555 Timer Circuit troubleshooting

[deepthaw]

Got a nice old NES controller with integrated turbo switches via what I assume to be a 555 (or rather, 556 since it's two buttons but one IC). Originally nothing related to the buttons at all functioned when turbo was enabled. I tracked this down to some cracked solder joints, and fixed that up pretty easily. However, now turbo doesn't work. I can switch it to turbo off/low/hi and it just registers as regular button presses.

 

Any tips on what to look at next? The controller is a bit of a pain to work on (it's a Sansui SSS which is made of two pcbs sandwiched together so I'll have to desolder stuff to get at the components) so I'd like to target any repairs as effectively as possible. I know 555 circuits use capacitors/resistors to setup the timing, would a bad cap cause it to act in this way or should I look elsewhere? Do I just need to order a tube of 556 ICs and see if that fixes the issue?

[Stephen Moss]

I hate 555 timers, they either work first time or are complete pain in the ass to get working, I prefer programming a PIC, they work and have more accurate timing even when running from their internal clock.

That said, without a schematic to see how it is all connect one can only speculate but as you said...

14 hours ago, deepthaw said:

Got a nice old NES controller with integrated turbo switches via what I assume to be a 555 (or rather, 556 since it's two buttons but one IC).

then, assuming each button is connected to one half on the 556 (if that is what it is) then a bad cap may stop one from work, while not impossible having two bad caps preventing both from working would seem unlikely.

1. Check the 556 has power by measuring the voltage across pins 7 & 14, if that checks out then
2. Measure the voltage on the reset pins (4 & 10), I would expect it to be either permanently at the supply voltage or at the supply voltage when turbo mode is enabled and 0V when not. 

If you measure the output on pins 5 & 9 are they at the Supply voltage, 0V/GND or somewhere in between?

 

The next question is how is it switching the two speeds, is it selecting one of two capacitors or one of two resistors, which seemsthe most likely or another method? If the switch configuration is Off in the centre position then...

1. Track whether the centre pin is connected to the supply voltage (probably switching resistors), or GND or Pins 2 & 6/8 & 12 (probably switching the capacitors) and
2. With no power connected use the resistance/continuity range on a multi-meter and check that switched to the high/low positions the centre pin in connect to the one of the pins either side of it (I am assuming the switch connection are in a standard row configuration).

 

See that that reveals.     

[deepthaw]

@Stephen MossThanks so much for the troubleshooting steps - I’ve got desoldering braid etc on order to get this thing properly taken apart so I’ll come back to check on this when I start actually working on it.

 

its really nice for an NES controller and I only paid $5 so if I can repair it, it’ll be a little jolt of happy juice.

 

 

Edited November 28 by deepthaw