Decisions...

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.

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.

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.

OK, so the counter is still dead. I know what's wrong with it and have ordered parts to repair it, they should arrive today if the post works. Probably needs a calibration too so I'll call in a favour for that.

Before I can proceed further I need to see signal into the radio, to that end I'm going to be using one of the DDS modules as a signal generator and will be picking up a couple of rotary encoders later today to build one of these:

http://www.g4jnt.com/AD9850-Controller.pdf

That should at least allow me to check through the receiver and if I get a signal there, I can make a reasonable assumption that the PLL is working correctly and all aligned correctly.

I've also managed to buy one of the Anytone/Quixiang rigs, a KPO DX-5000 (anyone know how to work out which software version?) so I'll be on air, probably listening more than chatting but the dog will still be front stage.

Gordon's Alive!!!!

VCO lock, mixer crystals wobbling as they should, it transmits.

Somewhere.

Frequency counter not working. Oh well, I'll fix that and then align the PLL, Loop crystal and the LO crystal.

It seems the dog isn't as much of one as I thought it might be.

It lives

Kind of.

Most static voltages are correct, the PLL supply is at 8.4 volts, nothing gets hot, current draw is right around 300mA on RX (depends on channel display at this stage, I can see segments changing the current draw as I select channels) so I'm happy to put the MB8719 into it when it arrives.

Headache now so I'll check the crystals are oscillating somewhere near the correct frequencies tomorrow when I dig out my 'scope and counter.

The post came.

And it delivered these:

Which I forgot I'd ordered. There are two AD9851 versions on their way as well.

Good grief...

The Tone switch was wired as a Roger Beep switch, connecting or disconnecting one leg of C146, but whenever the mod was active, you got HF cut on the received audio because the original function was still connected. Odd.

The SWR/Power/Cal switch was wired to short out the AM Power preset (the Red and White/Violet wire in the 'as was' pictures), VR11 and had another unknown function but the Blue and Yellow wires were floating loose inside the case and it will remain a mystery now.

Other mods found? Umm, so far, D8 shorted with a wire link to cure 'lazy' needle. Not sure if this mod will stay or not, it's a minor one that won't affect the operation of the radio much and may actually improve it.

I'm sure there may be other 'mods' to be found, likely just as inept but it's time for sleep now.

More tomorrow if I get time.

Somebody stop me..

This is getting a bit addictive, started stripping and cleaning the front panel now. The wiring has been chopped about quite a bit, bizarrely, the three front panel switches I'd have expected to have been re-purposed to add 'functions' still have what looks like original wiring and the only 'useful' switch, the SWR/Power/Cal switch had been bodged for some as yet unknown purpose.

Hopefully the wiring diagram will be accurate.

Channel switch, before:

And after:

Ready to put back in now.

(I know, the flux isn't left on purely out of laziness on the manufacturer's part, it's there to help protect the board a little too, or that's how it's been explained to me in the past, so I'll be giving it a light spray of a PCB lacquer before it goes into service)

Drying out.

The board is almost clean, it's actually in really good shape as well, no unpleasant poker work soldering (wait 'til you see the iron I use, you'll shudder but it really is the best I've ever used) and only a couple of track cuts which have been done neatly so should be easy to sort out.

The component side of the board is a bit mucky still and I've yet to strip off the front panel so I can clean up the switches but I'm feeling oddly confident and enthused with this project.

One thing I do like about this radio, the option for dual outputs. Now I have a mind to do a MOSFET conversion on it as well but I'm getting a bit ahead of myself there I think.

Cleaned and dried nicely, it really is in pretty good condition under the muck.

PLL removed and the tracks around it aren't too bad either considering it's been hacked about (all that damage was already done)

Cup of tea time.

Now to start getting it back to stock, thanks to a great guy, (Andy) I have a PLL winging its way to me so by weekend I hope to be able to transmit.

I think now I'm going to give it some power and see if it lights up, if you see smoke, call the fire brigade.

More updates as they happen.

Am I mad?

Or do I have too much time on my hands?

Well, probably a little of both.

A bit of history and a confession first, many years ago I used to be a 'rig doctor'.

Not your average run of the mill screwdriver expert, hey, I was a trained electronics engineer (I suppose I still am but I don't do that for a living any more) and I used to modify, repair and 'tune' (I hate that description) radios, from your average 27/81 40CH FM set all the way up to and over Somerkamps, Yaesu, Icoms and once upon a time some really lovely Israeli Military stuff that I'm still not really allowed to discuss.

The repairs were the bread and butter stuff which kept me in beer and cigs when I used to consume those things but I *really* enjoyed modifying radios, developing EPROM conversions, adding bands, making them do things they were never supposed to do essentially and I specialised in the Cybernet PLL02A chassis sideband rigs with the occasional Cobra, Uniden or President thrown in for good measure. 


So, digging around in my shed for some components, I found a pile of old rigs. Most are complete and working, one or two are donor scrappers and then there was the Cobra.

A PC-879AB Chassis Cobra 148GTL-DX.

Looking very sorry for itself.

A Cobra that I'd been given to repair almost 20 years ago.

Now, while I do sometimes put things to one side and forget about them, this particular Cobra's owner had a history of being a bad lad but somehow he managed to persuade me that it was essential he was 'on-air', being a soft touch, I lent him a mint Multimode 2 so he could be on air.

Which he sold.

For £30.

To say I was a bit pissed off was an understatement, so I kept hold of his Cobra and told him that once he got my radio back I would give his back to him.

Of course, I never saw the Multimode again and the Cobra was dumped in a corner of my workshop, I moved house, gave up on radio because I discovered computers, had kids, all that and forgot about it.

Until a few weeks ago when I found it in a very soggy box of old books.

It's definitely had it's day, it's very sorry for itself and had been hacked about by some screwdriver expert who knew enough that Cobra's have a Motorola PLL. Except, they don't, not all of them anyway. This one needs an MB8719 but someone has put an MC145106 in place. Obviously, it doesn't work.

So, what to do? Chuck it in the bin? Yeah, that's probably best.

Except, well, I'm an idiot. I've decided to resurrect it and have a little fun with it so, here, on these pages I hope I'm going to be able to document my efforts in getting this old lady back on air.

I've still got a load of old test equipment, meters, a 'scope, frequency counters and multimeters but I need a few new bits on this quest because, I've decided, back on air isn't good enough.

This radio is going to be something a bit special. Cosmetically, it's bloody ugly but the PCB? Looks pretty  good.

So, first order of business is to get it back up and running, transmitting on frequency then let's see where it goes after that.

Right, I've bored you enough with words now, some pictures so you get an idea of my folly. (note, this is pretty much as it came to me, no knobs, ratty front bezel, extra wires. Band switch missing, that was me, can't remember why but it was 20 years ago and the corrosion, yeah, my shed.)

Pretty nasty eh?

 Lovely faceplate. Might need a new bezel. Or not. I'm not really interested so much in the looks of this rig, I want function and if I can get that then I may put some effort into making it pretty.

 But, mostly complete and not actually too bad from an electronics point of view

 A couple of track cuts and bridges, not entirely sure those red, white, yellow and blue wires are meant to be there but there's a service manual I need to read...

 Ah, PC-879AB, it's a Cobra then.

 Umm, that's not right, at least not in this radio...

 And no loop crystals in two of the places they should be. Hmm. This thing has been modded, reckon there's a board missing.

A TIP32!!! It must be a Jentec! Oh, what? Yeah, ermm, I was fitting TIP42 and TIP32 regulator transistors  to Cybernet Export rigs and Cobras years ago and I'm not called John.

A BDX54C Darlington transistor. I'm going to have to ask my mum if I was called John.

All in, a bit sad and bedraggled. Next job, take out that MC145106P, power it all up and see if I get the relevant static voltages, life from the loop oscillator and other crystals (yeah, not the PLL one until I get an MB8719)

Next post, probably this weekend at some point, let's see if I can coax some life out of her.