replacements for obsolete electrolytic caps

Hey guys, I'm going to re-cap one of my old tape-decks and ran into a problem. The record and playback amp cards are full of 6.4uf 25v lytics that are quite bloated and need replacing. Searched everywhere and cannot find this value. I did, however, find some 6.5uf 20v solid tantalum caps. In audio circuits, would this be a good sub-in for those old electrolytics??? I've read they are better but I've never experimented with changing composition.

Similar thread - at the sluts' place:
http://www.gearslutz.com/board/geekslutz-forum/725999-where-68uf-caps.html

Jakob E.

6.5V seems close enough to 6.4 for government work...  The breakdown voltage depends on the power supply and actual max voltage the cap will see across it. What is the circuit's PS voltage?

JR

http://www.memotronics.com/product?pid=554 That will do the trick. Don't know the actual voltage but I'm quite sure it will probably never see 25v.

"both phase response and distortion at audible frequencies tends to improve with larger value DC-blocking electrolytic capacitors."  As Jakob pointed out. Maybe I should go with a higher value AND voltage as well?

What I was wondering was if anyone here has tried out solid tants in place of old lytics and had experienced an audible difference?

Or any other composition, for that matter.

Dave.

6.8uf is a far more common value, you will find many more choices.

http://ca.mouser.com/Passive-Components/Capacitors/Aluminum-Electrolytic-Capacitors/_/N-75hqt?P=1z0wqsjZ1z0vhw8Z1z0sq5yZ1z0z819Z1z0z7l5&Keyword=6.8uf&FS=True

Regards,
Mark

Cool Mark, thanks.

I actually have an BOM, for another project, being put together for Mouser, browsed again and found some axial-lead 50v 10%s I'm throwing in. http://ca.mouser.com/ProductDetail/Xicon/140-NPAL50V68-RC/?qs=zemNgm%252bvN97tl%2fC%2fDkxSRlesHLaQo6MLkaggSCnTyBE%3d

Dave.

analogstud said:

I actually have an BOM, for another project, being put together for Mouser, browsed again and found some axial-lead 50v 10%s I'm throwing in. http://ca.mouser.com/ProductDetail/Xicon/140-NPAL50V68-RC/?qs=zemNgm%252bvN97tl%2fC%2fDkxSRlesHLaQo6MLkaggSCnTyBE%3d

Click to expand...

Hey Dave,

Those are bi-polar caps, generally used in audio path applications where the polarity is undetermined.

Are you sure you don't need polarized caps? Do the old caps have a polarity marked?

Regards,
Mark

Yes, the old ones were. Do I NEED polarity sensitive caps ONLY? These new ones are directional nevertheless and was planing on adhering to the orientation in the circuit. Looks like most are just coupling. Wish I had a schematic for this old deck, or at least be able to trace the leads from the rear of the termination. Maybe I should look around some more? Just to be safe?

Dave.

I'm fairly certain that bi-polar caps will work for coupling purposes where a polarized cap was before. Some even claim they are an upgrade over polarized caps. I haven't determined if that is the case.

I would stick with what was in there if possible.

Regards,
Mark

analogstud said:

What I was wondering was if anyone here has tried out solid tants in place of old lytics and had experienced an audible difference?

Click to expand...

Electrolytics are sometimes used in places where they may see a small reverse voltage for an instant.  eg the decoupling cap for the feedback resistor in power amps.

A tant in that position will die INSTANTLY giving large DC offsets and other cool effects.  Tants usually OK for Power supply decoupling.  Not sure about reliability for that.

Thanks Guys.

I'm not messing with that crap in an unknown circuit. I just went ahead and ordered polarized lytics at the nearest value and voltage. Probably should, however, take pen and paper, tear appart the chassis, and make a schematic if I'm gonna put the deck in service :-\

Dave.

Hey guys, I've replaced the 6.4uf 25v's with the 6.8uf 50v's and I'm now getting an overdriven signal when feeding audio into the line-level and mic inputs as well. These polar caps are around a block of 2N3391 transistors http://pdf1.alldatasheet.com/datasheet-pdf/view/50004/FAIRCHILD/2N3391.html According to the data sheet, the Vceo and Vcbo are 25v. Might the problem be that I've put in 50v caps rather than the original 25v caps around these? I've managed to find someone on the net that actually had a schematic of my machine and took a low-quality snapshot for me. Here is the pic. The caps are the polarized ones in the "record amplifier" section.

No, the higher voltage rating will have no significant affect on operation at less voltage.

I would suggest troubleshooting that simple circuit with DC voltmeter, to confirm that the DC biases and operating points for every stage are reasonable.

One backwards or mislabeled cap could throw off DC operating points.

JR

The 50V caps will have no affect on the circuit, wrong polarity will.

Recheck your work.

You will most likely have to realign the machine, this includes your line level receivers, etc.

Regards,
Mark

hmmmmmm......polarities are good. Didn't think the voltage would make a difference as that has to do with the cap, itself but that's the only value that has changed since the previous, un-distorted audio state (however, the bass-response seems a bit tighter). I'm just working with the amp, no transport, so alignment is irrelevant. Just putting audio in and monitoring it at the outputs unless there is a ground-connection or something else, that needs to happen in the transport? Doesn't appear to be. I'll fiddle with it when I hook it back up to the transport and run some tape at the studio.

analogstud said:

Hey guys, I've replaced the 6.4uf 25v's with the 6.8uf 50v's and I'm now getting an overdriven signal when feeding audio into the line-level and mic inputs as well. These polar caps are around a block of 2N3391 transistors

Click to expand...

You've almost certainly caused a short, either by solder blob or a cap lead touching an adjacent pad.

As JR says, you need to take DC voltage readings to verify where the bias is being thrown off.

Nope. No shorts from my work.

Well what the heck! Hooked it up again, and now it sounds fine? Maybe the caps had to charge up and sit for a few days? strange......

Sometimes caps need to be formed in, or slowly charged up, but this is generally an issue with large and very old caps that have been sitting unused for a long time.

If it's OK now, it's OK now...

JR

Thanks for the help guys. Couldn't have been anything else. Very warm sounding amplifier. Lots of harmonic-distortion. Smooth, musical top-end. Sounds really good with Rock-n-Roll. Hitting the tape could only improve on those characteristics (I hope).

Dave.