[sdiy] Symetrix A-220 troubleshooting

[Trevor Page]

Tavys,

I'm no expert on power supply repair but I occasionally repair them. Someone
will be able to offer much better advice than me, but here's how I'd tend to
go about it.

Assuming this is a tranistorised amplifier (I could be wrong!) I would
personally check the final amplifier stages first, and then onto the power
supply board. It's good to make the assumption that if the power supply has
blown, a fault within one of it's loads has caused it.

Since the power amplifier devices dissipate a lot of power they tend to go
first and then take the power supply with them. If they're discrete
transistors (usually mounted to a large heatsink) disconnect and check them.
If one of these transistors dies and one of it's junctions appears as a
short (e.g. between collector and emitter terminals) the device can
effectively present a short circuit to it's collector supply, damaging the
PSU (or simply blowing a protection fuse). Alternatively it can just sit
there drawing excessive current from the PSU, possibly causing a ripple upon
the DC rail, and maybe the hum that you hear?

Checking the power supply components is straightforward. You've probably got
at least one bridge rectifier in there, or four discrete rectifiers - test
these. Capacitors do fail, but such a severe fault would probably due to
some other cause. No, there will be a more servere cause. Though it's never
bad practice to fit replacement capacitors to complete the repair.

Trev



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----- Original Message -----
From: Tavys Ashcroft <bigtex at cwnet.com>
To: Synth-DIY list <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, July 21, 2001 9:36 PM
Subject: [sdiy] Symetrix A-220 troubleshooting


> Not exactly a synth...but I'd use it to listen to one.
>
> I just got a Symetrix A-220 stereo amplifier dropped on me (ow) and
> it looks nice and clean inside and out.  Unfortunately, there's a not
> so nice hum and no audio.  It's the same volume (loud) no matter
> where I turn the volume knobs.  I'm assuming this is the power supply
> capacitors?  It doesn't look to have any sort of voltage regulator in
> the power supply, just big fat 6800uF capacitors and a bridge
> rectifier.  I haven't tried leaving it on long enough to crank the
> input to it to see if I can get any audio out at all.  The hum is
> really loud and I live in an apartment.  I'm hoping that it's just an
> ungodly level of ripple on the DC going to the amplifier and that's
> all.  Any thoughts?
>
> -Tavys
>