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A high efficiency inverter for f luorescent tubes This high efficiency inverter will power either an 18W or 36W slimline fluorescent tube from a 12V battery. It can be used for camping, emergency lighting or as part of a solar powered lighting installation for remote areas. By JOHN CLARKE Fluorescent lighting has many benefits over incandescent lamps. Fluorescent tubes use far less power than the equivalent light output incandescent lamps. They also provide a relatively diffuse light since the light is emitted from a large surface rather than from the virtual point source of a light bulb. Battery powered fluorescent inverters are very common these days. You can find them in small portable lamps, in caravan, bus and boat lighting and in automotive inspection lamps. In most of these, a self-oscillating single transistor inverter steps up the voltage 26  Silicon Chip from the battery to a high AC voltage sufficient to start the tube. Once the tube is lit, the inverter transformer then provides current limiting for safe operation. This is a simple system that works but it does have a few problems. Firstly, these simple inverters are not very efficient. This is because the inverter must provide a very high voltage (usually in excess of 1000V AC) in order to start the tube but only deliver 100V or less once the tube has fired. This means there are considerable losses in the inverter transformer and to a lesser extent in the transistor drive circuitry. Because of this, simple inverters are rarely practical for tubes of more than 20 watts output. Another problem with simple inverters for fluorescent tubes is their lack of voltage regulation. This makes no allowance for the fact that the voltage across a battery falls as it becomes discharged. Consequently, the tube may be over-bright on a fully charged battery and become noticeably dimmer as the battery discharges. A consequence of brute force starting and overdriving when running is shortened tube life. For maximum life, they must be started correctly and some form of regulation must be included to avoid overdriving the tube when the battery voltage is high. Our new inverter design overcomes the above shortcomings and has high efficiency. It can be made to suit 18W and 20W tubes or 36W and 40W tubes. The tube filaments are preheated for cor­rect starting and the circuit incorporates voltage regulation so that the tubes will have long life. Furthermore, MOSFET DRIVERS AND CONTROLLER FEEDBACK START-UP CIRCUIT +340V 12V BATTERY STEP-UP TRANSFORMER AC RECTIFIER AND FILTER The 340VDC is applied to the fluorescent tube driver cir­cuit. This is essentially a free-running oscillator once the tube is running but a start-up circuit is required to allow the tube to fire. The start-up circuit applies a pulse train to the oscil­lator and if a tube is connected, the oscillator runs at a fre­quency set by the series inductor (L) and resonant capacitor (C) across one end of each tube filament. The resulting current through the resonant capacitor heats up the tube filaments and allows the tube to fire. The circuit then changes to a different operating mode. Inductor L limits the current to the tube and the operating frequency becomes lower as set by a saturable transformer. The AC capacitor is used to prevent DC being applied to the tube. DC can cause mercury migration to one end of the tube which will ul­timately reduce the operating life. 0V OSCILLATOR WITH SATURABLE TRANSFORMER FLUORESCENT TUBE DRIVER DC-DC CONVERTER Circuit details AC CAPACITOR INDUCTOR (L) FLUORESCENT TUBE RESONANT CAPACITOR (C) Fig.1: this block diagram shows the main circuit features of the fluo­rescent inverter. Note the feedback to maintain a constant DC voltage from the rectifier output. This ensures constant brightness with varying battery input voltage. since the tubes are run at a very high frequency, there is no flicker, either at start-up or during running. Nor is there is any hum or audible whistle and radio interference is low. The inverter is designed to be housed in a standard 18W or 36W batten fitting so that the fluorescent inverter and lamp are an integral unit. Block diagram Fig.1 shows the block diagram of the fluorescent inverter circuit. It comprises a DC-DC converter (which steps the 12V up to 340V DC) and a fluorescent tube driver circuit. The DC-DC converter employs a step-up transformer which is driven by two Mosfet transistors at a frequency of around 120kHz, as set by a switchmode controller IC. The resulting high voltage AC output from the transformer is rectified and filtered to provide DC. Feedback is applied from the output to the switchmode controller IC to maintain the DC voltage at 340V. • • • • • • • • • Main Features Suitable for 18W and 20W or 36W and 40W tubes High efficiency Fast starting without flicker Filaments preheated Constant lamp brightness from 11-14.4V supply Light output equal to conventional mains-powered lamp Reverse polarity fuse protection Fuse protection for faulty tube Low electromagnetic radiation The full circuit for the fluorescent inverter is shown in Fig.2. At the heart of the DC-DC converter is IC1, a TL494 pulse width modulation (PWM) controller. It contains a sawtooth oscil­lator, two error amplifiers and a pulse width modulation compara­tor. It also includes a “dead time” control comparator, a 5V reference and output control options for push-pull or single ended operation. Oscillator components at pins 5 and 6 set the operating frequency of the pulse width control at about 120kHz. This fre­quency was selected to obtain the maximum power output from the transformer. The PWM controller generates variable width output pulses at pins 9 and 10, to ultimately drive the gates of Mosfets Q1 and Q2 via paralleled buffers in IC2. Mosfets Q1 and Q2 drive the centre tapped primary winding of transformer T1. The centre-tap of the transformer’s primary winding connects to the +12V supply while each side of the prim­ ary winding is connected to a separate Mosfet. Each Mosfet is driven with a square wave signal so that when Q1 is on, Q2 is off and when Q2 is on, Q1 is off. With Q1 on, 12V is applied to the top half of the transformer primary winding. Because of transformer action, the lower half of the transformer primary winding also has 12V across it. Similarly, when Q2 turns November 1993  27 Q4 BUK457-600B S G N3 330  1W 330  ZD3 12V 1W on, 12V is also impressed across the top primary winding. The resulting 24V peak-to-peak waveform on the primary is then stepped up by the secondary winding. High speed diodes D1-D4 rectify the AC output from trans­former T1, while a 0.1µF 250VAC capacitor filters the rectifier output to provide a stable voltage. We can get away with such a small value filter capacitor here because the operating fre­quency is so high. 36W AND 18W FLUORESCENT INVERTER 8.2k 3T 6T 0.4mm DIA ENCU N3 N2 24T N1 F2 400mA 200mA F1 5A 2A FL1 18W 4 E1 9 36W 11 1k 7 9 14 1k 10 E2 16 7 IN(+) .001 5 15 13 4.7k 2 1M 0.1 47k 14 5V IN(-) FB GDS 4.7k 6 IC1 TL494 1 8 11 12 3 GND 16T 12 8 15 2 3 1 IC2 4050 5 0.1 6T 470 25VW G 6 S D Q2 MTP3055E 10 S D G Q1 MTP3055E 4 0.1 ZD1 16V 1W 470 25VW 82  F1 +12V 28  Silicon Chip 3T 0.25mm DIA ENCU 470 25VW S2 4T F2 L1 10uH S1 4T F1 T1 0.1 FEEDBACK D3 270k 270k 0.1 63V DIAC1 ST2 22  680pF N1 3kV N2 D4 150k D5 1N4936 150k 330  1W 330  330  1W D1 D2 150k 150k 136T 0.1 250VAC 4x1N4936 +340V T2 330  1W ZD2 12V 1W L2 900uH Q3 BUK457-600B D G S D .001 3kV F2 0.1 250VAC FL1 Fig.2: the complete circuit of the fluorescent inverter. The DC-DC inverter section runs at about 120kHz while the fluorescent driver section runs at 65kHz for 36W tubes and 110kHz for 18W tubes. Feedback Feedback from the high voltage DC output is derived from a resistive divider (two 270kΩ and an 8.2kΩ resistor) and applied to the internal error amplifier in IC1 at pin 1. If the DC vol­tage becomes greater than 340V, the pulse width drive to the Mosfets is reduced until the correct voltage is obtained. Simi­larly, if the voltage drops below 340V, the pulse width is increased until the correct voltage is achieved. The DC gain of the error amplifier is 213 times, as set by the 1MΩ and 4.7kΩ resistors at pin 2. The 47kΩ resistor and 0.1µF capacitor across the 1MΩ feedback resistor provide fast AC re­sponse from the circuit. Power to IC1 and IC2 is supplied via an 82Ω resistor from the +12V battery supply and filtered with a 470µF capacitor. A 16V zener diode protects the circuit from high voltage tran­sients. To eliminate RF noise generated by the switchmode DC-DC converter from being radiated by the supply leads we have includ­ed a filter comprising inductor L1 and a 0.1µF capacitor (at the input). Fluorescent driver The fluorescent tube driver comprises Mosfets Q3 and Q4, transformer T2 and associated components. The fluorescent tube is driven via inductor L2 and the N1 winding of transformer This photo shows the gate drive pulses to Q1 & Q2 in the DC-DC converter when driving an 18W tube. The gate pulse width will be greater when the circuit is driving a 36W fluorescent lamp. T2. The N1 winding drives the gates of the Q3 and Q4 Mosfets via the N2 and N3 windings which are antiphase connected. When power is first applied, there is 340V DC between the drain of Q3 and the source of Q4. The 0.1µF capacitor adjacent to Diac1 begins to charge via the two series 150kΩ resistors. When the voltage reaches about 30V, the Diac fires and discharges into the gate of Q4. Zener diode ZD3 protects the gate from overvol­tage. Mosfet Q4 is now switched on and current can flow from the +340V supply via the fluorescent tube top filament, the .001µF 3kV capacitor, the second tube filament, the 0.1µF 250VAC capaci­tor, inductor L2 and transformer T2’s N1 winding. Current flow in N1 will then apply gate drive to Q3 via N2 and switch off gate drive to Q4 via N3 (due to the polarity of the windings). If this oscillation does not occur, the 0.1µF capacitor again charges up and the Diac fires to switch on Q4 again. Ul­ t imately, oscillation will occur with Q3 and Q4 switching on and off in alternate fashion. The frequency of operation is set by the combined inductance of L2 and the N1 winding and the .001µF capacitor across fluorescent tube FL1. The oscillator current now passes through the fluorescent tube’s filaments and allows the normal mercury discharge to take place inside the tube. When this happens, the .001µF capaci­tor across the tube is effectively shunt­ed out by the mercury discharge. These are the starting pulses present at the drain of Q4 with no tube in circuit. Pulses from Diac1 drive the base of Q4 and switch it on. Note that a 10:1 probe was used for this meas­urement. This takes place at a peak voltage of about 100 volts. The frequency of oscillation is now determined by the prop­erties of the core of transformer T2. As the current builds up in winding N1, the core begins to saturate. When this happens, the flux in the core stops changing and the gate drive to Q3 or Q4 ceases. The flux now collapses to drive the opposite Mosfet and this process continues to maintain oscillation. Current through the tube is limited by the current at which the T2 core saturates and the L2 inductance. These two components provide the same current limiting function for the tube as does the ballast in a conventional fluorescent lamp fitting, except that the frequency is many times higher than 50Hz. The start-up circuit, comprising the 0.1µF capacitor and Diac1, is prevented from interfering with the normal operation of the circuit by diode D5. The diode discharges the 0.1µF capacitor every time Q4 is switched on, thus preventing the Diac from fir­ing. The gate drive to Q3 and Q4 is limited using two parallel 330Ω gate resistors and 12V zener diodes which clamp the gate voltage to a safe value for the Mosfets. The 330Ω resistor from gate to source provides a load for transformer T2 so that the saturation characteristic for the core can be accurately set. Note that while Mosfets Q1 and Q2 in the DC to DC converter are fitted with heatsinks, Q3 and Q4 switch only small currents and therefore they do not require heatsinks. However, during the switch-over process, when one Mosfet is switched off and the other turns on, the Mosfet which is turned This is the waveform at the drain of Q4 when driving an 18W tube. The overall amplitude is 330V peak to peak & the fre­quency is 110kHz. November 1993  29 12V off commutates whereby its internal reverse diode briefly conducts. This commutation can lead to high dissipation in the Mosfets. To reduce this dissipa­ tion to almost zero we have connected a snubber network to the output (ie, the junction of Q3 and Q4). The snubber network consists of a 680pF capacitor in series with a 22Ω resistor. The two 150kΩ resistors connecting from the 680pF capacitor to the +340V supply act as a load for the circuit if the fluores­cent tube is not present. F1 0.1 82  470uF L1 .001 47k 1M 1k 1k IC1 TL494 4.7k 0.1 ZD1 1 4.7k 0.1 1 IC2 4050 270k 470uF T1 GND TERMINAL 1 470uF Q1 270k D1-D4 0.1 250VAC 0.1 150k 680pF 150k 150k 22  ZD3 Q4 330  330  150k ZD2 Q3 N2 330  T2 N3 330  330  330  N1 0.1 250VAC L2 F2 1 .001 3kV TO TUBE END TO TUBE END 30  Silicon Chip Fig.3: the PC board layout. Note that transistors Q1 & Q2 are fitted with heatsinks & note also that high voltages are present on the board when power is applied. Q2 ST2 6 F1 S2 F2 S F 1 10 PRIMARIES: 4T, O.5mm DIA. ENCU SECONDARY: 136T, 0.4mm DIA ENCU Fig.4: the winding details for transformer T1. Note that the primary windings are bifilar. Circuit changes 8.2k 0.1 T1 S1 5 There are a few changes to be made to the circuit, depend­ing on whether it is to be used with an 18W or 36W fluorescent tube. These are shown in the table on the circuit. The input fuse (F1) is 2A or 5A and the winding details of transformer T2 are varied. The reason why transformer T2’s windings are varied is to vary the frequency of the fluorescent driver circuit and thereby set the current through the tube. For 18W tubes the frequency is 110kHz and for 36W tubes the frequency is about 65kHz. While the frequency for the 36W tube is not quite halved, the changes to transformer T2, combined with the fixed inductance of L2, means that the current is doubled. High frequency operation Before concluding the circuit description, we should make a comment WARNING! This project develops potentially lethal voltag­es. At no time should any part of the circuit be touched while power is applied. This project should not be attempted by inex­perienced constructors. about operating fluorescent tubes at high frequencies. In some technical literature, fluorescent tubes are stated to be much more efficient at high frequencies. This is not true. There may be a small difference between operation at 50Hz and, say, 1kHz but above that, the light output from a fluorescent tube is directly proportional to the current through it, although there are limiting factors above which the tube becomes overheated and its life is shortened. Therefore, the efficiency of the circuit is much the same for the 18W and 36W tube versions of the cir­cuit, regardless of the fact that the operating frequencies are different. Note also that the 18W version of the circuit will work with a 20W tube and the 36W version will work with a 40W tube. The slightly higher rated (and thicker) tubes have the benefit that they are easier to start but they are more expensive. We should also make some comments about the circuit effi­ ciency. We have set the current through the respective 18W and 36W fluorescent tubes to be close to the value it would be if running in a conventional 50Hz ballast circuit. This results in the A piece of blank PC board material is used to prevent direct contact with the underside of the components board through the large cutouts in the batten base. The hole for the starter (in the batten cover) should also be sealed. The assembled PC board fits neatly in one end of the batten, as shown in the photograph at top. Make sure that the board is properly secured before fitting the cover & the fluorescent lamp. 18W version of the circuit drawing 1.5 amps from a 12V supply and the 36W version drawing 3 amps from a 12V supply. Does this make the circuit 100% efficient? The answer is clearly no since an 18W fluorescent tube does not dissipate 18 watts – a significant amount of power in an 18W fitting is dissi­pated by the ballast. Hence, while we cannot quantify the overall circuit efficiency, we can state that it is quite high and cer­tainly higher than other inverter designs intended for driving fluorescent tubes. Our estimate of the efficiency is “better than 80%”. Construction The PC board for the circuit is coded 11312931 and measures 286 x 46mm. It will fit inside a standard 18W or 36W fluorescent tube batten. Construction can begin by winding the toroids and the transformers. Let’s start with L1, the larger of the two toroids and brown in colour. Wind on 18 turns of 0.8mm enamelled copper wire with even spacing around the toroid. L2 is not a toroid but is the smaller ferrite assembly comprising a bobbin, two core halves and two clips. Before wind­ing this you will need to set the gap in the centre leg of the core halves. You will need a small file (a points or hobby sized file would be ideal) and a set of feeler gauges. Initially, place the two core halves together and observe that there is no gap between the mating surfaces of the core halves. Now file only the centre leg of one core half, making sure that you are filing squarely and evenly across the face. The required gap is 0.15mm and can be accurately measured with feeler gauges when the two halves are held together with your fingers. The whole process should not take longer than 5 minutes since the ferrite material is quite soft. Now wind 60 turns of 0.4mm enamelled copper wire onto the bobbin, with the start end soldered to pin 6. Wind each layer neatly side by side across the bobbin and insulate between each layer with a length of insulating tape. Solder the end of the winding to pin 7 on the bobbin. The inductor can then be complet­ed by fitting the core halves into the bobbin and securing them with the clips. Transformer T1 is the larger of the two ferrite assemblies. The ferrite RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 2 4 1 1 2 2 2 1 1 Value 1MΩ 270kΩ 150kΩ 47kΩ 8.2kΩ 4.7kΩ 1kΩ 330Ω 82Ω 22Ω 4-Band Code (1%) brown black green brown red violet yellow brown brown green yellow brown yellow violet orange brown grey red red brown yellow violet red brown brown black red brown orange orange brown brown grey red black brown red red black brown 5-Band Code (1%) brown black black yellow brown red violet black orange brown brown green black orange brown yellow violet black red brown grey red black brown brown yellow violet black brown brown brown black black brown brown orange orange black black brown grey red black gold brown red red black gold brown November 1993  31 Fig.5: this is the PC artwork reduced to 70.7%. To reproduce it full size, use a photocopier with an expansion ratio of 1.41. cores should not be gapped in this case since we want the gap to remain at zero. Fig.4 shows the winding details. Wind on the secondary first using 0.4mm enamelled copper wire. Termi­nate the start of the winding at pin 2 and neatly wind on one layer of wire across the bobbin. Insulate this with a layer of insulating tape. 32  Silicon Chip Note here that the start and finish of the insulating tape should begin on the underside of the bobbin (ie, the pin side of the bobbin). This will ensure that the ferrite core halves will fit over the completed windings. Continue wind­ ing until 136 turns have been wound on in several layers with insulation tape over each layer. Terminate the finish of the winding at pin 1. The two primary windings are wound bifilar (ie, two wires at the same time), with one end of each winding starting at pins 4 and 5 and finishing at pins 6 and 7, respectively. Wind on four turns, making sure that the two windings do not cross over each other. Note that there will not be sufficient room to cover the windings with insulation tape. These windings can be clearly seen in one of the photos accompanying this article. When the bobbin is completed, fit the core halves and the retaining clips. Toroid T2 is wound as detailed in the table on the circuit diagram (Fig.2). If you are making the 36W version, use 0.4mm enamelled copper wire. If you are making the 18W version, use 0.25mm enamelled copper wire. Wind on the N1 winding, keeping the windings tightly packed toward one side of the toroid. The N2 and N3 windings must be wound in the same direction as the N1 wind­ing. With the transformers and inductors complete, assembly of the PC board can proceed. Before installing components, check the board for shorts or breaks in the tracks. Also check the holes for correct sizing for each component. You will need 3mm holes for the PC board mounting, transformer T2 and for the heatsink mounting feet. Two 3mm holes are also required for a cable tie to hold down L1. Start the board assembly by inserting all the PC stakes plus the four 2AG fuse clips. This done, insert the resistors, links and diodes, followed by the two ICs. Make sure that the diodes and ICs are cor­rectly oriented before soldering. The same comment applies to the electrolytic capacitors. The ST2 (Diac1) can be installed either way around. Now install transformer T1 and inductor L1 onto the PC board, taking care that pin 1 marked on the bobbin is oriented correctly. Transformer T2 is mounted using a transistor mounting bush together with a Nylon screw and nut. L1 is held in position using a small plastic cable tie. Mosfets Q1 and Q2 are fitted with small vertical heatsinks using machine screws and nuts. Apply a smear of heatsink compound to the mating Below: this close-up view shows how transformer T2 is secured to the PC board using a Nylon screw, a transistor insulating bush & a nut. Coil L1 at the other end of the board is secured using a plastic cable tie. Be sure to install Q3 & Q4 with their metal tabs adjacent to the edge of the board. surfaces before screwing the Mosfet body to the heatsink. Each heatsink is secured using the integral mounting feet which pass through the holes in the PC board. When they are inserted into the board, use a pair of pliers to twist the feet and hence lock them into the board. This done, solder the Mosfet leads to the copper pattern. Mosfets Q3 and Q4 can also be mounted at this stage – they do not require heat­sinks. Finally, fit the fuses into the fuse clips and the board is complete. Installation We recommend that the PC board be installed into the fluo­rescent batten before testing, because the voltages developed by the circuit are potentially lethal. Before installation, the existing ballast, starter and terminal strip will need to be removed from the fluorescent batten. Now drill holes to accommodate the PC board and drill out a hole for the cord grip grommet suitable for the 12V lead entry. We mounted the board on top of a piece of blank PC board material to cover the copper tracks (any other insulating material would do), while the hole for the starter was covered using a piece of plastic and a metal clip. This will prevent direct contact with the underside of the PC board through the large cutouts in the batten base and cover. The board mounts onto transistor mounting bushes, used here as low profile spacers, and is secured at six points with screws and nuts. Connect up a length of polarised twin-lead to the 12V input and connect the wires from the tube ends to the PC board as shown on the wiring diagram. The negative terminal of the PC board is connected to chassis using a short piece of hook-up wire soldered to a solder lug. Testing Once the PC board has been installed in the batten, you are ready for testing. Insert a fluorescent tube into the fitting and apply power. The tube should initially glow with a bluish tinge for a half second or so and then come on with full brilliance. There should be no flicking during the startup phase (as is the case with normal fluorescent lights) and there should be no discernible flicker at all once the tube is at full brilliance. PARTS LIST 1 PC board, code 11312931, 286 x 46mm 1 blank PC board, 336 x 46mm 1 18W or 36W fluorescent tube batten 1 EFD25/13/9 3F3 core (no air gap), former and clips (2 x Philips 4312 020 4116 1, 1 x 4322 021 3524 1, 2 x 4322 021 3516 1) – T1 1 RCC12.5/7.5/5 3F3 ring core (1 x Philips 4330 030 3792 1) (T2) 1 RCC17.1/9.8/4.4 2P90 ring core (1 x Philips 4330 030 6031 2) –L1 1 EFD20/10/7 3F3 core, former and clips (2 x Philips 4312 020 4108 1, 1 x 4322 021 3522 1, 2 x 4322 021 3515 1) – L2 2 battery clips (1 red, 1 black) 2 vertical mount TO-220 heatsinks (Jaycar Cat. HH-8504) 4 2AG PC mount fuse clips 1 5A 2AG fuse (36W version) 1 2A 2AG fuse (18W version) 1 400mA 2AG fuse (36W version) 1 200mA 2AG fuse (18W version) 1 cord grip grommet 7 transistor mounting bushes (6 for 4mm PC board standoffs) 1 3mm Nylon screw and nut 1 small cable tie 2 3mm dia x 6mm long screws & nuts 8 3mm dia x 12mm long screws, nuts & washers 1 solder lug 1 80mm length of 0.8mm tinned copper wire 1 600mm length of 0.8mm enamelled copper wire If the inverter does not power up the fluorescent tube, switch off power immediately and check for faults. Check that all the components are located correctly and that the transformers and inductors are wound and oriented correctly. Transformer T2 must be wound with correct phasing or the oscillator will not function. You can check that the DC-DC converter is functioning by measuring the voltage between the GND terminal and F2 fusehold­er. It should be 340V DC. Use your multimeter set to read 1000VDC and check that the multime- 1 10.5m length of 0.4mm enamelled copper wire 1 500mm length of 0.5mm enamelled copper wire 1 800mm length of 0.25mm enamelled copper wire 1 2m length of twin automotive wire (polarised) 7 PC stakes Semiconductors 1 TL494 switchmode IC (IC1) 1 4050 CMOS hex buffer (IC2) 2 MTP3055E avalanche protected N-channel Mosfets (Q1,Q2) 2 BUK455-600A, BUK457-600B high voltage N-channel Mosfets (Q3,Q4) 5 1N4936 fast recovery diodes (D1-D5) 1 ST2 Diac (DIAC1) 1 16V 1W zener diode (ZD1) 2 12V 1W zener diodes (ZD2, ZD3) Capacitors 3 470µF 25VW PC electrolytic 2 0.1µF 250VAC metallised polypropylene 5 0.1µF MKT polyester 1 .001µF MKT polyester 1 .001µF 3kV 1 680pF 3kV Resistors (0.25W, 1%) 1 1MΩ 2 1kΩ 2 270kΩ 4 330Ω 1W 4 150kΩ 2 330Ω 1 47kΩ 1 82Ω 1 8.2kΩ 1 22Ω 2 4.7kΩ ter probes are in good condition before making this measurement. The voltage is potentially lethal. Further tests can be made using an oscilloscope. You must connect the oscilloscope probe earth connection to the GND termi­nal on the PC board. Keep your probe set on 10:1. You should be able to see the starting pulses applied to Q4, by measuring the waveform at the drain (metal tab) of Q4 when the tube is out of circuit. The oscillation should be observed at Q4’s drain when the tube is installed SC (see oscilloscope photographs). November 1993  33