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FM Tuner Alignment Issue

R

RossW

Addicted Member
I'm still fairly new to tuner alignments and this one has me stumped.

I'm trying to align a National Criterion FM mono tuner. I am doing a dual sweep alignment with a ST 1000A.

I get a nice flat and symmetric pattern on the scope, and the tuner receives stations very well BUT only up to ~105MHz. Everything is great 88-105, the dial calibration is spot on, reception is strong, etc. but there's a "wall" at 105MHz. No reception, no sweep pattern, nothing. It's not a gradual transition either; the sweep pattern doesn't go from good to bad, it goes from good to not there.

The schematic is available here:
http://akdatabase.org/AKview/albums/userpics/10004/National Criterion Owners.pdf

What am I missing?
 
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Do the tuning capacitor plates short above that frequency?
 
Well, I checked it out and there's nothing shorting but I did discover a cold solder joint on a ground connection. I'll redo that joint tomorrow and see if there is an improvement!
 
Likely a tired mixer/oscillator tube (6X8). Tube tester won't tell with any certainty,so you'll have to sub with a new one.

EDIT: Hey,that's quite the machine! FM: Cascode,3 if's and 2 limiters,AM: RFamp & 2 if's,this was certainly made for getting some distance. Nice
 
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It is a magnificent radio, supposedly it has a spectacular AM section because it was designed for binaural stereo. They tried to squeeze every ounce of performance out of the AM section so that it wouldn't sound funny compared to the FM signal.

I ended up with two of these a few years back, I cleaned up and recapped this one, the other one is still in storage. It's too bad National killed the product line before they released an MPX unit (the tuner actually has an output specifically for one).
 
Agreed on the binaural stereo. The AM section on my McIntosh MR-55A easily rivals most FM tuners.I have a daily driver Fisher R 200 that also has a good AM section (RF & 2 if's) despite being a multiplex tuner. Great for long distance oldies stations,ha ha.
 
Getting back to this...
The bad solder joint certainly wasn't doing it any favors, if I poked it reception went to zero at all frequencies. Even after fixing that however, the signal still disappears around 105MHz. If I tune close to that and put my hand near the 6X8 tube the signal also disappears so I think the next step will indeed be a new 6X8 tube (which unfortunately I don't think I have any of).

Stay tuned!
 
The 6X8 is pretty common and goes for give-away prices.Just for the heck of it have you checked the voltages around the mixer/oscillator stage?

Stay tuned? Really!?!? Gee Ross,people have been shot for less than that:p
 
Yeah, I just bought one on e-bay for $3.50 shipped, should be here this weekend. If that doesn't fix it I'll start checking voltages.

I actually wasn't even thinking of making a pun when I typed that, sometimes things just work out!
 
I still don't understand how the heck e-bay does it; if I were to send you a tube through the mail,the postage alone for a small parcel would probably cost 4 to 5 times what you just paid for the tube AND shipping. Which really sucks,because I have so many of this type that I will never use,it would be nice to send some to a good home rather than have them sitting in a box collecting dust:(
 
I was wondering that myself. First class shipping from CA to MA has got to be more than I paid for the tube :dunno:
 
It may seem odd,but it's a very common occurrence in RF and pulse/timing circuitry; a tube tests as ok on a good quality machine,but just can't function at the higher frequencies.In many of these applications,test by substitution was accepted practice.Glad you found the problem:)
 
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