Loudspeaker LS4621 is a loudspeaker with built in audio amplifier. The loud speaker can be permanently installed with a bracket but can also be temporarily installed on a metal surface by the three magnets on the backplate. The magnets are for temporarily stationary application and are not intended to use in moving vehicles. For vehicle use, the use of the bracket is mandatory. The loudspeaker is registered with NATO Stock Number (NSN) 5820-17-052-4058. 5820 is for the group "Radio and Television Communication Equipment, Except Airborne", number 17 is for The Netherlands and 052-4058 is the detailed number for this loudspeaker. The Philips' code for this loudspeaker unit is "9551 690 56300".
There's a knob  for adjusting the audio level. There's also a three position switch  available. The middle position of the switch is the 'off' position. The power to the amplifier is disconnected and no audio line is selected. With the switch in the bottom position, only the audio from the selected channel is available like the audio from a specific receiver of the intercom for example. (The desired channel has to be selected on the IC-4620 intercom unit.) With the switch in the top position, all audio is amplified and fed to the loudspeaker. So it's possible to 'reject' audio from a not selected channel.
The main difference between the LS-3621 and the LS-4621 is the knob used. The older LS-4621 has a bolt visible on top of the volume adjuster knob. The newer LS-4621 has a knob with a plastic cap placed over the bolt.
The ten contact plug used on the loudspeaker is the U-77/U plug model that fits on the U-79/U jack. There are many variations in the wiring of this connector. The specific pinout that applies to this loudspeaker is is shown below.
|audio (selected channel)
|audio (all audio)
|+18VDC for audio amplifier
The inside of the loudspeaker unit is accessible by unscrewing four bolts and removing the backplate. The device is waterproof, so there's a seal placed in the backplate to prevent intrusion of water. In the housing is the loudspeaker, the amplifier module, a terminal plate for wiring, the toggle switch and the volume pot.
The amplifier module is clamped in position by the rear plate. So the module can be pulled out of the holder easily. The module contains two printed circuit boards. One board is the audio amplifier and the second board contains the two push/pull 'power' transistors for audio amplification.
The housing of the module is asymmetrical, so the module can be placed only in one direction. The holder is marked with the number '140' corresponding with module number 140.
As shown below is the design rather minimalistic. The terminal board contains only one resistor, the rest is used for wire links.
On the image below are the module board shown in more detail. All the electrolytic capacitors are glued to the printed circuit to prevent damage due to extreme shocks ans vibration. This loudspeaker is built to last...
The knob of the speaker is equipped with illuminating paint. The white(ish) paint starts to glow faint green when uv-light is applied. Older tactical radio's like the RT-77, A-510 and radio's from this era are painted with (highly) radioactive Radium paint so that the knobs and text on the panels are visible in the dark. Luckily the RT-3600, RT-4600, SEM-25 and for example SEM-35 have no radioactive paint on them! The illuminating paint is free of radioactive materials, so no health risk due to radiation there. The effect of applying uv-light is shown below.
Sidenote: The disadvantage is that Radium paint is a huge health risk when inhaled/ingested! (Therefore apply clear lacquer over the paint to prevent spreading of the radioactive material. Note: The radiation is still there though.) I measured on a A-510 transceiver over 40 uSv/h where 0,1 uSv/h the normal levels of background radiation is. Thus the radiaton level is at least 400 times higher than normal radiation levels...
Unfortunately I don't have the schematic of the LS-4621, but I have the schematic if the LS-3621. I assume that the schematics are the same since the mechanical design is the same/very similar. Details are different like module number 140 for the LS-4621 and module number 11 for the LS-3621. Click on the image for the full size version.
Luckily I was able to find a couple of brand new LS-4621 loudspeaker in it's original box! The boxes were labelled with the NATO code NSN 5820-17-052-4058 and the Dutch product name "luidspreker eenheid" or "loudspeaker unit" in English. The production date is 09-09-1994. So after 28 years these speakers 'saw' daylight again. The speakers are in good condition. Only the electrical wire had some 'white wash', probably due the ageing of the plastic or the conservation material to prevent sticking of the rubber insulation.
Military equipment is usually very well documented. Also for the (similar) LS-3621 is there a lot of documentation available. Next to the schematic and drawing are there mainly spare part lists, so nothing very interesting. For reference is the drawing of the unit added below.