Weller soldering iron add-on.

Now I'm a great fan of the Weller TCP "Magnastat" iron; robust, simple, great parts availability. Yes, there are better irons out there, but these are usually available secondhand for peanuts.

They are available in various wattages, and I have two, both 50W, one with a nice fat screwdriver type tip, for valve stuff and tagstrip etc, and a slender one, for more delicate work.

The tip has a temperature rating stamped on it's base. This contains a special magnet, which loses it's magnetism at a certain temperature (known as Curie temperature). This magnetism is used to operate a magnetic switch (like a reed switch, but a little more robust) to switch the element on and off, which heats the tip.

As bench space is limited, I only use one power supply, and got sick and tired of pulling one iron plug out of the power supply, and plugging the other in when I wanted to switch between irons. What was needed was a switch box....

... then I got a bit geeky (well, there's a surprise!). Let's have an led on the box, to show when the iron is heating. Useful to know if the iron is up to temperature, or if a fault has occurred (occasionally the switch or element fail , see here ) , so I devised a simple circuit to put an LED on when the iron is drawing current.

Simple. When the iron is drawing current, there is a small voltage dropped across R1, which causes the op-amp IC1A to attempt to drive it's output to rail. Now the current flowing in R1 is obviously AC, so the output is a 50Hz (or 60Hz, depending on your mains) waveform, but it's enough to light a red LED. The power supply for the op-amp is a crude supply derived from the 24VAC from the weller PSU. It's half-wave rectified by D1, and D3 and R2 Clamp the voltage peaks to 5.1V. It's smoothed by C1.

It's crude, horrible but works!

Vintage test gear - Heathkit Harmonic Distortion Analyzer IM-5258

A quick description of the Heathkit Harmonic Distortion Analyzer (Analyser!)

Great bit of vintage 70's 80's test gear.

I've tried a couple of times in my life to build a THD analyser, before I gave in and just bought one... I think I'd be OK today, but I have a reliable instrument, so it saves me the grief!

It takes the input signal, a sine wave, and buffers it, matches it for voltage with it's own internal oscillator, then notches out the input signal. What then remains is the harmonic distortion and noise. Simple to say, a little more complex to put into practice.

The Q-factor of the notch filter must be high enough so it doesn't notch out the 2nd harmonic (or 3rd, 4th, 5th etc) , which at audio frequencies can be difficult to achieve. The resultant output needs to be measured with a high bandwidth AC RMS meter, sensitive to microvolts, which can also be a tall order!

It's also tricky to keep the whole thing from oscillating.

Thankfully, I picked up this:

It's all discrete, not an op-amp in sight, and just works! It's also auto-tuning, but you still need to set the range and adjust the notch, so I'm not sure how "auto" it really is!

So, when we're designing an oscillator or amplifier, we can check how much distortion it really is producing, and where in the frequency spectrum it is! Useful stuff....

Here's a quick video of the thing in action...

The MK87 Dreadnaught capacitor reformer - an explanation.

A terrific young lady called Elise has emailed me regarding my capacitor reformer. She'd like some more details on it's function, and, perhaps more importantly ... it's name.

WARNING. Do NOT try this at home. This project calls for some high voltages. It will floor you or kill you if you come into contact with it. It will not warn you, blow a fuse, nor pull your safety trip. It may not give you a second chance. RESPECT IT'S AUTHORITY.

Reforming capacitors carries the risk of explosion.

I will not, under any circumstances, accept any liability if you decide to re-create this project for yourself.

Now we've got that out of the way....

It's not a pretty looking thing. It's had many iterations, hence the state of the box. It used to have a 1uA meter movement (with adjustable shunts), but that suffered so many accidents, the needle ended up shaped like a banana, and eventually the meter went open circuit, due to a major overload. At this point I grafted in a digital meter, salvaged from scrap equipment. It directly reads 0- 999.9 uA, and is powered from a separate power supply (a redundant mobile phone charger)

The front panel also has the terminals for the capacitor under test, and a discharge switch.

On the side is mounted a selector switch, which switches in varying values of current limiting resistor. Nice labelling , eh?

The main power is supplied from a variable transformer (variac) which is built into by bench supply. It's variable from 0-350 VAC.

The sensible option would be to power the unit from a dedicated, isolated multi-tap transformer, but I don't have one...

It's wired up like this: -

So, what is this "reformation" anyway?

Well, when we're servicing or restoring a piece of ageing electronics, the electrolytic capacitors may have begun to lose their polarity. It's a chemical thing. My daughter would know. Anyway, the practical upshot is the capacitor will draw current if woken abruptly from it's slumber. (This even applied to "NOS" capacitors, even if they've never been used)

Say we have a 200uF 450V smoothing capacitor, and it's drawing 10mA at 400V, It's really pretending to be a 40K resistor, and it's going to start to dissipate power (Power = Voltage x current = 400 x 0.010 = 4 watts), and not only that but it's probably not being much of a capacitor either.

This power is going to be dissipated as heat. Now 4 watts doesn't sound like a lot, but this is inside a sealed can (usually aluminium) full of electrolytic gunge and tin foil. Pressure starts to build up inside, the resistance gets lower, more current is drawn, more heat is generated, and the whole situation just gets worse until the can explodes, and showers the workshop in bits of tin foil, and acrid smoke. If you ever experience this, just open a window with your now trembling hands, and leave for 5 minutes to allow the smoke to clear!

What the reforming process does, is let the plates regain their "form" or polarity. We need to gently apply power, and raise the voltage to the capacitors full rated voltage over a period of time. I usually do this for a period of a few hours, sometimes overnight.

So, having made our reformer, how do we use it?

Firstly we need to set our maximum voltage. Move the selector switch round to 0 ohms, and measure the voltage at the output terminals. Don't connect your capacitor as yet. Adjust the variac to give the required voltage (I usually use the rated voltage less 10% to allow for any variations in mains voltages).

Now switch off the supply and hit the discharge switch for 1 min. This will discharge C1. You can now connect the capacitor to be reformed to the terminals. Select the 12 megohm position, switch the discharge switch off, and switch the supply back on. The meter should show a few uA as the capacitor charges. Now decrease the resistor until the cap is drawing approximately 100uA and leave it there. Pop back in an hour or so, and the meter should have dropped again. Reduce the resistance again, and the meter will once again rise.

Repeat the above process until no improvement is made. Take your time. If the can is isolated, feel it to see if it's getting warm. It shouldn't. If it is, increase the resistance and just wait. Once the electrolytic has reformed, remove the supply, and hit the discharge switch. Wait for the capacitor to discharge.

What sort of leakage can we expect from an old electrolytic?

Good question. It depends on the value of capacitance; bigger values leak more. And voltage rating; Higher voltages *tend* to leak more. This is where experience is key. If your 22uF, 400V capacitor is drawing 1mA, forget it. If it's drawing 400uA it may be OK. If it's drawing 200uA or less, put  it back in the kit and switch on!

Modern capacitors have very low leakage values. Not so with the ancient stuff I deal with, but even 60 year old electrolytics that haven't seen power in decades can be reformed successfully. It's not always the case though. Some are beyond redemption, and replacement is the only option.

I sometimes choose to monitor the voltage across the capacitor during reforming, but we need to be aware that our meter will have some resistance of it's own, and will effect the reading on the current meter (it will increase slightly) and will drop the voltage across our capacitor slightly (as it discharges through the resistance of our meter). A good quality digital meter will have an internal resistance of 10 megohms. A moving coil meter , like an AVO 8, will have a varying resistance of 20 Kilohms per volt. Keep this in mind.

I'm lucky enough to have a capacitance meter (for measuring the actual value in uF) and an E.S.R. (equivalent series resistance) meter, and I'm always amazed how some of these old devices shape up after a successful reforming.

Now, here's the warning bit again....

A variac is NOT isolated from the live or line of your mains. Use an isolation transformer for your safety. Reforming a high voltage (more than 50v) electrolytic capacitor poses a very real shock hazard. It may shock or kill you. 

Be safe. There's only one of you, and I need all the readers I can get!

Oh yes... the name....

It was a joke. It started out as the dreadnaught , got blown up, became the dreadnaught MKII, then after so many failures and improvements, I lost count...

Workshop video rack.

Whilst I don't get as much time as I'd like to "play tellys", the state of the workshop video sources has been plaguing me recently.

For those outside the UK, I'll explain a little. All TV transmissions in the UK are now digital. They're delivered in Band IV UHF.

In times gone by, there used to be a 405 line black and white only service (System A). This existed  from 1936 until 1984 and started out in Band I VHF, and then expanded into Band III when ITV started in 1955. 625 lines (System I) was introduced for the start of the BBC2 service in 1964, in UHF only (it went colour (PAL) in 1967, and all services moved to UHF 625, both in bands IV and V (Band V has now been flogged off to the mobile phone operators for 4G etc.) )

Now, to run my old tellys we need to generate at least some of these signals. The UHF 625 line colour stuff is easy, just get a DVB receiver (Freeview) with a modulator built in. 405 lines is a little more tricky, but thankfully there exists a small box called an Aurora, which is a 625 line to 405 converter (there are other types available too, to convert between almost anything!)  with a built in VHF modulator. It's superb. Auroras (Auroae?) can be found here. If you are inside the EU, you can order them from Crowthorne tubes here.

The big problem is this motly collection  of boxes and power  supplies, the odd DVD player etc, has just been jury rigged as required. Not a satisfactory situation.

So, I designed myself a little rack to house it all.

 All connections are made on the front panel. This is a workshop piece of equipment, and you don't want to go fiddling around the back. The signal from our aerial (with our digital TV signal on) feeds the top Belling Lee socket, this is passed to a "4G" filter to remove any signals from band V. The RF then feeds the two freeview boxes and the outputs from these (in Band V) are mixed and sent to the mixer in a UHF modulator. The band I output from the Aurora standards converter feeds an attenuator (it's output is hot!) and a low pass filter, this is then combined with the UHF RF and passed (finally!) to the lower Belling Lee locket for connection to the set or sets.





Video to the standards converter is switchable from nothing (which causes the Aurora to output test card C), the Aux input sockets on the front, the DVD or either freeview receivers. Video to the UHF modulator can be switched from the Aux sockets or the DVD player (The freeview is already modulated)





 Much neater and I'm rather pleased with how it's all come out.

I've subsequently added a small Sumvision Cyclone media player to the set up, which is great for playing video files and test cards from a USB memory stick.

A sad end ... and a new beginning.

Ladies and gentlemen, please. A moment of silence for the workshop's Weller TCP soldering iron.

Since 1995, it has provided many hours of soldering. Repaired countless electronic circuits. Outlasted two marriages. Been a loyal friend. Last month saw the need for a new tip. But, alas, the old one had seized into the element's housing. After a liberal soaking in Plus-gas (other penetrating oils are available), freezing & heating,  the element failed to recover from the procedure.

Thankfully, a new element was just a day away (Here eBay is not your friend. Elements on eBay were very expensive. I got mine from RS Components)

The iron-y (ouch!) is that you need a soldering iron to fit it. Anyway, I have several irons, but none so good as the venerable Weller!

Here's to the next 19 years of service!!