NuTone Apartment Intercom System Model 478
NuTone made apartment house entry intercom systems since the early 1970's and the last model in the product line was the Model 478. Apartment entry intercom systems are referred to by the model number of the amplifier. A model 478 system uses model 485 or N485 apartment speakers and there are also a variety of add on modules for the 478 amplifier.
The system components break down as follows:
NuTone Model 478 Control Amplifier: The heart of the system, a solid-state design that includes a built-in call tone signal, communication, entry door-release, and adjustable volume for call tone, entry door and inside speaker volume.
NuTone Model 466 Second Door-Release Module: This add-on module installs inside the Model 478 amplifier and allow, the resident to control 2 entry doors into the apartment building.
NuTone Model 467 Timed Door-Release Module: This plug-in module installs inside the 478 amplifier and adds a timing circuit for the main entry-door release mechanism. The 467 is adjustable from 3 to 20 seconds. The model 467 timer module is required if the Postal-Lock Door-Release feature is part of the 478 system.
NuTone Model 468 Second Entry Door-Release Module: This module is the same as the model 467 except it controls the second entry-door release mechanism.
NuTone Model 469 Auto-Call Transfer Module: The auto-call transfer module is added to the 478 amplifier for installation on 2 entry-door installations. It automatically routes the communication and door release functions to the entrance that initiated the call and locks out the communications and door-release for the second door. The lock out is adjustable from 30-60 seconds.
NuTone Model 485 & N485 Apartment Speaker: The original model 485 apartment speaker and the newer N485 speaker are interchangeable and will both work with the model 478 amplifier. The N485 features a revised and modern switch circuit board for greater reliability.
Problems with a 478 apartment intercom system are divided into 2 separate areas, problems with the N485 apartment speakers and problems with the 478 amplifier and its accessory modules.
NuTone 485 and N485 Apartment Speaker Problems:
N485 apartment speakers are a rugged and long lasting design. The primary problems revolve around problems with sticky switches inside the speaker station. The N485 has 2 communication switches which are labeled “talk” and “listen”. These are operated by the brown rocker bar on the speaker grill.
The switches on the circuit board have return springs built-in to them and they insure that the switch returns to the “at rest” position when the talk or listen bar is released. These switches can become sticky, mainly due to age or environmental conditions.
An apartment speaker station with a stuck switch can disable the entire communication system in a building. The application of some switch of contact cleaner will usually clean away any residue and allow the switch to operate freely again.
The door release switch on the N485 speaker will have the same problem and it may become sticky with age. The same cleaner can be used to renew the action of this switch.
NuTone Model 478 Amplifier Problems:
The model 478 amplifier is a very well designed unit and it's not uncommon to have them in operation for 20+ years before they require service.
Time is the biggest factor with problems related to the 478 amplifier. A 20 year old 478 has accumulated 175,200 hours of “on time” and even in the best of conditions this is at the end of the normal life span for most common passive electronic components.
Fortunately there are only a few of this type of components in the 478. The typical problem that can be observed with a failing 478 system is the switch over between “talk” and “listen” will be slow and it can become intermittent. This is caused by the gradual failure of the components that allow the talk and listen relay to energize properly.
There can also be problems which are caused by the location where the 478 amplifier is installed. Many times the 478 will be installed in the wall cavity behind the entranced directory. This type of location is exposed to a much greater amount of weather and temperature. These factors can cause damage to the circuit board inside the 478 amplifier which can lead to problematic operation.
The other common problem which occurs with the 478 is related to the door release circuit. The 478 was designed to operate a 16 volt, intermittent duty electric door release mechanism. The electric door release is controlled by a electronic compo net called a SCR.
Think of a SCR as an electronic switch. When it's activated, power for the electric door release flows through the SCR and the release is energized. When the release is the wrong type or if the wires to release are damaged (shorted) the SCR will fail and the door-release will no longer work.
Inside th ered box is where the SCR is mounted on the 478 circuit board
Here you can see what happens when a SCR is overloaded (the red arrow points to the exploded SCR). This 478 amplifier had been in operation for almost 15 years without any problems, when the build maintenance people replaced the electric door release with a new unit. When they did this, the door release wires were pinched inside the metal door frame and were shorted out. The SCR failed within 3 days.
NuTone 466, 467 & 468 Module Problems:
NuTone Door Release Modules 466, 467 and 468 are fairly trouble free and most problems are associated with the SCR just like in the 478 amplifier.
NuTone 469 Auto-Call Transfer Module Problems:
The NuTone 469 Auto-Call Transfer Module can have problems with identifying which entrance initiated the call originally and maintaining the lockout to the second entrance. 469 modules can be very difficult time consuming to repair and usually they require a mocked up communication system on the repair bench for final testing.