High Voltage Electrolytic Capacitor Reformer

The high voltage electrolytic capacitors in valve equipment tend to deteriorate if the equipment is not used for an extended period. This deterioration takes the form of reduced capacitance and greatly increased leakage current. In some cases, the capacitor will become virtually short circuit.

If an item of equipment with capacitors in this state was used, the high current drawn could cause the capacitors to overheat and possibly explode. This can also cause damage to the mains transformer and rectifier valve.

This unit is designed to pass a controlled low current through the capacitor, allowing the internal chemical composition to reform gradually without the risk of overheating.

Two output voltages are available. The 240V setting is intended for reforming 275V capacitors, while the 320V setting may be used for 350V and higher components.

The unit is also very useful in the early stages of testing and repairing valve equipment. This use is detailed elsewhere on this web site, and will not be repeated here.

This unit is designed for operation from a mains supply of 220-240V 50/60Hz. The design is not suitable for operation from a 110-120V supply (as used in the USA and some other countries). However suggestions from a visitor to this page, for producing a 120V version, is given at the end of the article.


This unit is connected directly to the mains and produces potentially lethal voltages. Do not allow this unit to be used by young children or persons who are not aware of the dangers. The unit must be prominently labelled with appropriate warnings. It is recommended that this unit is not permanently fitted with a mains plug, so that it cannot be inadvertently connected to the mains.

The unit is intended to be used in a workshop situation, by someone who knows what he or she is doing. Anyone who is familiar with working on valve equipment should be used to dealing with high voltages and therefore able to treat this unit with due respect. This project is definitely not suitable for beginners.

Circuit Operation

The mains supply voltage (220-240V AC) is half wave rectified by D1 and smoothed by C1. R1 is a surge-limiting component, intended to limit the initial inrush current to C1. R1 must be a wirewound component.

Circuit DiagramR2 will gradually discharge C1 and any capacitors that are connected to the output of the unit when the mains supply is switched off. This will take about 15 seconds with C1 alone, and much longer with additional capacitors connected.

The voltage is then regulated by D2, D3 and D4. Note that D4 is short-circuited by SW1 on the 240V setting. R3 is the current limiting component, and will run warm. The zener diodes are rated at 5W.

R4 is added in the negative output rail to allow the output current to be monitored without breaking the circuit. This allows a meter to be connected periodically to monitor the current, rather than committing a meter for the whole reforming process. A voltmeter connected across this resistor will indicate 0 - 3.5V for 0 - 35mA output current. Thus, the voltage reading should be multiplied by ten to obtain the current in milliamps. R4 could be replaced with a suitable DC milliammeter fitted to the case.

R3 will limit the current through the capacitor to a safe limit. If the capacitor leakage current is high, the output voltage will be low due to the voltage drop in R3. The zener diode chain prevents the voltage rising above the rated capacitor voltage when the leakage is lower.


The circuit is constructed on plain matrix board. Do not use Veroboard, as the spacing between the tracks is insufficient for the high voltages involved. A printed circuit board could be designed, but it is not really necessary for a simple design like this.

Circuit BoardA suggested circuit board layout is shown, although this may be varied to suit the components and case used. Do not pack the components too close together. R3 will get warm, and should be mounted a little above the surface of the circuit board.

Alternatively the circuit could be constructed on tag strip or tag board, in true valve tradition!

The unit must be mounted in a suitable enclosure for safety. I would suggest a metal case, because some of the components run warm. A low cost diecast box would be ideal, and is sufficiently durable to survive workshop use. The case must be earthed. A mains indication neon should be fitted to the case and connected to the mains input to provide a warning that the unit is live.

Make sure the mains polarity is connected as shown, with the neutral connection to the negative terminal of C1. As mains neutral is at a similar potential to earth, the negative output terminal and test meter terminals should be at a safe potential. Do NOT rely on this however.

The output may be bought to a pair of 4mm sockets or some other connector that has no exposed live parts. Use red for positive and black for negative. A pair of test leads should be made, having 4mm plugs on one end and insulated crocodile clips on the other.


Using a test meter on a low resistance range, check that the metal case in connected to the earth pin of the mains plug. Using a high resistance range, check that there is no connection between the metal case and any of the input and output connections.

Set the meter to a high DC voltage range (400V or greater) and connect it to the output terminals. Set SW1 to the 240V position and switch the mains on. The meter should read between 220V and 260V. Switch to 320V range, and it should read between 300V and 340V. Switch the mains off, and the meter should gradually drop to zero over about 15 seconds.

Set the meter to the 10V DC range and connect it between SK5 and SK6 (across R4). Switch the mains on and the meter should read zero. Using a piece of INSULATED wire, carefully short-circuit the output. The meter should read between 3V and 3.5V (equivalent to 30mA to 35mA). R3 will be dissipating about 11W and will get hot, so do not leave the output shorted for long. The unit would not normally be left connected to a short circuit for any time, so the 7W rating of R3 is fine for normal use.

If the unit passes these tests, it can be put into use. Make a point of labelling SW1 with the voltage settings, and also label the unit with a suitable high voltage warning.

In Use

The use of this unit for testing valve equipment is described elsewhere on this web site. The subject of capacitor reforming has also been mentioned, but a little more information is given here.

Make sure the capacitor is rated at 275V or higher. Set the switch on the unit to a suitable setting for the capacitor - 240V for 275V components and 320V for 350V components. Connect the unit to the capacitor, making absolutely sure you have the polarity correct. If there is more than one capacitor in a single can, they should be tested one at a time. You will need to use a capacitor mounting clip or something similar to connect the unit to the case of the capacitor if this is the only negative terminal. Place the capacitor on an insulated surface, and do not allow it to come into contact with anything including the case of the reformer. Connect a voltmeter between SK5 and SK6 to monitor the output current.

Do not touch the capacitor or output leads when the unit is switched on. Observe the reading on the meter. If the capacitor is good, the reading will steadily fall to almost zero within about 30 seconds or so.

If the capacitor is not so healthy the meter will show a higher reading and fall very slowly. Check the reading every 15 minutes or so, it should be a bit lower each time you look. If the reading is above about 5mA (0.5V), switch off and feel the temperature of the capacitor when you check the reading. If it is warm, leave the unit off to let the capacitor cool down before continuing. If the capacitor gets too warm the leakage current will increase.

With luck the meter should read almost zero after a couple of hours, but leave it for up to about ten hours before giving up. Some sources give the maximum acceptable leakage for a high voltage electrolytic as 1mA for each 10uF, but I feel that this is excessive. I would aim for about 1mA for each 30uF. When the capacitor is done, switch the unit off at the mains and wait for the meter to read zero. This could take a minute or more, depending on the value of the capacitor.

If you have a dud capacitor that cannot be reformed, do not throw it away. The tidy solution is to fit it back onto the chassis so it looks right, but do not connect it. Fit a modern electrolytic tidily below the chassis, and no one will ever know! If you are really enthusiastic you could dig out the innards of the old capacitor and fit the modern replacement inside the can - I do not have this much patience!


I strongly recommend the use of an earth leakage circuit breaker or RCD with this unit. While it will not prevent a lethal electric shock, it will trip out if there is a short circuit or current leakage between either of the output connections and earth.

Parts List


100R 2.5W wirewound resistor


330K 0.5W resistor


10K 7W wirewound ceramic resistor


100R 2.5W wirewound resistor


10uF 450V axial electrolytic capacitor


1N4007 rectifier diode

D2 & D3

1N5380B 120V 5W zener diode (RS 447-2465)


1N5374B 75V 5W zener diode (RS 447-2421)


SPST min toggle switch (rated for mains use)


Red 4mm socket (2 off)


Black 4mm socket (2 off)


Plain matrix board


Neon Indicator


Diecast Box


3 core mains flex

Update 1 - 110-120V Input Version

The following information was kindly supplied by Jim Tills, and may be helpful to visitors from the USA and other countries with 110V-120V mains supplies:-

I built a USA version of your HV electrolytic capacitor reformer. The following changes were made:

I used a 110V AC to 220V AC auto transformer on the input (it only cost $10).
The zeners were changed as follows:



130V 5W



100V 5W



100V 5W

I replaced SW1 with an SPDT centre-off type so either D4, or D3 and D4, can be bypassed (switch wiper connects to the junction of D4 and R4). This gives approx. 130VDC, 230VDC or 330VDC output. The 130VDC is good for USA AC/DC sets. The rest of the project is very similar.

I would like to thank Jim for this information, and for kindly allowing me to include it here.

Update 2 - 275V/350V Output Version

The following information was kindly supplied by Stewart, who originally posted it in this thread on the discussion forum. The details are compiled from two forum posts, a day apart. Photos of Stewart's unit are also included.

The isolation transformer was added, partly because I had it anyway, and because I was a little wary of one of the croc clips pinging off a capacitor and landing, unknown to me, on a metal chassis or such. Removing the link to the mains with the transformer just made me feel a little happier. The transformer was a Maplin one - I don't know if they still do them though: 0-240, 0-240 @12VA.

The Letraset looked lovely until I sprayed the panel with a clear protective coating - at which point they started to move and deform. Yes, time to try something else!

Having found my original notes and re-reading Paul's article I realise why I went for different zeners: At that time I was doing quite a bit of work with valve amplifiers - some of which had pretty high HT - and I wanted to run the capacitors nearer their working voltage. It was only after selecting alternative values that I ran in to the tolerance problem and had to adjust the values slightly once more.

Paul's unit offered 240 or 320 Volts I think - I increased this to 275 and 350Volts. I had always intended to use a transformer for added safety and originally had in mind an old HT transformer (300-0-300). In the event that got used elsewhere for a repair so I used the Maplin 240V one. This in fact means that on the 350Volt setting my unit actually only gives the rectified mains/ 240V value of 336V (and somewhat less if the cap has significant leakage) - somewhat defeating the point of my changes!

I used the following zeners - doubling up the numbers to share the load a little:

D2 became 1N5383B (150V) & 1N5375B (82V) in series
D3 became 2 x 1N5355B (18V) in series
D4 became 1N5358B (22V) & 1N5374B (75V) in series

The other change was to the 10k series resistor - I found this ran quite hot so increased it to an 11 Watt one.

I would like to thank Stewart for this information, and for kindly allowing me to include it here.

This website, including all text and images not otherwise credited, is copyright © 1997 - 2006 Paul Stenning.
No part of this website may be reproduced in any form without prior written permission from Paul Stenning.
All details are believed to be accurate, but no liability can be accepted for any errors.
The types of equipment discussed on this website may contain high voltages and/or operate at high temperatures.
Appropriate precautions must always be taken to minimise the risk of accidents.

Last updated 14th April 2006.