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Thorn Ultra 6816 B&W CRT TV Teardown

Thorn Ultra 6816
Thorn Ultra 6816 (Stock Photo)

The other day at the local canal-side waterpoint, this TV was dumped for recycling, along with another later model Colour TV. This is a 1970’s Black & White mains/battery portable made by Thorn. It’s based on a common British Radio Corporation 1590 chassis. Having received a soaking from rain, I didn’t expect this one to work very well.

Tuner
Tuner

Being so old, there is no electronic control of the tuner in this TV, and only has the capability to mechanically store 4 different channels. The tuner itself is a cast box with a plastic cover.

Tuning Lever
Tuning Lever

The mechanical buttons on the front of the TV push on this steel bar, by different amounts depending on the channel setting. This bar is connected to the tuning capacitor inside the tuner.

Tuner Compartments
Tuner Compartments

Unclipping the plastic cover, with it’s lining of aluminium foil for shielding reveals the innards of the tuner module.

Tuner Input Stage
Tuner Input Stage

Here’s the tuner front end RF transistor, which has it’s can soldered into the frame, this is an AF239 germanium UHF transistor, rated at up to 900MHz.

Tuner IF Mixer Stage
Tuner IF Mixer Stage

As the signal propagates through the compartments of the tuner, another transistor does the oscillator / IF mixing, an AF139 germanium, rated to 860MHz.

Tuning Capacitor
Tuning Capacitor

As the buttons on the front of the set are pushed, moving the lever on the outside, the tuning capacitor plates intermesh, changing the frequency that is filtered through the tuner. The outer blades of the moving plates are slotted to allow for fine tuning of the capacitance, and therefore transmitted frequency by bending them slightly.

Mains Transformer
Mains Transformer

Being a dual supply TV that can operate on either 12v battery power or mains, this one has a large centre tapped mains transformer that generates the low voltage when on AC power. Full wave rectification is on the main PCB. The fuse of this transformer has clearly been blown in the past, as it’s been wound with a fine fuse wire around the outside to repair, instead of just replacing the fuse itself.

Chassis Rear
Chassis Rear

The back of the set has all the picture controls on the bottom edge, with the power input & antenna connections on the left just out of shot. The CRT in this model is an A31-120W 12″ tube, with a really wide deflection angle of 110°, which allows the TV to be smaller.

Main PCB
Main PCB

The bottom of the mainboard has all the silkscreen markings for the components above which certainly makes servicing easier 😉 This board’s copper tracks would have been laid out with tape, obviously before the era of PCB design software.

Components
Components

The components on this board are laid out everywhere, not just in square grids. The resistors used are the carbon composition type, and at ~46 years old, they’re starting to drift a bit. After measuring a 10K resistor at 10.7K, all of these would need replacing I have no doubt. Incedentally, this TV could be converted to take a video input without the tuner, by lifting the ferrite beaded end of L9 & injecting a signal there.

Flyback Primary Windings
Flyback Primary Windings

The flyback (Line Output Transformer) is of the old AC type, with the rectifier stack on top in the blue tube, as opposed to more modern versions that have everything potted into the same casing. The primary windings are on the other leg of the ferrite core, making these transformers much more easily repairable. This transformer generates the 12kV required for the CRT final anode, along with a few other voltages used in the TV, for focussing, etc.

Rectifier Stack
Rectifier Stack

The main EHT rectifier stack looks like a huge fuse, inside the ceramic tube will be a stack of silicon diodes in series, to withstand the high voltage present.

Horizontal Output Transistor
Horizontal Output Transistor

This is the main switching transistor that drives the flyback, the HOT. This is an AU113, another germanium type, rated at 250v 4A. The large diode next to the transistor is the damper.

I’ve managed to find all the service information for this set online, link below!
[download id=”5616″]
More to come if I manage to get this TV working!

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CRT Flyback / Line Output Transformer Destructive Teardown

Small Flyback Transformer
Small Flyback Transformer

Here’s a small flyback / Line Output Transformer from a portable colour TV set. Usually these transformers are vacuum potted in hard epoxy resin & are impossible to disassemble without anything short of explosives. (There are chemical means of digesting cured epoxies, but none of them are pleasant). This one however, was potted in silicone, so with some digging, the structure of the transformer can be revealed.

Cap Removed
Cap Removed

The cap was glued on to the casing, but this popped off easily. The top of the core is visible in the silicone potting material.

The Digging Starts
The Digging Starts

A small screwdriver was used to remove the potting material, while trying not to damage the winding bobbin & core too badly. The bulge in the casing that I originally thought might house a voltage multiplier turns out to be totally empty. The white plastic bobbin is becoming visible around the core.

Bobbin
Bobbin

After some more digging & a lot of mess later, the entire transformer is revealed. The primary & auxiliary secondaries are visible at the bottom of the transformer, next to the pins. These transformers have multiple windings, as they’re used not only for supplying the final anode voltage of several Kilovolts to the CRT, but many of the other associated voltages, for the heater, grids, focus electrodes, etc. These lower voltage windings are on the same part of the core as the primary.
Above those is the main high voltage secondary winding, which looks to be wound with #38-#40AWG wire (about the thinnest available, at 0.07mm diameter. This is wound in many sections of of a few hundred turns each to increase the insulation resistance to the high voltage. The main anode wire emerges from the top of the bobbin.

Output Rectifier
Output Rectifier

Hidden in a recess at the top is the main HV rectifier, which on this small transformer is a single device (it’s probably not internally, most likely a series stack of diodes to get the PIV rating required).

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6-Point Ignition Transformer

Ignition Transformer
Ignition Transformer

Here’s a quick teardown of an ignition transformer, used on gas fired ovens & hobs.  This unit takes mains 240v AC & uses a transformer to step the voltage to several kV, at a low current to ignite the burners.

Bottom
Bottom

The transformer section is completely potted in Epoxy resin for insulation, but the driver circuitry is exposed, with a pair of leads from the primary winding exposed

Driver PCB
Driver PCB

The drive is very simple. The incoming AC flows through a series resistor through a half-wave rectifier to charge up a 2.2µF film capacitor. Once the voltage on the capacitor reaches a certain level, a DIAC in series with the transformer primary fires, discharging the capacitor through the primary.
The current spike induces a very high voltage on the secondary winding, this then arcs across a gap in the gas flow to start ignition.