Thermionic standards converter......?
Good progress over the weekend.
Video waveforms at the anodes of the sync and video mixer valve V20.
This signal is sent direct to the monitor terminal "VID"+.
Sync height needs slight adjustment.
More results. The picture was taken from the screen of a Ferguson phase II "Senator" 705T TV set.
Have replaced all the TCC paper capacitors in the mains lock chassis. All had leaks in varying degrees. The 'scope traces show the eight frame sync broad pulses are coincident through the 50Hz AC zero crossing point
The mains lock discriminator circuit.
The RF modulator assembly is constructed to a very high standard. The modulator utilises the properties of the Mazda 6F33 pentode. This valve has special suppressor grid characteristics which permit the baseband video to be supplied to the third grid. To prevent the G3 going positive I believe a diode is connected inside the valve to G3.
The lower oscilloscope trace shows the modulated video signal. The sync tips do not go down to zero or almost zero carrier level.
Locking the TPG11 from the 625 line field sync pulse.
The Murphy pattern generator uses the mains frequency as the reference to lock the master oscillator to the correct frequency. We need something more stable and the frame sync pulse of the 625 system is the answer. The counted down frame sync envelope pulse from the divider chain is coincident with the zero crossing point of the mains sine waveform and operates in a limited range about zero. To use the 625 sync pulse I'm considering using a variant of the circuit used in my standards converter. A CMOS 555 timer can be employed. The device is operated as a mutivibrator and triggered by the 625 field sync on the reset pin 4. The waveform across the ramp forming capacitor in the timing circuit can serve as the reference for the discriminator circuit. The CMOS 555 can operate at VCC 15volts so the waveform across the capacitor will be 1/3rd of VCC and used as a substitute for the reference derived from the 6.3V heaters supply.
Someone got a bargain this morning when a Murphy TPGII sold. It looks slightly different to mine as well as having many more of the 6SN7 in place of the ECC34`s
But will it be restored or simply used a source of Audiophool double-triode valves?
My guess is stripped for its value.
I reckon only a dyed in the wool engineer would appreciate the design intricacies of the Murphy TPG11 pattern generator. As I have often mentioned before it wasn't easy designing high performance circuits in the old days before transistors, TTL and programmable devices.