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Old 24th Jul 2017, 12:52 am   #30
1100 man
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Join Date: Apr 2016
Location: Ventnor, Isle of Wight, & Great Dunmow, Essex, UK.
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Default Re: Kolster Brandes RV 10 (Crown)

Hi Dom,
Have you managed to obtain a copy of the circuit yet? My R&TV servicing books are up in Essex so I do not have the circuit to hand but I assume it will be fairly conventional.
In order for the U26 heater to light, the whole line output stage needs to be working correctly. If it isn't, there is not sufficient energy to power the heater so it's a good indication that all is not well

The anode of the PL81 gets its volts via the primary winding of the LOPT from the cathode of the efficiency diode (PY81 or similar) The anode of this diode is connected to HT via a small inductor.

There is also a 'boost capacitor', one side of which is connected to HT, the other side goes to another connection on the LOPT. The boosted supply is often used to feed the frame oscillator, focus volts etc.
The boost is a useful place to measure what is going on in the output stage.
If I had the set in front of me I would proceed as follows:

1) Identify the boost cap from the circuit and ascertain which side of it goes to the HT supply and which side is the 'boosted' side.
2) Connect a meter to the 'boosted' side (1000V range) and switch the set on.
3) As the diode (PY81 etc) warms up you should get a reading of 200V or whatever the HT supply is.
4) As the PL81 starts to conduct and you hear line whistle, the reading on the meter should climb to about 650 to 750V. At this point, the U26 heater should light.

If you get no reading or a very low one on the meter, check that there is HT on the anode of the diode (PY81 etc). If there is, then the PY81 is probably faulty.
With the set running, check for volts on the screen of the PL81. As has been mentioned, the screen resistor (2k2 ono) can fail.
The PL81 itself could be faulty and as has also been mentioned, change the boost cap and also the coupling cap to the grid of the PL81.

If all this fails, then my suspicions would fall on the LOPT itself, but there are still a few more checks that can be done.

If someone would like to post the relevant part of the circuit, I could be much more specific as to where to measure and exactly what voltages would be expected!

Hope this helps and remember to be careful when measuring these high voltages.
All the best
Nick
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