NA8 Restoration

The NA8 is the utility power supply for the 100 Watt Transmitter (100W.S. / L.S.100/108). It was used in airports and other stationary positions where utility power was more practical to use than an Umformer or gasoline generator.

This specific NA8 was manufactured in March of 1944 judging by the date stamps on some of the components. It was manufactured for use by the Luftwaffe based on the contract number on the manufacturer label.

The device weighs around 70 KG due to the five transformers and chokes inside. Although two people could move it with difficulty, it is not recommended for one person to lift it.

This NA8 was complete inside, although some rust, debris and dirt covered most surfaces. It is definitely worth it to restore to working condition, since it would be expensive to build an equivalent power supply with modern components which delivers 1000V and 12V with the correct current.

The NA8 when it arrived. Looks much better then the other example I saw 10 years ago.
The glow-lamp is at the top left, ON/OFF switch is bottom left, anode voltage selector is at bottom right and voltmeter is top right.

Manufacturer info. NA8 / SAF Süddeutsche Apparate-Fabrik, Nürnberg

View from the top.

Left side of NA8

The right side of the NA8 has the utility power connector on the left, ground lug in the middle and transmitter power outputs of 1000v and 12 V on the right.

Closeup of the voltmeter, It shows filament voltage of 12 volts when power is on, and shows anode voltage when the blue button is pressed.

Cover off - the state of the power supply. It may have been sitting in a garage or outside for a long time. There's a lot of debris, dirt, and many surfaces are rusty. It's difficult to determine how extensive the damage is caused by time, without restoring each individual component.

Dirt, rust and dust cover the transformers and selenium rectifier.

Dirt is on the capacitors, wires and resistors.

These capacitors are completely caked in dirt.

Time to take it apart. The HV selenium rectifier gets removed first, which allows for access to the other components. It sits on spacers at the bottom of the chassis.

After cleanup, one section of the rectifier looks nice, but it still can't be trusted to work, since these types of rectifiers lose their properties over the space of about 20 years. There is a danger of combustion for these rectifiers which can set the whole device on fire.

But, I had to test it, just out of curiosity and it worked fairly well.

Next, the HV transformer came out. This is the heaviest part of the NA8.

HV transformer on the bench and is being cleaned and restored.

After cleanup, the transformer voltage taps are visible. The NA8 can be configured for utility voltages ranging from 110V to 220V, depending on the country being liberated. Only one primary voltage can be configured, and the front panel voltage selector can switch between the secondary voltages. In the United States, the voltage is approximately ~117 AC, therefore 125 volts will be used for testing and operation.

After cleanup and re-painting, the HV transformer looks a lot better.

Rear of the HV transformer. There are labels which indicate test voltage of 3,600 volts and some part number (V 7340). Tap wires are in good condition.

Front of the HV transformer showing intact protective tape.

Removing the wire connectors from the output filter capacitor.

The bleed-off resistor shown from the side reveals the extensive rust on the connectors. This resistor was defective and had to be replaced.

Socket holding the 220 V glow lamp

The 220 V glow lamp.

The output filter was used for both the HV and LV output circuits. It was manufactured by the Czech firm Telegrafia, which also manufactured the Lo70KL40 marine transceiver. The capacitor is 30uF at 1,200 V and checked out well when tested.

This is the filter capacitors for the AC block. They are unique in their construction from Bakelite. The connectors needed to be carefully soldered out and their shapes preserved. These capacitors suffer from the same problems as other German WWII capacitors where there are gaps between the box and the top covers. Air, dirt and moisture enter the capacitor and accelerate the decay of the metal plates between the paper separators. You can see bulges and gaps on the sides that indicate heavy capacitor decay.

After cleanup, here's the body of one capacitor. Manufactured by S.A.F., they are 400uF at 40V.

The capacitors are gently heated up with a heat gun, and the top cover removed with the connectors intact. Here is the tar used to seal the capacitors inside the box. The tar is now hard and brittle, and can be chipped away with small tools.

Under the tar you can see the paper capacitors and connectors.

After being taken out, you can see the messy oxidation of the metallic material through the paper. This is bad.

Cross-section of the capacitor to show how it was wrapped.

Another capacitor with the wax paper on it.

The empty and clean capacitor boxes.

LV filter capacitors. They are the same value as the other ones, and in the same horrible condition.

LV filter capacitors have been removed for restoration.

Outside covers have been removed to allow much better access to the right side of the NA8, which was necessary to remove the parts on this side.

The LV filter capacitors were in even a worse state than the AC capacitors. The expanding decayed capacitors here have cracked the Bakelite, which will have to be re-glued back together.

A "graveyard" of old NA8 capacitors. None of them measured even close to their original values.

LV filter capacitor boxes in some state of restoration.

New capacitors before installation within the old boxes.

Front of the NA8 with the control panel removed.

Siemens power switch and circuit breaker. It was made for 6 Amps, but since we reduced input voltage to 125VAC, we need to increase this to 10 Amps. Another similar switch was installed which was rated for 10 Amps.

Siemens switch with cover off.

LV filter choke specifications.

The rear of the voltage selector switch.

Voltage selector switch removed for cleaning and lubrication.

Filter chokes for HV anode current.

The HV anode current chokes being cleaned.

HV anode current chokes fully restored and painted.

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