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Electrolytic Capacitor Reformer & Tester, Pt.2

Last month we introduced our new electrolytic capacitor reformer and tester - ideal for anyone working with vintage radios, valve equipment or indeed the hobbyist who has gathered a collection of electrolytics over the years. Now you can not only test them, you can (usually!) breathe new life into them if they've suffered from the ravages of time!

Part 2: by Jim Rowe


Because the voltage source in this instrument can be set to provide quite high DC voltages (up to 630V) and can also supply significant current (tens of milliamps), it does represent a potential hazard in terms of electric shock. We have taken a great deal of care to ensure that this hazard is virtually zero if the instrument is used in the correct way – ie, with the lid closed and secured – even to the extent of quickly discharging any capacitor when the lid is opened.

However, if the safety switching is bypassed, especially when it’s set to one of the higher test voltages, it is capable of giving you a very nasty ‘bite’ should you become connected across the test clips or a charged high voltage capacitor. There are some situations where such a shock could potentially be lethal.

Do NOT bypass the safety features included in this design. We don’t want to lose any SILICON CHIP readers to electrocution.

With the exception of the power supply, microswitch (S6) and of course the capacitor under test/reforming, virtually all of the circuitry and components used in the Electrolytic Capacitor Reformer and Leakage Meter are mounted on a single PC board measuring 222 x 120mm and coded 04108101.

This is supported behind the transparent lid of the case – in fact, a modified storage organiser – which houses the instrument.

As you can see from the photos and assembly diagrams, the main board is suspended from the lid of the enclosure and label (which becomes the instrument’s front panel) via four 25mm long M3 tapped spacers.

Click for larger image
The completed reformer and tester built into its modified "storage organiser" case. The circuitry, including the test clips, is completely isolated when the lid is closed and any charge on the capacitor under test/reforming is bled away safely when the lid is opened. There is plenty of room inside the case for the 12V DC power supply and in this case its IEC lead, which in use emerges from a hole cut in the side of the case alongside the supply.

The LCD display module mounts just above the centre of the main board on four 12mm long M3 tapped Nylon spacers (or two such spacers if you use the Altronics LCD module).

The DC/DC converter’s step-up transformer T1 (wound on a 26mm ferrite pot core) mounts on the main board at upper left using a 25mm long M3 Nylon screw and nut, while voltage selector switch S1 also mounts directly on the board at lower left.

The only components not mounted directly on the main board are power switch S2, pushbutton switches S3-S5, the two test leads (fitted with alligator clips) and, as mentioned earlier, the microswitch. All switches are mounted on the front panel, with their rear connection lugs extended down via short lengths of tinned copper wire to make their connections to the board. All of these assembly details should be fairly clear from the diagrams and photos.

To begin fitting the components on the main board we suggest you fit the fixed resistors. These are all 1% tolerance metal film components, apart from the 0.27Ω, 2.2kΩ and 8.2kΩ 5W resistors and the 2 x 1kΩ 1W resistors.

When you are fitting all of the resistors make sure you place each value in its correct position(s), as any mixups may have a serious effect on the meter’s operation and/or accuracy. Check each resistor’s value with a DMM if you want to make sure of no mistakes.

It’s also a good idea to fit the 1W and 5W resistors with their bodies about 2mm above the top of the board, rather than resting on it. That’s because these resistors can become quite warm during an extended ‘electro reforming’ test run.

It’s logical to follow with the wire links, most of which are 0.4mm long, so they’re easily fashioned from resistor lead offcuts. There are quite a few of these links, of which five are located underneath the position where the LCD module is fitted later.

Next place the eleven 1mm terminal pins in the board – two for each of the three test point locations, two for the DC input connection and three for the high voltage output (to the micro-switch and capacitor). Follow these with the sockets for IC1 and IC2 (both 8-pin sockets) and IC3 (an 18-pin socket).

After these are in place you can fit 25-turn trimpot VR1 at centre left and trimpots VR2 and VR3 at upper right. Next are the small low-value capacitors, followed by the two larger 470nF/630V metallised polyester units and finally the two high voltage electrolytics, which lie on their side at the top of the board with their leads bent down by 90°. They are each held down using a Nylon cable tie which goes through the hole in the PC board and around the edge. Once the high voltage electros are in place you can mount the low voltage electros, three of which go at far right and the remaining 47μF unit at lower centre just near TP2.

Don’t forget to fit all of the electros with their orientation as shown in the PC board overlay diagram (Fig.3), as they are all polarised.

Next fit the two relays, making sure that they too are orientated as shown in Fig.3. Then you can solder in voltage selector switch S1, which as you can see mounts with its indexing spigot in the ‘1:30’ position. Before you fit the switch you should cut its spindle to a length of about 12mm and file off any burrs, so it’s ready to accept its knob.

After switch S1 has been fitted to the board, remove its main nut/lockwasher/position stopwasher combination and turn the spindle by hand to make sure it’s at the fully anticlockwise limit. Then refit the position stopwasher, making sure that its stop pin goes down into the hole after the moulded ‘11’ digits.

Next refit the lockwasher and nut to hold it down securely, allowing you to check that the switch is now ‘programmed’ for the correct eleven positions – simply by clicking it around through them by hand. You’ll probably need to temporarily attach the knob first to get enough grip to turn it. If all is OK, remove the knob for now.

The next step is to wind the step-up autotransformer T1. This might sound a bit daunting, but it’s not. You can find step-by-step instructions in the box titled ‘Winding Transformer T1’, which also explains how to fit the completed transformer to the main PC board.

The final components

With the transformer wound and fitted to the board, you’ll be ready to install diodes D1-D6. These are all polarised, so make sure you orientate each one correctly as shown in Fig.3. Also ensure that D1-D3 are the three 1N4148 diodes, D4 is the UF4007 and the two 1N4004 diodes for D5 and D6. When fitting the two zener diodes ZD1 and ZD2, note that they are NOT the same voltage – and of course they too are polarised.

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