Changing Capacitors on a Goodmans 40+40

mc_bob · 2 Jul 2006 at 22:33

Hi,
Today I dusted this Amp off and wired it up to my computer.
Sounds great!
Produces all the highs and lows.

However.
The balance needs to be turned slightly more to the right speaker since less power is being put through it so I opened up the Amp.

The two big capacitors have small holes in the bottom and some beige coloured substances coming out of them.
Also, turning the volume dial past 5 it begins to output dodgy signals and it distorts. It's definately not the speakers.

I want to replace these to see if it fixes the problem.

They are rated at 4000uF and 63v.
I can source 15000uF 63v caps at a reasonable price.

They will produce more power but I also realise turning the amp up very high may burn out other components.

Please post your thoughts on this.

Is it a

or a

?

MC_Bob

mc_bob · 3 Jul 2006 at 19:23

Anyone ?

MC_Bob

RainyDay · 4 Jul 2006 at 01:02

Hi,

I'm not familiar with this amplifier but in general:

These large electrolytic capacitors are either part of the power supply
or are DC blocking capacitors in the output of single rail power amps.

If these are power supply caps, fitting a larger value will often improve
perfomance. However, going from 4000mfd to 15000mfd will put a little
extra strain on the rectifier bridge at start up. At this time the caps will
(at first) appear as a short circuit, then begin to charge. With larger caps
the period that they appear as a short (or very low resistance) will be
increased. During this time the bridge will be stressed. If the diodes are
reasonably (over) rated they should be fine but you may have to increase
the inline fuse values slightly. Only do this if you get a lot of blown fuses
after the upgrade.
The above is more likely to happen if you use caps with a very low ESR.
To be sure either replace the diode bridge(s) with a larger rated part or try
to keep the replacement caps in the range 4700mfd-6800mfd.

Less likey, as this is a hifi amp:

If the caps are for DC blocking of the output a 15000mfd part will be too
large. The output transistors will probably fry at startup. It's true that
larger cap values will increase the low frequency response but going too far
out of the original design spec will likely overload the power transistors.
Again try to replace these with 4700mfd-6800mfd parts.

If you really can't get a lower value than 15000mfd, wire two in series for
each cap. This will give you a more reasonable value of 7500mfd. If you
do this also fit a 470K Ohm resistor across each of the (four) caps. This
will ensure that the voltage is evenly distributed across the caps.

Hope this helps

.....RainyDay

mc_bob · 4 Jul 2006 at 07:20

Ah, thanks a lot!!!

Really helpful.

I managed to find a place where I can get a 4700 uF for £1.75 inc postage!

So I'll see how this goes

Thanks a lot for your reply, cleared a lot up.

MC_Bob

Dr.EM · 4 Jul 2006 at 13:24

Thats good, make sure it is at least 63v like the originals though; getting capacitors with a voltage rating under the originals could be disasterous. 100v etc is fine, just not 50v or any under 63v.

mc_bob · 4 Jul 2006 at 17:18

Yeah they are 63v

Do you think this will cure the problems I am having?

MC_Bob

RainyDay · 6 Jul 2006 at 15:32

My pleasure

4700mfd caps are fine as a replacement.
The other poster is correct, you must keep
the voltage rating at 63V or higher. I should
have mentioned that.

From your description of the leaky caps and
the distortion you were getting, I suspect that
these will be the only components you will need
to replace for a fix.

Make sure you clear up all the "goo" that leaked
from the electrolytics. This is corrosive and
won't do you any good at all if ingested by
mistake.

Also make absolutely sure you've connected the
new caps correctly. These caps are polorised and
"will" explode if connected incorrectly.
Usually the "negative" lead is clearly marked.
Often (on larger caps) the positive terminal is marked
with red ink.

Finally, try to get capacitors with the highest "Ripple
Current" specification. Not all 4700mfd caps are made
equal

The ripple current will be listed in the spec
sheet. For a 40Watt amp (per side) about 1.5-2.5Amps
minimum would seem about right.

Hope this cures your problems, post your results.

.....RainyDay