania.
For a time base this unit has a mechan­i­cal timer that uses a syn­chro­nous AC motor, and can count up to 999.99 min­utes before rolling over it’s count. The “Count” on/off switch on the front pan­el starts both the timer and the counter when turned on, and retains the final count and time when turned off. The “Reset” dial can be used to reset the timer and count, or just reset the count by turn­ing the oppo­site direction.

The Picker mod­el 600010 scaler includes a high volt­age pow­er sup­ply capa­ble of 500 to 1,600 Volts for pow­er­ing either a Geiger-Muller tube or scin­til­la­tion detec­tor. The unit also has low volt­age BNC ter­mi­nals for Pulse Input and Output to and from the Pulse Amplifier board.

The Picker scaler uses a total of 9 Germanium tran­sis­tors of which three are used in the pulse ampli­fi­er (PA) board. The PA cir­cuit uti­lizes a sen­si­tiv­i­ty switch on the rear pan­el to set either 25 or 250 mV sen­si­tiv­i­ty for the input cir­cuit. I had some extra time while wait­ing for parts to arrive, so I reverse engi­neered sev­er­al of the cir­cuits in the scaler includ­ing the pulse ampli­fi­er board. After inspect­ing all of the com­po­nents in this unit, my best guess is that it was built in the first few months of 1964, mak­ing it at least 61 years old now.

The pur­pose of the PA board is to ampli­fy the input and shape the out­put sig­nal to dri­ve the Dekatron tube counter sec­tion. It uses a ‑48 volt sup­ply and pro­duces a minus 10 volt bias sup­ply on the board using a sim­ple Zener reg­u­la­tor. Two of the tran­sis­tors used in the PA are Philco 2N1754 MADT Germanium PNP devices in a TO‑9 case. The MADT tran­sis­tors used a new for it’s time Micro Alloy Diffused-base Tran­sis­tor process uti­liz­ing Cadmium elec­trodes in place of Indium, which increased heat dis­si­pa­tion, Beta, and speed.

The dri­ver cir­cuit board uti­lizes the Dekatron glow-trans­fer count­ing tubes as both a decade counter and count dis­play. It uses the remain­ing six Germanium tran­sis­tors for pulse shap­ing and prop­er phas­ing between the two guide pulse out­puts per tube. There is also a cir­cuit that uti­lizes the time coun­ter’s reset microswitch to force the Dekatron’s count to zero on all tubes. The Scaler used two types of Dekatron tubes. The first two decades of the counter use the Sylvania 6909 high-speed tubes and are capa­ble of count­ing from 0 to 100,000 Hz. The remain­ing three decades use the Sylvania 6802 low-speed tubes which oper­ate from 0 to 4,000 Hz.

The pow­er sup­ply is built into the chas­sis of the unit and con­sists of two main sec­tions. One sec­tion pro­vides pow­er for the Dekatron counter stages and ampli­fi­er cir­cuit­ry. The oth­er sec­tion pro­vides the high volt­age, and fil­a­ment sup­ply for the two 6BG6GA tubes.

The high volt­age sec­tion gen­er­ates around 1,800 Volts uti­liz­ing a Greinacher volt­age dou­bler cir­cuit to the anode of V106. There are also two iso­lat­ed 6.3 VAC sup­plies for the fil­a­ments of V106 and V107. The HV out­put requires both the Power switch and High Voltage switch­es to be on before out­putting high volt­age. There is a neon indi­ca­tor that lights when pow­er is sup­plied to the HV section.

The ampli­fi­er and counter sec­tion of the pow­er sup­ply uses one trans­former out­put wind­ing to gen­er­ate 540 Volts using a Full Wave series mul­ti­pli­er and then using a volt­age divider and 1 uF capac­i­tor pro­vides a 134 volt pulse for the zero reset cir­cuit. The sec­ond trans­former out­put is a cen­ter tapped 70 VAC wind­ing sup­ply­ing a full-wave bridge rec­ti­fi­er for a ‑43 and +35 out­put ref­er­enced to ground.

The high-volt­age selec­tor and reg­u­la­tor cir­cuit is com­posed of two 6BG6GA beam-pow­er pen­todes V106 and V107, along with V108 an OG3/85A2 cold cath­ode volt­age ref­er­ence tube at 85 volts, and a Neon bulb used to bias the cath­ode and heater on V106. Using a rotary wafer switch, and four pre­ci­sion resis­tors, 11 dif­fer­ent fixed high volt­age lev­els can be set in incre­ments of 100 Volts. A 100K poten­tiome­ter placed in series with the switch allows a 100 Volt range of adjust­ment between the set levels.

Other than replac­ing all the leaky elec­trolyt­ic capac­i­tors the only oth­er repair was the neon indi­ca­tor for the high-volt­age pow­er. The entire hous­ing was miss­ing from the indi­ca­tor with just the bulb and met­al retain­ing clip remain­ing. The orig­i­nal own­er did pow­er up this unit and was lucky that the clip was not across the bare Neon bulb leads and was most like­ly rest­ing on the insu­lat­ing sleev­ing as shown below.
In one of my old parts bins, I had a sal­vaged indi­ca­tor that fit­ted perfectly.

After sev­er­al pur­chas­es, I final­ly end­ed up with a work­ing set of Dekatron tubes. My first pur­chase was for a set of (NOS) New Old Stock tubes, which where quite the oppo­site. Two of the tubes had bro­ken index­ing posts on the octal bases, and all were very well used with inter­nal sput­ter­ing on the glass enve­lope caus­ing some dark­en­ing. The next tube I ordered looked great but would not count past 3 due to a slight­ly bent anode disk. With the next order I final­ly had two work­ing 6909 tubes for the first two decades. That was great progress as the unit could now count up to 39,999.

While wait­ing for the last 6802 Dekatron tube, I turned my atten­tion to the noisy mechan­i­cal timer. The unit most­ly just need­ed a good clean­ing and lubri­ca­tion. I did end up remov­ing the cov­er of the syn­chro­nous motor and used a nee­dle oil­er to get some lubri­ca­tion deep inside it’s plan­e­tary gear­ing. This helped quite a bit but there is no doubt that the timer is count­ing even when not look­ing at it.

The final 6802 NOS Decatron tube arrived along with a cou­ple of spares so the Picker Scaler is now ful­ly func­tion­al.
I tried it out with my DIY scin­til­la­tion detec­tor and it worked very well. It was able to eas­i­ly detect some ear­ly (cir­ca 1936–1942) Fiestaware plates and bulb can­dle hold­ers that used an ura­ni­um oxide glaze. In the con­fig­u­ra­tion pic­tured below I was read­ing over 5,000 counts per minute and a back­ground count of around 100 cpm.

DIY scin­til­la­tion detec­tor and radioac­tive Fiestaware 5,054 cpm