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NC-183D Bandwidth




 
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Author Topic: NC-183D Bandwidth  (Read 3787 times)
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AB3FL
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« on: January 09, 2011, 08:35:30 AM »

Hmmm...take 2

I am looking for a way to widen the IF bandwidth on an NC-183D with the crystal filter off.  I think stock is only about 4kc and I would like more like 8kc.  I think that the NBS-1 which is a modified nc-183d has an adjustable IF bandwidth.  If anyone knows how to widen the IF or has a schizmatic for a NBS-1 that would be great

thanks

Tom - AB3FL
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w3jn
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« Reply #1 on: January 09, 2011, 09:45:50 AM »

Tom, my post to your first attempt on this apparently corrupted the database somehow...  I can't even delete that other thread.

Anyhow, the difference between the NBS-1 and the -183D is that the NBS has tertiary windings on the IF transformer that are switched to provide the different selectivities.  Other radios with similar arrangements are the NC-400, R-390, and R-392.  See attached picture, which illustrates the tertiary windings in the R-390 IF.

Short answer is you can't do it the way the NBS-1 does it.  You have several options to widen the bandwidth:

1 - Stagger tune the IFs.  You adjust the primary to 454 KC and the secondary to 456 KC on each transformer, for example.  This will most likely create an asymmetrical IF response, however.  But it's easily reversible if you don't like it.

2 - Install Q-killing resistors across the primary and secondary of each IF transformer.  Try something between 47K-100K.  This will upset the gain balance and will require re-adjusting the IF gain by messing with those resistors that set the gain for each band on the last wafer of the bandswitch.  You'll also hafta re-align the IF.

3 - Increase the coupling - install a couple pF capacitor between the hot leads of the primary and secondary of each IF transformer.  Again this will upset the gain balance and will require re-alignment.



* r390IF.jpg (87.55 KB, 1085x708 - viewed 375 times.)
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KM1H
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« Reply #2 on: January 09, 2011, 09:29:06 PM »

Swamping the IF with 47K should work along with the stagger tune for at least 6kc.

Im not sure what John means by changing band resistors as its the IF that may need a boost and I wouldnt mess with hitting the mixer any harder even if you upgrade it to a 6BY6.  Since the manual gives stage gain measurements just change the screen resistors for the first 2 IF amps a bit lower and retest. I wouldnt go below 33K. If its a stock radio all the droppers in that set are way out of spec anyway.

Carl
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w3jn
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« Reply #3 on: January 09, 2011, 11:24:15 PM »

I'm talking about R1, R2, and R3 that set the band gain, Carl.
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AB3FL
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« Reply #4 on: January 10, 2011, 02:49:22 PM »

OK.....I put 56K (I had a bunch of them) across the primary and secondary of the 5 IF transformers.  I aligned the IF as in the manual for now to about 455KC.  The meter barely deflects now.  I lowered the value of R3(which is for band E) and was able to get it to move a little above 0 with the 455KC applied to the stator of the detector section of the tuning cap.  I removed the signal generator and hooked up the antenna.  I thuned to 1490AM which is a local AM station and used to pin the meter and now it only moves to about S5.  This is even after playing around with the values for R3.  Is it normal to lose this much gain with the resistors on the IF transformers?


Tom - AB3FL
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KM1H
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« Reply #5 on: January 10, 2011, 09:38:11 PM »

You dont want to go messing with those front end resistors and lowering R-3 just makes it deafer anyway.

I gave you some suggestions yesterday but Ive never shotgunned all IF's at once. In my case a Kiwa filter was added between T-1 and T-2 and just those were swamped. Dont remember if I removed L-23 and 32, its been ages since I did it.

Back up and try just T-1 and use 100K this time or a pair of the 56K in series. Record what it does for bandwidth and signal. Move to T-2 if all looks good, stop when youre satisfied. You can also try grounding V-5 cathode to get a bit more gain.

Has this radio been recapped and all out of tolerance resistors changed?

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AB3FL
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« Reply #6 on: January 10, 2011, 10:06:42 PM »

For now I removed all of the 56K resistors that I put in and realigned the IF.  I did replace all of the paper/electrolytic caps and all of the out of spec resistors like the infamous 47K ones.  I will have to try one IF transformer at a time tomorrow..

thanks

Tom - AB3FL

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WQ9E
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« Reply #7 on: January 10, 2011, 10:09:32 PM »

Carl,

Might it be easier to try some very small value caps across the transformers to widen the bandwidth somewhat?  I recall this was one of the approaches used by Gonset to compensate for the drifting oscillator in their G-76.  It sounds like the swamping resistor approach is going to get very involved.

Or better yet go for a "broad as a barn door" low end Halli receiver Smiley
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Rodger WQ9E
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« Reply #8 on: January 11, 2011, 09:38:58 AM »

I was thinking of the caps but didnt feel they were necessary in mine Roger.

I didnt delve too deep but it may be possible to bypass T-2 and T-4 as all IF's have the same part #. That would give a HQ-140X type of response.

Heading outside to finish the E-W Beverage before the snow hits. Its a balmy 15F outside.

Carl



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