BX645A

Dear Forum members

I am in the process of restoring the above set. In this set the band switch is motorised. I am unable to understand the working of this and I need following clarifications.

1. What is the function of the switch SK7? I presume this is fitted inside the motor assembly since it is not visible from outside.

2. I am uploading the 3 pages of the service manual which gives the details of the motorised band switch. The circuit shown is when the band switch is in KG2a position. So as per my understanding for every time the band switch buttons are pressed from KG2a to MW the switch segment SK6 moves one position clockwise ( if we are looking from the front of the chasis) so that the proper band contacts are made. The motor unit has a mechanical stop when either the KG2a band or the MW band is reached. So the motor cannot move beyond these limits. Now my question is when it reaches MW band, the mechanical stop has reached, and now if I select KG2a button how the motor runs in a reverse direction to reach KG2a band? I presume the motor is 6V ac motor because the supply is given from the filament winding of the main transformer.

Regards

P.Rajan [image] [image] [image]

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L.S.
Ref your first question; SK7 is a part of the motor.Its task is to keep the motor running if after
a bandchange the motorstopposition is not yet reached .
The bandselectswiches are conncted to the contacts that mate with the contactsegmet wit the one open space.So a bandchange causes the motor to run until the selected position is reached ,
(open bandswitch and open spot off selectorsegment).
The motor starts left/right depending on how he feels himself(like the little phlips citruspress) .
The springleaf on the middle gear arranges the reverse start.
rgds,
Sieme

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Dear Mr. Sieme

Thank you very much for your reply. You are correct regarding the automatic reversal of the motor when a mechanical stop comes similar to the Philips citrus juicer. I have also noticed similar motor reversal in one of the microwave ovens. Few more clarifications I require regarding the above set.
1. The switch SK7 contacts are open as shown in the schematic at the point when KG2a band is reached. Is it correct that as soon as next band button is pressed the switch contacts of SK7 gets closed till the next band is reached?

2. I am attaching a sheet of the service manual showing the top layout of the components. At the back of the chasis a plug is shown which I have marked with a black circle. What is the use of this and where it has to be plugged?

3. What should be the voltage rating of the capacitor C9 which is connected to the primary of the main transformer?

Regards
P.Rajan

[image]

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Dear Mr.Rajan,
Referring to SK7.
In my opinion it is:
A:part of the motorunit
B: contstant in closed position except for one short period per revolution.
The bandchange operation starts by pushing one of the selectbuttons,which will pass the power from SK6 to the motor.When The notch in SK6 reaches the line of the selected band button this circuit opens;the motor then continues on the SK7 to finetune the startingposition for the next selection.I think I am right about this as when manually rotating the motor knob with no bandselectswitch engaged it returns to the same position,left or right depending on the way it was turned of position and never more than halve a revolution.
The black plug is an onboard metal plate antenne(the alluminium circels on the ferrite antenna).
On the low band it acts as a screen when the ferrite is selected
As for the C, it is in the partlist as 0,1microF and there is a V in the partnumber which I think but am not sure of , stands for the voltage rating.
In my radio it is not(no longer?) present.
73
Sieme

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Dear mr Sieme

Thank you very much.
Is the plug which I have marked with a black circle to be inserted into the Antenna socket?

Regards
P.Rajan

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yes,it is.

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Addendum
What is the use of SK7 ?(I think)
SK6 is an unfit type of switch to interrupt the motorcurrent,it does its job in slow motion with sliding contacts resulting in a lot of sparking and contacterosion .Therefore the quick acting (microswitch like?)SK7 is parallel to SK6 taking over the actual interruption .In the short time it takes for SK7 to reach its open sector the contacts of SK6 are safely opened and in the correct position.
Open for comment
rgds
Sieme

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