Better way to mute an HRO?

[stevef]

My HRO-W mutes by interrupting the B+.   This causes a big thump/pop in the phones or speaker.  Is there another way to mute that won't cause this?  Can't find anything in the old handbooks that index a "muting" topic.

[KC4VWU]

Mute the speaker? Switch the speaker out and a suitable resistor in on tx; as well as simultaneously grounding the input of the rx. Works good.

I have a 5RA1 that I've been chomping at the bit to get started on since last year this time, but there is always something ahead of it on the bench (that is, when I actually have spare time to play radio). I believe the "W" model in the one in between the SR/JR's and the 5. Sweet! 


73, Phil

[stevef]

I suppose I could mute the speaker, but is there any "damage" being done to the front end by the transmitted RF?  Overload?  I am using the T/R relay in a Johnson Matchbox by the way.

[Jim, W5JO]

Get a two pole relay and mute the B+ and the audio as suggested. 

[KM1H]

The only thing that will damage the front end is RF from a bad relay.

You can also mute by biasing the audio stages well beyond cutoff. Other methods bias the IF stages.

Or the brute force tried and true methods already mentioned.

Carl

[Tom WA3KLR]

Some Hammarlund receivers kill the mixer and some of the i.f. stages as I recall.   This way the oscillators are not disturbed, nor are the audio stages disturbed.  Study their schematics.

I am in the process of very slowly restoring a 1935 HRO Senior so I will be interested in your findings, and the behavior of my receiver in this respect when it is finished, as I do intend to use it with my transmitters.

Good luck.

[KA0HCP]

I too have a 1935 HRO Senior waiting restoration.

Something to keep in mind is a caution in the manual to always keep either the speaker or a jumper plugged in to avoid damaging the audio amplifier tube.

I have no idea how far down the model line this applies.

[w3jn]

Do it the way most 50's receivers did it - insert a high resistance in the RF gain control when it's in standby.  Switching the B+ is a real recipe for a nasty shock and if something shorts out in your T/R switch, you can wreck the power supply.

According to the schematic I have, the rotor of the RF gain control goes to ground.  Simply break the connection at that point, put a 47K or so resistor to ground there, and re-wire the standby switch across that resistor so that when it's in standby the resistor will be inserted in the RF gain circuit and thus vastly reduce the RF gain.  You might have to experiment with the value a bit to completely mute the RX, but you should always have a DC path to ground in that circuit.

[AJ1G]

If you make the switched added resistance in the cathode return of the RF gain control you can use it to set a sidetone monitoring level in transmit.  Think this is illustrated in older ARRL handbooks on setting up for CW breakin keying....

[stevef]

Thanks gang.  I'll short the "BSW" connection inside the receiver which will free up the BSW terminals for the RF gain pot wiper.  The BSW terminals are connected to the relay-controlled "RCVR control" terminals in a EFJ Matchbox.  Will install a 50k or 100k pot across those terminals and try it out.

Steve, KK7UV 

[k4kyv]

The problem with just opening the B+ line (which IIRC is what the BSW does), is that the hf oscillator and bfo are turned off during transmit, and when you go back to receive, the oscillators have cooled down and have to warm up again, so there is a constant drift.  Probably not a problem with AM, but makes it difficult when working CW or a slopbucketeer.

I ran a pre-War one for years.  I remember keeping the B+ on all the time, and I think I shorted out the audio from the secondary of the output transformer that fed the speaker.  The original HRO does not have a built-in output transformer. It's been over 20 years since I used it so I am a little fuzzy on the details, but I know I kept the B+ on all the time, and silencing the receiver during transmit was never a problem.

Shorting out the audio to the speaker does not create the "pop" that breaking the B+ line does.  Another possibility would be to re-wire the BSW to turn off the B+ and screen voltage to the audio output tube, and incorporate a R-C network similar to a key click filter on cathode keyed CW rigs.

I tried to do as little modification to the HRO as possible, and the wiring changes I did make I tried to use the same style hookup wire and period components as  much as possible. Another mod, which I built into the separate power supply unit (rack mount version), was to regulate the screen voltage to the HF oscillator.  Otherwise, strong signals that activated the AVC tended to pull the oscillator frequency.  They corrected that problem in the HRO-5.

The pre-war HRO, with the bare metal german silver dial, S1-S5 meter scale, black interior and round i.f. cans, was my main station receiver throughout the 1970s and early 1980's, till I got the 75A-4.  On 160-20m, the HRO performed almost as well as the A-4. I added a reversible modification to use mechanical filters in place of the stock xtal filter, so the selectivity was just as good as the A-4, and the sensitivity was about the same. Once it warmed up it was pretty stable.  I could monitor SSB nets on 20m for hours at a time without touching up the tuning dial setting.
I still have the receiver, but haven't turned it on for years, so it would probably need some work to bring it back to life.

Roger, N4IBF (SK)  had a HRO-50.  It was a nice receiver, but I liked mine better. I eventually ended up using an outboard audio amplifier, the same one I now use with my 75A-4, for much improved audio quality. Don't recall exactly how I connected it up, but I think I took audio from the  headphone output, and the T/R relay silenced the outboard amplifier.

[KM1H]

Im still looking for a 35 dial for my E series. It has an R meter instead of S and it goes 1-5.

Carl

[flintstone mop]

Another 'brute force method"
Using a relay. When you key up the relay would activate and ground out the audio coming into the volume control. or the wiper.
OR wear headphones and monitor yourself while on the air........... Best way to critique your TX audio........sorta