25-09-2016, 08:44 PM
Hi.
Well I decided to pop up into the loft as something in the back of my mind made me think I have an X79 up there. Half an hour of looking through boxes that had not been recorded found a nice NOS X79 and a pull.
I then fitted the NOS valve into the tuner and I was rewarded with good results. Now I did see a mod to the FC base, a link from pin 7 to pin 9 which I removed, in fact the IC (Internal connection) in the X79 is actually between these pins, the removal though was done in case it caused stray capacitance.
With the X79 the tracking on SW1 & 2 is spot on now, on MW & LW it is perfect at the high frequency end of the band, better at the low frequency end of MW but not so good at the LF end of LW.
Now there is a common series cap to the oscillator tuning cap that is only switched in on MW & LW and I would think it has changed in value downwards, this would explain the mis-tracking on Medium & Long waves.
Tomorrow I'll remove the cap and measure it, there is no marking so I have no idea of its value, I'm hoping its a preferred value type that has gone low value.
I'll check and come back with my findings.
Well I decided to pop up into the loft as something in the back of my mind made me think I have an X79 up there. Half an hour of looking through boxes that had not been recorded found a nice NOS X79 and a pull.
I then fitted the NOS valve into the tuner and I was rewarded with good results. Now I did see a mod to the FC base, a link from pin 7 to pin 9 which I removed, in fact the IC (Internal connection) in the X79 is actually between these pins, the removal though was done in case it caused stray capacitance.
With the X79 the tracking on SW1 & 2 is spot on now, on MW & LW it is perfect at the high frequency end of the band, better at the low frequency end of MW but not so good at the LF end of LW.
Now there is a common series cap to the oscillator tuning cap that is only switched in on MW & LW and I would think it has changed in value downwards, this would explain the mis-tracking on Medium & Long waves.
Tomorrow I'll remove the cap and measure it, there is no marking so I have no idea of its value, I'm hoping its a preferred value type that has gone low value.
I'll check and come back with my findings.
