Wednesday, November 25, 2020

QRM-180 V3.41 Manual 07.4
Another Noise Reduction System
Unless you live far out in the countryside you likely have your share of RF interference from TVs, LED lamps, power lines, computers, solar panels, etc. A common method to reduce QRM (popular since the days of vacuum tube TVs) is to use a second antenna to collect a suitable sample of the noise, invert that signal and add it back to the main signal. It it is a fairly straightforward design that doesn’t rely upon clever filters and traps specific to the type of noise.

And the good news is that they can be reasonably effective. Don’t expect miracles but one of these can get rid of enough noise to make them well worth having. A number of commercial versions available today are targeted especially for amateur radio, and while they are designed with amateur radio transceivers in mind, they will work equally well on receiver only and SDR applications. Again, they can do a good job canceling a local noise source, but not atmospheric noise. There are a lot of reviews on line as well as a bunch of YouTube videos that give some useful information on tuning them and noise antenna design.

The QRM-180 came about because the commercial units actually worked but seemed to be missing some needed features. Unfortunately they are largely built with surface mount components and were difficult to modify. The QRM-180 board (with ground-plane top and bottom) managed to fix many of the shortcomings (my subjective opinion) of the commercial units. The circuitry is not all that original, it is a combination of what was deemed to be the best of the existing designs. The board is all through-hole components which makes it much, much easier than surface mount to build by hand and also makes it serviceable in the future. The circuit could have been laid out with surface mount and machine assembled (much cheaper) but selling the kits is a way to encourage more hands-on involvement in ham radio. Early hams had Heathkits, Allied Radio, Eico kits, EMC, and others, but now there are just too few useful kits on the market.
The case is a compromise between being roomy enough for fumble-fingered assembly and not so big as to take up valuable space next to the transceiver. It sits comfortably on top of most external speakers, or on top of the transceiver itself. It is thick aluminum so it is well shielded and heavy enough not to slide about during use. The electronics is easily removable from the enclosure for assembly and service. The front and rear panels are made from PCB material with copper ground planes on both sides to complete the shielding of the enclosure.

A separate noise antenna gathers what is hoped to be a good sample of the offending noise without a lot of your coveted DX signal. That sample is then run through a broad band amplifier before being inverted by transformer T1 and fed to a phase shifter. Two J310 FETs then act to buffer and combine the two basic signals. Several schemes were tested for phase adjustment, and this one was not only the simplest, it introduced less distortion.

The combined signal is then buffered by Q4 and sent to the receiver through a relay. In transmit mode, the relays serve to isolate the device and allow the transceiver to be connected directly to its antenna. The relay is large enough to comfortably handle 200w, but make sure that you put the QRM-180 between your transceiver and your linear if you use one. It will probably be toast if you try to install it following the linear. The PTT (really just the transmit signal not necessarily the actual PTT from the mike) input grounds the gate of Q2 which in turn turns on Q4 the power tab transistor. This not only energizes the relays, it supplies power to the rest of the circuitry. The relays energize on PTT low so that the default state is safe (circuitry bypassed) when the power is off.

There are fuses and protection diodes on the signal inputs. In spite of some odd suggestions on the Internet, don’t remove them. They are useful and do not degrade the performance at all (unless they are blown, duh). The pi filters are perhaps debatable but for the bands where the QRM-180 is useful they are a good thing.

The provided amplifier for the noise antenna is a commercially available pre-amp (LNA) that provides a nice flat, low distortion 30dB of gain that can be adjusted down using RV1. In very rare cases where the noise is very large and the noise antenna is also large, the LNA may not be required and could be bypassed. You can also try plugging the LNA into the 9V power connector. This reduces the gain and improves the flatness a bit.
The QRM-180 does have transmit-detect circuitry to disable the relays to try and protect things if the PTT is not connected when you transmit (C17 coupling some RF and eventually turning on Q1). Like the other devices with this type of circuitry, it sorta works. It is pretty good with AM but SSB will be intermittent at best and will quickly blow a fuse. It is useful in an emergency when you accidentally plug things in wrong, but for normal use ... You must connect the control wire from the transceiver.

On almost any transceiver built in the last 30 years there should be a connection on the back of the transceiver designated for operation of a linear amp. Look for something labeled TX GND, PTT or similar. If you are lucky it is a separate dedicated connector. That is probably what you want to connect to the PTT input of the QRM-180. But check it first: operate the transceiver while monitoring that output with a voltmeter. It is likely just relay contacts to ground so if you read no voltage, try measuring continuity. Whatever the case, it must go to ground when in transmit mode. Depending on your transceiver, you may have to obtain a suitable connector to reach this signal. The QRM-180 PTT input sets at 5V and needs less than 1 ma to ground to operate.

If your transceiver has a built in tuner, check the output when the tuner operates. The tuner should also pull this output to ground when it transmits. In nearly all cases, when you have a built-in tuner, the output does not change during tuning and you have to manually make sure that the QRM-180 is turned off when actively tuning. Failure to turn it off will eventually blow the fuse. Note, You may find that your transceiver’s autotuner has trouble on the higher bands with the QRM-180 installed, even when powered off. C17 is probably the culprit. That cap is just enough to annoy some autotuners, especially at the higher bands. Lifting one leg of C17 and disabling the auto-detect is generally the best fix.

At the worst, (on a very, very old transceiver or CB radio) you may have to just tap into the microphone connector and find the push-to-talk contacts. Possibly putting a male/female adapter between the mike and the radio. Check the Internet for instructions on how to connect your transceiver to a linear amplifier, the connection will be the same.

The power requirement is 12VDC (11 to 13VDC). The included wall wart will work (it is a switching type, and they all generate some sort of undesirable noise) or any other good 12V source. If you have a linear supply, use that. Most transceivers have a 12V output that could be utilized, but you should have at least 400ma available to power the QRM-180. Check your transceiver manual first. For automotive use, or anything over 13V you should drop the voltage by putting two diodes (1N4005 or similar) in series with the positive lead.

The trick to success is to get just enough of the noise signal (and without the desired signal) required to match the level of the noise component on the primary signal path. The kind of noise will dictate the requirements, but basically you will need a dedicated antenna for the noise and some time to experiment with settings at different times. Don’t expect to eliminate atmospheric noise-- you cant. But for a specific noise source you should expect a significant if not complete reduction. Read the reviews and manuals of the other devices (all available on-line) to get the full picture. The antenna should NOT be similar to the primary antenna you are operating on. You don’t want to receive the primary signal, you want (as much as possible) to capture only the noise.

If you somehow got this far and have realized that you don’t really have a specific noise problem, don't loose hope. There is generally a local noise component of what we all assume is atmospheric noise. With the right antenna, the QRM-180 might still find and eliminate enough noise to drop an S-unit or two.

Step one in deciding upon the noise antenna is to try and know a bit about your enemy. What time of day is it present, is it near or far, where in the band is it strongest? If you have multiple antennas, is the noise the same on each? Try to tune it in. Before you insert the QRM-180, use your receiver to find an antenna that picks up the noise without the desired HF signal (an old TV antenna is great place to start if you have one). Then install the QRM-180 and start on an easy band, say 20 or 40M connecting something to the noise antenna Some people swear by a long piece of wire laying on the floor. If that works, the noise is probably in the shack, and should be fixed with some grounding and ferrite beads not a phase inverter.

If you have an old 2m antenna, it is often a good choice, especially if mounted horizontally.

  • The GAIN knob is just that, you are trying to find a level that just matches the noise so that the result is zeroish. It is not to find the best listening level. Use the radio knobs to do that.
  • The two PHASE knobs jointly adjust the phase of the noise signal. You should adjust each for a minimum. Unfortunately they also mess a little with the level, so you have to go back and forth a bit between all three.
  • The NOISE Attenuator control is rarely used and is on the rear panel. When the LNA is installed it is used to get the noise signal in a range that can be matched by the GAIN control. Without the LNA it will probably just be set to max. It will depend on the size, type, and location of the noise antennas, as well as the strength of the noise source. If you find that it works best with the attenuator below half way, try reducing the gain by plugging the LNA into the 9V power source, or consider removing it completely.
  • The DELAY ADJ is inside on the pcb. This screwdriver adjustable trimpot is used to adjust the delay time of the relays after transmit. It is not critical if you just use AM or SSB, the time can be fairly long without being annoying. CW will require a shorter time and a little more fine tuning.

Begin routine adjustment by turning the gain down a little on the transceiver, if it has a pre-amp turn it off (with the gain of the QRM-180 it is unlikely that you will need it). Turn on the QRM-180 and turn it’s gain to about half, then adjust each of the both phase controls for minimum noise. Keep adjusting things until the noise seems to be minimized. You should find a very sharp null point. If the LNA is installed, start with the Noise Level control set to about 70%.

You have to use your receiver controls as well as the controls on the QRM-180 to get the best results. The signal will likely be a little different than what you are used to. Try with and without the transceiver’s notch filter, NB, and different width settings. It will take a little practice to get things right quickly. With a little practice, most noise can be killed in 3-4 seconds of knob tweaking. Changing frequency within a band should require minimal readjustment.

Try with and without the QRM-180 frequently to be sure that you are helping things. Try to ignore the level change and listen just for the change in the noise component relative to the signal. Use the transceiver volume control to match the levels for comparison. Again, you cant escape atmospheric noise, focus on the specific noise source.

For different bands you may get slightly better results with a different noise antenna. If it seems to work better with a different antenna on another band, try tying the two together. You may get away without an antenna switch. In any case, you should run coax from the back of the QRM-180 to the noise antenna, but pretty much anything will work. RG-59 is readily available and a good choice. It doesn’t need to be 50 ohm, you are not going to transmit on it. Try a short dipole, an old 2m antenna, a rain gutter, or a fence,... you never know what will resonate with your particular noise problem. Going vertical is a good option if your main antenna is horizontal, but try both. Remember you are trying to receive just the offending noise, not the HF signal. If you can set up an antenna with some gain, great, point it at the source. When you have established a decent reduction, note the knob settings. It will be pretty repeatable for the same conditions on the same band. A word of caution, whatever you choose, don’t go with a big noise antenna that is too near to the one you are transmitting on. The noise antenna could easily pick up enough RF to blow the fuse on the noise antenna line.

It is a good idea to keep spare fuses in the case (the small 6V bulbs). They do a good job of protecting the rest of the circuitry, so if you do blow one, just break out the soldering iron and put in a new one.

And while the QRM-180 works the 180-10M bands, the performance of the phase adjustment drops off at higher frequencies. If the intended use is 10-6M consider reducing the value of C6 & C7 to around 50pF, remove C17, and replace L4 with a jumper.

Other Thoughts/ Uses:
The QRM-180 is a collection of circuitry that provides a bit of amplification and phase shift. If you have a couple of vertical antennas you might try using the QRM-180 to combine them and use phase shift to adjust direction.
At least two users have modified the QRM-180 to work down around 475kHz with success. Thanks to Tom WB4JWM for his efforts supporting the 630M band. You can contact me for his recommended changes. It will however, disable the higher bands.
 Assembly Instructions
This kit may not be as challenging as building a HeathKit color TV, but it will still require a bit of time and skill.  If you have not built an electronic kit for a while, you should probably practice with a simple DIY kit of some sort from eBay.  Before you start the ‘180 assembly, thoroughly study the schematic and understand the function of each part of the circuit, it will be a help in construction and operation.  Review the instructions and be certain that you can identify all of the components.  Begin by sorting out the parts so as not to loose anything, get some good light, and a place to work comfortably for a couple of hours at a time.  Complete assembly is probably a two evening job; there are a lot of parts.

You will need some basic tools: soldering iron (about 40W) with a smallish tip, your favorite solder, sharp flush cutters to trim leads, screwdrivers, needle nose pliers, and if you are not 13 years old, a good magnifying lens.  A volt-ohmmeter, even a cheap one is pretty much a necessity (Harbor Freight has several for $10 or less, as does eBay).

You will notice that when soldering, that the ground plane is quick to dissipate the heat on grounded pads so you will have to stay on those pads a bit longer to get good flow to the top of the board.  3-4 seconds is usually about right.

It is generally best and easiest to install the smallest parts first, and gradually work up in size.  Don’t stress about getting the components close to the board.  It may look nice if they are all flat and neat, but keeping them a little above the board would make any rework easier and generally improve air flow.  This is especially true of all the small transistors.  Refer to the board layout drawing and install 6-7 parts at a time, then solder them.  Then trim the leads and put in the next group.  Checking them off of the BOM as you go will save time in the long run.  Don’t worry if you have parts left over, there are often a few extra parts in the kit (I don’t count so good, but I try hard to never be short).

When the board is complete, you can clean the backside with a little rubbing alcohol and an old toothbrush to remove the flux from soldering.  Check to see that all leads are trimmed enough so that they will not short to the case.  Check especially those along the edge of the board.

      [_]  1) It’s a good idea to install the Zener Diode D7 first, this will keep it from being confused with the 1N4148 diodes which look almost identical.   Then install the 1N4005 black rectifier diodes (D1 & D18).   Now you can put in the three 1N4148 switching diodes (D2, D13, D19).  As with all the diodes, watch the polarity and match the band with the silkscreen image.
      [_]  2) Install the two full wave bridges, they are in the 4 pin dip packages.  Note the + and – location on the board layout drawing.
      [_]  3) Install the inductors, be careful, there are two different values and they can be difficult to tell apart.  They look like resistors, but will measure nearly zero resistance on your ohmmeter.
      [_]  4) The resistors are next.  R9 and R7 will run warm in operation so mount them about 1/8” (3mm) or so above the board.  Save the potentiometers until later since they are large and get in the way.  Some resistors have 4 value bands (1%) and some have 3 bands.  It didn’t matter but the 1% types were supplied when available.
      [_]  5) Install the capacitors, just save C3 until later.  Only C2 and C3 are polarized, the rest can go in either way.  Each is marked with a very small 3 digit value code.
      [_]  6) Now is actually a good time to install the board connectors.  The pre-wired cables should be oriented correctly: The 4 pin cable should wind up with the RED wire in the square pad.  The two-pin cables should also be inserted so that the RED wire goes to the square pad.  The green connector and the SMA connector may require a little more heat.
      [_]  7) Take a break from soldering and wind the transformer.  Wind 10 turns of the enameled wire, and then three turns of the insulated wire.  The wires can exit the ends or sides of the core, it doesn’t much matter.  Just make sure that when it goes on the board the enameled wires terminate on the C7 side.  Trim the wires to about the right length to fit the board (not too short).  The insulated wire can be stripped with a good pair of strippers. The enameled wire must be prepped by scraping the ends with a razor blade or sandpaper, then tin the ends to make sure that all of the enamel has been removed before soldering.  Double stick tape will hold the core in place on the board.
      [_]  8) Install Q2,Q3, Q5, Q6.  Leave out Q1 and U2 until after testing.  Leave all the transistors well above the board, it will make removing them possible if that ever becomes necessary.  On Q5, install the provided heat-sink before you solder it in.
      [_]  9) Install Q4, the large PNP power tab transistor.  The metal side of the transistor goes toward the inside of the board.
      [_]  10) Add the bit of plastic shaft to the shaft of RV1.  Use the adhesive lined heat shrink provided.  This will allow this control to be adjusted from the back panel.  Use a heat gun or very small flame to shrink the tubing.

[_]   11) Install the ‘delay adj’ trim-pot, RV5.
      [_]  12) All of the larger parts can go in now.   Don’t forget to install C3, and the 2 fuses (lamps).  Solder in the controls and the relays.  Note that RV1 is a 1K pot.  The others are 5K.  Be sure that the pots are firmly seated against the board  so that the front panel will align with the enclosure correctly.  Some of the pots have a metal anti-rotation mounting tab that will interfere with mounting in the front panel.  Cut or break that tab off of the 5K pots before soldering.

[_]  13) You can now install the front panel.  Do a test fit then carefully bend the LED to line up the the hole. Attach the panel with the washers and nuts on the three controls, just finger tight for now.  Make sure that the LED is firmly snapped into its bezel and solder it in place.

[_]  14) This is not critical but it is a good idea to electrically connect the ground plane of the main PCB to the back of the front panel.  Just solder a bit of wire (a piece of resistor lead) from one to the other.  Doing this will allow operation of the unit outside of the case for testing.  (the only other ground for the front panel is through the case and the paint can insulate connection)

[_]  15) Cut one of the two pin cables (red/black wires) to about 2 inches and solder it to the power switch.  Polarity doesn’t matter.  Install the switch into the front panel and tighten the nut.  Plug the cable into the PWR SW connector on the board.

[_]  1) The rear panel is really a PCB.  It will provide a location for the fuse and protection diodes for the noise input.

[_]  2) You will need to surface-solder a full wave bridge pack to the back of the rear panel.  Note the orientation and match the + & -.  This is as close as we get to doing surface mount.

[_]  3) You should install the remaining 4 connectors as indicated.  Tighten well, and if you have a dab of Locktite TM  ,glue, or nail polish, this would be the place to use it.

[_]  4) Solder a small lamp (fuse) from the center pin of the BNC connector to the L-shaped pad next to the four diodes.

[_]  5) A 5.6uH inductor is also soldered between the ‘L’ shaped pad and ground. It is easiest to cut one leg to about 3/8“ (8mm) and prep it with a small bend.  Then you can hold the other long leg to solder it in place.  After the short end is soldered cut the long end and bend it into place and solder.  Depending on your noise source you may get better results without this inductor.  For now, a good plan is to leave the ground end floating so you can easily try it later.

[_]  6) On the four pin connector with wires, cut the white and red wires (don’t cut the yellow and black) to about 3.5 inches long.  Then wind 3 turns of both the yellow and black wires through the green ferrite ring.  This choke helps to block RF from getting in or out through the power wire.  It also a good idea to solder a 0.1uf cap across the power connector on the rear panel.  The yellow and black wires can now be soldered to the power connector on the rear panel.  The yellow wire is connected to the center conductor, positive terminal.  The ground lead should also be jumpered to the pad on the rear panel.

[_]  7) Solder the white wire to the center pin of the RCA jack.  Then, solder the red wire to the L-shaped pad.  You can also solder a 0.1uf cap across from the center pin of the RCA jack to ground if you like. Although not necessary, it can keep some RF out of the power.

[_]  8) Strip and prep the thin coax cable with the gold SMA connector, and using the pad next to the BNC connector, carefully solder the shield to ground and use the connectors ground lug as a strain relief for the cable.  The center conductor should be soldered to the L-shaped pad.  Use minimal heat to avoid melting the center conductor.

[_]  9) Using three pieces of about 2.5 inches of 18-14 awg wire, connect the SO-259 connectors to the three pin Phoenix connector.  Note, the center pin is ground (GND on the PCB silkscreen)

[_]  10) You can solder the remaining 2pin red and black wire connector to the LNA.  Run the wires through one of the mounting holes as a strain relief and connect the red wire to + and black to -.  Don’t install the LNA until you do some basic testing. 

[_]  1) It’s not a bad idea to check to make sure that the board slides nicely into the case and that the front panel holes line up.  You may have to loosen the controls or re-solder them to get a perfect fit, but don’t install the case just yet.  Double check the rest of your work and verify that all of the transistors are in correctly and that all of the diodes are in the correct direction.  The LNA can be left out for now.  Steps 2 thru 4 can be done with the boards out of the case.

[_]  2) Before the 4 pin power cable is plugged into the board, plug in the power supply to the rear panel and verify that 12V is present on the yellow and black wires and that yellow is positive.

[_]  3) Now plug in the 4 pin cable and the green 3pin connector.  When pressing the power switch, you should hear the relays click and the the LED should light in a yellowish color.  If it is green, the LED is reversed.  If nothing happens check for 12V power at the power switch, the orientation of Q4, 5V across D7, and the location and orientation of Q2.  If C3 gets hot and explodes violently, it was in backwards.

[_]  4) If everything works so far, and with power on, try grounding the control input (the RCA jack).  The relays should click and the LED should change to red.  If this fails, check D7, and the other diodes and components around  Q2.  When you are satisfied that the power and relay control is working, then install Q1 and U2.

[_]  5) You can put everything in the case at this point but leave the top off.  You can tighten the controls and install the knobs.  (It’s good time to put the rubber feet on the bottom as well) 

CAUTION: When the transceiver is connected, there is a possibility of high voltage and RF exposure. 
BE CAREFUL!  Turn RF power down on your radio and take care not to transmit.

[_]  6) Continue testing with the transceiver, the power supply, your main antenna, and a noise antenna, connected.  PTT is optional since you will not be transmitting.  Hook up a noise antenna to the BNC connector on the rear panel.  Maybe a 2m antenna or just 5-10 feet of wire in the shack for now.

[_]  7) The LNA board can be installed now, it simply sits above the main board held in place by a 90 degree SMA coupler to the LNA’s output.  Power is provided by one of the two, two-pin connectors.  Use the 12V connector for testing.  The signal input to the LNA comes from the rear panel cable (the one with the SMA male connector).

[_]  8)  When everything is in the case and connected, turn the QRM-180 power on and off.  Listen for the relays to click and observe the power LED.   When the power is on the LED should be yellow-ish.  Set the gain, and the rear attenuator to max.
[_]  9) Turn on the transceiver.  With the QRM-180 off, the receiver should work normally.
[_]  10) When you turn on the QRM-180, there should be a lot of noise in the receiver.  Play with the controls to get a feel for them and try it with and without the noise antenna connected.

[_]  11) At this point you should be able to adjust the two phase controls to get a sharp null point.  If not, recheck your work and use an ohmmeter to verify that the rear panel cables are connected and not shorted.  You can lift one end of L1 on the rear panel to get a check of the resistance of the SMA cable.

[_]  12) If everything works the unit is now complete!!  Go to the section on operation and begin testing your options for a good noise antenna.  If you still have issues, review the “list of common problems” and continue testing.

You can operate with or without the LNA as needed, depending on the noise level and size of noise antenna.  If you remove it just plug the SMA cable from the rear panel directly into the main PCB.   For less gain, you can also try plugging the LNA into the 9v power source. While normal is 12v, the LNA is a bit cleaner on 9V.  Try both.

It is fairly robust circuit so you should not experience any problems if you managed to get all the parts in the right holes and in the proper orientation.  But don’t panic if you do have an issue.  Should something not function, check the schematic for that area and recheck the parts related to that function.  It is surprisingly easy to miss-read a component’s value.  Start with getting the relays to click when the PTT input is grounded.  When that works, you know that the power circuitry is right. 

If you still have signal problems, try connecting to the receiver but without the antennas.  Use a small probe or bit of wire, and following the schematic, touch various points along the signal path.  Just your body as a signal source will introduce plenty of static to locate a fault.   If you can mess with the knobs and get a null in background noise, it is working properly.  You just need to get the best noise antenna and get good at fussing with the knobs.

Common Problems:              Power issue or relays don’t click:
    • 12V red/black wires switched on the rear power connector.  Check the power at the board.
    • The rear panel gets its ground through the green connector.  Be sure that it is plugged in when testing the PTT function through the RCA jack.
    • A 1N4148 diode switched for the Zener?  There should be approx 5V across the Zener.
    • Is the power tab transistor backwards?  Tab toward C3.
    • Check pads that have top traces, sometimes solder doesn’t flow to the top and the plated-thru hole can break leaving the top trace unconnected.
    • A few pads are very close to the edge of the board and can short to the case if the leads are not trimmed flush on the back of the board.
                              Signal issues:
    • Check the resistance of the SMA coax cable to the rear panel, it is easy to overheat the shield when soldering and short it to the center conductor.
    • Is the LNA in backwards?  RF in goes to the rear panel, RF out goes to the board.  Is the power correct to the LNA?  Plus voltage to VCC on the LNA?
    • Are the enameled wires of T1 sanded and soldered?
    • Check all the fuses with an ohmmeter.  It is difficult to tell when they are open.  Spare fuses are included but if you need more, they can often be found in hobby shops or on eBay by looking for ‘model railroad lamps’.  6V 40-100ma lamps are best but small 12V lamps will work.
    • Check the grounds between antennas and the radio.  A loose connector will cause lots of grief.                                                  
      All else fails, email me and I will try to help.

 As most of you know, I have been selling kits on Ebay.  The QRM-180 kit, the HV Diode board and parts for the SB-220, and the upgraded capacitor bank, also for the Heathkit SB-220.  I like the kits, and it feels good to promote some of the old hands-on experience that used to be the heart of ham radio.

Ebay is fine, but I get a lot of inquiries from folks (not hams) that have no idea what to do with the kits but want a lot of information.  Ebay is also expensive.  They make more on each kit than I do.  They are also pushing me to set up a business account which is even more expensive.

So...  I am not selling on Ebay, at least for a while.  If you are one of those that want one of those kits you can always contact me...

I still have the kits and can ship anywhere in the US by USPS priority mail ($8).

International is more difficult (and expensive) but if you give me an address I will try to get a shipping quote.

Payment will be by PayPal only.

Thanks for the interest.