Quote:
Originally Posted by FERNSEH
Hi Steve,
If your set has the VC1 chassis has the frame sync modification been done? Well worth doing if the frame hold is critical. The triode section of the PCL84 is rewired to serve as the frame sync amplifier and clipper.
All subsequent chassis have the modification.
DFWB.
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Frame sync modification (copied from an STC Consumer Products publication):
To convert the triode section of the PCL84 from functioning as a 405 vision interference limiter, to a frame sync pulse amplifier (as used in VC2 and all subsequent dual standard chassis):
Using the triode section of the PCL84 in place of the OA81 diode.
Components required:
680kΩ ½W 10% resistor
68kΩ ½W 10% resistor
2.2kpF 10% capacitor
Procedure:
1. Remove C66, R61 and OA81 diode
2. Replace R74 10kΩ resistor by 68kΩ
3. Replace R76 1.8kΩ resistor by 10kΩ
4. Connect cathode of PCL84 triode to junction of R71 and R72 and leave grid of valve connected to chassis.
5. Connect triode anode of PCL84 to junction of R74 and C72
6. Fit 680kΩ resistor from HT line to triode anode of PCL84
7. Fit 2.2kpF capacitor in parallel with R74
For the circuit of this change, refer to the VC2 chassis circuit diagram, V7 triode section.