Cold war electronics project....

[efiste2]

HI there, Im new to the forum and am hoping you guys can help me out with my project......I have a Carrier/Reciever unit from the EARLY WARNING SYSTEM of the 1960s. basically it was plugged into a phone line and if the Russians decided to attack it would sound a warning sound to alert local police/fire stations to sound the big AIR RAID SIREN in the local area.
Anyway I have adapted it so that I can plug an IPOD in to it and play the early warning sound through the speaker to demonstrate what its role was. The only problem is the sound coming from the speaker is very very quiet even with both the IPOD and the UNITS volumes turned up to full.....

The unit is basically a radio tuned to a certain frequency and then an amplifier/audio to push out the sound, I have disconnected the RF circuit board so only the AUDIO AMP part of the unit is working. The unit is powered by 4xD type batteries (6v)

So I am looking for a SIMPLE way to increase the audio level from the units loudspeaker. My electronics level is basic and I can solder. I have been told that it may be IMPEDANCE matching between the output of the IPOD and the input of the AUDIO PCB of the unit. I am looking ideally for a qucik fix that I can buy from Maplins etc..

 

[efiste2]

Could this be fitted between the source and the amplifier circuit board, to increase the audio output ?

http://www.maplin.co.uk/universal-preamplifier-module-37746

 

[efiste2]

Or this ?

http://www.maplin.co.uk/12w-universal-amplifier-module-46809

 

[john monks]

Do you have a make and model of this device?
Is there a schematic somewhere?

 

[efiste2]

Heres the old schematic



The red arrows indicate where its been disconnected from the RF receiver so that the Radioe frequency part if the unit is no longer in use.
The green arrows indicate where I have connected the audio signal.

would this item fit between the source and the input of the amplifier, as a sort of pre-amp....
http://www.maplin.co.uk/1w-amplifier-kit-220054

When connected to my PC audio output it does play slightly louder.....but I would like to be louder.

 

[john monks]

The green arrows is where you inject audio. I suggest connecting this point to a radio speaker to test the rest of your amplifier to make sure that is is working properly. The schematic came through sort of fuzzy and I have a hard time making out the components.

 

[efiste2]

Sorry for the delay in replying, im a little confused, do you mean wire a radio speaker (ie exactly that, a speaker from a portable radio) across the SIGNAL INPUT terminals ? and so if the radio speaker plays the audio track at the "same" volume as the UNITS speaker it proves the amplifier circuit isnt working.

I have to add that the unit has a volume switch, and moving this does increase the volume, but only slightly.....

Thanks for your efforts in assisting me.

 

[john monks]

Yes that is what I mean. Simply connect the speaker leads directly to where you have your green arrows.
The way the volume control is hooked up the volume will not go to zero. It can only be reduced. The other thing is that your circuit has a form of a squelch that kills the audio when the volume is very low. So this is why I suggest connecting your device to a radio so that you can run it at a relatively high volume so you can overcome the squelch circuit. It looks like there is a mute switch so you might want to try it both ways.

 

[efiste2]

Your spot on with your circuit analysis (as far as I know) the unit is supposed to squelch the output until a warning is recieved then it (in operation) wail like a banshee....it also played a "confidence tick" which proved the line connection back to the early warning radar control.
The master volume control knob is also the ON/OFF switch. Ideally id like to run the audio through the original speaker, so I am assuming I need to bypass this squelch circuit in some way.....any ideas

 

[john monks]

The squelch circuit should be no problem. The way it's hooked up it only suppress low volume like hiss or other small interference coming through the phone line. It works by shutting off the forward bias on the output transistors. But still a strong enough signal will still get through. If you want to disable the squelch circuit you just need to short your mute switch.

 

[KrisBlueNZ]

I'd like to see a readable version of that schematic...

 

[efiste2]

here is a link to the webpage that describes my UNIT very well it also has the CIRCUIT DIAGRAM.....about 2/3 of the way down....

http://www.ringbell.co.uk/ukwmo/Page212.htm

 

[efiste2]

I have been advised that a TRANSFORMER could be used to allow the low output of an IPOD audio source to drive the large speaker of the reciever unit......what do you guys reckon and if so could you link me to something like the part I would need.....

 

[john monks]

That sounds good to me although the audio may sound a little different, maybe too good. Driving the speaker directly certainly would solve a lot of problems.