RA-17 mute - UK Vintage Radio Repair and Restoration Discussion Forum

Steve_G4HTZ · 28th Feb 2019, 12:46 pm

Hi all
Just getting into AM on 160 and 80m .
Recently acquired a heathkit DX100U and a Racal Ra-17 ..both of which seem to be in working order .
I have made up a tx/RX antenna switch box which is controlled via the DX100U a/c socket .

How do I go about muting RA-17 ....looking at circuit diagram it seems you could break the DC link on the back with a relay on TX ....would have to measure current at this point to see if it is within rating of contact on relay RX/tx switch .

I'm guessing this has probably been asked before but couldn't find information anywhere .

Regards
Steve G4HTZ

m0cemdave · 28th Feb 2019, 3:10 pm

When I used an RA17 with a transmitter I just used the link terminals on the back, switched with a Tx/Rx relay.
Any relay capable of switching 300V DC at 100mA or more should be OK. In my case it was a rather heavy-duty Omron MK3 plug-in type that I happened to have available when I converted the transmitter (a KW Vanguard) to relay operation.

Silicon · 28th Feb 2019, 10:21 pm

In the Racal RA17L, the AGC line is connected to the terminal block on the rear of the chassis.

If you used a relay to connect a 25V negative supply to this terminal you may be able to mute the receiver. It only needs to supply half a milli-Amp to do this. You could try it with three PP3s wired in series.
If this works satisfactorily, you could use the internal -25V DC supply that is used for the AGC.

You could remove one of the 600 Ohm 3mW audio outputs and use one of the terminals to terminate the internal -25V DC supply.

Steve_G4HTZ · 1st Mar 2019, 7:44 am

Thanks for information....something to work with.

Steve.

G6Tanuki · 2nd Mar 2019, 5:40 pm

Achieving muting by breaking the HT line, though it works, is not ideal.

The relatively-unloaded HT rail will rise towards the peak-voltage delivered by the HT transformer, causing additional stress on the HT filter capacitors and then when you re-make the HT circuit this excess voltage gets 'dumped' into the rest of the circuit . In some radios (such as the AR88) this can be ruinous!

With no HT during 'transmit' periods there's a risk that local-oscillator valves will change in temperature, causing frequency-drift.

With the power-dissipation varying between receive and transmit, the insides of the radio will vary in temperature, again potentially causing frequency-drift.

The 'mute-by-bias' approach keeps the HT line loaded, and the local-oscillators running continually so they should be more stable.

There's also the advantage that by setting an appropriate level of bias the receiver is not totally muted, so you can use it to provide "side-tone".

28th Feb 2019, 12:46 pm	#1
Steve_G4HTZ Triode Join Date: Jan 2019 Location: ASHEN ESSEX Posts: 33	RA-17 mute Hi all Just getting into AM on 160 and 80m . Recently acquired a heathkit DX100U and a Racal Ra-17 ..both of which seem to be in working order . I have made up a tx/RX antenna switch box which is controlled via the DX100U a/c socket . How do I go about muting RA-17 ....looking at circuit diagram it seems you could break the DC link on the back with a relay on TX ....would have to measure current at this point to see if it is within rating of contact on relay RX/tx switch . I'm guessing this has probably been asked before but couldn't find information anywhere . Regards Steve G4HTZ

28th Feb 2019, 3:10 pm	#2
m0cemdave Octode Join Date: Feb 2009 Location: Bletchley, Buckinghamshire, UK. Posts: 1,219	Re: RA-17 mute When I used an RA17 with a transmitter I just used the link terminals on the back, switched with a Tx/Rx relay. Any relay capable of switching 300V DC at 100mA or more should be OK. In my case it was a rather heavy-duty Omron MK3 plug-in type that I happened to have available when I converted the transmitter (a KW Vanguard) to relay operation.

28th Feb 2019, 10:21 pm	#3
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,163	Re: RA-17 mute In the Racal RA17L, the AGC line is connected to the terminal block on the rear of the chassis. If you used a relay to connect a 25V negative supply to this terminal you may be able to mute the receiver. It only needs to supply half a milli-Amp to do this. You could try it with three PP3s wired in series. If this works satisfactorily, you could use the internal -25V DC supply that is used for the AGC. You could remove one of the 600 Ohm 3mW audio outputs and use one of the terminals to terminate the internal -25V DC supply.

1st Mar 2019, 7:44 am	#4
Steve_G4HTZ Triode Join Date: Jan 2019 Location: ASHEN ESSEX Posts: 33	Re: RA-17 mute Thanks for information....something to work with. Steve.

2nd Mar 2019, 5:40 pm	#5
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: RA-17 mute Achieving muting by breaking the HT line, though it works, is not ideal. The relatively-unloaded HT rail will rise towards the peak-voltage delivered by the HT transformer, causing additional stress on the HT filter capacitors and then when you re-make the HT circuit this excess voltage gets 'dumped' into the rest of the circuit . In some radios (such as the AR88) this can be ruinous! With no HT during 'transmit' periods there's a risk that local-oscillator valves will change in temperature, causing frequency-drift. With the power-dissipation varying between receive and transmit, the insides of the radio will vary in temperature, again potentially causing frequency-drift. The 'mute-by-bias' approach keeps the HT line loaded, and the local-oscillators running continually so they should be more stable. There's also the advantage that by setting an appropriate level of bias the receiver is not totally muted, so you can use it to provide "side-tone".