The Jentec Cobra Transceiver MKV that was found in a loft

[Ashtec]

There is one thing i will ask you radio pete how do we know which corba's was converted by john him self as i can not see note in my one to say he had done the work on it or not.................................

[radio pete]

There is one thing i will ask you radio pete how do we know which corba's was converted by john him self as i can not see note in my one to say he had done the work on it or not.................................

Hi Paul johns ones had all the screws & nuts sealed with a wine red nail vanish, mine where any colour I had @ the time they where all dated on the speaker cover & most of mine had a small vero board fet preamp fitted to help with the widebad receive hope this is of some help.

[beko]

mine was signed behind the main front panel and matching serial number..

[UGLY_BOB]

Corbywhoreslops have just ordered a case of red nail varnish, and a bag of small stickers.

[radio pete]

Corbywhoreslops have just ordered a case of red nail varnish, and a bag of small stickers.

LOL

[RD250]

There is one thing i will ask you radio pete how do we know which corba's was converted by john him self as i can not see note in my one to say he had done the work on it or not.................................

Hi Paul johns ones had all the screws & nuts sealed with a wine red nail vanish, mine where any colour I had @ the time they where all dated on the speaker cover & most of mine had a small vero board fet preamp fitted to help with the widebad receive hope this is of some help.

Hi Pete
Looking at the images of Paul's radio I did wonder what was being done to expand the receiver sensitivity. Most of the expanded 148s I saw in the old days (I left the scene before these Jentecs appeared) had nothing done to the front end to expand the response of the front end.

If the front end is tuned/peaked up as per the service manual (ignoring the mistakes in the manual) then these radios become very narrow indeed. They can be off tuned to improve this but even then the radio starts to go deaf at the edges of 120 channels let alone with 2 to 4MHz of coverage.

I used to alter the cap in the two stage BPF and also rebias the front end amp on radios that had LL to HH and this worked well across 5 blocks of 40. I also modded the IF stage in a couple of places. I saw lots of radios with front end preamps fitted usually on piggy boards soldered to the tops of the transformer cans. The problem with preamps is they ruin the dynamic range and several of the ones I saw were dual gate mosfet designs with way too much gain that were unstable or borderline unstable.

There is another issue that I suspect this radio will suffer from. If you mega expand a 148 to also try and cover the 10m band then the spurious terms from the Tx mixer will start to get very large. From memory the worst one is 2*LO-10.695MHz

I got past this on my 12 block 148 (26-30MHz) by switching in a capacitor on the lower ranges on the post mixer tuned circuit to repeak the response to provide selectivity against the spurious terms. Otherwise the radio will produce huge spurious terms within 2-3MHz typically. So you end up with a very dirty transmitter unless you overcome this.

I don't see any mods in this area on Paul's radio so I'd guess it will produce some very big unwanted terms from the transmitter along with the wanted when using it on the top end of the 10m band.

It won't affect the performance on the wanted channel but it will mean the radio will cause interference on other frequencies.

My other query was how you overcome the different clarifier gain on the different modes. eg the clarifier has a different range on AM vs LSB vs USB so did you have to put some fudgery in somewhere for this? Otherwise I'd expect the accuracy of the 1kHz steps to suffer according to mode selected?

[RD250]

BTW you can overcome the Tx mixer issue above by tapping off a logic level from the main counter on the Jentec board and use it to switch in and out a capacitor to repeak the tuned circuit after the Tx mixer. Pretty easy to do and it will make the transmitter a lot cleaner if used up at the top part of the 10m band.

[Hammyman]

I know it may be a bit of a dumb question but if the goal was to get a VFO so you could reverse engineer it, wouldn't one of the many external ones for old Kenwood and Yaesu HF sets that crop up do the job?

Also a quick Google shows this basic design was put on the internet in 2000: http://www.pan-tex.net/usr/r/receivers/vfo.htm

In fact there are VFO designs from simple through to very high spec digital ones all over the internet.

And as you seem to be quite technically competent with the Cobra boards, surely you could work out how to insert that VFO?

[Crusader]

There is one thing i will ask you radio pete how do we know which corba's was converted by john him self as i can not see note in my one to say he had done the work on it or not.................................

The ones i have seen where usually had a signed sticker on the eprom or the lids signed.

[RD250]

Just noticed an old thread appeared asking about the alignment of a Belcom CB radio.

I followed the links to the schematics and the circuit for the Belcom LS102 display and PLL control is incredibly similar to the Jentec.
Same principle except the Belcom resolves down to 100Hz.

A bunch of MC14510 counters acting as a reference counter. Also the BCD to 7 segment decoding looks identical.

Also the Belcom method of decoding the count to drive a 'weighted' homebrew DAC to control the fine steps on the clarifier (PLL loop) VCXO is the same too.

I suspect the Belcom designers must have spent a fair bit of time designing the VCXO for the 100Hz steps as this would have to be aligned up very carefully to avoid issues with drift and tuning accuracy. Even so I'd expect the Belcom to be poor wrt accuracy and drift looking at the schematic.

Either way, if you want to know the principle of how the Jentec board works just look at the Belcom circuit for the counters and the decoders and LED drivers. It may be a coincidence that they are almost identical but either way you get to see a schematic that isn't a secret

http://www.cbtricks.com/radios/belcom/l ... fo_sch.pdf

[Ashtec]

Aother question for you radiopete what this sticker about on the lid of this MK5 i know there's a date but whats the
00052 stand for........................

[radio pete]

you have the 52nd Mk5 that John did it's not one of mine hope this is of some help

[UGLY_BOB]

Reprints of that sticker are now available from 'Cobledrawerspotz.cum' on e-bay.

[Ashtec]

you have the 52nd Mk5 that John did it's not one of mine hope this is of some help

I did think this radio pete but i thought i best ask you about it just to see what it was about + it all add's to the resale value if it go's back up for sale.......................

[radio pete]

you have the 52nd Mk5 that John did it's not one of mine hope this is of some help

I did think this radio pete but i thought i best ask you about it just to see what it was about + it all add's to the resale value if it go's back up for sale.......................

resale I can give you a £100£ for it PMSL