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Old 16th August 2013, 02:00 PM   #11
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It seems your grasping. Changing random parts won't do you any good. Do you own, or do you know anybody with an osciloscope?? It can tell you where the noise originates from. Maybe it's a massive PSU ripple, maybe a faulty component. I'm still betting on a bad ground connection/bad ground design/ground loop type issue.
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Old 16th August 2013, 02:16 PM   #12
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I don't own an oscilloscope but could see myself buying a used one. Would you start at the transformer and move up the chain? I'm afraid this amp is dead because I removed a second small push-in IC in case that was the source of the noise. Wasn't. I lost it. But this made in China amp deserved to be put out of its misery. I see no bad soldering joints. It's annoying because the board is so small, very few components. Could be that the lost IC surfaces if i look for it.
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Old 16th August 2013, 02:35 PM   #13
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Hi,
Check the DC voltage using the voltmeter in AC. It should read close to zero. Any voltage means that you need to replace the filter capacitors.
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Old 16th August 2013, 02:46 PM   #14
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Quote:
Originally Posted by strawberry View Post
But this made in China amp deserved to be put out of its misery.
hahaha

It depends where I would start measuring, but a clean DC supply for the PSU is easy to check. Just probe it at e.g. the supply pins of the opamps and you're done. Next, I would check where in the signal path the noise would become apparant. etc. etc.
I wouldn't want to miss my scope. Vital piece when you'er serious about working on electronics. Second hand, there are some real gems at a very reasonable price.
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Old 16th August 2013, 05:32 PM   #15
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LM1875 amplifier, heavy noise at higher input voltage levels

Perhaps there is a substitute.

Last edited by strawberry; 16th August 2013 at 05:37 PM.
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Old 16th August 2013, 05:47 PM   #16
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Quote:
Originally Posted by funk1980 View Post
Second hand, there are some real gems at a very reasonable price.
You are'nt kidding!
Just looked for fun...
The Tektronix I bought 30 some years ago when I started that cost me $900 can be had for $250.

The one I wanted, but was $2,000 (2465DVS) is on a BIN for $350 for ome that was calibrated less than a year ago from an avionics firm.

It would be great if you could borrow one to have a peep at your power, and then work your way into amd through the amp.

The fact that you say everything quiets down when you plug a guitar in and either touch the strings or turn down the guitar does sound like something poorly grounded at the input stage.

Which could be a simple design flaw.

I think a very expensive cable coule be the solution to all your problems, but only if you buy it from me!

We make a special one with two conductors and molded ends...

Or is that moldy ends?

Last edited by headstack; 16th August 2013 at 06:01 PM.
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Old 24th August 2013, 06:28 PM   #17
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If it goes away when you touch the strings, your instrument may not be properly grounded inside.
Check the continuity (ohms) between the ground of the 1/4" jack (the outer edge) and the bridge. Zero means it's grounded. No response means it's not. If it's not, it needs to be.

Also, move closer and away from the computer monitor or TV. If it changes, you may need shielding as well.
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Old 24th August 2013, 06:41 PM   #18
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Quote:
Originally Posted by drmcclainphd View Post
... and the bridge.
a few days ago i rewired a bass guitar, and forgot to connect the ground wire coming from the bridge ... now I know why its there

but I still always make sure I touch the strings, and turn down volume att before letting go of instrument
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Old 24th August 2013, 11:15 PM   #19
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'Musician Safety | No~Shock~Zone'

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Old 25th August 2013, 02:41 AM   #20
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a few days ago i rewired a bass guitar, and forgot to connect the ground wire coming from the bridge ... now I know why its there
Never leave a previously connected wire unconnected. It increases the probability that the smoke will leak out. Then you'll need to get a smoke technician to seal the leak and refill the system with smoke. Oh, you'll rarely see smoke coming from a guitar. They run at a very low level (ie. very little smoke) and it's mostly spread through the gobbazillion winds of those teentsy wires wound up in the pickups. It could take a long time for it to leak out, so you'd never notice. Ever seen someone replacing transistors and such in failed circuits? New smoke. In the old days we used vacuum tubes to keep a vacuum on the system, keeping the smoke in. If one failed, the smoke might escape, so we used several tubes in most designs. But we also used things like capacitors filled with paraffin so we could occasionally enjoy a good smoke-letting, and to keep the users scared, and in awe of our spectacular courage for sticking our hands in places where smoke recently came out. Now, we didn't really have to crouch down on the floor behind the old DuMont console to do this, but when we saw that it all started with their kid dropping pennies through the vent slots like it was a big piggy bank, we had to hide our red faces lest we laugh out loud. And THEN when we saw that apparently mom already knew what happened, and tried to get the pennies out, but in so doing saw all the dust whiskers growing on all the hi-V components, well she was CERtainly not going to let ANYone think she was a poor housekeeper, and so she took a vacuum cleaner to the insides. Which explains why several of the tubes are broken off at the base. So rather than laugh at her right out loud we had to hide behind the TV and take a hit off of our smoke refill canister to calm down. Which is WRONG, just WRONG, you should never take a hit off your smoke canister. So we'd take two, to even things out.

Nowdays everything is digital. It's all ones and zeros. If a leak happens, the ones leak out (because zeros are, after all, nothing, right?). Then you have to get a ones specialist to re-insert the ones in the right places. That takes an awful lot of specialized knowledge. You have to be able to count up to one, over and over. And sometimes it takes kids with weird eyes standing in a corn field saying " L i n u x i s e v i l . R e i n s t a l l W i n d o w s ." over and over, But I digress, which is the opposite of congress, or maybe not. And congress is part of politics, which is derived from poli (poly, meaning many) and tics (ticks, blood sucking parasites). How they got into my smoke refills, I have no idea. I hope they didn't get into my pudding. Nurse? NURSE! Where's my pudding? The ticks stole my pudding again! Wait, I think I found it. They hid it in my Depends. Oh ... that's not pudding.

-- Smoke technician since 1962. Some of these stories are true. The names were changed to protect the innocents from me, or the other way around. Wait. What's my name? Oh, here it comes, in digital: onezerozeroonezerooneonezeroone... crud, I lost count. Can we start again? Hey, did you ever notice that ones have a pointy part on one end, and a small perpendicular crossbar on the other? Kind if like a diode symbol. I think maybe ones have polarity. Someone call Art Bell.
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