18-12-2023, 12:43 PM
The tuner is from a Decca Decola radiogram, the kind with stereo EL34s and a VHF only tuner. The valve linup is ECC85 in a separate tuner box and two EF41 as IF amps with EB91 detector. there is a magic eye of the elongated circular kind, dont thow the numbr its rubbed off and the display is a bit dim too.
The front end was well out of tune and stations are coming in much better now after trimming the presets.
The modulation has come back on my signal generator now. - I'd better have a look at that shortly.
I'll check the IF alignment next because I'm still seeing only about 5 volts across the detector capacitor (which has been changed and is the correct way around)
I have injected 10.MHz into each of the IF grids but never get more than a few volts on the capacitor - My signal generator gives a few hundred millivolts maximum output.
The IF valves have about 75 volts on the screen grids and neary full HT on the anodes (~220v) There are no cathode bias resistors but there is a resistor network giving some negative bias onto the grids - perhaps from the detector I'm not sure yet.
I've not managed to find a schematic of this particular tuner anywhere yet. but it isn't very complicated.
Kind regards, David.
The front end was well out of tune and stations are coming in much better now after trimming the presets.
The modulation has come back on my signal generator now. - I'd better have a look at that shortly.
I'll check the IF alignment next because I'm still seeing only about 5 volts across the detector capacitor (which has been changed and is the correct way around)
I have injected 10.MHz into each of the IF grids but never get more than a few volts on the capacitor - My signal generator gives a few hundred millivolts maximum output.
The IF valves have about 75 volts on the screen grids and neary full HT on the anodes (~220v) There are no cathode bias resistors but there is a resistor network giving some negative bias onto the grids - perhaps from the detector I'm not sure yet.
I've not managed to find a schematic of this particular tuner anywhere yet. but it isn't very complicated.
Kind regards, David.
