Pal Decoder Diagnostics

Below you will see a fault finding table for the two basic decoding faults, no colour or unsynchronised colour.

dec_block

Incorrect ident, the PAL switch operating in the wrong phase gives green faces symptom. If this is a permanent condition check the ident amplifier tuning. If the fault is sometimes present, sometimes not, check the ident amplifier output, the feed circuit to the PAL switch, and the setting up of the burst discriminator.

An inoperative PAL switch results in severe blinds across the picture, most noticeable where the R-Y component of the chrominance signal is of high amplitude. The usual cause of this fault is a defective transistor in the bistable circuit or a faulty IC, depending on decoder design.

Absence of one of the colour difference or RGB signals can be deduced from the colours displayed: voltage checks in the appropriate channel generally reveal the source of the fault.

Colour casts where not due to incorrect CRT first anode supplies are generally caused by a faulty output transistor, a fault in one of the clamps in the output stages or a leaky electrolytic signal coupling capacitor ( this can also result in streaking).

No Colour
|
First Action – Override Colour Killer
|
Result
|
(A) No Colour (B) Unscynchronised Colour (C) Correct Colour
Check: Check: Check:
Chroma Amplifier Burst Amplifier Colour KIller
Burst Peaking Burst Gating Ident amplifier and Emitter-Follower
Delay Line Driver Burst discriminator and following DC amplifier
Reference Oscillator and Emitter-Follower Reference oscillator tuning

Reference Oscillator Faults

Faults in the reference oscillator circuit account for a high percentage of decoder troubles, either no colour ones or “rainbowing” i.e. unsynchronised colour.

Tuning drift and loss of output signal amplitude are the main issues. Any cheap and nasty capacitors should be replaced. This applies particularly to oscillator tuning and coupling caps, good quality 1% types should be used.

Transistors can cause drift and must be replaced with the type specified by the manufacturer or the equivalence. Carry out tuning using an oscilloscope. A good final test of the oscillators pull-in range is to remove the aerial plug and then with a test card being received, reconnect and disconnect  the plug to the socket about 25 times. If the colour content of the picture remains stable after this repeated interuption of the signal, the oscillator can be assumed to be operating correctly. Some setmakers suggest using a test meter to tune the oscillator, however an oscilloscope gives more reliable and exact readings.

Ident Stage

The ident amplifier generates a 7.8kHz sinewave and is another stage that can give a fair amount of trouble. The main faults are no output or low amplitude output.

No output usually means that the transistor or i.c has failed. Low amplitude output is usually the result of the 7.8kHz tuned circuit coil being out of tune or a faulty decoupling capacitor. The coil can be adjusted by the visual method, tune it for complete colour resolution over the entire screen. Alternatively a far better method is to use an oscilloscope, tuning for maximum amplitude output.

Chroma & Burst Channel

The stages in the chrominance and burst channels are resonably straightforward, faults being easily traced with an oscilloscope and the help of many waveforms for these are given in all the manufacturers service manuals and Radio & Television Servicing books.

However the tuning of the burst channel is critical and again the use of 1% capacitors is recommended. Interstage supply rail decoupling caps can also cause trouble, the fault usually being colour patterning in the region of 1MHz all over the screen.

Delay Lines

The glass chrominance delay line is utterly reliable. The only problems likely to be encountered in this part of the circuit concern the tuning and setting of the balance/phasing control(s) in the associated matrixing circuit. These adjustments can be carried out visually, tuning for minimum Hanover bars on a test card, but yet again it is recommended using an oscilloscope and the waveforms given by the manufacturer.

The delay line in the luminance channel gives little trouble though there is a tendency for it to go open-circuit, recognisable by only chrominance information reaching the screen. Dry joints here cause luminance ringing.

PAL Switch: RGB Channels

The faults in the bistable (PAL switch), preamplifier and output stages consist mainly of the failure of transistors and i.c’s. Straightforward voltage checks will soon locate the source of trouble. Repeated failure of RGB output and driver stages can be due to a faulty CRT. The reason? Interelectrode flash-overs in the tube and the d.c. coupling used between it and the RGB output transistors.

Reduction of the CRT first anode voltages can help, also the use of good qualirty spark-gaps on the CRT base panel. Regunned tubes have a habit of flashing-over so it is well worthwhile watching out for this

Wire-wound RGB output transistor load resistors also give problems, usually in the form of dry joints which result in either red, green or blue colour streaking. Beam current control is sometimes applied on the decoder panel. It is good practice to check operation.

Miscellaneous Troubles

Wiring harnesses, colour controls, plugs and sockets and cracked printed boards are other sources of trouble. Careful inspection and probing are required to deal with these. The use of good quality intruments and tools together with a logical approach to faults makes decoder fault finding much easier.

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