I wish to repair an old 80s\90s Sanyo tape\radio 'boombox'. The unit uses some custom (or at least, very obscure and obsolete) end-actuated slide switches in a 6P3T configuration, one for mode selection and one for radio band selection.
The mode selector switch switches, among other things, the main power rail(s). This first section of the switch (which I have called SW1A) has burnt out after some years and no longer functions. The second section of the switch, (now called SW1B), has only one function - to pull low some part of the tuner circuitry when switching from AM to FM mode.
See attached schematic I have drawn up for how the original design was intended to operate.
To clarify, the 9.6v rail comes from the tape deck mechanism, as an output from the supply to the motor, switched on only when a function button on the deck is pressed. (ie: the 9.6v rail is at 0v when the tape is doing nothing)
The 12v rail comes straight from the DC\AC selector switch, which switches between the battery compartment and the mains transformer\rectifier diodes.
SW1A allows either the 9.6v or 12v rails to continue through to the main supply filter capacitor, a 2200uF 16v part, which then supplies the rest of the circuitry.
As a new switch was impossible to find, my repair idea consisted of taking the good contact from SW1B and putting it in SW1A, then using SW1A to activate some relays to perform its (and SW1B's) functions, with the idea being that SW1A will now last much longer as it is only switching a small current to some relay coils, and SW1B gets replaced with a relay as well since I stole its contact.
I used a 7805 to tap from the main 12v rail and power three 5v reed relays, RL1, RL2 and RL3.
The circuit I designed worked as intended during testing but failed immediately in practical use - both RL1 and RL2 failed with their switching contacts shorted together. They were rated for 1A which seemed plenty given the size of the transformer, but now I think the surge current to charge up the 2200uF capacitor may have been too high for them.
Is it possible to use my relay circuit with the addition of some kind of protection to prevent the relays from being damaged or do I just need heavier relays?
The problem with larger relays is that space and mounting options are limited, and their cost may make the entire thing uneconomical.
I also thought about using MOSFETs instead, but would this fall down another rabbit hole? - if RL1 and RL2 were N-channel MOSFETs (with NPN BJTs for inverting the gate drive), then whichever one was off would get its Source pin pulled high by the output of the other. I don't know if this also has the potential for damage.
The mode selector switch switches, among other things, the main power rail(s). This first section of the switch (which I have called SW1A) has burnt out after some years and no longer functions. The second section of the switch, (now called SW1B), has only one function - to pull low some part of the tuner circuitry when switching from AM to FM mode.
See attached schematic I have drawn up for how the original design was intended to operate.
To clarify, the 9.6v rail comes from the tape deck mechanism, as an output from the supply to the motor, switched on only when a function button on the deck is pressed. (ie: the 9.6v rail is at 0v when the tape is doing nothing)
The 12v rail comes straight from the DC\AC selector switch, which switches between the battery compartment and the mains transformer\rectifier diodes.
SW1A allows either the 9.6v or 12v rails to continue through to the main supply filter capacitor, a 2200uF 16v part, which then supplies the rest of the circuitry.
As a new switch was impossible to find, my repair idea consisted of taking the good contact from SW1B and putting it in SW1A, then using SW1A to activate some relays to perform its (and SW1B's) functions, with the idea being that SW1A will now last much longer as it is only switching a small current to some relay coils, and SW1B gets replaced with a relay as well since I stole its contact.
I used a 7805 to tap from the main 12v rail and power three 5v reed relays, RL1, RL2 and RL3.
The circuit I designed worked as intended during testing but failed immediately in practical use - both RL1 and RL2 failed with their switching contacts shorted together. They were rated for 1A which seemed plenty given the size of the transformer, but now I think the surge current to charge up the 2200uF capacitor may have been too high for them.
Is it possible to use my relay circuit with the addition of some kind of protection to prevent the relays from being damaged or do I just need heavier relays?
The problem with larger relays is that space and mounting options are limited, and their cost may make the entire thing uneconomical.
I also thought about using MOSFETs instead, but would this fall down another rabbit hole? - if RL1 and RL2 were N-channel MOSFETs (with NPN BJTs for inverting the gate drive), then whichever one was off would get its Source pin pulled high by the output of the other. I don't know if this also has the potential for damage.
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