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Author Topic: Too much RF gain  (Read 10232 times)
K7MDO
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Posts: 325




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« on: April 23, 2012, 11:43:16 AM »

I recently finished recapping and cleaning up a NC-190 and it operates quite well, except for one thing.

It seems to exhibit too much RF gain, particularly when operated at 125 VAC as opposed to 115 VAC for the line supply.

There is what seems to be intermodulation or distortion on strong stations unless the RF gain is turned WAY down and it seems to worsen at full line voltage. 

Any ideas how to "desensitize" the little beast?  Or is this just a failing of a low end receiver?

73, T
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AD4U
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« Reply #1 on: April 23, 2012, 11:54:51 AM »

I am not familiar with your particular receiver.  However in the old days when the BFO was used to tune SSB signals (in the receivers that did not have product detectors) one had to back off the RF gain quite a bit to receive SSB signals.  If not SSB signals were distorted. 

If your receiver does not have a product detector, then what you are experiencing is normal.

Dick  AD4U
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KB4QAA
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« Reply #2 on: April 23, 2012, 03:00:59 PM »

What you are experiencing is absolutely typical for radios design before the early 1960's.  They did not have modern AGC circuits and it was necessary to ride the RF Gain and AF Gain knobs.  Unlike modern receivers one doesn't just crank the RF Gain to Full.

One useful method when dealing with strong interfering signals from crowded band conditions or from thunderstorms is to:
1. RF Gain - to minimum
2. AF Gain - to maximum
3. Slowly increase RF Gain to useable level and adjust as necessary.

p.s.  Do this in the wrong order and you blow your eardrums!  Smiley  This technique is still perfectly applicable with modern radios (with AGC set to Fast/OFF).

Enjoy the differences, and appreciate the improvements we enjoy today!

bill

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K7MDO
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« Reply #3 on: April 24, 2012, 01:54:31 AM »

Thanks for the comments.  What you describe works fine and is what I too have found works...it just may be this model exhibits the problem worse than others I have worked with in the last few years. 73
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G3RZP
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« Reply #4 on: April 24, 2012, 03:06:27 AM »

To get reasonable distortion from a diode detector on AM, you need a minimum of about 5 volts of RF. More if the AC and DC load lines are different. Even more is often needed on the AGC detector. However, too much BFO injection on CW actually reduces the amount of AF output, and some prewar receivers even had adjustable BFO level to allow the operator to get best performance. For minimum distortion on SSB, however, the BFO level needs to be appreciably above the signal level,  so with a more optimum level for CW, the signal level at the detector has to be wound way back. If the BFO level is too low, the signal with added carrier - the BFO - has a very high modulation depth, at which the diode detector distorts - badly!

So that's why the RF gain needs winding back. Where there's a separate IF gain control, the RF gain is set for optimum Signal to Noise Ratio (SNR) and the IF gain wound back to minimise the distortion.
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K7MDO
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« Reply #5 on: April 24, 2012, 09:31:54 AM »

What worries me is "who is going to know this stuff in 50 more years?"....  I realize things have moved on in electronics to the extent that maybe no one needs to know the intricacies of a 50 year old receiver design....  but I find it refreshing to know someone still does!

Thanks for the education and help, 73, Tom
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G3RZP
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« Reply #6 on: April 24, 2012, 09:55:34 AM »

Tom,

There is an awful lot of basic radio and even basic electronics not taught today - at least in the West. You find people 'doing  RF design' who have no real idea of things like coupled tuned circuits and Miller effect and so on. OK, they don't need it because the computer does all the work - which is fine until something doesn't work.

A good example of this was some time back when one of our labs was looking for RF IC designers. A number came for interview but it was very obvious that in the boom times, if you could see lightning and hear thunder, you would get sat in front of a Unix work station and told what to simulate. But understanding what you were doing and why was another matter...

About 18 months ago, I went to the optician. 'What work do you do?' he asks.
I say 'I'm a radio engineer'.
The reply was ' But everything is digital now, so there isn't any need for radio, is there?'.
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K8AXW
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« Reply #7 on: April 24, 2012, 10:40:25 AM »

MDO: 
Quote
I recently finished recapping and cleaning up a NC-190 and it operates quite wellI recently finished recapping and cleaning up a NC-190 and it operates quite well

I was recently given an old NC-183D receiver.  I finally powered it up yesterday (brought it up slow with a Variac for about 45 minutes)  The hum is terrible and not controlled by either the RF or AF controls.

Need to do a recap so was wondering if you'd share your sources for the twist-loc power supply cap and what did you use for the other electrolytics throughout the receiver?

Any other bits of information on this job would also be appreciated.  If you wish my email address, please let me know or send it through the ARRL address listed on QRZ.com.

Al
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WB4SPT
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« Reply #8 on: April 24, 2012, 12:15:36 PM »

Several ways of "recapping" the electrolytics.  I know of no new, factory built multi-sections out there.  For my Drake 4-line, I removed the twist lock cans and cut, with a dremel cut-off wheel, the can off, just above the bend at the base.  Remove and toss the guts.  I bought 105deg caps from Mouser and drilled small holes thru the base phenolic that could pass the small leads, which then were soldered to the lugs.  The top was put back on using JB weld epoxy and masking tape to keep things looking good.  Sometimes using lead insulation, epoxy inside the can as required.
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KD0REQ
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« Reply #9 on: April 24, 2012, 01:37:17 PM »

the Mallory FP line was bought by some employees, and is cranking out FPs that look to be sellers.  a bunch of the TV/phonograph stuff that had all sorts of bypasses in them are no longer made.  find them through www.tubesandmore.com as a selling agent.  the company is CE manufacturing.

I have replaced some LV stuff by putting a piece of perfboard in the capacitor chassis hole, pulling the leads through, and connecting them when historical considerations don't count.

most replacements are made with today's 1/4 and 1/5 sized capacitors wired under the deck, and the FP terminals are disconnected to prevent booms and confetti.

//edit// doing a little further Webifying, it would appear that the provenance of the newly built FPs is a corporate asset... and that CE manufacturing is a piece of CE distributing, which appears to equate by address to the same ownership that is Antique Electronic Supply aka tubesandmore.com.  I've had good luck with the parts and AES, but for accuracy, the sticker on the cap is the distributor's trade mark.
« Last Edit: April 24, 2012, 02:13:41 PM by KD0REQ » Logged
KB4QAA
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« Reply #10 on: April 24, 2012, 04:11:44 PM »

http://www.hayseedhamfest.com/

Hayseedhamfest carries can type capacitors single and kits, and says he can make caps to order.   
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W5JO
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« Reply #11 on: April 24, 2012, 04:42:24 PM »

What worries me is "who is going to know this stuff in 50 more years?"

I hope you and others you educate will.  This is like counting change at a cash register.  There aren't many who know how unless the computer tells them.  Pass you knowledge along.
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WA5VGO
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« Reply #12 on: April 26, 2012, 06:30:02 AM »

Sounds like the receiver has AVC problems.

Darrell
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W4OP
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« Reply #13 on: April 26, 2012, 07:17:54 AM »

For receivers that use a BFO for CW/SSB the AVC  has to be disabled when using the BFO. If iot were not, the BFO injection would swamp the AVC rendering the Reciever deaf. Same reaon the S meter is inoperative when the BFO is used. The advent of the product detector overcame these shortcomings.
His 109 is performing as it should.

Dale W4OP
« Last Edit: April 26, 2012, 08:04:54 AM by W4OP » Logged
WA5VGO
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« Reply #14 on: April 26, 2012, 11:36:37 AM »

All superhet receiver use a BFO to demodulate SSB or provide a beat note for CW. The AVC needs to be turned off when there is no way to isolate the BFO signal from it. A product detector is the most common solution, but not the only one.

Where in the original post does it say he's attempting to receive SSB or CW?

Darrell
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