Audio/Power transformers
Since we started out as a repair shop for historic/ vintage German equipment, our biggest worry was always the condition of the transformers inside these historic modules.

Most of the times it is easy to see what's going on with power transformers, they usually work or they don’t.
But even with these, at times you may have a transformer that isn’t quite fully functioning; maybe the voltage is off, maybe there is overheating, etc.

It is harder to determine the condition of audio transformers without the use of the proper equipment.
It isn’t sufficient to test an audio transformer simply by judging their bypass signals, and they don’t tend to get hot unless they are the output transformers of a tube amp.
The problem often lies with faulty insulation, as at times they can slowly start leaking signal from the primary- to secondary windings or sometimes to the core acting like a frequency filter. In most cases they have either electric or magnetic shorts, but still passing signal.

Possible problems

Phantom Powering
One of the biggest concerns with these historic microphone pre-amp input and output transformers, is the phantom powering. Developed in the beginning of the 60s, and initially a sub-standard of powering microphones, this has now moved on to become the industry standard, applying 48V via two resistors to each leg of the modulation (pin 2/3) so there is no potential difference and using the shield as return path/0V.

Theoretically this shouldn't pose a problem for a none center taped transformer, but in practice there is always the capacitive property; primary to core or the Faraday shield, then also the primary windings to secondary windings etc. These will become charged if 48V DC is applied and the transformer (standard tube input x-former and tube mic outputs have ratios from 1 to 10 and even higher) acts like a ignition coil creating approx. 1 to 2KV!! on the transformers secondary. Mic pres with no damping resistor like the V76 (input transformer has 1 to 30 ratio) can even create up to 3KV!!!! and this is enough for a millisecond energy burst that is powerful enough to burn microscopic holes into the insulation.

It is necessary to keep in mind that in the 1940/50 manufacturers only guaranteed the PVC coated magnet wire for approx. 24V AC, which is why every single layer was insolated with additional material like oilpaper, etc.
Therefore when you dump 48V into a input or output transformer you engage in a slow process of killing your historic transformer.
DC blocking capacitors do not help at all because the time required to charge the capacitive property is still too much, and every time the microphone gets disconnected while the phantom power is on, the entire energy of those caps is dumped charging the windings of the transformer and pre-magnetizes the core.

If you want to use phantom power then it is necessary to use a ramped 48V supply with a time constance of more than 5sec.
This will take care of the problem posed by this energy burst, simply by having a slower time constance than the capacitve property of the transformercan charge.
But still there is a slight chance that something will get burned in the process, as you are dealing with historic/vintage parts, that had a maximum shelf life of aprox. 25 years.

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Composition/ Storage
Another thing to be aware of if you have no experience with historic transformers, is to make sure that you do not touch the leads because they might break off, they are most quite fragile due to the chemical break down of PVC wire or the tape that was used to make the transformer, etc.
There also tends to be decomposition of bobbin material, mounting foam etc. so if your vintage x-former still works, be very careful measuring it.

If your audio equipment has ever been stored in a damp environment, there is a change that the core material could have started to decompose. This is easy to spot, as it will most likely appear as rust or flaking and crusting on the metal core of the transformer.
This becomes harder if you own a power and audio transformer that was made before 1950, as mostly silk and similar organic materials were used as lamination isolation material. Once 20% of the silk is gone, the magnetic path is compromised and eddy-currents will then load down the transformer.

The main thing to bear in mind is that regardless of the reason behind the breakdown of your transformer, the steps needed to get it working again are both difficult and complicated.

Bobbins/ Lamination
Most power transformers will burn out when they reach the end of their life and therefore in 8 out of ten cases a new bobbin is needed.
Also as the insulation of the lamination is most likely to be compromised it too will need to be replaced, and in these cases simply rewinding is not an option.

98% of the lamination needed for the vintage/ historic American power transformers are still available, but the original lamination needed for European and especially German modules are almost impossible to come by if the module was made before 1950.
In these cases the historic lamination needs to be remanufactured and freshly insulated.

It is also very common for audio transformers to have insulation problems due to electric shorts.
In 99% of these cases the bobbin can be reused, but in 5 out of 10 cases the lamination is so far gone that new one is needed, and as written above replacing these with originals is hit and miss and sometimes this option is not available.

Applying a new hydrogen coat by heating treating the metal in a hydrogen atmosphere furnace is enough, sometimes (mostly American lamination) a special chemical coating procedure is needed.

Our Services
Today in most cases NOS replacement or new replacement parts are no longer available, so in 1995 we decided to develop the techniques that would enable us to build the dedicated machines we now have, which are needed to rewind these historic magnetic components.
Indeed this is very time consuming, but sometimes it is the only option.

We can re-manufacture almost every European or German made power and audio transformer and in some cases we can offer a new production version.

Rewinding is a time consuming process that can take up to 4 months, just for the soaking process to get rid of the 30 year plus coating. For more details on this or the re-manufacturing of vintage/ historic audio or power transformers please contact Oliver; Oliver@tab-funkenwerk.com either by e-mail or phone.

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