Choosing a capacitor for a power supply application

[172pilot]

I have a yamaha DX7 Synth from the 80s.
I have heard horror stories about caps in the power supply blowing up, and I think it's time I do some preventative maintenance on it.  The stories I've heard describe the power switch not working anymore, and being unable to turn off the synth while it's billowing smoke, forcing you to go unplug it from the wall. 
I have attached a snippet from the schematic.  I believe based on this description, that cap C1 must be failing shorted, thereby bridging the power switch.  Does this make sense?

So - I'm from what I'm reading that I should be looking for a "Safety" cap so that the replacement would fail open, not shorted, but I'm not sure how to identify that.

Secondary to that question is a more basic question - My understanding of caps is that they let AC flow, but block DC, so I'm wondering how this cap isn't seen as a dead short across the power switch during normal operation in the first place.  I'm sure it's my bad/limited understanding of caps, and that it comes down to the value of the cap not being able to pass 60hz, but I'm wondering if someone could tell me just for my own comfort zone, what the frequency of AC this cap would pass vs. block?

In the schematic, I have included the European version because that's what shows the entirety of the DC side of the supply.  The top of the US one is shown below, and is identical for the parts in question, and just has a different transformer for the different input voltage..

Thanks in advance!!!
-Steve in Maryland

[ledtester]

Found this video which points to a 0.022uF RIFA cap:

YAMAHA DX7 Safety Warning (Capacitor issue)


RIFA caps should be replaced with a modern safety cap:

https://eevblog.com/forum/chat/old-rifa-capacitors-and-a-disaster-story/

Update: I have a feeling that you should use a Y-class safety cap in this case, but perhaps someone with more knowledge in this area would like to comment on that.

[bdunham7]

Yes, C1 failing short would account for not being able to shut off your flaming synth.

You should replace it with a decent  X1 or X2 rated capacitor, perhaps with a bit higher voltage rating, like this:  https://www.mouser.com/ProductDetail/EPCOS-TDK/B81141C1223M000?qs=pGJ4H8VyKtWghOgrwqiA%2FQ%3D%3D

As for AC vs DC, the capacitor will have an impedance dependent on frequency.  For a .022uF (22nF) capacitor at 60Hz, that is about 120k (120,000 ohms).  So when the switch is off, there will be about 1mA of leakage current.  You could probably replace it with a .01uF (10nF) model and that also might help if there are any physical size constraints, since a higher-voltage rated X1 capacitor is likely physically larger than what you have now.

[srb1954]

I have a yamaha DX7 Synth from the 80s.
I have heard horror stories about caps in the power supply blowing up, and I think it's time I do some preventative maintenance on it.  The stories I've heard describe the power switch not working anymore, and being unable to turn off the synth while it's billowing smoke, forcing you to go unplug it from the wall. 
I have attached a snippet from the schematic.  I believe based on this description, that cap C1 must be failing shorted, thereby bridging the power switch.  Does this make sense?

Assuming it is a RIFA brand capacitor, as shown in the video, these can fail emitting copious quantities of smoke but they rarely go totally short circuit.

In any case it is still best to remove or replace any RIFA capacitor with a different brand to prevent the dramatic release of the magic smoke. In this location you can just totally remove it from the circuit and your synth will continue to work OK. The only downside is that your power switch will have a shorter life and you might have to replace it a little sooner than otherwise expected.

Just be absolutely sure to unplug your synth from the mains socket before starting any work in this area so you don't get a fatal shock!

So - I'm from what I'm reading that I should be looking for a "Safety" cap so that the replacement would fail open, not shorted, but I'm not sure how to identify that.

The correct type to use is a class Y safety cap. These are designed and tested not to fail to a short circuit condition. Finding a replacement 22nF (0.022uF) class Y safety cap may be a little bit more difficult as these type caps are not normally used in values larger than 4.7nF (0.0047uF) in normal circuit configurations. Using a 4.7nF in this location would still provide some protection for the switch but not as much as the original 22nF value.

Secondary to that question is a more basic question - My understanding of caps is that they let AC flow, but block DC, so I'm wondering how this cap isn't seen as a dead short across the power switch during normal operation in the first place.  I'm sure it's my bad/limited understanding of caps, and that it comes down to the value of the cap not being able to pass 60hz, but I'm wondering if someone could tell me just for my own comfort zone, what the frequency of AC this cap would pass vs. block?

Capacitors pass all AC frequencies but by varying amounts so there is no sharp cut-off AC frequency below which no current is passed. The AC current passing through the capacitor is proportional to both the frequency and the capacitance value. For any given capacitor it will pass 60x as much current at 60Hz than it does at 1Hz. For DC, which is equivalent to 0Hz, no current is passed except for an initial charging spike (but that is another story altogether).

In the schematic, I have included the European version because that's what shows the entirety of the DC side of the supply.  The top of the US one is shown below, and is identical for the parts in question, and just has a different transformer for the different input voltage..

While you have your unit open it would pay to closely inspect the electrolytic capacitors (1000u/35V and 4700u/16V) in the power supply. If these show any signs of bulging or leaking then they should be replaced immediately, or you could just chose to replace them as a precaution against their inevitable future failure.

[172pilot]

Thanks to all of you who replied..   This is great info..  I'm obviously not very experienced in analog electronics, and would be much more comfortable troubleshooting some old TTL digital circuit!   Analog electronics, plus wall current makes it even more 'exciting' so getting some info was definitely great!

I was looking on amazon, but I think I'm going to take the link given to Mouser, and not be cheap and be worried if I'm installing another cheap cap!  I will also do the other caps while I'm at it.  I'm more comfortable on the other side of that transformer, and understand the DC side capacitors much better!

-Steve

[abdulbadii]

just get on market hi decent quality ones:

Nichicon.
Rubycon.
Nippon/United Chemi-Con
Sanyo/Suncon.
Functional Polymer Capacitor
Panasonic.
Hitachi

for audio signal, polypropylene is the same qualities DF (dissipation factor ) and dielectric absorption (DA) as polystyrene