Sansui AU-919 Polystyrene Capacitor

[tinkerer]

I am rebuilding the F-2839 Amp Board on a Sansui AU-919. There is a 33pF Polystyrene Capacitor that the Sansui glue has eaten a leg off. First what is the advantage of a Polystyrene Capacitor on an Amp Board and what can I replace it with?

 

[JyriK]

Is it an infamous Black Flag cap?
In any case, a c0g ceramic cap would be a good choice, if polystyrenes are hard to come by.

 

[tinkerer]

I'm not familiar with the Black Flag Cap, and I don't see anything indicating a black flag on the markings.

I had thought the WIMA Film would be a good sub but I can't find it in a Non-Polar version.

Very much appreciate the feedback JyriK. Thanks

 

[painttoad]

I'm not familiar with the Black Flag Cap

Black Flag Caps

Black Flag Caps

conradhoffman.com

WIMA Film would be a good sub but I can't find it in a Non-Polar version

wima film caps are non polar.

go with a c0g ceramic cap as suggested

 

[Hyperion]

Just for information Polystyrene is the best capacitor dielectric you can get audio-wise, this is what Black Flags are, unfortunately especially after ~40 years they have become unreliable. The only type capacitor that is freely available today, and comes close to being a substitute in this application is an 'NPO' or 'C0G' type ceramic capacitor (NB: not an 'ordinary' ceramic capacitor).

 

[tinkerer]

Thank you so much for that explanation. So, on these vintage amps and receivers would you recommend replacing all the Polystyrene caps since they are probably out of spec?

 

[tinkerer]

I have read and discovered what the Black Flag Caps are from the link painttoad sent. And even though that's more work I do appreciate learning and avoiding blowing up a nice amp after lots of work and expense. I was wondering what those black suckers were since I've never run across them before.!!!

 

[Hyperion]

Thank you so much for that explanation. So, on these vintage amps and receivers would you recommend replacing all the Polystyrene caps since they are probably out of spec?

No, only replace the 'Black Flags' unless any of the other polystyrene capacitors are physically damaged or faulty, they will need to be replaced - treat the polystyrene capacitors like the 'Family Silver' - they are a small part of what makes the AU-919 special.

Also note that polystyrene capacitors are sensitive to the heat from soldering activities - so no blanket solder re-flows in pursuit of a fault, or prolonged application of heat on joints where polystyrenes are involved.

I was wondering what those black suckers were since I've never run across them before.!!!

We don't know for sure but we think they were specially made for Sansui at a time when they were trying to increase the 'speed' (i.e. 'slew rate' coupled with frequency response), of their amplifiers in order to defeat 'TIM' (=Transient Intermodulation Distortion). They managed to design them to be so fast they needed a lot of strategically placed damping capacitance - hence the requirement for the 'Black Flags'

It was Sansui's insatiable appetite for innovation in Audio Amplifiers and trying something new that got me liking Sansui around 48 years ago, 'Diamond Differential' 'Ground Free' 'SFF' (=Super Feed Forward), 'X-Balanced' and so on... it didn't seem at the time that other manufacturers were doing anything comparable.