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ChristianFletcher 11th Sep 2006 8:00 pm

Operation of a valve line output stage
Please could someone explain what is meant by the term efficiency diode feeding the LOPT?
Is this just another term for a rectifier or is it performing another function?

Thanks in advance Christian.

jim_beacon 11th Sep 2006 11:28 pm

Operation of a valve line output stage
The boost diode is used to recover energy from the line output transformer during flyback. It makes the line ouput stage more efficient (I'll let the experts explain how - then I might learn something as well!).


ChristianFletcher 12th Sep 2006 7:37 am

Operation of a valve line output stage
Thanks Jim

Would I be right in considering the capacitor is there to correct the power factor of the inductive circuit?

Thanks Christian

Mike Phelan 12th Sep 2006 8:19 am

Operation of a valve line output stage
1 Attachment(s)
OK - here we go, Chris (and Jim :) ). This will be a bit long, so you may fall asleep. Keep away from the PY top cap. :o

Assuming that the LOP stage is already running, and the spot is in the RH side of the raster, and the combined scan coils/width control/LOPTx are treated as a single inductor.
  • The PL is turned hard on due to the grid drive, and its anode voltage is about 40. The current through the LOPTx is increasing linearly - the sawtooth for the forward scan. The PY is turned on and feeds the HT to the PL via the LOPTx. CBoost receives a charge of about twice HT with normal autotransformer action.
  • At the end of the scan, the PL is cut off by the grid drive so the current in the LOPTx is cut off.
  • The LOPTx and Cstray start to go into oscillation - the LOPTx energy is passed to CStray, whose voltage rises to several Kv; autotransformer action means that the PY cathode and the EHT overwind follow. CBoost is effectively earth. We are halfway through the flyback, and the PY is cut off by the high voltage on its cathode.
  • This is an unstable state - CStray discharges into the LOPTx in the opposite direction - the current is reaching the end of the flyback. Left to its own devices, this would all keep oscillating until losses intervened.
  • However, as the voltage spike falls, when the PY cathode reaches HT, the PY turns on and effectively puts CBoost across it and passes current through the LOPTx from CBoost but in the opposite direction, starting the forward scan.
  • This current would only last for about half the scan, but the PL is turned on at about 40% before the current from CBoost becomes non-linear.
  • So, there is a saving because the LOP stage is operating at about 600v and the first part of the scan is provided by CBoost charged during flyback.
Make sense? I shall be asking questions! ;)

ChristianFletcher 12th Sep 2006 10:51 am

Operation of a valve line output stage
Could you please clear up some of the terminology, is this just a naming convention related to this type of television circuit?


Many Thanks.

beery 12th Sep 2006 12:39 pm

Operation of a valve line output stage
Hi Chris,
the PL is the line output valve V10 (PL81), the PY is the effieciency diode V12 (PY81).

There will be two types of Cstray. Cstray is stray capacitance, ie. there is not actually a physiscal capacitor there. Remember that whenever you have two wires near each other or a wire near a metal chassis there will be stray capacitance between them. The line output transfomer will have stray capacitance across the windings and also stray capactitance between the windings and the surrounding metal work/coil core material etc..

The line output transformer (T2 in this case) can be called LOP transformer, LOPT, LOPTy, HOPT (horizontal output transformer) or the flyback transformer. These are all the same thing.


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