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Monday, September 12, 2016

Beogram 4002 (5501): Gold Plating of the Carriage Position Switch Spring Terminals

I am enjoying myself exploring the design of the early issue Beogram 4002 (5501) turntable that I am restoring right now. It is fascinating to see how the design evolved from the original Beogram 4000 design. After I rebuilt the arm lowering mechanism, I decided to take on the carriage position switches that are an exciting design feature of this 4002. The design is similar to the 4000 carriage position switches, but a bit improved. One of the weaknesses of the 4000 design is that the 7" record set-down point switch is a break switch, which has a tendency to cause problems when the switches oxidize. The reason for this is that a break switch depends on the spring force of the contact terminal to make the contact initially. A bit of oxidation, and the switch is permanently open, which severely disrupts the entire control system (usually the arm sets down immediately after pressing ON).

This potential issue was alleviated with the design under investigation here. Now the 7" detection is done with a standard make switch.
However, since these switches are the most likely sources of headache in these units, it is genrally a good idea to gold plate all their spring terminals. This prevents oxidation and so strongly reduces the probability of control system issues down the road. In this 4002 the switch actuators were also stuck due to hardened lubricants. So I took everything apart and lubricated the actuators and gold plated the terminals. Here are a few impressions:

This shows the switches after taking of the plate with the springs that push down the switches:

Point in case: The rightmost switch is stuck in down position.

Since the 550x Types were built before board to board connectors became prevalent in consumer electronics, one can only try to flip the board up for access to the switches and the plastic tabs that hold the actuator assemblies to the PCB. For this to happen the aluminum profile to the left needs to be pushed out of the way, which can be done after removing the two screws that hold it in place.
This shows the board after removal of the profile.
After unsoldering the leads of the carriage motor and of the end groove detection sensor, I lifted the board up and removed the actuators
and then the contact terminals (it is an excellent idea to do it in this order since it is very easy to damage the transparent plastic parts with the desoldering gun when taking out the terminals):
After cleaning the terminals with a fiberglass pen, I coated them with gold:
Nice and shiny!!

Then it was time to reinstall everything. First, I soldered the tabs back in:
They need to be flat against the PCB. Then I lubricated the red actuators and pushed their tabs back through the PCB (it helps to put a small dab of grease on each of the tabs, that prevents the necessity of applying much force, which keeps the small tabs from breaking off). 

This shows everything back in place:
All switches are up and they now move smoothly. I tried to run the turntable after this, but it turned out that there are still some major operability issues. The carriage motor only runs for a brief while after startup and the tracking sensor LED flickers while it does that...this indicates some severe power supply issues...this will be the topic of the next entry!






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