Making Yesterdays Electronics Work Today
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A note of caution on A823.
The power board has a coin shaped thermistor, remove the board and check the connections to this thermistor make good any sign of dry joints and ANY sign of carbonisation. These two things can cause a spectacular flash over. What lives on the other side of the power unit? The Decoder and you know what happens to semiconductors when this kind of thing happens.
The main cause for the i.c’s on the decoder to fail is if the two earthing screws on the PSU are not tight. The thermistor (VA1104 if I remember) should be replaced if it sparks on switch on. A company produced a replacement PSU for this chassis which used a soft start circuit which did away with the thermistor. These PSU’s were very poor design and gave poor H.T. regulation i.e. picture bounce on bright scenes.
I also forgot to mention the earthing screws on the decoder pcb must be tight, I have also seen the thermistor shorted out by twisting the two legs together The BT106 objects to this *mod*. Another quick warning if the line stage does not start up those big electrolytes can hold a charge for a long. There should be bleed resistors across both smoothers 47k or 470k rings a bell. Also make sure the 56-68 ohm wire-wound dropper behind the smoothers has not been bodged.