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servicing, repairing and replacing While this article is primarily concerned with servicing a V6295-type vibrator, the general advice would apply to many mechanical vibrator units found in vintage radios and other contemporary equipment, mainly those with three sets of contacts. I will also present a straightforward Mosfet-based circuit that acts as a solid-state replacement for a vibrator. It even fits in an original-looking can! Part 1: by Dr Hugo Holden T he inspiration for this article was my NZ-made ZC1 Mk2 military communications radio, designed to run from a 12V battery. Like many battery-powered radios, the HT supply was provided by an electromechanical switching device with a vibrating reed and contacts known as a split-reed synchronous vibrator (or just ‘vibrator’). In this case, it is a 7-pin unit, type V6295. The circuit of the “vibratorpack” power supply is shown in Fig.1. The vibrator is within the circle; the other components are external to it. The V6295 has a pair of contacts to switch the primary winding of transformer T3 and another pair to switch the secondary winding for synchronous full-wave rectification. One extra contact in the unit is used to switch the magnet coil on and off, to sustain mechanical oscillations of the vibrating reed at around 100Hz. This system was quite efficient, as the coil in the unit only consumed about 2W, and the contacts, when closed, had very low resistance. However, in common with all mechanical contacts which switch an inductive load, the contacts wear and burn, degrading after tens to perhaps 100 hours of use. Another significant problem is due to the latex rubber inside the unit, described later. There are numerous articles on how to repair the V6295. It involves cleaning the contacts of all oxides, ensuring their surfaces mate in perfect opposition when they close, and adjusting the contact gaps. The small contact for the vibrating reed is usually adjusted for maximum oscillation amplitude, consistent with good starting; however, it also has a role in very fine adjustment and contact switching symmetry. If the primary side contact gap is too Fig.1: the ZC1 Mk2 radio power pack with the V6295 vibrator in the centre. The 12V DC supply from the battery at lower right is converted to a 200V+ HT output on the left, mainly due to the interaction of the vibrator and transformer T3. 86 Silicon Chip large, the power pack output voltage drops off as the duty cycle is reduced. If too narrow, the contacts arc over. In addition, if the contact gap is too large, there is an excessive voltage overshoot on the leading edges of the transformer’s primary winding connections. The primary contacts must also have a slightly longer duty cycle than the secondary contacts and overlap when the secondary contacts are closed – see Fig.2. Thus, there is a brief time when no contacts are closed, and the transformer’s field is collapsing. The transformer’s tuning capacitors are chosen so that the voltage overshoot is as low as possible, thereby minimising the contact arcing and voltage spikes. Restoring an original V6295 vibrator (or a similar type) involves four main steps. The first is checking its mechanical integrity and, if necessary, performing any repairs. The second is Fig.2: the secondary contacts are closed for a shorter duration than the primary contacts, and there is a gap between one set of primary contacts opening and the other set closing. Correct timings and symmetry are essential for reliability and low output ripple. Australia's electronics magazine siliconchip.com.au Photo 1: an extension socket like this is invaluable for checking and adjusting vibrators. The loops in the wire make it easy to attach oscilloscope probes. Photo 2: the vibration-dampening natural rubber parts of the V6295 are its downfall. They degrade over time, fouling the contacts. static contact adjustment, while the third is dynamic contact adjustment. The final step is using an oscilloscope to check that it works perfectly. Let’s take these one by one. metal surfaces of the contacts oxidise from being exposed to air, and metal oxides are insulators. Also, any contact arcing produces very corrosive gasses, which are trapped inside the housing. #1 mechanical considerations Restoration Suppose you don’t have an extension plug/socket to support the unit while out of its housing and making adjustments, simultaneously giving you access to the electrical connections. In that case, you will need to make one. Mine is shown in Photo 1. The V6295 needs to be in a condition where it can be disassembled and reassembled without damage. Surprisingly, the main reason a V6295 will not run after a period of storage is due to the latex rubber inside the housing, not contact oxidation. However, the latter is also a factor over longer time frames. As latex (natural rubber) ages, it melts and turns into a tacky brown liquid, then a vapour – see Photo 2. In a closed container such as the metal housing, the liquid goes into equilibrium with the vapour. The vapour is deposited as a sticky brown liquid on the contacts as months and years pass. High storage temperatures speed up this process. For example, an immaculately cleaned and adjusted V6295 was put into storage. Two years later, it would not run. Taking it out of its housing again, brown deposits had appeared on all the contact surfaces, insulating them and causing them to stick together. This material is identical to the areas of melted latex. Therefore, all this old latex needs to be replaced. Even without this latex problem, the The best way to remove a V6295 or similar vibrator from its housing is by gently prising up the zinc material, working around the can very slowly until the lip is unfolded. Next, carefully smooth it to remove any marks. You can replace the rubber inside the unit with various soft, rubber-like products that do not break down as quickly. One example of a very stable, soft material that can withstand high temperatures without breaking down is silicone. It isn’t rubber (which comes from a tree), even though people often refer to anything with similar properties as ‘rubber’. One thing to note is that the zinc canister is a little short, and there is only a minimal amount of room between the mechanism’s top surface and the inside of the zinc case top area. A 1-1.5mm thick silicone rubber sheet is suitable for this top area. For the remainder, the material from an ordinary 4mm-thick soft Yoga mat (usually PVC foam) is suitable and easy to get. When the old latex is removed around the base area, it frees up a siliconchip.com.au metal washer that can be separated from the base by a new felt washer (green) shown in Photo 3. When the unit is reassembled, it is important that you can feel the mechanism shaking back and forth in the housing when held upright. Excessive mechanical coupling of the mechanism to its housing results in mechanical vibrations being coupled to the entire radio, making it noisy. #2 contact cleaning and static adjustments First, clean any latex deposits off with contact cleaner, passing paper strips between the contacts. Clean them further with a fresh piece of 15mm-wide 800 grit abrasive paper folded in half, with a sharp fold, placed between the contacts. With gentle pressure closing the contacts, both faces are cleaned simultaneously. A final wash with contact cleaner is required to remove any fine debris. Never file the contacts under any circumstances, as this ruins their flat faces, and it will not be possible to have a unit with good output and any longevity after that. It is essential that the mechanical alignment of the contacts is such that when their faces meet, their entire surface areas are touching, and the faces are parallel, as shown in Photo 4. Felt washer between metal washer and base Australia's electronics magazine Photo 3: a silicone rubber disc and pieces cut from a PVC yoga mat replace the natural rubber and are much more stable over time. June 2023  87 Photo 4: before adjusting the contact gaps, ensure the contacts are clean and close to perfectly parallel. Photo 5: here I am operating the vibrator in the radio (in this case, a ZC1 Mk2 communications receiver) to test it before replacing it in its metal can. You can now make the static adjustments. A first approximation of contact settings is achieved by setting their gaps to 0.1-0.15mm for the primaries and 0.22-0.28mm for the secondaries. #3 dynamic contact adjustments The best results can only be obtained from the V6295 after a dynamic contact adjustment. This involves running the unit out of its housing while monitoring three things with an oscilloscope and voltmeter. The points to monitor are the primary connections on the transformer, the DC output voltage and the ripple voltage at the filter output (the left end of filter inductor L9B in the case of the ZC1; see Fig.1). Photo 5 shows a V6295 running via the extension, taken at a moment when the vibrating arm was deflected. A pair of secondary contacts can be seen to be closed, with the other pair wide open. When the primary contacts are correctly set, each contact is closed for a nearly identical period. You can place a slight bias pressure on the contact (with a plastic tool, be mindful of the voltages) to check the effect while viewing the scope. It is important to only deflect them close to their bases, to keep the contacts as parallel as possible. If both the primary contacts are too closely or too widely spaced, arcing will be seen between them. If there is asymmetry, one will have a slight arc and the other not. A minor adjustment on the vibrating reed contact can correct the centring of the mechanical motion. More significant corrections must be made by moving both contacts. There are the contact gaps to consider, plus the symmetry of opening and closing comparing one contact to another. Scope 1 shows the waveforms with correct primary contact adjustments, with the scope probes connected to pins 6 & 1 on the V6295, effectively across the transformer primary. The time that each contact is closed, t1 & t2, is in the order of 4ms. When the primary contacts are closed, the voltage on the corresponding trace is 0V. You can see that the periods are very close to equal in this case (t1 = t2). Scope 2: if the secondary contacts are not adjusted correctly, the ripple on the DC output will vary on every second pulse like this. Scope 3: when the secondary contacts are correctly adjusted, the DC output ripple will be reduced in amplitude and more consistent, as shown here. Secondary adjustments The secondary contact spacing and symmetry profoundly affect the ripple voltage superimposed on the DC output (as well as the DC value itself). If the secondary contacts are too closely spaced, arcing and flash-over occur. Again, the effect on the ripple voltage can be seen by placing a slight bias on the contact with a plastic tool while the unit is running. Scope 2 shows when the secondary contacts are out of adjustment, resulting in a very asymmetrical ripple voltage. That is with 11.6V DC into the vibratorpack’s input, the sender switched on and the ZC1 in receive mode RT. Scope 3 shows the t1 t2 Scope 1: probing the two primary contacts in the vibrator (which connect directly to either end of the transformer primary) should reveal a symmetrical waveform. If it’s asymmetric, adjust the contact gaps. 88 Silicon Chip Australia's electronics magazine siliconchip.com.au Scope 4: with the two transformer primary waveforms (middle and bottom) along with the DC output (top), we can see that the secondary contacts close for shorter periods than the primary contacts. correct adjustment of the secondary contacts, resulting in a symmetrical ripple voltage. Scope 4 is a triple trace, showing both the primary voltages and the output ripple with a well-adjusted V6295. Note how the time that the primary contacts are closed is a little longer than the secondary contacts due to the wider secondary contact gaps. The multiple overlaid traces is an artefact of the photographic timing. Scope 5 shows an electronic V6295 replacement plugged in place of the mechanical V6295. This unit runs at 60Hz rather than 100Hz and makes for an interesting comparison. Notice the absence of spikes and transients in the electronic unit and the differently shaped ripple voltage, which is still about 2V peak-to-peak. Scope 5: with a Mosfet-based vibrator replacement instead of the mechanical V6295, the waveforms are somewhat cleaner (at 66Hz rather than 100Hz), but the DC ripple on the output is similar in magnitude. two Mosfets, is among the easiest to build, works exceptionally well and is quite efficient, being slightly more efficient even than the mechanical type (which has a standing power draw of around 2W). If you don’t have an existing housing suitable for this device (eg, taken from a failed mechanical vibrator), you can use a readily-available round aluminium housing. This commercial air intake pipe joiner, 75-76.2mm (3in) long and 38mm (1.5in) in diameter, is available on eBay – see Photo 7. It uses a standard Amphenol 7-pin base, also usually available on eBay, shown in Photo 8. This unit produces very clean switching waveforms and will start from voltages as low as 8V, even when the supply is loaded. Unlike units driven by independent oscillators, it does not require a tuning capacitor on the transformer primary. Also, it is intrinsically short-circuit protected because if the supply is overloaded and oscillations stop, both Mosfets turn off. The circuit is shown in Fig.3. Two Mosfets replace the primary contacts of the vibrator, while pairs of series-connected BY448 1500V diodes replace the secondary contacts. This might seem like overkill, but it #4 reinstallation You can solder a brass wire ring into position to re-fit the unit to the housing, as shown in Photo 6. This way, it can easily be removed later for more repairs/adjustments. Do not re-crimp the zinc can, or it can only be cleaned and repaired once, as the zinc casing will fracture. Solid-state vibrator replacement I have built several different solid-­ state circuits to replace a mechanical vibrator, including two using Mosfets, one using Darlingtons and one using bipolar transistors. The one presented here, using little more than siliconchip.com.au Photo 6: the vibrator can be held in its can using a C-shaped piece of brass wire. This makes it much easier to open again later. Photo 7: the vibrator replacement looks very similar to an actual vibrator, but is made from all-new parts. If you have a defunct vibrator in a suitably-sized can, you could possibly reuse it (and maybe its base). Australia's electronics magazine June 2023  89 OD = 35.3mm ID = 29.4mm Height = 8mm Photo 8: the spacer is held in the centre of the base/plug with a 10mm CSK M3 screw, and the PCBs are, in turn, held to the spacer using two M2 machine screws through holes drilled in it. The cylindrical spacer is used to attach the can to the base. Scope 6: the Mosfet-based vibrator replacement generates waveforms with rounded edges, as they do not switch super fast (to avoid RFI). is necessary to have a very high PIV (peak inverse voltage) diode rating. If the unit is unplugged while running (or there’s a bad connection to one of its socket pins), the undamped collapsing field of the main vibrator transformer can produce a peak voltage high enough to break down and destroy a single 1N4007 rated at 1000V. Each Mosfet is switched on by a positive spike coupled from the opposite end of the transformer when the opposite Mosfet switches off. This pulse is coupled via a 470nF capacitor with a 1.6kW series resistor. The Mosfets switch off after a defined time due to the gate discharge resistors; the result is alternating oscillation. 10nF gate-drain capacitors and 300W gate resistors slow the switch-on and switch-off times of the Mosfets to prevent RFI, while 18V zener diodes prevent the gates from exceeding their ±20V Vgs ratings. When one Mosfet switches on, the rapid drop in its drain voltage will couple through to the gate of the other Mosfets switch more-or-less simultaneously. The fact that the coupling capacitor values are relatively low (under 1µF) assists in making a unit that will slide easily inside the pre-made 38mm aluminium tube, a similar size to a standard vibrator can. Mosfet via the 470nF capacitor and 1.6kW resistor, ensuring it switches off simultaneously. The exact oscillation frequency will depend on the transformer characteristics. Scope 6 shows the drain voltages with the unit in operation in the ZC1 in receive mode, while Scope 7 shows the gate voltage of one of the Mosfets when conducting. The 470nF capacitor charges via the fellow Mosfet’s drain voltage (24V) and 10kW gate resistor until the charge current drops off and the gate voltage approaches the threshold Mosfet’s voltage. By that time, some transformer core saturation is beginning, so the feedback rapidly falls away, the Mosfet turns off, and the fellow Mosfet is driven into conduction. The unit runs at 66Hz in my set. Looking closely at the switching transitions on the transformer primary (drain connections) at 10V/div (Scope 8), they are free from radio frequencies and excessive voltage overshoot with this circuit. You can see how the Construction Various Mosfets will work in this circuit. While I used TO-3 case versions, TO-220 case versions could be used with some lead bending, such as the ubiquitous IRF540N, available from Jaycar and Altronics. Suitable TO-3 case Mosfets include the IRF130, IRF230, IRF350, 2N6756 and 2N6758. The 2N6758s made by Harris, available on eBay, are particularly good quality. It is based on two small, simple PCBs that sit back-to-back, as shown in Fig.4. They are not identical but very similar, with the only difference being the routing of one track. The holes for the components all fall on a Fig.3: this self-oscillating Mosfet-based replacement for the vibrator is nice and simple, needing just two Mosfets, four regular diodes, two zener diodes and 10 passive components. The properties of the external transformer set the oscillation frequency to around 66Hz. Note that the 18V zeners were 20V types in the original design but the voltages have been lowered slightly to allow for part tolerances. Photo 9: the base with the tapped spacer, BY448 diodes (note that the other two diodes are hidden inside the holes in the plug pins) and tinned copper wires already attached. 90 Silicon Chip Australia's electronics magazine siliconchip.com.au Scope 7: a Mosfet drain waveform (top) and its corresponding gate waveform (bottom). You can see how the gate voltage decays during each cycle until the Mosfet switches off and the opposite Mosfet switches on. 2.54mm grid, except for the TO-3 transistor holes, which do not land exactly on the grid due to the geometry of a TO-3 package. Assembly of each PCB is straightforward, with just six components on each board. Use Fig.4 as a guide to mounting the components on both, including the TO-3 Mosfets, which should be bolted down before soldering and trimming the leads. Solder the BY448 diodes directly to the base, as shown in Photo 9, with the second diode in each pair down in the appropriate pin recess. You will also need to cut and drill a metal hexagonal tapped spacer, as seen in Photo 8, plus a 3mm countersunk hole in the centre of the base to attach this spacer later. To ensure the 3mm diameter hole in the plug is drilled on-centre, a temporary 3mm spacer can be placed in the ¼in recess to guide the drill. The hole is then countersunk from the pin side of the plug. The hole for the 3mm countersunk Scope 8: a close-up of the Mosfet drain waveforms, showing how one Mosfet switches on (rising gate voltage) just after the other (with a falling gate voltage) switches off. screw needs to be centred in the well in the plug where the hex brass spacer fits and it is easiest to drill it from that side (opposite side to the pins). With the temporary spacer in the well to act as a guide, run the 3mm drill down the centre of that spacer to make the hole. Then once that hole is made, flip the plug over and use a countersinking tool on the material for the head of the screw. A larger sized drill should not be used as the drill could pass through by accident. The spacer’s end needs to be rounded off a little to fit into the deep hole in the UX7 plug. Additionally, a cylindrical spacer is needed to help fit the finished unit into the aluminium tube. This has an outer diameter of 35.3mm, an inner diameter of 29.4mm and can be 8-10mm tall. I cut the one shown in Photo 8 out of a piece of phenolic plate with two hole saws, then trimmed it to size. This spacer can also be made of metal, like aluminium. Attach the drilled, tapped spacer to the base as in Photo 9, and solder three solid-core wires to pins 1, 6 & 7 to connect to the PCBs later. You can use 0.7mm diameter tinned copper wire with insulating tubing slipped over the wires. Glue the cylindrical spacer to the Amphenol base using 24-hour epoxy (eg, Araldite). The two PCBs are mounted with a 5.4mm gap between them. The wire for the Earth connections passes between the PCBs. Two other ‘crossing’ wires are required, visible in Photo 10. These Fig.4: the two PCBs are similar but with some parts rotated or swapped as they mount back-to-back. Points X & Y on the two boards are joined (X to X and Y to Y), while both GND points are wired to pin 7 on the socket. Pins 1 & 6 are wired to the metal cases of the two TO-3 package Mosfets (not shown here). Photo 10: the vibrator replacement is now operational, with the two PCBs assembled, wired up and attached to the base via the vertical spacer. Australia's electronics magazine June 2023  91 Pin 7 (Earth) Pin 1 Pin 6 Photo 11: a short spacer over the top screw that holds down the two TO-3 Mosfets (insulated from their cases) keeps the PCBs apart. Photos 13 & 14: views of the finished vibrator replacement sans can. are Teflon-covered wire wrap types; however, any light-duty hookup wire would work. Ideally, the PCBs should have plated through holes. In the absence of those, for this hand-made prototype, I used small brass eyelets for the connections between the PCBs. Use a screw and nut to secure the Drain connections from pins 6 and 1 of the Amphenol base to the lower TO-3 transistor mounting holes. Rotate the PCB assembly so that the gap between the PCBs is over pin 7. This allows the wires from the base to pass in a very direct and orderly way to the Earth and two drain connections. Secure the upper mounting holes between the transistors with a spacer and some insulators, as shown in Photo 11. The two boards are joined at the top by using two transistor insulators, a 5.4mm high and 3.5mm diameter spacer, a wave washer and M3 nut plus a 4-40 UNC by 3/4-in (or M3 20mm) binder head screw (shown at the end of the article); Photo 12 shows the result. Photos 13 & 14 show the finished assembly. The final procedure is to fit the completed unit into the pre-made aluminium tube (it might be a good idea to check that it works first!). The top of the tube can be sealed with a 35.3mm diameter, 6mm-thick disc glued into place. This is a firm press fit. I made the disc shown in the photos from Bramite, a fibreglass-like insulator; however, it could be made from aluminium, Paxolin or any other material. The base is a firm fit into the tube. You could glue it in, assuming you have already tested it, because it is unlikely ever to require repairs. However, it can be retained with a 1.21.4mm spring clip made from spring 1.4mm spring wire clip Photo 12: the assembly is a relatively tight fit in the can, but it does fit. If you’re having trouble inserting it, try slightly filing the edges of the PCBs, careful that you don’t encroach on the copper tracks. 92 Silicon Chip Photo 15: like the real vibrator, the best way to retain the vibrator replacement in the can is with a C-shaped spring wire clip. It can be made by bending a piece of spring wire around a cylindrical former (the outside of the can, if necessary). Australia's electronics magazine Photo 16: the replacement (right) doesn’t look exactly like the original (left), but a casual observer probably wouldn’t notice the substitution. siliconchip.com.au steel wire. The clip engages the existing groove in the aluminium housing, then varnish is applied. This allows disassembly if required one day (see Photo 15). Photo 16 shows the finished replacement unit next to an original V6295. The air intake coupler is about 1.5mm longer than the original housing, so I trimmed 1.5mm off the lower edge (at the base end), but this is not necessary; it still fits well in the socket without doing that. Efficiency I measured the output voltages and efficiencies of the original V6295, this design and several other replacements (some of which will be described in upcoming issues). I made these measurements with a 12V DC supply, a 3.75kW load and a 47µF capacitor across the load resistor. The original unit delivered 267V DC at an efficiency of 66.6%, while the Mosfet replacement unit described here managed 276V DC at 67% efficiency. The most efficient unit with the highest output voltage is the somewhat more complicated oscillator-driven Mosfet version, at 72.7%. That is to be expected because of the low power drive requirements for the Mosfet gates and the low RDS(on) figure of the Mosfets used in that design. That is one of the designs to be described in a future issue, likely later this year. Positive-ground radios For positive ground radios, it’s possible to use the same design by using complementary devices (ie, P-channel Mosfets instead of N-channel Mosfets) and reversing both zener diodes. No other components in this design are polarity-sensitive. That is a great advantage of circuits using discrete parts rather than ICs; they are easily flipped to the opposite polarity if SC necessary. Some of the hardware used to assemble the vibrator replacement. siliconchip.com.au Parts List – V6295 Vibrator Replacement 1 Amphenol 7-pin base [eBay 115461595962] 1 76.2mm-long, 38mm diameter air intake pipe joiner [eBay 261366805060] 1 35.3mm diameter, 6mm-thick disc (eg, made from aluminium or FR4) 1 35.3mm OD, 29.4mm ID, 8-10mm high spacer (see text and Photo 8) 1 100mm length of 1.4mm diameter spring wire 1 double-sided PCB coded 18105231, 34 × 53mm 1 double-sided PCB coded 18105232, 34 × 53mm 2 TO-3 package N-channel Mosfets (eg, IRF350, IRF130, IRF230, IRF350, 2N6756, 2N6758) [eBay, AliExpress etc] 2 18V 1W axial zener diodes 4 BY448 1.5kV 2A axial diodes 2 470nF 63V axial plastic film capacitors 2 10nF 400V axial plastic film capacitors 2 10kW miniature ¼W axial resistors 2 1.6kW miniature ¼W axial resistors 2 300W miniature ¼W axial resistors 1 24mm+ M3-tapped metal hexagonal spacer (cut to 23mm long) 1 M3 × 20mm panhead machine screw 1 M3 × 10mm countersunk head screw 2 M3 × 5-6mm panhead machine screws 3 M3 hex nuts 1 M3 copper crinkle washer 2 transistor insulating bushes (the type used for TO-220 package tabs) 2 M2 × 10mm panhead machine screws 2 M2 hex nuts 2 solder lugs 1 5.4mm untapped spacer, 3.2mm inner diameter 1 300mm length of 0.7mm diameter tinned copper wire 1 200mm length of 1.5mm diameter heatshrink or insulating tubing 1 100mm length of light-duty hookup wire 1 small tube of 24-hour epoxy The NZ-made ZC1 Communications Radio The photo below shows the New Zealand-made ZC1 Mk2 Military Communications Radio. This radio was a masterpiece of electronics and mechanical engineering. The ZC1 Mk1 was created by the Collier & Beale Company of New Zealand, while the Mk2 upgraded design is attributed to J. Orbell of Radio Ltd. I’m very proud that this extraordinary radio was created in New Zealand, my original home. Practically every person in New Zealand learning the art of electronics & radio in the post-WWII period would have come across this radio, because they turned up in great numbers in the 1950s, ‘60s and ‘70s in surplus stores throughout New Zealand. They formed a structure on which the radio enthusiast could experiment and modify and, at the same time, learn about radio reception and radio transmitting. As a result, many of these sets were subjected to extreme modifications. It got to a point where unmodified and original units became quite rare. Many of the parts from them formed the cores of other electronics projects. The 6.3V tube heaters were connected in series pairs. As there are 11 valves in the set, one required a series resistor for its heater ballast. A photo of the front panel of the ZC1 Mk2 from the Author’s collection. For more photos of the ZC1, visit: www.radiomuseum. co.uk/zc1inside. html Australia's electronics magazine June 2023  93