Transmission1

Well guys, some of you may have seen this particular radio in the "What CB/11m Radio stuff Did You Buy Today" section.

It's a late Mk1 Cobra 148 GTL-DX (13 serial number) which has had a 3900 front fitted. Now it's had plenty of mods done. Switchable bleep on the CH9 switch, +10KC on the noise blanker and the tone switch, from what I can tell so far, does absolutely nothing. The SWR CAL rotary acts in a very strange manner indeed. I think it's supposed to be variable RF power as it's been hooked up to where the preset was. But it doesn't adjust the power, it just turns the AM modulation up and down (to completely off). There's also what I would describe as a rather large eprom board.

OK, here's the good bit. Remember that this is a MK1 PC879-AB boarded radio. The original three position band switch has been replaced with a ten, yes ten, position band switch. I expected the five positions after the UK40 to be empty or repeats of one of the other bands. No, not quite.

I had the radio hooked up to my El Cheapo frequency counter earlier and discovered the radio has a quite a considerable coverage. 26.065 through 29.695 in perfectly running bands of 40. Almost as much as a Lincoln!

So, does anyone have any ideas on this? I've never known a MK1 to cover this many frequencies. Or a MK2 for that matter (Jentec's aside).

The radio needed a good clean internally. It looked like it had been sat for a very long time gathering dust. It also needed a new signal meter and bulb. Surprisingly near enough bang on frequency in the notch (I'm not a fan of those things and usually remove them).

Anyhow I've had a good ramble so I'll finish off with a few photo's. What do you think guys?

A couple more pics.

I assumed that this IC on the back of the board was the MC145106 which had been used to replace the MB8719. However upon lifting the eprom PCB I discovered that the 145106 is actually located there. So I have no idea what IC has been used on the reverse side.

So, nine bands of 40 channels plus alpha's and UK40.

Rear of band selector and labels inside the radio. Looks like 'Jim' was the OP on one. On the other 'J N' possibly? Nah, it couldn't be.

Looks like its switching caps in to allow the vco to operate over the wider bandwidth,

have they done anything to brodband the tx ?

what about the receiver front end, are they using varactor diodes to tune the front end like Simon used in his 148 & i used on my 9fdx ?

Hello

JD Custom JDC GPE 1 ISS1

viewtopic.php?f=29&t=22597

http://www.radiomods.co.nz/forum/downlo ... c8c1d9705e



http://cobraworxshopz.com/jd_custom_sx-1_40.html

@+
Claude

14CS06 wrote: 06 Oct 2020, 12:40 Hello

JD Custom JDC GPE 1 ISS1

viewtopic.php?f=29&t=22597

http://www.radiomods.co.nz/forum/downlo ... c8c1d9705e



http://cobraworxshopz.com/jd_custom_sx-1_40.html

@+
Claude

Thank you Claude. Many questions answered with one picture.

wa10 wrote: 06 Oct 2020, 11:36 Looks like its switching caps in to allow the vco to operate over the wider bandwidth,

have they done anything to brodband the tx ?

what about the receiver front end, are they using varactor diodes to tune the front end like Simon used in his 148 & i used on my 9fdx ?

The radio has continuous TX across all the bands it covers. It does drop off though towards the very top end.

I'll dig deeper regarding the front end mod. As I recall Simon used to use the SWR CAL control so you could tune the radio as you moved around the bands.

I had a few of those eprom boards here at one time [Unused not pulls] and fitting instructions, They all got binned in the end though, The MC145106 piggyback onto the MB8719 is the kind of thing Dave [Zodiac] used to do, It may be possible it is one of his

Whatever it is , it looks pretty scary .
I was temped to say that you might be better putting it back to stock , but given the amount of work that's been done to it , that might not be possible .

cb4ever104 wrote: 06 Oct 2020, 16:58 Whatever it is , it looks pretty scary .

Capture.JPG

I was temped to say that you might be better putting it back to stock , but given the amount of work that's been done to it , that might not be possible .

It's actually working fine Paul. Wonderful RX on SSB. Nothing like the tales you here of the Mk1 being deaf. There's also no sign of a lazy signal meter.

It came with a 3900 front fitted, but I'm considering getting a bespoke decal made to include the extra bands etc.

Black Spirit wrote: 06 Oct 2020, 14:29 I had a few of those eprom boards here at one time [Unused not pulls] and fitting instructions, They all got binned in the end though, The MC145106 piggyback onto the MB8719 is the kind of thing Dave [Zodiac] used to do, It may be possible it is one of his

So there are two MC145106 PLL's in the radio. That's something different. I was thinking about sending it down to you for a tune up at some point.

Yes simon did use the cal knob to tune it and it worked, i did a 9fdx with 2 extra bands & same kind of front end tuning,
The idea came from i think an early clear channel AR schematic simon showed me many years ago.

astradyne wrote: 06 Oct 2020, 17:44
So there are two MC145106 PLL's in the radio. That's something different. I was thinking about sending it down to you for a tune up at some point.

The MK1 uses an MB8719 PLL and it has an MC145106 piggybacked which is why it is capable of more bandwidth, Im surprised it has got down to 26.065 though, Usually a MC145106 will only go down as far as 26.085, Maybe the 4070 is giving it that little extra help ?

If it is working fine then i wouldn't suggest having a tune up, I would just leave it as it is.

Thanks for sharing that Nick .

Re. the decal, you'll need more letters than there are in the alphabet to put all those blocks around the band knob