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Author Topic: NC-183D oscillation  (Read 2874 times)
K7MDO
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Posts: 325




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« on: September 05, 2012, 10:26:22 AM »

I may have inherited a problem from the last guy who owned this NC-183d.  It plays and acts very well on the ham bands from broadcast through 80 meters but when you switch it to 40 meter bands and above and then advance the RF gain the set will squeal.  This occurs up near the 9-10 area of the RF gain.  At points below the oscillation (7 or less on the dial) on the RF gain control the set will receive fine though with commensurately lower sensitivity.

The oscillation is in the audio frequency and no matter where I put the scope on the rf or if chains it is present.  It sounds like "feedback"...  I have wiggled every wire, changed or moved every tube... no luck.

It seems like the second converter is a possible culprit as that is the only thing that gets switched into the circuit (1720 kc) in the 40 meter and higher positions.

Ideas?

73, Tom
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W8JI
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« Reply #1 on: September 06, 2012, 02:41:19 PM »

I'd suspect open capacitor(s) in the AVC system or an open bypass capacitor somewhere up in the front end, poor tuning capacitor grounding or bypassing, or poor shield grounding.

If it is band sensitive and RF gain sensitive, it has to be an RF amp or mixer area feedback problem. It may have more issues than that, but certainly the front end is involved.

Look at shields and grounding, and then you might try jumping bypass caps with a good ceramic disc. A ceramic disk is about 50 times better than tubular foil capacitors for bypassing.
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K7MDO
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« Reply #2 on: September 06, 2012, 04:51:13 PM »

Thanks so much for the advice....  I will do as you suggest....  I did last night find that of the two diagrams I have from downloads show that I have the earlier version of the set and it sure looks to me like someone may have installed new resistors and cap on the 1720 mixer ....  the component values are way off compared to the schematic so I will also set that back to original at the same time... it seems to me too that the mixer may be oscillating due to poor bypassing... we'll see .... and will post if I have success.  Thanks, Tom 
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K7MDO
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Posts: 325




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« Reply #3 on: September 06, 2012, 09:33:21 PM »

EUREKA! fixed it!  It did turn out to have some poor parts selections in the mixer but at the same time I checked all of the bypass caps on the IF strip.... some were different values and I also noticed that the leads were quite long.  I removed two of the worst offenders and shortened the leads and rerouted them to closer grounds...  immediately the 40 meter (C) band no longer oscillated but when I moved it to the (D) band for 20 meters I still had some pumping type of oscillation going on....  then I noticed that when I would reach into the chassis to add the "test" bypass cap the squeal stopped... when I pulled my hand out it started again...  so went to a wood stick and started moving some of the wires around and found the IF can leads on one IF can, if moved a 1/4" stopped the feedback.  All there was to it!  I think the BFO oscillator and B+ leads were too close together... the routing is very critical and whoever worked on the set before moved things enough to cause the problem.  Very gratifying to have success after a lot of head scratching.

73, Tom
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W5JO
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Posts: 59




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« Reply #4 on: September 07, 2012, 08:25:57 PM »

The placement of the coupling cap from the last IF to the ABC tube is critical.  Move it to the best location.
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K7MDO
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Posts: 325




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« Reply #5 on: September 07, 2012, 08:59:53 PM »

I found the position of the wires to the IF cans to also be incredibly touchy.  There are red and blue wires that cross on their way to three of the IF cans and each pair must be "just so"or oscillation starts at high RF sensitivity settings.

"ABC" tube?

Tnx Tom
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W5JO
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« Reply #6 on: September 08, 2012, 08:18:23 PM »

I found the position of the wires to the IF cans to also be incredibly touchy.  There are red and blue wires that cross on their way to three of the IF cans and each pair must be "just so"or oscillation starts at high RF sensitivity settings.

"ABC" tube?

Tnx Tom

Spell check ...ABC is supposed to be AGC tube 6AH6.  I am referencing a small tubular cap between the 3rd IF 6BA6 to the grid of the 6AH6.
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K7MDO
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Posts: 325




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« Reply #7 on: September 09, 2012, 11:47:57 AM »

Ahhh... yes, I found this to be very sensitive area... after an evening of "adjusting" wires and cap positions I have the thing working very well....  in fact I have even found a 6 meter station....  pretty darn good as when I got this radio it only played broadcast...  another thing that is surprising about this radio is its stability.... after a 5 minute warm up it seems to stabilize perfectly...  pretty decent radio and though not as good as my Icom 7600 it is a great fit for my "alternate" AM station along with my Eico 720/730 that I restored last year....  thanks for the note and take care, Tom
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W5JO
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Posts: 59




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« Reply #8 on: September 09, 2012, 05:27:14 PM »

There is a service note about replacing the mica variable caps in the oscillator section with small Johnson air variables to improve the stability.  Would require a bit of work but a nice change.
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N8CMQ
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Posts: 386




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« Reply #9 on: September 09, 2012, 06:33:44 PM »

With these old tube sets, the caps are usually very poor. Even the carbon resistors have drifted as well.
Have you looked at the voltages from the power supply? The filtering caps located throughout the set could also allow feedback.
On my SX-101 restoration, I am replacing ALL the caps, resistors and tube sockets. The only components I am not replacing, unless I have to, are the trimmers and tuning coils.
You might say I am going overboard, but that is what I like to do at times!
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K7MDO
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Posts: 325




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« Reply #10 on: September 10, 2012, 10:23:38 AM »

Yes, I have replaced 80% of the resistors and now, all of the bypass caps and coupling caps.

One piece of advice (one I didn't take and should have) is to stop during the restoration at stages and see "what's happening" to the operation of the radio.... play it at stages if possible.

If you are doing the whole shebang that can't work but it you are only going to replace "as needed" then frequent checks will help to identify what good or bad that "change" you just completed may have done.

Also keep records for the next guy... the set I just finished had at least three errors in the circuitry due to inexperience of the previous restorers.

I think the biggest help I had was an oscilloscope and RF generator for IF frequencies.  My URM 25 F really worked well!
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K7MDO
Member

Posts: 325




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« Reply #11 on: September 10, 2012, 10:32:02 AM »

I forgot a very helpful thing in my previous post!  Find, if you can a pair of "plastic" hemostats.  With these you can adjust capacitor and wire positions without introducing the capacitive effects of the your "hands".  It also will prevent a nasty shock or two.

I also used an old plastic writing pen with a metal core as a wand.  This worked like a "sensor" to find particularly sensitive areas to added capacitance just my poking around in the vicinity of the IF and RF tube bases.  Once found, I changed tools to the plastic hemostats and started moving wires to see what position was better.  This whole process took an evening but learning that I needed to do it took all summer!

73, T
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N4NYY
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Posts: 4816




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« Reply #12 on: September 10, 2012, 03:48:44 PM »

You can get an Elecraft RF generator. It just does not do 455 KHz. You either need a divider, or one of those cheapies for 455 KHz. The generator seems like a good deal.

http://www.elecraft.com/XG3/xg3.htm
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W5JO
Member

Posts: 59




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« Reply #13 on: September 11, 2012, 06:37:59 PM »

Should you be planning to do more receivers I would look for a function generator at the next hamfest.  Be sure it covers to at least 1 megacycle and has a sweep function.  The one I have cost me all of 10 bucks and is solid state with button activation of all functions including the sweep.  Frequency coverage for it is from 5 cycles to 10 meg which covers all the IF possibilities and the lower bands for RF.

Then using it with an oscilloscope you can sweep align the IF sections which is much easier and better than using a VTVM across the output or the S meter.
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