Beamec Series 2 C.R.T. Test and Re-conditioner

This manual is split into 3 sections. User Instructions Pages 1-6 Technical Manual Pages 1-10 Figs 1-4 Accessories Page 1 Figs A1/A2 Further notes Pages 1&2 Figs S3/1-S3/10

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INSTRUCTION BOOK FOR USE THE BEAMEC SERIES II(PATS. PEND)

* CATHODE RAY TUBE TESTING

AND RE-CODITIONING INSTRUMENT *

GRUNTHER INSTRUMENTS LTD LONDON

GENERAL DATA

Size: 10" x l4" x 7". Weight: 25 lb. approx. Mains Input: 200/250 volts. 50/60 cycles. Consumption: 60 watts max. Fuses: 1.5 amp. Input. 500 m/a. anti-surge H.T. External accessories: 5-base Multi-Adaptor for I.0./M.O./

B8A/Bl2A/E.M.I. Tubes. Test Prods for use on external volts and ohms ranges are an optional accessory. Input and secondary circuits isolated from chassis, which should at all times be earthed as is normal practice with portable equipment. Test voltages are derived from a stabilised source, giving long term reliability and compensating for various input voltage changes. WARNING. The lead to the final anode coming from the adaptor plug is alive to the pins inside the adaptor box during A2 TEST (see instructions). Care must be exercised to avoid contact while making this test.

SETTING-UP AND USING THE BEAMEC We have endeavoured to make the following instructions as clear and concise as possible and have assumed that the instrument will normally be used by a service engineer who will be familiar with various tube bases and heater voltages. Time spent in reading the instructions will be well rewarded by enabling quick and certain work to be done with what is a very useful instrument. Set the mains voltage selector at the rear of the instrument to its appropriate tapping, turn all three rotary switches to their extreme

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left positions, and plug the adaptor into the C.R.T. socket on the front panel. Fit the correct adaptor base to the tube, taking the flying lead to the tube anode cap, plug the BEAMEC into the supply and switch on. NOTE.-The CLEAR LEAK toggle switch should always be up in the NORMAL position during testing, and the TRIODE/PEN-TET toggle switch set correctly for the tube under test. The suggested sequence of tests and repairs are as follows.

HEATER CURRENT

Set HEATER VOLTAGE switch to its correct position, centre switch S2 to NORMAL and right-hand switch S3 to HEATER CURRENT. The meter will now read the current passed by the tube heater while operating at its correct voltage, and will show whether it has developed a partial short-circuit or is completely open-circuit. The meter deflection is 1 amp. full-scale on 6.3, 7.5, 10.5 and 12.6 volt. tubes, and 2 amps on 2 and 4 volt tubes. Gently tapping the tube base with the fingers while switched to this position will bring to light any intermittent heater faults that may be present, but the engineer should exercise care and not damage the tube by prolonged hard knocking. A period of 10 to 15 seconds is normally required from a switching on for the meter reading to fall to its final figure. As an average example, a tube rated at 6.3 volts at .3 amp. which gives a reading on the Beamec of .5 amp. When switched to 6.3 volts on the HEATER VOLTAGE switch, would have a typical partial short, giving every appearance of low-emission if used in a set with series heaters. This of course is caused by the heater failing to develop sufficient heat to raise the cathode temperature to its normal working level, but in a number of cases, by supplying the tube heater from a transformer this fault can be overcome.

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ELECTRODE LEAKAGE

Leaving the heater switch at its correct position and S2 at NORMAL, turn S3 to H-K. The meter will now be showing any leakage between heater and cathode to l0 megohm. Check each of the other three electrode combinations in turn, K-G Cathode-Grid, G-A Grid-Anode I, and K-A Cathode-Anode I.

CLEARING LEAKS

Should a leak be shown between any pair of electrodes, leave S3 switched to that position, turn S2 (centre switch) to CLEAR LEAK and change the top right-hand toggle switch from NORMAL to CLEAR LEAK. After one or two seconds return S2 and the toggle switch to their NORMAL positions and the meter will indicate whether the leak has been removed. In persistent cases, two or three applications of this procedure may be required, and in the case of very heavy shorts it may help to reduce the heater voltage to a point where the short or leak gives the highest resistance reading. Experience will give the best guide in determining what leaks can and cannot be removed, lightly tapping the tube between attempts will often work wonders with difficult cases. The correct manner for clearing leaks is to change S2 from NORMAL to CLEAR LEAK before changing the toggle switch, and it should be noted that although the meter ceases to indicate with S2 in the clear leak position, this is only part of the design and the fault cannot be cleared except by use of the toggle switch. The provision of an OFF position on the HEATER VOLTAGE switch enables cold tests for leakage to be carried out.

EMISSION Leaving the heater voltage set to the correct tube voltage and with S2 in the NORMAL position, turn S3 to EMISSION. Check that the TRIODE/PEN-TET toggle switch is correctly set. The readings now shown on the coloured 250 micro-amp scale are an indication of the usefulness of the tube in terms of Cathode emission and this reading, used in con junction with the A2 TEST about to be described, enable the performance and life expectancy of the tube to be forecast.

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TESTING THE FINAL ANODE

A point to mention, in tubes with a focus anode the final anode in all correctness should be called A3, but the average engineer usually refers to the final anode as A2, so to avoid any misunderstanding the final anode will be designated as A2 in these instructions. HEATER VOLTS and S2 set as for EMISSION, turn S3 to A2 TEST. The reading now obtained on the 250 micro-amp. scale is the actual beam current flowing in the final anode and will indicate any faults such as O/C or intermittent final anode. The current reaching the final anode is influenced by the presence of gas or air in the tube to a larger extent than the readings obtained during the EMISSION test are, therefore comparing the two readings will enable a useful gas check to be made. A "soft" or gassy tube will normally give a high emission reading but a much lower A2 reading, while high readings on both tests show a first-class tube. On ion trap tubes, the trap magnet should be adjusted to give the highest A2 reading obtainable.

PROCESS

If, when taking an emission reading the tube was low, the application of PROCESS will usually remove the most common cause of trouble, cathode poisoning and contamination. The following instructions should be carried out carefully:- Set HEATER VOLTS to correct tube voltage, S2 to NORMAL and turn S3 to PROCESS. The meter should be giving a very low reading or none at all, but some time during the two minutes following the switching to PROCESS the meter should (if the processing is successful) suddenly show a much higher reading, and will be observed to be fluctuating sharply towards zero at 5 to 6 second intervals. Processing is complete when 4 or 5 of these fluctuations have been noted, and S3 should then be turned hack to EMISSION to check the improvement obtained. If the emission now approaches 200 micro-amps or over, the process has taken effect. Assuming the tube is free from gas, which accelerates the formation

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of impurities on the cathode, the tube should now give an indefinite period of renewed life. In the case of tubes which initially show very low or almost non-existent emission and trying to PROCESS does not seem effective, it may be necessary to apply an increase of heater voltage during the processing time. Accordingly, after first switching to PROCESS change S2 from NORMAL to RE-ACTIVATE l or 2. Position l should be tried first as this only gives a 25% rise in Heater Voltage and is unlikely to Cause premature heater failure. S2 should be returned to NORMAL after processing is complete and before checking emission.

RE-ACTIVATION

Two positions of reactivation have been provided for use on tubes where low emission is caused by oxide coatings on the heater preventing the Cathode from receiving the heat needed for its correct working. This is applied in this manner:- Heater volts to correct working voltage. S2 to RE-ACTIVATE l and S3 to RE-ACTIVATE. The meter will show any increase in emission, and this should be allowed to reach a steady level for about one minute before switching back as for EMISSION to check improvement. RE-ACTIVATE 2 may he used if deemed necessary, but the possible risk of heater failure should be borne in mind. RE-ACTIVATE 2 applies 50% heater increase.

AGEING

Experience has shown that a period of ageing is some-times very useful in ensuring that a processed tube will keep to the standard achieved. This is very simply carried out by switching HEATER VOLTS to correct voltage, S2 to RE-ACTIVATE 1, and S3 to HEATER CURRENT. Five to ten minutes will be sufficient and a slight increase in emission should be observed after this time. However, ageing should not be carried out on tubes which show an emission reading of 200 micro-amps or over after processing, it is only useful on tubes which did not respond very well and are giving emission readings in the top or middle section of the red arc.

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The preceding notes should have given the engineer a clear insight into the range of useful functions that the BEAMEC is capable of carrying out, and the experience gained with use will enable him to make full use of it. Manufactured by- GRUNTHER INSTRUMENTS LTD. 14. ORIENTAL STREET, LONDON. E.14 Telephone EASt 4879

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GRUNTHER INSTRUMENTS LTD

* TECHNICAL MANUAL FOR THE BEAMEC SERIES 2 * CATHODE RAY TUBE TESTING AND RECONDITIONING EQUIPMENT

GENERAL SPECIFICATION (SERIES 2). 1. SIZE: 10" x 14" x 7" 2. WEIGHT: 25lbs approx. 3. FINISH: Grey hammer finished case. Black anodized front panel, leather carrying strap. 4. MAINS INPUT: 200/250 volts, 50-60 cycles.

(Input and secondary circuits isolated from case).

5. CONSUMPTION: 60 watts max. 6. FUSES: 1.5 amp mains. 500 M/A HT cartridge loading. 7. VALVES: 85A2 Reference tube

EN91 Thyratron 8. METER: Open Scale, knife edge pointer. ACCESSORIES 1. Five-way multi-base adaptor for International octal, Mazda octal, B8A, Bl2A and E.M.I. Cathode Ray Tubes (1 off complete Pt. No: B2047). 2. Test-prods and adaptor which allow the BEAMEC to be used for 0-250v DC and 0-10M.ohms measurement. (SUPPLIED SEPARATELY- PT. No: B2048) 3. Special high voltage test probe and adaptor which allows the BEAMEC to be used for EHT measurement up to 25Kv. (SUPPLIED SEPARATELY - Pt. No: B2049).

*****

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MAINTENANCE. The instrument will normally require very little attention as all components are operated well within the maker's specified ratings. Failure to operate after continuous service may, however, be due to more obvious faults, for example:- (a) Disconnected or broken mains leads. (b) Blown Fuses. (c) Mains voltage selector plug adrift. (d) Valve failure. etc. etc.

To facilitate the location of a more obscure form of failure a brief explanation of each of the circuit functions is given, together with voltage checks for each function. A circuit diagram (Fig: 1) and component layout diagram (Figs: 2 & 3) are appended. * VOLTAGE MEASUREMENT SHOULD BE TAKEN WITH A AVOMODEL 8 OR

A METER WITH A SIMILAR OHMS/VOLT CHARACTERISTIC. (20,000 Ohms/V).*

REMOVING THE INSTRUMENT FROM THE CASE. Removal of the TWO SCREWS FROM THE UNDERSIDE OF THE CASE and the TEN SCREWS FROM THE FRONT PANEL will release the chassis and front panel complete. Tilt the top of the front panel forward and lift the chassis clear. Care should be taken to see that the mains lead is fed through from the back of the instrument before completely removing the chassis. MAINS SUPPLY CIRCUIT.

The mains cable feeds both transformers (Tl and T2) via the Fuse Fl (neutral lead), ON/OFF switch and voltage selector panel (line) at the rear of the chassis. * * NOTE: The earth line of the three core mains cable should be earthed via a three pin plug. This line is internally connected direct to chassis. HEATER VOLTAGE CIRCUIT.

Heater supplies for CRT's are derived from the secondaries of T2 via the selector switch S1 and the special series resistors Rl and R2. When in operation the heater current of a CRT under test is measured by the voltage drop across Rl and/or Rl in parallel with R2 (see note 3). This derived voltage is rectified by the diode

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XI And applied to the meter as a smail negative PC voltage (S4 in NORMAL position), the return path of the meter being via switch S3(H), contact 1. The meter is then indicating directly the current being drawn by the heater of the CRT under test. NOTE: THE RESISTORS Rl AND R2 ARE INDIVIDUALLY CALIBRATED, AND ANY MODIFICATION THERETO WILL RESULT IN INACCURATE HEATER CURRENT READINGS. With the selector in the 2 TO 4 VOLTS position, full scale deflection of the meter will be obtained for a current of 2 amps and on ranges 6.3v and upwards the meter will show full scale deflection for a current of 1 amp. Increased heater volts are obtained on any of the selected ranges by switching the primary taps of T2 via S2(A). With S2 in RE-ACTIVATE 1, a 25% increase is obtained and with S2 in RE-ACTIVATE 2, a 50% increase is available VOLTAGE CHECKS. For a nominal mains voltage of 240v (no load conditions). Secondary T2 Terminals A and G - 12.6 volts S2 in RE-ACTIVATE 1 Pos: Terminals A and G - 16.6 volts S2 in RE-ACTIVATE 2 Pos: Terminals A and G - 19.0 volts INTER-ELECTRODE LEAKAGE TESTS.

Leakages of up to 10M.ohms may be read off directly on the meter, the voltages for these tests being derived from,the stabilized. 85v supply. The 85A2 stabilizer valve VI is supplied via S3(A) through a 68K.ohm resistor from MR1, R3 and C1, the voltage for leakage testing being taken from the 85A2 stabiliser via VR1 and R20. The meter SET-ZERO control VR1 is for adjustment of the zero ohms meter reading when the instrument is being used for leakage testing. This control, however, is pre-set before leaving the factory and should, not normally require further adjustment. If a 'leak' is registered between any pair of electrodes the switch S2 may be set to CLEAR LEAK. This disconnects the electrodes under test, whereupon they are re-connected directly to the NORMAL/CLEAR LEAK switch 4. With S4 in the CLEAR LEAK position, the capacitor C2, which is normally connected directly to the 280v HT rail and is therefore being continuously charged, is now discharged through the CRT electrode assembly.

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With S3 in the H/ALL position, the grid, cathode and anode

1 of the CRT under test are connected together. * * NOTE: During leakage tests the polarity of the 85v supply is reversed so that electrodes under test will not pass current due to emission. VOLTAGE CHECKS. Anode of 85A2 and Com: -Ve rail..............85v +DC ± 1 volt Positive Terminal of C2 and Com: -Ve rail...280v +DC approx. Junction of R3 and C1 and Com: -Ve rail

(or across C1)......280v +DC approx

NOTE: CONNECTION TO THE COM: -VE RAIL MAY BE OBTAINED AT THE STAND-OFF SOLDER TAG INDICATED IN FIG: 2. THE COMMON NEGATIVE RAIL IS TAKEN TO CHASSIS VIA THE CAPACITOR C3 THE CHASSIS MUST NOT, THEREFORE, BE REGARDED AS BEING AT NEGATIVE DC POTENTIAL.

EMISSION TESTS.

* Emission Triode *

With S3 switched to EMISSION and S5 in the TRIODE position, a voltage from the potential dividing network of R15 and Rl6 is applied between cathode and grid of a CRT under test. The current flowing at the junction of those two resistors should generally provide a reading lying between 200 micro-amps and full scale deflection. Normally any reading within the GREEN section of the meter scale is indicative of good emission.

* Emission Tetrode or Pentode *

With S5 switched to PEN-TET, voltages for CRT electrodes are derived from the potential dividing networks composed of R18-R19 and R5-R6. A total current of 250 micro-amps is available for the cathode and first anode and a further 250 micro-amps for the cathode and Gl (control grid) thus providing for a total of 500 micro-amps cathode current. In all emission tests the meter is connected in the cathode circuit of the CRT under test, A2 TEST.

For emission test with A2 of a CRT, the entire voltage of 660v from T2 secondary is applied via S3(I) to the voltage doubler circuit comprised of MR1, MR2, C4 C5. This provides a total DC voltage of 1.8Kv, which is fed to the final anode of the CRT via the 3.6M.ohm resistor R14 (See note 2). A total

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VOLTAOE CHECKS.

Junction R11-C6...............115v rising from zero voltage at 2-10 second intervals as the 85A2 strikes (measured with valve voltmeter only). Pin 3 CRT socket on front panel and Com: -Ve rail........................................240vAC

RE-ACTIVATE.

The method of applying voltage to a CRT is substantially the same as for the emission tests, except that the voltages, are derived from the potential dividing network of R12 and R13.

VOLTAGE CHECKS. :

Same as for emission tests.

* MISCELLANEOUS VOLTAGE CHECK POINTS *

TEST POINT CIRCUIT VOLTAGE

Pins 5 and 6 Heater voltage AC. 2v, 4v, 6.3v CRT. socket acoording to 7.5v, 10.5v,

position of S1 12.6v. ditto Heater voltage 25% increase on

according to each of the above position of S1 readings.

but with S2 on RE-ACTIVATE 1

ditto As above but 50% increase on

with S2 on each of the above RE-ACTIVATE 2 readings.

NOTE: The above tests are not affected by other switches.

+ Terminal C2 With. 34 set to +DC 200v and Com: -Ve normal rail

Pin 7 of V2 S3 in PROCESS -DC potential _ EN91 Thyratron position between 1 and and Com: -Ve 2v falling to 0.5 rail to lv as 85A2

strikes.

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CRT ADAPTOR BOX. This is supplied complete with leads and octal base plug for

connection to the CRT socket on the BEAMEC front panel. A circuit diagram of the internal wiring of the box and cable connections is given as Fig: 4.

* * * NOTE: The colour coded wires are those of the main

cable only, the internal wiring of the box being carried out in common colours.

The inspection cover may be taken off by removing the four

self-threading screws.

* * * * * * * * N O T E S :

1. On some BEAMEC Series 2 models the 1OK.ohm potentiometer VR2 is replaced by a 50K.ohm potentiometer paralleled by a lOK.ohm resistor or a 5K.ohm potentiometer in series with a lOK.ohm resistor as shown by the dotted component on the layout diagram.

2. On some models the 3.6M.ohm resistor R14 is substituted by a 3.3M.ohm with a 500K.ohm or 330K.ohm resistor in series.

3. On instruments fitted with type M (Pt. No. B2046) and type T (Pt. No. B2045)front panel meters, the two special resistors Rl and R2 are mounted behind the front panel,

adjacent to the meter.

--------oOo--------

WIRING COLOUR CODE Some of the wiring in the BEAMEC Series 2 instrument is carried

out in certain colours which are indicated in the circuit diagram. All other wiring is effected in a common colour such as Violet or Red etc.

The following are B.S. colours and abbreviations:-

BK Black YL Yellow EN Brown WH White RD Red GN Green PK Pink BL Blue OR Orange VI Violet

GY Grey

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BEAMEC (SERIES 2) COMPONENTS LIST Component No. Component Beamec Part No. V1 Mullard 85A2 B2001 V2 Mullard EN91 B2002 Rl Special Resistors B2003 R2 Special Resistors B2004 R3 2.2K.ohms 1/2w. B2005 R4 340 ohms 1/4w. B2006 R5 l00K.ohms 1/2w. B2007 R6, R9 1 M.ohm 1/2w. B2008 R7, R13 68K.ohm 1/2w. B2009 R8, R10, R21 500K.ohm 1/2w. B2010 Rll 1.5M.ohm 1/4w. B2011 R12, Rl6, R22 150K. ohm 1/2w. B2012 R14 3.6M.ohm 1/4w. B2013 R15, R19 20K.ohm 1/2w. B2014 R17 l0K.ohm 1/2w. B2015 R18 330K.ohm 1/2w. B2016 R20 38 ohms 5w. 20% B2017 C1, C6 16mfd.(EL) B2018

350v DC wkg.

C2 32mfd.(EL) B2019 350v DC wkg.

C3 0.lmfd. 500v B2020

DC wkg. C4, C5 0.lmfd. l000v B2021

DC wkg. VR1 50K. Potentiometer B2022 VR2 10K. Potentiometer B2023

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Component No. Component Beamec Part No.

S1 HEATER VOLTS Selector 7-way 2-pole B2024

S2 NORMAL-CLEAR

LEAK-RE-ACTIVATE Selector 4-way 2-pole B2025

S3 PROCESS Selector

10-way 9-pole B2026

S4, S5 D.P.D.T. Toggle Switch B2027 S6 S.P.S.T. Toggle Switch B2028 Tl H.T. Transformer B2029 T2 Heater Volts Trans- B2030 former MR1 Rectifier

16 RC1-1-16-1 B2031 MR2, MR3 Rectifier K3-25 B2032 X1 Crystal Diode (GEX.34) B2033 Fl Cartridge Fuse

1.5A 1¼” standard B2034 F2 Cartridge Fuse

500 m/A 1¼” standard B2035 Fl, F2 Fuseholder B2036 Pilot Lamp 6.5v 0.3a B2037 Pilot Lamp Holder B2038 CRT Socket (International Octal) B2039 V1, V2 Valveholder B7G B2040 V1, V2 Valve Screens B7G B2041 Mains Voltage- Selector Panel B2042 Tagboards 1 and 2 B2043

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Component Beamec Part No.

Panel Meter 5" Open Scale Type G 3 hole fixing B2044

do. Type T 4 hole fixing B2045 do. Type M 4 hole fixing B2046

Mains Cable Clamp 1 off B2053 Grommetts 2 off B2054 Control Knobs 3 off B2055 Solder Tags 6 off B2056 --------------------------------------------------------------

ACCESSORIES

CRT Adaptor Box 9complete) B2047 Test Lead Adaptor and Prods (volts and ohms measurement) B2048 Test Lead Adaptor and Probe (EHT) B2049 Octal Plug (for B2047 and B2048) B2050 Octal Plug (for EHT Probe lead only) B2051 CRT Anode Cap (Adaptor Box lead) B2052 --------------------------------------------------------------

* * CIRCUIT DIAGRAM AMENDMENT * *

ON THE CIRCUIT DIAGRAM Fig: 1 THE 150K.OHM

RESISTOR MARKED R20, SHOULD BE R22

- - - - - - -

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* ACCESSORIES TO THE BEAMEC (SERIES 2) * CRT TESTING AND RECONDITIONING EQUIPMENT.

The BEAMEC Series 2 equipment may be used for D.C. voltage measurement from 0-250v, for EHT measurement up to 25Kv and for continuity and resistance measurement up to 10.0M. ohms. DC AND OHMS MEASUREMENT. The 5" open scale meter is directly calibrated from 0-250v (centre scale) and from 0-10M.ohmo and to infinity (lower scale). The test prods are complete with leads and octal connector which is plugged into the CRT cutlet on the front panel (Fig:Al) (PtrNo.B 2048). * For voltage measurement set S3 to VOLTS * For ohms measurement set S3 to LEAKAGE AND EXTERNAL M.OHMS RANGE (KA) The ohms range SET-ZERO control is accessible through the front panel of the Series 2 BEAMEC. Connect the test prods together and adjust for zero ohms meter reading. ALL OTHER SWITCHES ON THE BEAMEC SHOULD BE SET TO NORMAL. The voltage for the ohms range is derived from the instruments internal 85v stabilized DC supply Each test prod is fitted with a 1 amp Bulgin Type F96 5/8” x ¼” dia. protecting fuse. To replace fuses, unscrew the two sections of the test prod. EHT TEST PROBE. This comprises a high voltage test probe with an integral 50M.ohm series resistor and allows EHT measurement of up to 25Kv. The wander lead with the crocodile clip is for negative or earth connection. (Fig:A2). (Pt. No:B2049). The probe is complete with high voltage lead and octal connector which is plugged into the CRT outlet on the BEAMEC panel. The meter will read 25Kv FSD. for a current drain of 500 micro-amps. When measuring EHT on a Television receiver, the brightness control should be reduced to zero. * For EHT measurement set S3 to VOLTS.

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Beamec Series 2 Further Notes Added July 2015

Capacitor C3 is rated at 500Vdc. Shorting the A2 connector to

earth during the A2 test can expose the capacitor to nearer

1000V. C3 should be up-rated to a 1000V part.

Note 2 on page 7 of the Beamec manual refers to an additional

resistor in series with the A2 output. In this version the

wire on pin 1 of the CRT socket is moved to pin 7. The extra

resistor (R21) is wired from pin 7 to pin1. R21 is shown in

both the parts list and in Fig. 2.

NOTE. During the A2 test there is 1.8kV (±900V WRT Com –ve)

applied between the tube’s A2 and K connections. EXREME CARE

IS REQUIRED WHEN CARRYING OUT THIS TEST.

On some version of the Beamec the mains fuse (F1) is wired in

the neutral lead. Safety can be improved by re-wiring it to

the live side in series with the mains switch.

Some versions of the Beamec are wired for H-K rather than

H/all leakage tests. In these the wiring from S3G/2 to S3F/2

and from S3F/2 to S3E/2 is omitted.

The Beamec manual quotes a few internal voltages. Here are few

more

Where Function Voltage Comment

Junction Divider 76V R16/R17 feed

Junction Grid voltage 4.8V R18/R19 Tet-Pen

Junction Grid voltage 8.9V

R15/R16 Triode

Junction Grid Voltage 23V

R12/R13 Re-activate

Junction A1 voltage 250V The resistance to this point is

R5/S5 1.1Mohms so the meter resistance

will affect the result.

All readings WRT Com –ve using 10Mohm input meter. S3 set to

Emission. No tube connected.

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In Process mode, the AC fed to the bottom of VR2 is in anti-

phase to the AC present at the anode of V2. With a

semiconductor diode in series with a 12kohm resistor

(explained in the next paragraph) connected to the Beamec and

V2 conducting the signal is a series of flat topped sine waves

with amplitude 12volts peak.

No set up procedure is given for potentiometer VR2. VR2 sets

the on time (meter needle at zero) of the pulses in Process

mode. During this time the tube is passing a high current

between its cathode and grid and grid connections. The pulse

length should be set on the shorter side of the available

range to help avoid possible tube damage. Should there not be

a suitable tube available when setting the pot, a

semiconductor diode with a Vrrm of greater than 250V in series

with a 100kohm 1 watt resistor can be connected between the

cathode and grid terminals of the Beamec. Diode cathode to

Beamec cathode.

Although the meter is scaled 0-250 micro-amps it is actually

measuring 0-500 micro-amps. No shunting resistor is fitted.

Reference is made to this fact in the Emission Tests section

of the Technical Manual.

The following diagrams show the full version of the schematic

then stripped down versions showing the valid circuitry in

each position of S3.