big caps

>>>"as this project is an amp, nothing is critical"

The (m) probably meant that they were a "matched pair" of caps for that stage.  The values of the caps determine filtering characteristics, so 6800uf vs 7500uf would normally mean a higher cutoff frequency.

The 71V rating is directly proportional to the internal electrolyte thickness and breakdown voltage.  Normally you can put a higher voltage rated cap as a replacement (if it fits), but not a lower one.

Good luck,

Scott

Thanks Scott!

If I can't find a 71v (or higher) rated cap at 7500uf, do you have an opinion on if I swap out with something I can find like 8000uf or 6900uf?

/R

And this just came to me, could I add (71v+) capacitors together to get the 7500uf value?

/R

It's hard to say without looking at a schematic.  If those caps are part of a notch filter,  you may get some 50/60hz noise if you change the value too much.

71V is kind of an odd value, 6800uf 100V caps are common and inexpensive, try a couple and see if it's a problem.

Scott

Where are they in the circuit.

At 71v and 7800uF, I'm picking they are across the suppy rails.

Since capacitors have decreased in size over the years, you'll probably find some even large at 100v will fit in the same physical space.

It's unlikely the supply is an inverter, so you should be fine.

mark

I agree with Mark's suggestions. This website confirms that these are across the supply rails, and it looks like a linear supply., it shows that the 7500uF capacitors are used for the supply. You could use the same value or slightly higher but within that ballpark (e.g.  8200uF or 10,000uF) since thay have 20% accuracy or worse often, as long as the voltage rating is the same or slightly greater, i.e.100V or so (anything higher, for the same size, you risk that the ripple rating of the capacitor may be too low - which would be dangerous, popping electrolytics vent a lot of smoke and unhealthy material). This one may be a suitable candidate (and the cost looks reasonable for this type of capacitor) if the size is close to the existing one on your board.

Thanks Mark, Thanks Shabaz,

shabaz, that cap is UK ($20 to deliver it/them).

From what I see here http://www.aikenamps.com/AddingComponents.htm It seems that I could add capacitors in parallel, with the lowest voltage rating equalling voltage rating for that part of the circuit.

I would assume that the lowest lifetime @ thermal rating also applies to the group, true?

Do you-all think that 85c is a good rating for an (old) poweramp?  I have been erring on the sode of caution looking for 105c, figuring that they would last longer if they didn't need to be rated that high.

Upon colese inspection it seems that just the short caps (6800uf @24v) seem swolen (and not as bad as I have seen) so since these are much easier to find

(http://www.amazon.com/Electrolytic-Capacitor-6800mfd-Temperature-degrees/dp/B009KUSCGK/ref=sr_1_fkmr1_3?ie=UTF8&qid=1378055104&sr=8-3-fkmr1&keywords=6800uf+24v)

I think that I will try to replace just these first.

Also: since the regular (non-power amp?) componet seems to be fine is there a more modern (and cheap, power efficient) solution to drive (as many as 8) old speakers, I use my audio system to play audio (podcasts mostly) throught the house and in my shop, I don't care too much about quallity.

Additionally, my web brower just crashed and this reply box had a recover option, awesome!  I tried several times to fix the word 'modern' because it looked too much like 'modem', funny.

Thanks all for your help!

/Richard

Hi Richard,

Sorry, this one will be better value from the US if you need to replace it.

The website mentioned earlier states that the 6800uF devices are 42v (not 24v) so it's worth double-checking, just in case. If the supply part of the entire amplifier is faulty then yes, it may be worth just replacing the swollen ones first (and keep away when powering up, because they can burst and have the contents exiting with a lot of force).

You're right that they don't last as long as desired, the life at 80degC or 105degC may just be a few thousand hours, but they will last longer at a lower temperature, and even 10 years powered up is not unusual (maybe a couple of decades is possible).

A lower power option would entail a different type of power supply and possibly a different amplifier design. It is not like-for-like, but these are quite cheap and are ok for powering two speakers with a small 5v or 6v DC supply of about 500mA or more. The quality is average, and power consumption is low.

They have a high input impedance so in theory you could connect several of them driven from the same input. It probably wouldn't sound anywhere near as good as what you have though. A single one may be worth testing as an option.

Richard/Shabaz

The temperature is usually higher for tighter packed devices, and I doubt that your power supply ever gets near the 85deg.

The higher temp capacitors generally have a lower ESR (better high frequency filtering), with the hours ranging from 1000 to 5000 hours, and a corresponding change in price.

I have replaced many caps in monitors and surprisingly they are only 1-2000 hours at their maximum temperature.

Newer caps need the vent, but in the old days they used to go off like a bomb with bits if internals and the corresonding liquid damaging many other bits.

Your voltage and time assumptions are correct, however the ability to pass ripple current (the bit they are filtering) will need to be considered.

If one is doing the bulk of the work, its likely to be exceeded or give up earlier, so where possible stick with a single capacitor option.

Swollen caps are often one symptom, along with gunk on the top by the vent, but generally they dry out and their capacitance drops off, and therefore they don't work.

You may wish to check the link in the earlier post. It seems those supplies gave problems, and this is a suitable replacement.

The element14 part numbers usually are the same for each branch/country, so often you can get it elsewhere.

Mark

Shabaz,

Wow, lysdexia got the better of me again, thanks it is 42v not 24v!

Nice save!