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.
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.
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.
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.
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.
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.
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.
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.
to conserve! just kidding, I know this thing doesn't draw enough current to concern me. I like things to work properly before I hack them up. 'Just that I bought it and it doesn't function up to my expectations is a peave.
Update- the problem seems to stem from the tone controls. When powering down with the tone defeat switch off (tone controls active) the offset is much lower, it peaks for a second at 430mV instead of the 5V I was getting with the tone controls disabled.
Update- the problem seems to stem from the tone controls. When powering down with the tone defeat switch off (tone controls active) the offset is much lower, it peaks for a second at 430mV instead of the 5V I was getting with the tone controls disabled.
I wonder if I just bypassed the tone control stage all together if this thing would work properly. I don't like the sound as much with the tone controls active, it sounds noticeably more detailed with the tone controls disabled.
In the tone control section there are a couple ic's (look like op amps) witht the numbers scratched off them , I wonder what they are.
In the tone control section there are a couple ic's (look like op amps) witht the numbers scratched off them , I wonder what they are.
Because it came that way. I bought this unit out of curiosity, and, for the most part, am pleasantly surprised. My other preamps are a Dynaco PAS2 which I tore all control circuits out of and a passive pre.
I'll bypass the tone control circuitry as soon as I get a chance, if the problem at power down remains it'll be easier to track down.
I'll bypass the tone control circuitry as soon as I get a chance, if the problem at power down remains it'll be easier to track down.
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