17-09-2016, 07:52 PM
(This post was last modified: 17-09-2016, 07:53 PM by Geordie McBoyne.)
I've just started the restoration of this 14" GEC BT310 which was made in 1959. This set is electrically identical to the model BT1156. The only differences between the two models is that the later BT310 has a fake wood grain effect on the cabinet and the CRT mask is painted gold.
As the variac is not at present in the workshop the usual "just plug in" method was adopted for the initial tests. No bangs or flashes and it wasn't long before the valve heaters started to glow and in fact minutes later a very dull raster appeared on the screen although that didn't last very long.
Clicks could be heard when the channel selector knob was rotated so we know the sound circuits are working. Surprisingly, the turret tuner is loaded with only two sets of coils, channel 5 BBC and channel 8 for Tyne-Tees Television.
Resetting the Aurora for channel 5 resulted in having a very loud test tone from the loudspeaker, but still no picture.
The CRT electrode voltages were 100V on the grid 110 on the cathode and only 115V on the first anode, that's too low, should be >350volts.
Clipping off the 0.5mfd boost smoothing capacitor resulted in only a slight increase of first anode volts. Time to look for faults elsewhere in the circuits.
We did have a battle to remove the chassis from the cabinet, the chassis was stuck fast but eventually the chassis yielded to our brutal methods.
It was feared that the CRT might be low emission because it would seem a booster transformer might be fitted. However, there isn't one. There is evidence of some strange servicing practises having been carried out though. Just check out that twin cable connected to the tube base, it is about twelve inches long and simply reconnects back to the heater chain.
For a brief period a decent picture appeared and then disappeared again.
The U25 wired ended EHT rectifier valve has been badly soldered, all three wires have become disconnected. The valve has been resoldered and this action has restored some activity on the screen. The line oscillator is running on the low side of 10,125Hz. The most likely cause of this fault will be the 100Kohm resistor which connected from the line hold control to HT.
The fourth picture shows the results so far.
Geordie McBoyne.
As the variac is not at present in the workshop the usual "just plug in" method was adopted for the initial tests. No bangs or flashes and it wasn't long before the valve heaters started to glow and in fact minutes later a very dull raster appeared on the screen although that didn't last very long.
Clicks could be heard when the channel selector knob was rotated so we know the sound circuits are working. Surprisingly, the turret tuner is loaded with only two sets of coils, channel 5 BBC and channel 8 for Tyne-Tees Television.
Resetting the Aurora for channel 5 resulted in having a very loud test tone from the loudspeaker, but still no picture.
The CRT electrode voltages were 100V on the grid 110 on the cathode and only 115V on the first anode, that's too low, should be >350volts.
Clipping off the 0.5mfd boost smoothing capacitor resulted in only a slight increase of first anode volts. Time to look for faults elsewhere in the circuits.
We did have a battle to remove the chassis from the cabinet, the chassis was stuck fast but eventually the chassis yielded to our brutal methods.
It was feared that the CRT might be low emission because it would seem a booster transformer might be fitted. However, there isn't one. There is evidence of some strange servicing practises having been carried out though. Just check out that twin cable connected to the tube base, it is about twelve inches long and simply reconnects back to the heater chain.
For a brief period a decent picture appeared and then disappeared again.
The U25 wired ended EHT rectifier valve has been badly soldered, all three wires have become disconnected. The valve has been resoldered and this action has restored some activity on the screen. The line oscillator is running on the low side of 10,125Hz. The most likely cause of this fault will be the 100Kohm resistor which connected from the line hold control to HT.
The fourth picture shows the results so far.
Geordie McBoyne.
