Claymore Transistor Intercom

[Sparks]

This device arrived this morning, clearly old and well-used. It's making some dreadful noises though. Once the 9V battery and the connecting lead are plugged in the units squeal and whine for tens of seconds at a time. Adjusting the volume on the master or pressing either of the Call/PTT buttons makes a small difference for as long as the buttons are pressed. Eventually the noise stops, save for the odd burst.

I think the units do otherwise work as they should but it's difficult to tell. What would be likely to be causing the noise and is there any way I can stop it ? I know very little about how the components function but I guess some of them are past their service life (the units must be well over fifty years old) and haven't been powered up in a very long time. I'm not too bothered about the slave but I would like to have a fully functioning amplifier that I can use in another comms application, perhaps as an intercom for two or more headsets.

Thankyou.

[dragonser]

Hi,
I wonder if some of the electrolytic caps have dried out and gone low in value ?
At least you have both units !

[Sparks]

I guess it's a possibility but, not knowing much about transistors, capacitors and the like, I can't say for certain. However, I have also obtained a similarly vintage Eagle transistor intercom, unused but I guess technically identical to the Claymore underneath the plastic and grilles. It has the exact same problem!

[bigfathairyvika]

If I remember rightly, they make a horrible squealing noise when the button is pressed until the connecting cable is connected between the two units.
So maybe look at the wiring at the jack sockets.
Also if the battery was a bit flat they used to do weird things.

Maybe wrong but that was how I remember them.

Also, don't forget to point the front towards the enemy....

Mark

[Sparks]

The battery is fine but I'll look at the audio sockets and connecting cable. Would someone knowledgeable be kind enough to identify the capacitors that are likely to need replacing. Thankyou again.

[Sparks]

By the way, this is a picture of the circuit board of the Eagle unit. I think it's a fair assumption it's essentially the same as the Claymore, just a slightly different layout.

[Cobaltblue]

I seem to remember the way these work is to make them "call" the blocking capacitor is shorted so maybe the blocking capacitor has gone very leaky.

Or perhaps the cable was remade at some point back to front so the capacitor is being reverse biased.

Does it still squeal when the remote unit is disconnected?

Cheers

Mike T

[duncanlowe]

We had similar units when I was young, used as a baby monitor but later as a toy. The horrible squealing noise reminds me of the noise they emit when the 'call' button is pressed. So the already mentioned suggestions such as a faulty blocking capacitor, or maybe a sticking switch would make sense.

[Sparks]

Thankyou for your responses folks. I'll have a more in-depth look in a few days when I have more time. Having posted pictures of the circuit boards, would any of you be able to identify the blocking capacitor or indeed any other components that may need replacing ? Thankyou.

[Graham G3ZVT]

I take it you have tried the units as intended, ie connected together and in separate rooms with the doors closed so they don't feedback / howlround?

As for the capacitors, why not replace all five electrolytics and the one in the slave?

[Sparks]

I have done as you describe, Graham. Still noisy! I see you have noted there are five electrolytic capacitors. How would I identify them please ?

[Dave Moll]

I would imagine that they are the five metallic cylinders wrapped in plastic (for want of a better description). Two are to the left and below the transformer in the middle. The other three in the top right corner.

[Sparks]

Much obliged Dave. I can identify a resistor but that's about my limit so that's a big help.

[Dave Moll]

I need to revise the above description, as the positions were based on the Eagle circuit. I hadn't spotted that this was not the one at the start of the thread. I'm sure, however, that you can identify the similar components in the Claymore unit. Apologies for any confusion caused.

[Sparks]

I've not had a proper look at either unit yet Dave but thanks all the same.

[Cobaltblue]

Can you confirm if it still squeals with the slave disconnected?

Cheers

Mike T

[Sparks]

Mike, no it doesn't. The squealing begins in earnest the moment I connect the slave.

[TonyDuell]

How close is the slave to the master? It's not just acoustic feedback between the units, is it?

[Sparks]

I've separated the units, Tony, and the noise is still there. Definitely not feedback.

[bigfathairyvika]

Time to do some actual repair work I think.

1.Replace the capacitor in the slave unit. Identifying marks should be on it.
Once this has been replaced, retest.
If this does not fix it:-
2. Continuity check the switches , jacks and cables.
3. Remove the master board from its unit and check all wiring and soldered connections for dry joints.
If this still does not work,
4. Note all ids for the capacitors on the main board and replace with equivalents. Then test again.
No luck, move on to the transistors.
5. Some where in there is a transistor or two, you need a multimeter with to do some checks on them.
Check each one on the diode setting, check for open and short circuits.
6. May also want to check transformers, again with a multimeter.

Once you have done all of these come back with the results.

Notes on IDing components: - Capacitors will usually have two markings: Voltage and Capacitance, hopefully voltage marked with a V and capacitance with uf.
From the pictures they look probably all in the range of 1uf to 10uf and probably 10-16v

Hope this helps.
If not I think you need to pass these on to someone who can repair them.
I will volunteer if no one else does.

Mark