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Command Set Repair




 
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WU2D
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CW is just a narrower version of AM


« on: May 14, 2011, 09:39:14 PM »

Every once in a while something craps out in the shack - well why not the mobile too? I lost the receiver in my 75M ARC-5 mobile station on the return trip from Nearfest. Suddenly I lost all sensitivity. I had IF noise and the outboard Q-multiplier and BFO and audio worked fine so the problem was likely in the front end. Any way I ripped the little receiver off the plywood and tore into it today.

This receiver is nor stock by any means. It was modified by a ham long ago and it has a pretty classy setup in the detector, AGC and audio. The 12SG7 detector AGC tube was replaced by a 6AL5 and a 6SL7. The 6AL5 lives inside underneath on a little metal strip suspended between the two sides of the receiver. This conversion was documented and it is essentially the "ARC-5 Repackaged" circuit discussed in Electric Radio last month. With the Q-multiplier, a homebrew addition I wound for 1415 kc, it really makes a fine mobile receiver. Without it, the ARC-5 is simply not selective enough for realistic mobile work.

Anyway I tried swapping the 12K8 tube and the 12SK7 RF tube. The scope said that the issue was that the LO was not "O"ing. Darn - a dead oscillator. First I pulled the triple coil assembly and inspected the contacts and buzzed out the oscillator coil. All looked good. Then I measured the various voltages on the grids of 12K8 converter tube and they were normal. No bad bypass cap.

So next this meant tearing into tuner box. 20 or so screws later I had it apart and applied voltage. I saw where I had replaced a cap in the oscillator section a couple of years ago (why is it that the problems are always hiding in a place where you do not want to go?).

There are several trimmer caps on the top of the three gang variable that tunes the radio. Three of these are tweekeble through holes in the cowel which you can access with the top cover removed and two you can not because there are no access holes. You must remove the tuner box to get at them. Sure enough, the exposed trimmers turned nicely but the front two made a snapping sound when I attempted to move them and don't you know immediately the oscillator started up. It seems that these front bandspread/bandset trimmers are set at the factory and are not touched for decades and after a while they become galvanic and even form a semiconductor junction which has the effect of killing the oscillator. I have seen this effect on the oscillator set cap on the VFO of the Command transmitters many times as well. After spraying deoxit and working all of the trimmers I decided to add two more access holes to the tuner box so all trimmers could be tweeked with the cover in place.

While I have it apart, I think I want to add a simple Noise limiter and switch. Who knows when I will have it on the bench again?

Mike WU2D
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WU2D
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CW is just a narrower version of AM


« Reply #1 on: May 16, 2011, 10:05:17 PM »

Well I reverse engineered the ARC-5 mods and came up with the following schematic. The conversion artist cleverly added a 6SL7 and a 6AL5 in place of the single 12SR7 Detector/BFO, thus giving more gain and a better BFO and a classic dual diode detector/AGC system. There is no noise limiter. This produces a HiFi sound that is quite impressive in the car. A mobile receiver really needs an effective noise limiter however, especially in my Maxima.

I want an ANL so I will be doing a rework based on the circuit in September 2003 Electric Radio Updating the Automatic Noise Limiter. by K6STI, Brian Beezley. His circuit uses the same 6AL5 with only a couple more parts.

Mike WU2D


* BC455_Sch.gif (22.83 KB, 1034x557 - viewed 372 times.)

* OriginalMod.jpg (296.33 KB, 2544x1809 - viewed 378 times.)
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WU2D
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CW is just a narrower version of AM


« Reply #2 on: May 17, 2011, 10:43:23 PM »

Progress! After reading further about the ANL circuit, I came to the conclusion that that it was not what I was going need to kill the nasty Maxima fuel pump noise effectively. That ANL circuit is more of an overmodulation or thump or crash limiter circuit. I wanted something that was made for reducing periodic narrow pulses. There have been many complex circuits that sample these pulses somewhere in the chain - from a separate noise antenna to the front end right after the mixer and then process and feed this to a gain controlled stage or gate switch in the signal path to blank the pulses.

But I saw something really simple looking in the old Editors and Engineers handbook that got my attention. It is called a Bishop limiter.

The Bishop Limiter uses a 6AL5, 3 caps and two resistors and it is essentially a full wave shunt limiter with a time constant; that is put directly across the primary circuit of the last IF stage. That's right - from plate of the last IF tube to B+. So it is in effect a pre-detection IF envelope limiter.

My noise source is an old 12V relay wired to open and close a contact in series with the supply, coupled directly into the antenna with a few turns of gimmick. This thing makes a disturbing variable narrow RF pulse generator. You get a pulse on the make and break. It looks good on the scope. When this thing is on, even CHU Canada is wiped out (a very strong AM station here). It utterly disturbs the conventional 6AL5 AM detector and AGC.

But I did not want to shoehorn in yet another 6AL5 so I stuck in a pair of good old 1N4148's. NO GO. The the distortion was nasty. Unless the RF gain was backed off some, CHU was clipped severely, so this was not going to work. The diodes were coming on sharply and with too low a p-p voltage. I thought of trying some low voltage Zeners then something hit me - what if the 6AL5s were more of a linear diode (if there is such a thing)? The closest thing I could think of was a pir of RED LEDs. So I checked the VF of the LED's and they were softer and the voltage drop was at least double. That was the ticket. The circuit works like a champ. With the Bishop clipper off, CHU is wiped out with the buzzer, but when I flip the switch, it clears right up and I get CHU back Q5. 

I will optimize the values and shoot a little YouTube video to show the thing in action with the scope.

Mike WU2D
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WA1GFZ
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« Reply #3 on: May 18, 2011, 07:04:06 PM »

Yup red nice and slow. Blue fast I'm told.
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WU2D
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CW is just a narrower version of AM


« Reply #4 on: May 19, 2011, 10:02:15 PM »

Frank,

I settled on the RED LEDS and put a couple of 10 Ohm resistors in series with them to keep them off with stronger stations. To the ear, the reduction is amazing but you can see some real improvement on the scope too.

This weekend I am headed up to Canton NY and will have the mobile in the car so listen for me on 75M.

Mike WU2D


* LimiterOff_CHU.jpg (85.7 KB, 720x480 - viewed 354 times.)

* LimiterON_CHU.jpg (88.78 KB, 720x480 - viewed 366 times.)

* BishopANL.jpg (92.57 KB, 610x655 - viewed 363 times.)
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AJ1G
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« Reply #5 on: May 21, 2011, 08:23:39 AM »

Your 20W mobile was sounding great this morning on the MILRAD net after the QRN backed off later on in the session.  Very interesting on the ANL circuit.  I should try to build up a command set mobile system myself - I have enuff of them and probably could power  the transmitter with a PE103.  Also thinking about making a pair of 1625 shoes to pump up the volume of the K1 on 40 and 20 CW.  Would be a good mix of new and old tech....
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Chris, AJ1G
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WU2D
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CW is just a narrower version of AM


« Reply #6 on: May 24, 2011, 08:30:41 PM »

Chris,

The system worked flawlessly on the trip. Both ARC-5 TX and RX with the Q Multiplier and new noise limiter performed well. I tried to check in on CW on Sunday evening but could not get the Hustler to resonate at 3.570. I have had it with this toy antenna. I heard everybody no problem.

Next I need to work on the Antenna and the High Voltage in that order. I have to get more signal out. I have already found the place to attach the antenna mount. My Maxima has a shipping tie down that came off with some WD-40 and a ratchet, and it is a perfect place to fasten an arm that will give me a stable base just to the left of center and just below bumper level.

There was not a peep of an AM signal on the trip home Monday afternoon- guess we are in the AM doldrums on weeknights. Plenty of strong SSB and W1AW blasting CW. I am headed back north Thursday night to clean up a wet basement situation in the rental and hope to return to New England Saturday for the OMRN run. Listen for me again.

Mike
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