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Old 24th September 2004, 12:14 AM   #1
Illusus is offline Illusus  Canada
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Default Usher pre-amp woes...

I recently bought a used Usher p-202 preamp. I put it in my computer room to test it out before trying it out in my listening set up. It sounded great and I listened for a while just enjoying it. I then turned it off and arrrrgh! My speaker cones pushed out to their limits and then slowly retreated.
With the pre on, I measured the offset at the outputs and it was ~4mV, but when I turned the switch to the off position it climbed rapidly to 5V!! then slowly dropped to 0.
What is causing this?!

BTW, I can not find any info on this pre anywhere, anyone know anything about it offhand? I still haven't contacted Usher directly.
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Old 24th September 2004, 12:24 AM   #2
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Oh, here's a pic of the guts- Usher P-202 Super Operational Stereo Pre-Amplifier.
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File Type: jpg usherp-202.jpg (82.8 KB, 235 views)
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Old 24th September 2004, 12:38 AM   #3
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I think you can be sure that it's caused by capacitors discharging. This type of effect can be reduced, if not eliminated, by the judicious choice of time-constants.

The severity of the effect indicates that something is wrong. Maybe a faulty cap on one half of the psu regulators causing asymmetrical collapse of the supply volts, assuming a split-rail supply. Who can tell with no information.

If you get no information, do some checks to see if you can find the part of the circuit that causes it. Power up and down while monitoring it. Is it power supply, input stage, output stage etc.

Hope this helps

edit;- just seen the pic. Those modules don't look fixable to me, hope it aint one of them.
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Old 24th September 2004, 03:28 AM   #4
Illusus is offline Illusus  Canada
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Yeah, I recognized that pattern to be a discharging capacitor, just how that voltage was getting from it to the outputs(both, left and right) bothered me. I also hope it's not one of those funky fet modules...I've never seen any like that before (my experience with electronics is not extensive). I'll keep poking around, I'll post what I find.
One more thing- this thing is full of these little pink resistors with a single black band- what are they? I've never seen the like of these either.
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Old 24th September 2004, 03:34 AM   #5
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by Illusus
...little pink resistors with a single black band- what are they?..
I would expect them to be diodes.
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Old 24th September 2004, 03:46 AM   #6
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the black band is right in the middle of the part therefore not indicative of anode/cathode orientation. I don't think it's a diode.
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Old 24th September 2004, 03:47 AM   #7
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This " Super ........." probably have not output signal relay, what is in this case " supermistake " - you must get it there or never switch off .
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Old 24th September 2004, 04:09 AM   #8
Mr Evil is offline Mr Evil  United Kingdom
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They could be jumpers, or ferrite beads.
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Old 24th September 2004, 04:20 AM   #9
Illusus is offline Illusus  Canada
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Well it's not the power supply, I measured at the PS -/0/+ where the output fet modules are fed from and the voltages are rock solid at power up and both neg and pos voltages drop evenly at power down.

Upupa Epops - there is no recognizeable relay anywhere in this thing, so you may be right. But how can a product reach mrketing if it is potentially dangerous to connected equipment? would the designer not see this a problem?
If this is the problem, how do I retrofit it with an output signal relay?

Mr.Evil- I think they're jumpers as well.
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Old 24th September 2004, 04:20 AM   #10
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a resistor with a single black band is a "zero ohm" resistor.
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