Think I’ve messed up buying old intercom

[MurdMac]

Bought a set of 3 Realistic FM Intercoms from eBay. Great condition all bagged abd boxed and instructions too

Can’t get them working though and after reading the following post I think I’ve figured out why

Old School Realistic (Radio Shack) Intercom ???'s

Old School Realistic (Radio Shack) Intercom ???'s (Pic Added) Picked up a working pair of these at a local thrift store. It was a gamble at $25, but when I got them back, I realized they were 5-watt tx power and they actually worked. I think these things are circa 1970. What kind of range...

forums.radioreference.com

Seems they transmit through the power cables

I’m on the uk 240v 50HZ AC
To power these I’ve had to buy step down transformers 110v/60hz

I’m now guessing the transformers are killing the signal going through the house power cables

No call tones … no nothing

Any ideas?

Turns out when I half pull out the plug and touch the other units plug pins to the one half plugged in it does in fact work but it is then technically a wired unit
Transformer definitely stopping the transmission

Apologies these are Realistic Model 43-214

 

[RFI-EMI-GUY]

Bought a set of 3 Realistic FM Intercoms from eBay. Great condition all bagged abd boxed and instructions too

Can’t get them working though and after reading the following post I think I’ve figured out why

Old School Realistic (Radio Shack) Intercom ???'s

Old School Realistic (Radio Shack) Intercom ???'s (Pic Added) Picked up a working pair of these at a local thrift store. It was a gamble at $25, but when I got them back, I realized they were 5-watt tx power and they actually worked. I think these things are circa 1970. What kind of range...

forums.radioreference.com

Seems they transmit through the power cables

I’m on the uk 240v 50HZ AC
To power these I’ve had to buy step down transformers 110v/60hz

I’m now guessing the transformers are killing the signal going through the house power cables

No call tones … no nothing

Any ideas?

Turns out when I half pull out the plug and touch the other units plug pins to the one half plugged in it does in fact work but it is then technically a wired unit
Transformer definitely stopping the transmission

Apologies these are Realistic Model 43-214

You would have to put a coupling capacitor like a 0.5 uf disc cap non polarized, 500 volt or so between the primary and secondary windings. Perhaps on both sides. Look inside to see how the RF was coupled to the lines inside and replicate in your step down transformers.

 

[MurdMac]

That sounds easier than what I’ve just been planning. The user manual has a schematic on the rear which lists input as 120V
And below that It is asterisked *220V and there are 10 components *starred on the schematic as alternatives consisting of (capacitors, diodes, resistors and 2 red power LED’s) I was considering replacing all these on all 3 intercom units

When you say on both sides do you mean capacitor between :
+ve primary to +ve Secondary?
&
-ve Primary to -ve Secondary?

Many thanks

 

[MurdMac]

I’m not seeing the RF being coupled back to the power supply
appreciate you’re eye here

 

[RFI-EMI-GUY]

That sounds easier than what I’ve just been planning. The user manual has a schematic on the rear which lists input as 120V
And below that It is asterisked *220V and there are 10 components *starred on the schematic as alternatives consisting of (capacitors, diodes, resistors and 2 red power LED’s) I was considering replacing all these on all 3 intercom units

When you say on both sides do you mean capacitor between :
+ve primary to +ve Secondary?
&
-ve Primary to -ve Secondary?

Many thanks

That s what I am suggesting so that RF passes from winding to winding. however if the schematic has a 220V mod, you should consider it. is there a transformer inside?

 

[MurdMac]

It’s not a 220v mod - I think they’re just listing the components that are different in the 220/250v uk or eu models that may or may not have also been available at the time
Can you see anything coupling the internal transformer in the attached schematic?

 

[wtp]

would you happen to have a parts list also ?

 

[RFI-EMI-GUY]

I’m not seeing the RF being coupled back to the power supply
appreciate you’re eye here

OK This helps. Did not see it before. For you to do a 220 mod you would have to change T2 only. Not all the other * parts which as you mentioned are probable same parts sourced differently in other markets. Changing T2 may be impractical unless you can scrounge them.

Circuit explanation: Above T2 on the primary (120V) side look for C1 and C2 that bridge the RF to and from T1. This is how BALANCED RF is conveyed from the circuit to the AC plug and from there to all of the outlets in your home that are on the same leg of the mains source. For your step down transformer you need to replicate capacitors across the transformer windings so that the signal is not blocked by the inductance of the transformer. I had suggested two 0.5uF . Any value 0.1 uF or larger will get the signal across 0.5 uF should be ideal. Just make sure they are ceramic discs with 500 V or higher rating as they will be on your mains.

 

[RFI-EMI-GUY]

Like so....For your outboard step down transformer. You build them into small utility boxes with a 220V cord and 120V outlet.

 

[MurdMac]

Brilliant thanks for this
What’s your feeling about running 2 x non modified units in series connected to 240v. These would be in my 2 bedroom garage Annex

The other 2 would be in my house (on the same leg of the grid transformer power supply). These 2 also in series as they are literally located one in the ground floor and the other one directly above on the First floor?

Could that work?

 

[MurdMac]

Option to replace T2

Currently voltage in from my transformer is 120V. v out is 25V ac
You’re saying if I replace it with a 240V to 25V then I can leave all the other components untouched?

Any recommendations on good replacements for the T2 transformers to achieve this?

 

[MurdMac]

25V Voltage taken at the board ends of the yellow wires

 

[RFI-EMI-GUY]

You would need a Transformer of correct turns ratio V In/Vout and VA rating. You will have to figure out how the white wires are attached at C1 and C2, seems to be a safety design.

I don't think putting two 120 V intercoms in series on 240V line would be safe and the signal will be impaired.

 

[MurdMac]

Interesting
Re the series wiring
Do maths and science both not say it should be harmonious
What does is your concerns? As they seemingly rely on interconnection to even work (hilariously advertised as wireless ) 😆

 

[RFI-EMI-GUY]

Interesting
Re the series wiring
Do maths and science both not say it should be harmonious
What does is your concerns? As they seemingly rely on interconnection to even work (hilariously advertised as wireless ) 😆

You can always try it.

 

[MurdMac]

I’m not dismissing your doubts and appreciate it may be a bit unorthodox wiring them in series which may throw up some unforeseen issues

For overall ease I’m considering going with the replacement of T2 transformer

Found this ones on Ali express :
10W. 220>24V ac. (240>26.2V)

https://a.aliexpress.com/_EJ6tLZ8

I think they should do the trick
Have to check size of the original though and consider if it’ll fit ok in the casing?

 

[MurdMac]

Hmm size is the issue
The original one is about 35mm (1.5”) cube
Fixing holes at 47mm centres
A 240v > 25v transformer is always going to be a bigger unit than a 120v > 25v unit

 

[RFI-EMI-GUY]

Hmm size is the issue
The original one is about 35mm (1.5”) cube
Fixing holes at 47mm centres
A 240v > 25v transformer is always going to be a bigger unit than a 120v > 25v unit

Somehow the OEM 240V transformer fit. the VA rating will ultimately dictate dimensions and weight.

 

[RFI-EMI-GUY]

I’m not dismissing your doubts and appreciate it may be a bit unorthodox wiring them in series which may throw up some unforeseen issues

For overall ease I’m considering going with the replacement of T2 transformer

Found this ones on Ali express :
10W. 220>24V ac. (240>26.2V)

https://a.aliexpress.com/_EJ6tLZ8

I think they should do the trick
Have to check size of the original though and consider if it’ll fit ok in the casing?

link no worky

 

[MurdMac]

Obviously you’re correct
I’ll try and sort the link
According to the schematic the transformer was the same on the 240v?
Only 10 x components changed
Resistors/Capacitors and the 2 LEDs