F.M. Rx problem with Mk1 Cobra 148 GTL-DX
Posted: Monday 26th May 2008, 6:34
Hi all,
I blatantly need help with an ill Mk1 148GTL-DX, PC879 mainboard with the 8719 PLL - has a 20-year-old JD Custom Band Converter Eprom fitted, with a 145106 PLL piggy-backed across the original....it's worked fine for me for 20+yrs until the following..
After a slight mishap (1ft drop off a shelf onto a wood bench
) it unsurprisingly developed a few faults...all of these except one I've managed to put right (was off by a channel and a bit, which was down to a dry joint on the mod PLL, damaged eprom ribbon and delta sw. wires)...
The only problem I can't seem to nail is with the FM receive - with another radio in close proximity on TX, I can hear audio OK, although with a little more hiss than I'd expect, but although the signal meter shows I'm receiving signals down an antenna normally, I'm getting no audio with them (just FM background noise).
So radio works perfectly in TX mode on AM, FM and SSB, but I have normal RX audio only in AM and SSB modes, with just static in FM.......
The only things Ive messed with up til now are L5, L9 and L10, which I just tweaked a little with the second radio on TX...this didn't help at all for far-off signals, just tidied up the audio from my second radio a bit....
I have taken voltage measurements from the detector IC2 while in FM Rx mode, they are as follows:
1 - 1.3v
2 - 1.3v
3 - 6.75v
4 - 0v
5 - 3.13v
6 - 3.17v
7 - 5.1v (this fluctuates a little...)
According to the paperwork I have here on the radio, these figures are ball-park apart from pin 7, which it says should be at 0v in this mode....?
Anyone out there have any ideas as to what to try next? I suspect I've just knocked it out of alignment (I've also had a frequency counter on the oscillator and carrier offset testpoints, they're all spot-on across the bands).This was my first multimode, and I'd sorely like to get it fixed....
Anyone have any ideas?? I have limited test gear here (DVM, counter, scope etc), but ready and willing to poke!!
Thanks!
Dan
I blatantly need help with an ill Mk1 148GTL-DX, PC879 mainboard with the 8719 PLL - has a 20-year-old JD Custom Band Converter Eprom fitted, with a 145106 PLL piggy-backed across the original....it's worked fine for me for 20+yrs until the following..
After a slight mishap (1ft drop off a shelf onto a wood bench
) it unsurprisingly developed a few faults...all of these except one I've managed to put right (was off by a channel and a bit, which was down to a dry joint on the mod PLL, damaged eprom ribbon and delta sw. wires)...The only problem I can't seem to nail is with the FM receive - with another radio in close proximity on TX, I can hear audio OK, although with a little more hiss than I'd expect, but although the signal meter shows I'm receiving signals down an antenna normally, I'm getting no audio with them (just FM background noise).
So radio works perfectly in TX mode on AM, FM and SSB, but I have normal RX audio only in AM and SSB modes, with just static in FM.......
The only things Ive messed with up til now are L5, L9 and L10, which I just tweaked a little with the second radio on TX...this didn't help at all for far-off signals, just tidied up the audio from my second radio a bit....
I have taken voltage measurements from the detector IC2 while in FM Rx mode, they are as follows:
1 - 1.3v
2 - 1.3v
3 - 6.75v
4 - 0v
5 - 3.13v
6 - 3.17v
7 - 5.1v (this fluctuates a little...)
According to the paperwork I have here on the radio, these figures are ball-park apart from pin 7, which it says should be at 0v in this mode....?
Anyone out there have any ideas as to what to try next? I suspect I've just knocked it out of alignment (I've also had a frequency counter on the oscillator and carrier offset testpoints, they're all spot-on across the bands).This was my first multimode, and I'd sorely like to get it fixed....
Anyone have any ideas?? I have limited test gear here (DVM, counter, scope etc), but ready and willing to poke!!
Thanks!
Dan