17-04-2012, 04:41 PM
Hi Joe,
The procedure that is being referred to is called sweep alignment. I'll explain the general idea. It's a method of realignment that sweeps the test signal from the signal generator over a defined range of frequencies and the detected R.F. is displayed on a 'scope in synchronism with the sweep. So you get an instantaneous picture of the passband on the 'scope. Compared to the traditional 'single freq. input and tune for max. signal' method, this method literally gives you the 'full picture' of the I.F. amp. response curve and you can thus see the effects of any adjustment as you make them - in real time. A 'ganging oscillator', a 'sweep generator', a 'sweeping signal generator' and a 'wobbulator' are basically all the same animal.
Having said all that, that method of alignment is rarely necessary for domestic broadcast radios - except for 10.7 MHz I.F. amplifiers, where it is essential. The reason for this difference is the required bandwidth of the I.F. amplifier. For A.M. broadcasts, a bandwidth of about 6 kHz at - 3dB and 25 kHz at -20 dB is quite adequate; the bandwidth is relatively small. However with F.M. broadcasts, we require a much wider pass-band. It is the superior audio quality that is achieved with F.M. that requires this broader bandwidth.
With comms. receivers (more my line of interest) - especially those in the 'complicated / expensive class' - sweep alignment is usually de rigueur; the 1st. I.F. of a Racal RA-17 being a good case in point.
Some of what I have written above is a bit simplified in places (I expect the purists will pull me up on one or two points
), but the overall concepts are valid.
O.K., lecture over!
So to your set: I would think it's highly unlikely that the I.F. cores have been tampered with. I would only attempt I.F. re-alignment (and that using the single freq. method too) as a last resort. If there is a fault in the I.F. strip, then defective decoupling capacitors and / or changed value resistors - along with the possibility of faulty AFxxx transistors - seems the most likely cause . . . if the fault is in the I.F. stages; you may have an A.G.C. fault. An A.G.C. fault on sets like this can play havoc with the I.F. stage - including causing dynamic mis-alignment (and that can wait for lecture no. 2) . . . which brings me back to where I came in.
Al.
The procedure that is being referred to is called sweep alignment. I'll explain the general idea. It's a method of realignment that sweeps the test signal from the signal generator over a defined range of frequencies and the detected R.F. is displayed on a 'scope in synchronism with the sweep. So you get an instantaneous picture of the passband on the 'scope. Compared to the traditional 'single freq. input and tune for max. signal' method, this method literally gives you the 'full picture' of the I.F. amp. response curve and you can thus see the effects of any adjustment as you make them - in real time. A 'ganging oscillator', a 'sweep generator', a 'sweeping signal generator' and a 'wobbulator' are basically all the same animal.
Having said all that, that method of alignment is rarely necessary for domestic broadcast radios - except for 10.7 MHz I.F. amplifiers, where it is essential. The reason for this difference is the required bandwidth of the I.F. amplifier. For A.M. broadcasts, a bandwidth of about 6 kHz at - 3dB and 25 kHz at -20 dB is quite adequate; the bandwidth is relatively small. However with F.M. broadcasts, we require a much wider pass-band. It is the superior audio quality that is achieved with F.M. that requires this broader bandwidth.
With comms. receivers (more my line of interest) - especially those in the 'complicated / expensive class' - sweep alignment is usually de rigueur; the 1st. I.F. of a Racal RA-17 being a good case in point.
Some of what I have written above is a bit simplified in places (I expect the purists will pull me up on one or two points
), but the overall concepts are valid.O.K., lecture over!
So to your set: I would think it's highly unlikely that the I.F. cores have been tampered with. I would only attempt I.F. re-alignment (and that using the single freq. method too) as a last resort. If there is a fault in the I.F. strip, then defective decoupling capacitors and / or changed value resistors - along with the possibility of faulty AFxxx transistors - seems the most likely cause . . . if the fault is in the I.F. stages; you may have an A.G.C. fault. An A.G.C. fault on sets like this can play havoc with the I.F. stage - including causing dynamic mis-alignment (and that can wait for lecture no. 2) . . . which brings me back to where I came in.Al.
