National NC-88 questions
 

I'm working on an NC-88 and a couple things aren't sitting right with me.

1. Overall 'oomph' seems to be missing. It receives well enough but
even benchside listening requires that the volume be up to about 75%.
Doesn't seem right. An AA5 would blow it away. Finger on the volume
pot wiper gives a very healthy hum so the problem must be further back.

2. BFO. Works but is really weak. I tried more coupling but no
increase. The Sensitivity control has to be almost at the floor to hear
it. Its really too weak to be functional.

Recapped, many resistors found out of tolerance, voltage checks are all
ok. Some previous owner had the IFs badly mistuned. Found an
intermittent factory joint on the bandswitch.

I guess what I'm asking are these 'features' typical of the NC-88? I
wouldn't think so. Maybe the two symptoms are related?

Thanks for any comments.

-Bill WX4A

National NC-88 questions
 

On Thu, 25 Feb 2010, Bill M wrote:

I'm working on an NC-88 and a couple things aren't sitting right with me.

1. Overall 'oomph' seems to be missing. It receives well enough but even
benchside listening requires that the volume be up to about 75%. Doesn't seem
right. An AA5 would blow it away. Finger on the volume pot wiper gives a
very healthy hum so the problem must be further back.

2. BFO. Works but is really weak. I tried more coupling but no increase.
The Sensitivity control has to be almost at the floor to hear it. Its really
too weak to be functional.

I don't have a clue, but before SSB (and I can't remember whether this
receiver predates it, it likely doesn't matter for this purpose when it
came out), BFOs were relatively weak. Their purpose wasn't to provide a
good mixing action, just a beat note.

Which is why there were all those articles about turning down the RF gain
when receiving SSB on such receivers. I'm sure a lot had terribly weak
BFOs even then. And given that the same general design in general
coverage receivers carried over into the age of SSB, the age of the
receiver may not be that important.

There may be an issue, but perhaps your expectations are higher than the
design had in mind.

Michael VE2BVW

Recapped, many resistors found out of tolerance, voltage checks are all ok.
Some previous owner had the IFs badly mistuned. Found an intermittent
factory joint on the bandswitch.

I guess what I'm asking are these 'features' typical of the NC-88? I
wouldn't think so. Maybe the two symptoms are related?

Thanks for any comments.

-Bill WX4A

National NC-88 questions
 

"Bill M" wrote in message
...
I'm working on an NC-88 and a couple things aren't sitting
right with me.

1. Overall 'oomph' seems to be missing. It receives well
enough but even benchside listening requires that the
volume be up to about 75%. Doesn't seem right. An AA5
would blow it away. Finger on the volume pot wiper gives
a very healthy hum so the problem must be further back.

2. BFO. Works but is really weak. I tried more coupling
but no increase. The Sensitivity control has to be almost
at the floor to hear it. Its really too weak to be
functional.

Recapped, many resistors found out of tolerance, voltage
checks are all ok. Some previous owner had the IFs badly
mistuned. Found an intermittent factory joint on the
bandswitch.

I guess what I'm asking are these 'features' typical of
the NC-88? I wouldn't think so. Maybe the two symptoms
are related?

Thanks for any comments.

-Bill WX4A

All of this points to something in the
detector/avc/noise limiter area. Have you made tube socket
resistance measurements and voltage measurements? Those may
show something up. When doing extensive recapping there is
always the chance of getting something in wrong or one of
the new caps being bad.
While the BFO voltage is generally low in conventional
receivers its not _that_ low and should give a strong beat
note with the RF gain in normal position although it will no
be linear enough for SSB.


--
Richard Knoppow
Los Angeles
WB6KBL

National NC-88 questions
 

Richard Knoppow wrote:


All of this points to something in the
detector/avc/noise limiter area.

A big ole DUH for me. ANL switch was open in either position. Amazing
any audio got through at all.

Sounds pretty normal now.

-Bill

National NC-88 questions
 

"Bill M" wrote in message
...
Richard Knoppow wrote:


All of this points to something in the
detector/avc/noise limiter area.

A big ole DUH for me. ANL switch was open in either
position. Amazing any audio got through at all.

Sounds pretty normal now.

-Bill

Contratulations!!! Its so easy to overlook obvious
things. It can be very helpful to have someone else look at
a thing you have been busting your head over. The NC-88 is a
neat little receiver.



--
Richard Knoppow
Los Angeles
WB6KBL

National NC-88 questions
 

Richard Knoppow wrote:


Contratulations!!! Its so easy to overlook obvious
things. It can be very helpful to have someone else look at
a thing you have been busting your head over. The NC-88 is a
neat little receiver.


Yeah, I had to step away from it for it to become obvious.

I've had a fun time tuning around with it. I bought the thing on a
nostalgic bent and its nice to cruise around on a relatively simple old
tube receiver. Most of my SWLing in recent years has been solid-state
and homebrew stuff. Thats fun too but its just not the same :)

73,
Bill

National NC-88 questions
 

On Fri, 26 Feb 2010 23:16:48 -0400, Bill M
wrote:

Richard Knoppow wrote:


All of this points to something in the
detector/avc/noise limiter area.

A big ole DUH for me. ANL switch was open in either position. Amazing
any audio got through at all.

Sounds pretty normal now.

-Bill

I think we've all had our DUH moments. Mine had to do with an HQ-145
which was given to me when I bought a Heath SB-200 from a ham who was
selling the stuff on behalf of an estate from a deceased ham. He said
it didn't work...and it didn't. I recapped it after an inital "smoke
test" was positive. I also bough a 3rd part reproduction manual for
it.

Several instances of cursory testing didn't improve matters and the
receiver was set aside for other matters. Later when I noticed some
minor varients between my receiver and the schematic, I downloaded
another schematic and noted that this new schematic had a note on
it that my orignal schematic did not have. The note said that you
have to have either a send receive relay in place or you have to
jumper the line cord socket on the back panel intended for the
send-receive relay. Otherwise, the receiver will think it is set to be
deadened for transmission. I quickly ginned up a jumper, plugged it
in and of course the receiver worked perfectly. But talk about
feeling dumb.

I think I lost the import of that connection in my tracing the circuit
through some switching.

We all do it.

Jon W3JT