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Author Topic: -Low Sound in A1200  (Read 9130 times)

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Offline Castellen

Re: -Low Sound in A1200
« on: November 23, 2009, 04:38:32 AM »
I've written plenty of detail on this subject in this article.  It describes the A4000 analogue audio stage, but the A1200 circuit is mostly the same.  The main difference is the physical layout.
Replacement parts are available from the likes of Farnell, Radio Spares Digi-Key.

If you can't repair it yourself, I operate a comprehensive Amiga repair service from New Zealand.  Contact details on the site mentioned above.
 

Offline Castellen

Re: -Low Sound in A1200
« Reply #1 on: November 23, 2009, 05:20:25 AM »
Quote from: rkauer;530869
Non-polarized capacitors leaks, too!

 It is strongly recommended swap the original polarized units for this other type because the original circuit is an alternate current circuit. Common capacitors will last much more than those originally installed.


@rkauer
Just read your comment regarding capacitor polarisation.  In this application that statement is simply not true.  Bipolar capacitors would only be required if the applied frequency is low enough so that the reactive impedance of the capacitor becomes significantly higher than the impedance of the load you're driving, and the AC amplitude is high enough, therefore causing the capacitor to be momentarily reverse biased to a level high enough to cause damage.

In fact I do recall explaining the same detail in the past, refer to this thread.

If capacitive coupling of AC signals using polarised capacitors was indeed a problem, then pretty much every piece of analogue electronic equipment on the planet would be affected; they all use the same techniques.

Some rare occasions where you do actually require non-polarised capacitors for audio coupling include:
- 2-wire telephone circuits, as the 50V line polarity could be either way
- Speaker passive crossover networks, where the low frequencies, high voltages and low impedance loads concerned are actually an issue with coupling capacitors

Have a read on basic audio amplifier theories for more detailed explanations.

The electrolytic capacitors in question typically begin leaking due to poor long term stability of the electrolyte.  For the same reason, many low quality capacitors of unknown brand are frequently seen leaking in modern electronic designs.  For example, it's a common problem on the DC reservoir capacitors in some modern computer boards, often less than a year or two old.  The parts are correctly polarised in relation to the DC supply, but they leak due to the unstable electrolyte.

That's why product from renowned manufacturers such as Panasonic and Rubycon are generally more expensive; because they've spent a good deal on research to obtain a stable chemical compound and have years of experience to verify it.  Sometimes you do get what you pay for.

So when providing advice *PLEASE* do try to give correct advice and research the facts if you're unsure.

Let me know if you need clarification on anything I've mentioned.
 

Offline Castellen

Re: -Low Sound in A1200
« Reply #2 on: November 24, 2009, 05:54:56 AM »
The above notes about tantalums are indeed correct.
It's also worth pointing out that they will work fine for audio coupling and will be OK in the Amiga if you want to use them.  The problems you might have is the mechanical mounting of them since the PCB pads for SMD electrolytics are different to those used by SMD tantalums.

Tantalums should generally not be used on DC power supply rails for AC decoupling.  The reason is that the low equivalent series resistance of the tantalum capacitor results in a very high momentary inrush current when the power supply is turned on.  The high current flow causes the capacitor to heat up internally, which can result in it going short circuit and sometimes exploding.  Google 'explode' and 'tantalum' to find out more.