noisy intercom

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Poor old sod

Joined Jul 25, 2017
145
set of 3 vintage NOS vox intercoms [KAPO WI-353A] with fm signal on mains, very noisy hum in speakers when receiving. 100hz? psu filter caps, on all 3? no motors/microwaves active in house, all same phase, call tone same as noise. call tone seems low hz, impure sound. utterly drowns voice.
instructions are set each one on its own channel, to call: select channel of called unit. i have all 3 on same channel in 3 rooms, not sensitive enough for audio feedback.
edit #1
several caps have leaked in one unit. expect others likewise. what's good to wash the mobo with, and preserve text?
 
Last edited:

LowQCab

Joined Nov 6, 2012
2,052
Without knowing anything about the situation,
( other than "nos", which could mean anything depending on who You ask ),
You need to replace ALL the Capacitors first.

Some small Ceramic Capacitors may be just fine,
but the cheapo-Electrolytics are toast,
along with any Capacitor that is sealed with Wax.

Use Clear Spray-Paint on the Speaker-Cones before they disintegrate,
front and back,
and especially if one or more of them will be mounted outside.
.
.
.
 

BobaMosfet

Joined Jul 1, 2009
1,979
set of 3 vintage NOS vox intercoms [KAPO WI-353A] with fm signal on mains, very noisy hum in speakers when receiving. 100hz? psu filter caps, on all 3? no motors/microwaves active in house, all same phase, call tone same as noise. call tone seems low hz, impure sound. utterly drowns voice.
instructions are set each one on its own channel, to call: select channel of called unit. i have all 3 on same channel in 3 rooms, not sensitive enough for audio feedback.
edit #1
several caps have leaked in one unit. expect others likewise. what's good to wash the mobo with, and preserve text?
And is this NOISE 60Hz? Ubiquitous, eveyrwhere in the states? Or are you elsewhere?
 

Audioguru again

Joined Oct 21, 2019
4,687
Since 100Hz was mentioned then the noise is poorly filtered 50Hz electricity that is full-wave rectified.

A proper FM detector is supposed to ignore AM modulation but a poor FM detector picks up 50Hz and 100Hz interference buzzing from light dimmers.
 

MisterBill2

Joined Jan 23, 2018
11,880
OK, a line frequency hum is ususally failed filter capacitors gone bad, but it nay also be poor rectifiers diodes in the supply. So the first check is for AC ripple on the DC supply voltage.
If you are able to show a circuit of the device we can identify the parts to replace.
AND, "NOS" means New Old Stock.
 

MrSalts

Joined Apr 2, 2020
1,762
AND, "NOS" means New Old Stock.
Obviously true but, define "old"? Stored more than 5 years? Stored more than 50-years?
Define "new". Sealed in a box and Never used? Repackaged in original box and looks new? What if the manufacturer sold the item in a cardboard box without a seal? What evidence do we need to prove "new".

NOS - whatever that means.
 

MisterBill2

Joined Jan 23, 2018
11,880
If the polarized capacitors are the "Rubycon" brand then replace them all, because from an older time they were all known to fail. Try it with one unit first and verify that the hum is gone. In addition, there were silicon diodes that for some reason produced little line frequency spikes of almost a volt on the filtered, rectified DC output. Those required the addition of a 0.1MFD filter capacitor right after the rectifier. That should get rid of most of the buzz/hum. It would probably not be an interference issue, but a power supply problem, and so the quality of the FM detection system should not matter, unless the signals are quite weak.
 
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