[NLRS] Parts is parts

[Dan Larson]

On Thu, 05 Feb 2004 14:05:09 -0600, Dr. Gerald N. Johnson, electrical engineer wrote:

>The ceramic block is likely a filter, could be replaced by another to
>cover 902 or a home made one such as in the VHF/UHF chapter of some ARRL
>handbooks or early 902 articles in QST by W0EYE.

TRUE! (I need to learn to keep my mind open better!) DUH, I don't have
to keep it or even replace it with the same type filter!

>
>For transmitter output tests a roll of 100' of RG-58 terminated by a
>pilot lamp gives a flat load and a relative power indicator. The
>brighter the lamp the more power is going into the coax. Start with a
>#48. When it burns out go to a #47, then a #44, then a dome lamp bulb.
>Not very expensive test equipment.
>
>Yes you have to take control of the PLL/VCO, possibly tweak the hardware
>rest frequency of the VCO.

At least my frequency counter goes up to 1.3G.... I can also use my
various scanners and HT receivers which cover the band, to some degree 
or another for testing.

>
>I'm not kidding about using NBFM. It worked, and it can work again.
>

I just found your description very interesting. I've heard of slope detection
of FM before, but you explained why it works so much better. Where do the second
order modulation products come from? V to F nonlinearities in the VCO? I can see how
keeping deviation low reduces that.

>Offhand I'd guess the first IF frequency is at 45 MHz, and likely the
>mixing is still fairly linear down to 10.7 MHz.

I did see a small 45Mhz rock on the board. Not sure what it was there for.

>
>Two places to look for a schematic or service manual, (well maybe
>three). E.F. Johnson at Waseca, the local two way shop that cares for
>E.F. Johnson products, and epay. There are several hams at E.F. Johnson
>that should be relatively willing to help other hams use their products.

Hey! the sticker said it was made in Waseca, MN!

>
>If they are the 8615 radios with a 45 MHz split, they probably use the
>same transmitter frequency for receiver LO so the first IF is truly 45
>MHz.

Would that 45MHz rock be a filter then? 

Should I be looking for a 55.7 or a 34.3 rock on the board for the second 
mixer to the final IF frequency at 10.7 MHz?

>I have some Micor that do precisely that. Actually it looks like
>most of the receiver is made of low band boards with one crystal for 45
>MHz. And the transmitter is the exciter from a 450 MHz radio. Then they
>split the 450 MHz power and run separate doublers for receiver and
>transmitter and they have a circulator on the transmitter side to
>protect the fragile PA transistor from high SWR at the antenna. Working
>simplex seems to demand a pair of radios or rapid switching of the same
>multiplier string (the Micor has room for several channel elements) from
>transmit to receive channel elements, and my thought was to make up for
>one being off tuned by adding a MMIC buffer stage, maybe 0404 or 1104 to
>one of the sides. One of those projects that hasn't made it very far.
>
>Possibly even with the off tuned ceramic filter, a preamp say with
>cascaded 0604 MMIC before it would still receive well. There'd be a need
>for that preamp to have some filtering in it.

The paging systems at 930MHz & up are pretty loud around my QTH!

Now, you have me thinking that there *IS* hope for some fun out of 
these radios rather than just scrapping them for parts. Although,
I think the 45MHz offset will be a *slight* problem in the Amateur
33cm band (HI HI). I'll have to change the PLL frequency between rx/tx.

Well, these things will more than likely sit for a while around my QTH,
but I think I *will* make an attempt now at getting some info from 
E.F.Johnson. Esp. if there are HAMs there.... I am always planning 
several projects ahead of what I am actually working on. Gotta finish 
my 20m SSB rig first. 

I want to build a 2m linear amp to use with my Tentec 2m transverter next. 
Do you think a Mocom-70 PA section can be biased class A or class AB?
They seem pretty cheap to get. Working at 2m will hopefully pave the way
to 33cm for me a bit.

Thanks a bunch! 

73

de Dan KC0LUY