Still awaiting help-F.M. Rx problem on Cobra 148 GTL-DX MK1

[Stimpy]

Hi all,

I can't believe that there isn't anyone on here that can help with this problem...

I still 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....

Thanks in advance!!

Dan

[The Defpom]

Pin 7 is the detector output, it feeds the output to a point just after the AM detector, before the squelch circuit, so does the radio have normal audio in all other modes ?

If only FM is effected then I would just replace IC2 and go from there, if other modes are also effected then I would be looking at a squelch amp problem.

[Rick]

First check for cold solder joints from L9 to IC2 it maybe that simple.

The voltage at Pin 7 is OK it should be around zero in AM and SSB.

Try replacing filter FL2 it maybe O/C.

Check coils L10 and L11 maybe O/C.

Also check the voltages on TR13, TR14, and TR15

All check in FM RX.
TR13 Base 0.70 vdc, Collector 7.80 vdc, Emitter 0.10 vdc.
TR14 Base 0.70 vdc, Collector 2.80 vdc, Emitter 0.0 vdc.
TR15 Base 2.40 vdc, Collector 7.80 vdc, Emitter 1.80 vdc.

If you have a scope you should be able to see the signal at Pin 1 and 2 of IC2, it will look like a compressed AM signal.

[The Defpom]

Similar circuit, it may be of help

[The Defpom]

Cool, I got the attachments working again (it was broken) !

[Rick]

Scott your circuitsample.gif is not for a 148GTL-DX Mk1 (PC-879).

[The Defpom]

Yep, it is just a similar setup from a sister chassis, I do not seem to have a diagram for the PC879