Improved Receiver Muting and Protection

Discussion in 'Technical' started by W5HRO, Aug 19, 2014.

  1. W5HRO

    W5HRO Administrator

    One of the final things I need to work on is optimizing the muting of my receivers during transmit. I currently have a control box with relays inside that plugs into the back of my HB transmitter cabinet. I used it years ago to mute my 3 National receivers at my old station back in Tulsa. The box also contains 8-ohm load resistors the receivers output to during transmit. Basically I tried muting the HRO receivers and/or the NC-183D by their normal muting method first which basically turned the B+ off. Unfortunately that led to very loud speaker feedback and popping before and after mute happened so I added the 8-ohm resistors with a slight delay to the other B+ on/off relays to eliminate it by disconnecting the speakers first, etc. However, every time I went back to receive the receivers had drifted some. I soon realized it was a very bad idea to shut the B+ off in the old tube receivers during transmit. Even if it doesn't shut off the VR tube going to the oscillator tube's screen many components still have to warm back up again each time.

    Nowadays I'm just using the relays to switch to the 8-ohm load resistors then back to the speakers and I'm no longer muting anything else. I'm using the simple protection circuit attached below mounted at the receiver output connector inside of my HB antenna coupler which contains a large vacuum T/R relay. This prevents any large RF dumps from getting to the receivers during transmit across the open T/R relay contacts.

    Anyway, my goal is to try and eventually eliminate all mechanical relays at my station except for the main vacuum T/R relay inside of the antenna coupler. The problem is they would need to be SPDT and not SPST to disconnect the speakers like I am currently doing. I could maybe use SPST solid state relay bricks to externally control the CPC1788 relays Jim referenced in another topic instead, but I want to keep it simple. Maybe lift the AF tube cathode connection and short the 1st IF tube's grid input to ground or something similar during transmit. Basically I want no speaker noise or frequency drift to occur and shutting off the B+ is NOT an option.

  2. W5HRO

    W5HRO Administrator

    Here's the control box I've been currently rebuilding. I had already removed the extra relays for the old B+ muting and I installed new terminal strips. Right now it still has two radio shack relays, but all they do is switch the speakers to the 8 ohm loads during transmit. The box plugs right into the back of my HB transmitter and 110VAC gets applied to it during xmit.

    The hot glue has turned a little yellow after about 18 years, but it's still solid and holding.

  3. M0AYF

    M0AYF Member

    Hi again,

    I dont know if this will help but way back in 1975 I was talking to a chap who had exactly the same problem as you. His hb tube/valve receiver would drift a little if he turned the HT/B+ on/off between overs.

    To get around the problem he decided to leave the HT on all the time but included loosly coupled power rectifier diode into the VFO (a BY127) which served as a poor mans varicap diode. The idea was that when in TX the RX VFO would be "pulled" off-frequency by just a few kHz for the duration of the transmit period by the application of a bias Voltage to the diode in TX.

    While this is not a complete solution in itself you could use it conjunction with other precautions to mute the RX(s) without having to turn the B+ off each time.

    Hope this helps.


    Des (M0AYF)
  4. W5HRO

    W5HRO Administrator

    I've actually heard of people doing that before so it could possibly work. My main concern is the S meter slamming all the way to the side because of not turning the B+ off so that might be a possible solution.

    I was thinking more about the SS relays Jim posted too and just using one to short the grid of the 1st RF amp tube to ground during transmit may also solve that. The only thing is those MOSFETs are very sensitive and may get damaged being connected to the antenna unless my protection circuit prevents it. If I was to short the grid of the 1st IF amp instead then the last RF stage would be looking into a short which may be bad. Maybe just doing something at the detector instead might be best. Anyway, its going to require some thought.

    Right now my relay box will work. There are two DPDT relays in there for switching two speakers on and off along with my external T-368 exciter. That will get me by for now. The only change I may make is instead of having the transmitter apply 110Vac to the box on xmit is leaving the 110Vac on all of the time and just grounding the negative lead of the internal relay supply instead. That would speed up the receiver muting, but everything is still pretty fast now even like it is.
  5. M0AYF

    M0AYF Member

    Hi again,

    Just another though, would some sort of -ive rail be possible? That is a negative rail of say -100 Volts (ballpark figure) with lots of de-coupling (bypass capacitors/filtering etc) fed through a set of relay points when the PTT is active in tx.

    The -ive supply could be applied to the control grids of the RF, 1st detector and perhaps 1st IF stage to "mute" the RX's when in TX.


    Des. (M0AYF)
  6. W5HRO

    W5HRO Administrator

    Well, most have grid leak resistors from all of the grids to ground, but pulling one of them down real far negative during transmit could actually work. I don't think you would want to do more than one stage because you wouldn't want long wires connected from all of them. You would want to use the solid state relay Jim posted and place it right at the grid connection. Second, you would need to create the negative supply.

    There are a lot of different possibilities.
  7. M0AYF

    M0AYF Member

    Hi again,

    W5HRO wrote:
    I don't think you would want to do more than one stage because you wouldn't want long wires connected from all of them.

    I dont think that would be an issue if each lifted grid resistor was locally bypassed. The -ive "mute" supply could be derived from the AC heater supply with a Voltage multiplier and fixed regulator. The current required would be minimal so it would not upset the heater chain supply.

    Another option might be to add a few LDR's in key circuit locations arranged as improvised opto couplers. LED's driven from the PTT line would cause the LDR's to fall in resistance and effect some circuit change or changes. e.g. One LDR to "short-out" a control grid to ground and perhaps another to cause a reduction in screen Voltage.

    There are indeed a lot of different possibilities.

  8. W5HRO

    W5HRO Administrator

    So your are saying to lift all of the existing grid leak resistors in the receiver then apply 0V on receive and a negative voltage on transmit? If you are talking about using separate 1M resistors connected to the grids then it would require a falily large negative voltage to overcome what's already at the grids via the grid leak resistors. Not saying you couldn't make something like that work, but I'm looking for a more simple solution and only messing with the IF or the detector's output and then maybe the final AF amp stage.

    I looked at the NC-303 and HRO-60 receiver schematics and it looks like the IF stages already have too much other circuity at the grids so what I think would be best is just pulling the +150V on the screens down to ground instead or to a reduced voltage. Probably just doing it on one of the IF stages would be sufficient.

    Anyway, attached below is the idea. You just pull the screen voltage down during transmit. This is probably what I will wind up doing. All it would require is adding one solid state relay and one resistor. The +150V, 2.2k or 47K resistors and the caps are already there at the screens.

  9. KM1H

    KM1H Guest

    The only stages needed to be biased into deep cutoff are the first RF amp, stage controlling the meter, and the audio output.
    Some sets have the RF gain control on the first RF and one or more IF stages which makes that an easy job if you want to bias or lift the cathodes.

    I dont see the need to open the speaker lead if the audio stage is effectively inoperative.

    Since the grid bias is part of the AGC line and already bypassed in several places, inserting the control circuitry at the detector/AGC tube there shouldnt be any RF pickup. Ive brought out AGC to external devices such as a BC-453 and SSB adaptors from several receivers.
  10. W5HRO

    W5HRO Administrator

    I had a hunch this topic would get your attention :razz:

    Well, the HRO-60 is really bad about going POP! through the speaker when it switches back to receive as the B+ reconnects. That was the reason I started switching the speaker to the 8-ohm load then back to the speaker again. It was so loud that it always woke people up in the house. I didn't have the NC-303's back then, but I still like the idea of removing the speaker so I'm sticking with it for now.

    I think pulling the screen down on the IF stage would do what I want. The NC-303 has a 2.2K screen dropping resistor and the HRO-60 has 47K resistors. That would kill the S-meter, etc.

    Or, maybe pull the 1st IF screen down in the HRO-60 and the 1st RF amp tube screen down in the NC-303. The 303 has the 47K screen dropping resistor on that stage. The problem with doing the 1st RF amp stage in the HRO-60 is that it doesn't have a separate +150V supply, the screen is powered via the plate through the coil and dropping resistor and it doesn't have the other 2.2K in series with the coil for the extra protection. I don't like the idea of the increased current going through that section and possibly opening up the coil winding.

    I think every receiver is going to require a bit different approach.
  11. KM1H

    KM1H Guest

    Ive been flat out on outdoor projects and little time for the PC or radio.

    I use my HRO-60 with a Viking II CDC barefoot and a few times with the AF-67 and a 350W carrier linear. The relay is a Dow Key by itself with the V II and also add the built in relay in the amp when that is used. This is mostly on 10, 12 and 15M. The Dow Key has the built in spring loaded RX isolator which gives a good 60dB+ isolation on 10 and more as the frequency goes down. The amp relay is good for 30dB on 10M. Ive used Dow Keys daily from the 50's to 70's and havent fried any antenna coils at any power level on any band and there was a bit of haywire involved also....

    I use headphones and havent heard any pops; I wonder if that is rectified RF getting into the RX and maybe even gassy 6V6's. Did you change C-80 across the primary of the audio xfmr? It is there to snub the collapsing field and prevent insulation breakdown. Are both 6V6's functioning in circuit?
  12. W5HRO

    W5HRO Administrator

    I have not looked at it in a long while, but I doubt I ever changed C-80. Anyway, I'm sure the audio transformer is where the problem lies when the B+ gets reconnected. It does the same thing switching the B+ switch off and then back on again on the front of the receiver. All of the old tube receivers make a little noise when switched and I don't want any, I want silence. My HRO-60 just happens to be the worst one for that along with frequency drift when it switches back to receive. What I did was add a cap and a resistor on the coil of the relay so it would add a slight delay before the speaker was reconnected. The B+ went back on first and then the speaker after. Now the B+ stays on and only the speaker gets switched off and on. I just want to add some additional muting in the receivers to kill the IF or the detector. My protection circuit will keep the receiver front ends from being damaged. It's not that big of an issue with tube receivers anyway.

    Anyway, I'm going to modernize the muting process and leave the B+ always on for all them, but using no mechanical relays to do it in the end.
  13. M0AYF

    M0AYF Member

    W5HRO Wrote:
    So your are saying to lift all of the existing grid leak resistors in the receiver then apply 0V on receive and a negative voltage on transmit? If you are talking about using separate 1M resistors connected to the grids then it would require a falily large negative voltage to overcome what's already at the grids via the grid leak resistors.

    Sorry, I see now my schematic was confusing. The 1 Meg resistors are intended as examples only. What I was trying to describe was to "lift" the ground end of any existing control grid resistors in a couple of stages only (say RF stage and 1st detector) and to fit a decoupling/bypass capacitor to ground. Then connect a switchable -ive supply to these two points such that when the TX line is active then the -ive bias comes in to cut-off the two valves/tubes. In RX the lines are returned to ground.

    Making the kathode of one or more tubes more positive is also another option since making a kathode more positive with respect to the control grid is the same as making the grid more negative so the valve/tube would again be driven towards cut-off. An existing positive rail may be easier to "borrow" and pot' down as oposed to adding an extra -ive rail.


    Des (M0AYF)
  14. KM1H

    KM1H Guest

    Most radios have AGC applied thru those 1meg or often 470K resistors in the RF and IF amps. They are already bypassed which along with the resistors form part of the AGC time constant.

    Three 9V batteries in series will provide sufficient bias to cut off the tubes and will last their shelf life since no current is involved.

    Simple and cheap and no fancy circuitry needed.

  15. M0AYF

    M0AYF Member

    Hi Carl,

    Carl (KM1H) wrote:
    Most radios have AGC applied thru those 1meg or often 470K resistors in the RF and IF amps. They are already bypassed which along with the resistors form part of the AGC time constant.

    Thats a good point and much less invasive than lifting resistors etc. 9 Volt batteries would save having to build a dedicated -ive rail. I think that would be a viable and practical solution.


    Des (M0AYF)
  16. W5HRO

    W5HRO Administrator

    It's easy to say that, but every receiver will be different. What you would have to do is modify the AVC line to create like a max AVC limiting position during transmit. In some cases it will in fact be easier said than done.

    I don't like the idea of using batteries. It would be better to just build a small supply inside of the receiver or build it in your relay control box and apply it externally. Batteries always go dead and would need to be continuously replaced. I can just see it now, it's Thanksgiving weekend or Christmas day and you go to fire up the station and you find out the batteries are dead. Then you have to run down to the 7-Eleven and pay triple because it's the only place that's open :icon_thumbdown:
  17. KM1H

    KM1H Guest

    Then quit buying Chinese batteries. Duracell and Energizer (not counterfeits) claims a 5 year shelf life for the 9V.

    Since I use them in so many products from household to test equipment I always have at least 3-4 on hand.
  18. W5HRO

    W5HRO Administrator

    Hey, some of those Chinese products are pretty good. Some of the crappiest stuff made today comes from the U.S. It's sad but true. What world have you been living in :icon_problem:

    The only thing I have that uses 9V batteries (Duracell and Energizers) are my two smoke detectors and they usually only last about year or so.

    BTW, some of the longest lasting batteries I've seen are Panasonics. The cheap ones usually come with the equipment you buy and they seem to last longer than most all of the other name brands. Their Evolta line are the best though.

    Attached Files:

  19. W5HRO

    W5HRO Administrator

    Here's the fix for the HRO speaker POP!

    The Issue:
    The HRO's have a front panel "B+" switch and a rear panel "BSW" terminal used for receiver muting during transmitting that control the B+ for a good portion of the receiver's circuitry. When the B+ is switched either on or off, a large "thump" emanates from the speaker. The problem is annoying and potentially harmful to the speaker.

    Circuit Analysis:
    A scope on the switched B+ line confirmed that a large voltage spike (>20V peak) runs line when the B+ is switched either on or off. The spike is fed to the audio amplifier after the AF gain control through the plate supplies of V-13 (the 1st audio amplifier) and V-10B (the audio amp phase inverter).

    The HRO also has an unswitched B+ line that supplies the OB2 oscillator voltage regulator, V-16, the CW oscillator V-11 and the screens of the audio output tubes V-14 and V-15. All other B+ voltages in the receiver are fed by the switched B+ line (an exception being the plates supplies to V-14 and V-15 are fed through an unfiltered unswitched B+ to provide a HV load when the B+ is switched off).

    A review of later National receiver designs showed that the last National receiver that used B+ switching for muting seems to be the NC-400. In that receiver, only the RF chain's B+ was switched. The entire audio amplifier circuit remained energized during transmit period.

    Lift the two 47K plate resistors for V-13, R47 and V-10B, R52 from the switched B+ line and feed the B+ for these two tubes from the unswitched B+ line. The closest point for unswitched B+ is the screen, pin 4, of V-14. A new tie point was created by added a small terminal strip to the side of the chassis between V-13 and V-10 fastened by an unused hole in the chassis. A new wire was run between pin 4 of V-14 and the tie point. The original R47 and R52 were replaced with new 47K resistors routed to the tie point.



    Switching the B+ on and off was now quiet and no other discernable difference in performance was noted.

    This fix was found here:
  20. W5HRO

    W5HRO Administrator

    I received a PM about the HRO-60 and/or HRO-50 mod via the previous post in this thread and the complaint was that it worked, but the S-meter slams to the side when the B+ gets turned back on. I don't seem to recall that happening with mine, but I have not used either receiver in several years. Don’t know if it happens after doing the mod found on Emmitt’s Fix It Shop page or if it’s a separate issue, but adding a 0.5uF to a 1uF cap from pin 3 of the Xtal calibrator or FM adapter socket to ground may correct it. Do not use an electrolytic cap though. Make sure it’s a non-polarized cap. Pin 3 on those octal sockets is a direct connection to the AVC line and adding a cap to ground will also improve the AVC anyway.

    If anyone does these mods please let me know if it fixes the issue, I'm going to do them when I get ready to re-cap both receivers.

    P.S. I also fixed the incorrect pic in the previous post. Looks like the After pic was the same as the Before pic and I never realized it until now.
  21. Mikeinkcmo

    Mikeinkcmo Member

    I use an old Johnson T/R switch for the antenna duties, with the receive port followed by a diode clipper similar to yours, then a line amplifier, and splitter. The result being all receivers are "on line" so far as the antenna is concerned.

    I built a station controller which handles the distribution of mic audio and the Key line, in addition to muting sll receive audio, and a CMOS level common "mute" line to all receivers. I installed a small interface perf board in each receiver to handle the "stock" muting chores, and as you noted it seems each of the old receivers are unique. Rx audio goes to select panel then on to the controller, which handles the Tx/Rx switching, and transmit side tone.

    All the amps are COR operated with all the normal internal antenna switching bypassed. Amp outputs are switched using a patch panel.

    A bit elaborate but no pops or chirps, and all my ancient boxes are plug-n-play. KE0ZU/Controller/AntDistBlokDiaTxOnly_zpsrlamihhy.jpg.html?sort=4&o=10
  22. W5HRO

    W5HRO Administrator

    I see from the below this is what you have in each of the receivers.

    StaConR5 1_zpszitk8sg1.jpg~original.jpeg

    What you have done is a bit more elaborate than what I need, but I'm sure it works pretty good :icon_thumbup:

    The old HRO receivers would require a bit more work though because they have really primitive muting systems. Some are probably worse the others, but my HRO-60 pops very loud through the speaker when the B+ is turned back on via that primitive muting. It happens when the B+ is suddenly applied back to the first and second audio stage tubes again and it happens because the final push-pull 6V6 stage is still on and is not switched. When the other tubes get hit with the B+ the 6V6's greatly amplify it resulting in the loud pop! through the speaker. The guys fix in this thread should correct that by leaving the B+ unswitched to those stages, but the other guy who sent me the PM never did tell me if the S-Meter slam issue happened after he did the mod so I don’t know if that was the cause or not. Sometimes fixing one problem can create another, but it could have been a non-related issue.
  23. Mikeinkcmo

    Mikeinkcmo Member

    Switching B+ seemed to be national's favorite approach to muting, Hallicrafters, RCA, and others sometimes did the same. None of the old iron has any real sophistication by today's standards, but all worked, and a "thud" was pretty much a part of the deal. The thud here, is handled by delaying the station speaker mute release just enough for things to settle.

    The only common thread in all of my interfaces, is the +12 mute signal, after that, most if not all are variations on one theme or another.

    Another approach would be a 3 or 4 step sequencer, be it in simple logic or even done with relays, of all things.:icon_shh: It would be the simplest solution if your units are stock, but still not a fifteen minute project, and it would only be "good" for one Tx/Rx at a time since there would not be any switching provisions.

    The real issue is that IF you have more than one receiver or transmitter running on the bench, and use only one speaker/phones and antenna, then some sort of switching scheme enters the picture, and the more versatility you want, the worse it gets.
  24. W5HRO

    W5HRO Administrator

    The other problem with switching off the B+ to mute which was already mentioned in this thread is frequency drift. The last thing you really want to do is switch off the B+ during transmit. When you switch back to receive the receiver often needs to restabilize again.

    Many of the old tube receivers really need some type of improved internal muting modifications instead of turning off the B+ to do it. The B+ should really be left on. One of the ideas was to pull the control grid of the first RF amplifer tube or more into cuttoff then another idea was to pull the screen voltages down. Another idea was to use relays to lift the cathodes from ground, but that would just add to the instability and drift problem unless it was only done to the 1st RF amp stage.
  25. W5HRO

    W5HRO Administrator

    This is kinda a rough idea and I'm not sure if it would work, but I analyzed it on the simulator and it appears to work.

    Something like the below may work at the cathode of the first RF amp tube V1, the 6AB6 in the HRO-60 for example. All you would have to do it get a HV transistor in a TO-92 package, a diode, 5 resistors, 5 capacitors and everything could be mounted right at and near the tube socket pins. It would just need to be tied to the +250V line that is always on and when that supply drops to 0V at the base of the transistor it places a much higher resistance at the cathode of V1 to ground thus turning it off. Again, I'm not sure it would work, but I think it should. The caps values below I selected on the simulator round off the edges nicely to protect against sharp spikes when turning on and off and pads it. It should also help bypass and keep noise out of it.