Should I restore and make it pretty, back to original Cobra state or go further?
Well, I've got the DDS boards, I think a little fun should be had.
I'll run through the radio properly, I'm going to replace all those horrible electrolytics, they were never particularly good quality from the factory and after 30+ years, they'll be way past best.
A full alignment from front to back and back to front first, make sure there are no nasties lurking, a few good squirts of switch cleaner and then the DDS boards. Not possible to FM a DDS chip without some serious coding and a good fast microcontroller so I'm planning on using one as an offset crystal replacement, I'll work out the practicalities of it and post my thoughts later.
Wednesday, 10 July 2013
Friday, 5 July 2013
VCO, Varicaps and venting
Onwards, now with the frequency counter in action again I can see where the dog is transmitting.
On the same damn frequency on every channel.
Hmm, N-Codes correct from the channel change switch, crystals oscillating at the right frequencies, let's see what the VCO is doing.
Oh....
Freewheeling.
Oh dear, it seems that there have been some mods to the board around the VCO.
Looking around the web I can see that the capacitors on the rear of the board are too numerous and there's a diode missing:
The two 'extra' that have been fitted turn out to be C78, 33pF and C74, 330pF.
For some reason they've been moved to the copper side, I don't like that, I prefer them on the correct side of the board so I moved them back. Had to melt off the wax around the VCO (L15) and actually remove the VCO coil to get reasonable access to the original positions but now they are living where they should do.
The missing diode turns out to be a varicap, 1S23339G. Well, unless I find one in another radio, I'm pretty stuck for one of those. I can find the spec, that's easy. Matching it with an equivalent, hey, I want it now, not in a week, this radio has been sat idle for too long.
An old board from a Storno PMR radio had a BB409 on it, it's twice the capacitance of the 1S2339G at the same voltage but nothing ventured, nothing gained, I tried it.
A quick tweak of the VCO later, I have a radio that changes channels and transmits on the correct frequencies. Excellent.
I'll test receive tomorrow using the DDS generator from the last post.
On the same damn frequency on every channel.
Hmm, N-Codes correct from the channel change switch, crystals oscillating at the right frequencies, let's see what the VCO is doing.
Oh....
Freewheeling.
Oh dear, it seems that there have been some mods to the board around the VCO.
Looking around the web I can see that the capacitors on the rear of the board are too numerous and there's a diode missing:
The two 'extra' that have been fitted turn out to be C78, 33pF and C74, 330pF.
For some reason they've been moved to the copper side, I don't like that, I prefer them on the correct side of the board so I moved them back. Had to melt off the wax around the VCO (L15) and actually remove the VCO coil to get reasonable access to the original positions but now they are living where they should do.
The missing diode turns out to be a varicap, 1S23339G. Well, unless I find one in another radio, I'm pretty stuck for one of those. I can find the spec, that's easy. Matching it with an equivalent, hey, I want it now, not in a week, this radio has been sat idle for too long.
An old board from a Storno PMR radio had a BB409 on it, it's twice the capacitance of the 1S2339G at the same voltage but nothing ventured, nothing gained, I tried it.
A quick tweak of the VCO later, I have a radio that changes channels and transmits on the correct frequencies. Excellent.
I'll test receive tomorrow using the DDS generator from the last post.
Wednesday, 3 July 2013
DDS Lives
A little progress, the counter is working now, Thandar make a good piece of kit. I need to have the calibration checked against a frequency standard but it seems fairly close against the frequency sources I have here.
Talking of which, a little exercise with the PICKIT and breadboard yields this:
Which, in turn, using the blue wire as an antenna, gives this:
Now I've got a signal source I can rely on to be available I can carry on with the Dog.
Talking of which, a little exercise with the PICKIT and breadboard yields this:
Which, in turn, using the blue wire as an antenna, gives this:
Now I've got a signal source I can rely on to be available I can carry on with the Dog.
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