Ekco AD65. Audio input connection? - UK Vintage Radio Repair and Restoration Discussion Forum

TopTip · 1st Nov 2018, 1:42 am

Another thing. The introduction of devices like Echo, Siri, etc., opened new horizons for old radios. I have the attached “baby monitor” from 1937. It is a transmitter / receiver combo which communicates over the power line at 300kHz. I only have the “famous” nurse head, not the transmitter. To find a use for it, I looked inside. It has three valves, a rectifier diode, demodulater triode and a pentode for output. After some experimentation I decided the best way to use it is driving the pentode over a 1:35 (30dB gain) transformer, connected to an Alexa Dot on one end and the volume potentiometer at the other — you say, “Alexa, play BBC 3” and it does.

http://www.uv201.com/Radio_Pages/zenith_radio_nurse.htm

I want to do the same (or similar) for the AD65. The instructions that I have say, “you can connect a grammaphone across R10 (over capacitors or an isolation transformer. I would use a power isolation transformer instead).

But I do not quite understand why R10? Would it not be better somewhere before so the radio volume would be useable?

Silicon · 1st Nov 2018, 10:29 pm

Are you referring to the resistor R10 on Trader Service Sheet 649?
That would be the conventional place to introduce the audio signal.
It would also be the conventional place to put a volume control.

The AD65 is a bit different. It alters the volume by changing the DC bias on the variable mu RF and IF valves and hence altering their gain.

The volume control potentiometer R3 alters the DC bias on the cathodes of the mixer and IF valves.
Since the audio goes through V2, R3 should alter the gain of the first audio stage.

TopTip · 2nd Nov 2018, 6:36 am

Quote:

Originally Posted by Silicon

Are you referring to the resistor R10 on Trader Service Sheet 649?

Yes, exactly there. Thank you for your explanation.

Two more questions:

— When the same sheet says “grammaphone” signal, is it referring to the output of a crystal pickup? I think today that would be similar to what is called “line” or about 775mV.

— The sheet indicates the filter capacitors C17 and C18 as 24 and 8 microF. On my radio, which looks unadulterated, the very old looking cardboard-clad capacitor reads 8+8 microF. I know they had variations between units but mine looks like the one they have in their pictures yet this is a bit of a big difference. Should I renew it with 24+8? (I would put them in the original case).

Many thanks.

1st Nov 2018, 1:42 am	#1
TopTip Triode Join Date: Feb 2006 Location: New York, NY, USA. Posts: 39	Ekco AD65. Audio input connection? Another thing. The introduction of devices like Echo, Siri, etc., opened new horizons for old radios. I have the attached “baby monitor” from 1937. It is a transmitter / receiver combo which communicates over the power line at 300kHz. I only have the “famous” nurse head, not the transmitter. To find a use for it, I looked inside. It has three valves, a rectifier diode, demodulater triode and a pentode for output. After some experimentation I decided the best way to use it is driving the pentode over a 1:35 (30dB gain) transformer, connected to an Alexa Dot on one end and the volume potentiometer at the other — you say, “Alexa, play BBC 3” and it does. http://www.uv201.com/Radio_Pages/zenith_radio_nurse.htm I want to do the same (or similar) for the AD65. The instructions that I have say, “you can connect a grammaphone across R10 (over capacitors or an isolation transformer. I would use a power isolation transformer instead). But I do not quite understand why R10? Would it not be better somewhere before so the radio volume would be useable?

1st Nov 2018, 10:29 pm	#2
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,171	Re: Ekco AD65. Audio input connection? Are you referring to the resistor R10 on Trader Service Sheet 649? That would be the conventional place to introduce the audio signal. It would also be the conventional place to put a volume control. The AD65 is a bit different. It alters the volume by changing the DC bias on the variable mu RF and IF valves and hence altering their gain. The volume control potentiometer R3 alters the DC bias on the cathodes of the mixer and IF valves. Since the audio goes through V2, R3 should alter the gain of the first audio stage.