siemens: FT75 (FT 75); Bildmeister
Hello, old electronic friends'
I have this beautiful old Siemens TV, in very good condition,
All tubes are original from a factory, including picture tube-Siemens made.
Only problem is bad reception on UHF band.
Is it a problem with AF139 transistors,and what else can bee the reason.
Does anybody has a electrical diagram of this set.
Thanks in advance,Peter.
A very common cause for bad UHF reception in sets of the same generation as your Siemens set, is that the series (dropper) resistor for power supply to the UHF tuner has got open circuit or changed value. Typically, this is a 2 or 4 W resistor of perhaps 33 or 47 kohm resistance. This will cause too low supply voltage to the tuner, normal voltage uses to be 9 - 12 V.
Another possibility could be a problem with the VHF/UHF switching. In UHF mode, the RF amplifier and local oscillator of the VHF tuner should be stopped, usually by disconnecting the anode voltage from the respective valves. In some sets, the grid of the mixer valve PCF801 is connected to AGC voltage at UHF (it is running as first IF stage in this mode), but it should be at ground potential at VHF reception, where the RF valve is instead controlled by AGC. The main reason for this design was that the AF139 transistor is not suitable for AGC control.
Anyway, circuitry around the tuners was relatively well standardized in the mid-60's , and you would certainly be helped by studying a schematic for any european tv with a similar tuner layout.
I tried everything You suggested and found the problem.
It was the VHF-UHF switch.