MK IV Chassis
System: Wired and UHF 625 Line
Original List Price : £000.00 ( to be ascertained)
Intergrated Circuits: Yes
Had to get one of these as I live just four miles from where this set was originally made. See here to read all about the factory.
The set is in remarkable condition and the tube very good so I had quite high expectation this one would be fairly OK to power on. Power was applied, sound came up followed by a nice bright snowy raster. I fed in the test card signal and then set about trying to tune it in. Selecting channel 1 no amount of sweeping the frequencies pulled in the test card, hmmm odd.
This set is wired, meaning it can be used both on the Rediffusion cable system or over air broadcasts via UHF input. Not being familiar with this I wondered how it determined which input to use, does it detect a signal on wired or UHF and present to the screen. Whilst ponmdering what my next move was I powered down and went of to make a brew. When I came back I powered the TV on again but this time nothing! Dead!
That’s where there fun now begins.
Thanks to forum member “Red_To_Black” who made me aware of a very good article in the January 1994 edition of Television. It covers many of the possible causes for this scenario, most residing in the power supply, which can be powered via the mains out of the set with a load between 4L5 and chassis.
It won’t start with R2 open but now to ascertain what caused it to open.
The Chopper transistor is OK, wondering if the fault is in the line output stage not the PSU
Out with the timebase to check the output transistor
Nothing wrong with the BU208 output transistor…… What on earth can it be tripler or something I’ve missed.
TR2 BD433 is innocent
However the culprit is THY1 the 2p5m thyristor. I tested it on the DCA55 and it could not find a component.
The DCA55 spec states it can test thyristors that require gate and holding currents of less than 5mA, the 2p5m is around 2mA so I think its toast. To be certain I got my home built thyristor tester out and it confirmed that the thyristor cannot latch.
Thanks to Malc’s comment below I think he’s sussed my tuning trouble the tuner has been set to wired.
New 2P5M tested and fitted, the set is now working. great picture too.