Lowe Pr-150 preselector..........finally repaired
 

I finally got this one fixed. It must have taken a static charge. The fellow
I got this from picked it up in a package deal with an HF-150 that had a
blown 1st mixer. One of the PIN diodes was leaky. Instead of trying to
figure out which one or two or three of four.....................you get the
drift. I replaced all 16 of them with 1N5767 PIN diodes, and the unit is
working well. I measured the broadband insertion loss............right
around 10dB as specified. With the preamp engaged, gain is +3dB, also with
spec. It will make a nice mate for the HF-150. We'll see how long it holds
up............

Pete

"Pete KE9OA" wrote in message
...
I finally got this one fixed. It must have taken a static charge. The
fellow I got this from picked it up in a package deal with an HF-150 that
had a blown 1st mixer. One of the PIN diodes was leaky. Instead of trying
to figure out which one or two or three of four.....................you get
the drift. I replaced all 16 of them with 1N5767 PIN diodes, and the unit
is working well. I measured the broadband insertion loss............right
around 10dB as specified. With the preamp engaged, gain is +3dB, also with
spec. It will make a nice mate for the HF-150. We'll see how long it holds
up............

Pete


Good going Pete

Better to do the job more then right.

I use a MFJ-959B Preamp/tuner with my 150 when necessary. So far, I only
have one AM BC station that overloads but it occurs on most of my other rigs
also.

Lucky

I know what you mean...............the '150 has a pretty good front end. As
long as I've got mine apart, I might change those I.F. filter switching
diodes to PIN diodes. I was talking to Craig at Kiwa the other day and he
was telling me that it does help. He seems like a pretty good guy.
If I am not feeling too lazy, I might pop them in when I replace that
divider chip.
Oh, the preselector.............I made some detailed measurements after I
reassembled it. Insertion loss varies between -5dB to -12dB across the whole
range. The worst case is at 150kHz. This could be the 1N5767 PIN diodes that
I used. These devices have a minority carrier lifetime of 1.5uSec, making
the usable to about 200kHz on the low end. I suppose I could bump up the
bias current, but I am not sure how much current that decoder chip can sink.
Lowe is running them at about 25mA of current, which give them an "on"
resistance of around 5 ohms. I think most of the loss occurs in the LC
circuits themselves, because there is only -1dB loss in the broadband mode.
With the preamp engaged, gain varies between -0.8dB and +4dB, essentially
making the unit a lossless preselector. Once again, the worst case
measurement occured at 150kHz, again maybe being the PIN diodes, but I think
that the LC tank circuits are the culprits. Still, less than 1dB of loss on
the LF band is pretty ok. The folks at Lowe did a nice job when they
designed this unit. If I don't end up selling it, I might wind some coils
for the LF bands using Litz wire. This will give a sharper tuning
characteristic and have lower loss. A plastic frame tuning capacitor is
being used..........if I replace it with a high Q unit that has ceramic
standoffs, I might pick up a couple dB of gain.

Good going Pete

Better to do the job more then right.

I use a MFJ-959B Preamp/tuner with my 150 when necessary. So far, I only
have one AM BC station that overloads but it occurs on most of my other
rigs also.

Lucky

I used to have an MFJ 1040 years ago. Nice unit. I think I've still got my
old Grove TUN-1 preselector. I had the preamplified version for awhile and
gave it to a friend of mine. That wasn't bad either, but they used some sort
of MMIC. I should have kept that one; this way, I could have yet another
piece of equipment on my shelf!

Pete