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Sherwood AM 7040

Dire

Apr 30, 2018
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I've got that amp and there is a buzz coming out of the speakers when a CD player is connected to the amp, and the buzz gets louder when the volume increases. When there is no input connected the buzz disappears. Any ideas what I can try to do to get rid of the noise coming through the speakers?
Thanks
 
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kellys_eye

Jun 25, 2010
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Any ideas what I can try to do to get rid of the noise coming through the speakers?
Use a different signal source!

The 'buzz' is some form of interference from a poorly designed power supply associated with the CD player. Is the CD player powered from an AC source? If it can be powered from batteries then the 'buzz' may well disappear.

Check for good earth connections between the two devices too.
 

BobK

Jan 5, 2010
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Could be a ground loop. Are the amp and CD player plugged into different outlets?

Bob
 

Dire

Apr 30, 2018
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Upon further investigation I've discovered someone has removed a switch that was used to choose between "variable/fixed" input. Only the "variable" input is functioning and the part of the PCB where the switch used to be has solder points that only enables that one input. From the pics is it possible to know if the mod has been done correctly and is it possible the mod is the cause of the buzz, and is it possible to find a replacement switch? Thanks
 

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73's de Edd

Aug 21, 2015
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Sir dire . . .(straits ? )

The solder blob "switch contacts have been done correctly, and is thereby making the unit a variable input function but with it coincidingly, having a now dead set of fixed input connectors.

Your hummmmmmmmmmmmmmmmmm "problem" should be with a ground loop or two different AC line supply points or hum from CD player AC power supply, the already mentioned . . . . . battery only supply . . . if possible .. . .for the CD player should isolate it in a hurry.

TECHNO REF . . . .

upload_2018-5-1_5-8-9.png


73's de Edd
.....
 

Dire

Apr 30, 2018
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Maybe I was thinking about Dire Straits when I came up with "dire" or a combination of a dire situation and the group? Hm, not sure what was going through my noodle at the time.

Thanks for your replies. I have checked with a CD walkman and a tape player; no problem like when an AC-powered CDP is connected. Any ideas what next I can check, a most likely culprit?
 

73's de Edd

Aug 21, 2015
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Sir dire . . .

That then points to either a power induced hum or a ground loop.
I'm assuming the the CDP has a conventional AC line cord rather than a separate plug in line power wart .
In that case you would have two AC line cords involved one on the CDP and another on the MAIN amp.
Now I remember VERY VERY VERY few of those devices having a full blown, 3 prong AC line plug.
Specifically, they would tend to have two prong plugs , and if any attention to polarization of was involved,
that would be fulfilled by one blade being wider, for the ground oriented connection.
Somehow . . . . I want to think that both of your plugs will having twin smaller blades, so that they potentially
could be plugged in at either 180 degree reversed positions .
Confirm if that is being true and then, if so:
Plug in both units and evaluate for the "hummmm" presence and then reverse the plug 180 on the CDP and evaluate.
Then reverse the plug 180 on the MAIN amp system and then listen to the sound and evaluate and then
. . . . . .finally reverse the plug 180 on the CDP and evaluate.
See if one or two possibilities seriously abate or eliminate the problem.



73's de Edd
.....





.
 

Dire

Apr 30, 2018
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The amp has a 3 prong AC cable and the CD player has a 2 prong cable with left side being wider than the right. The tape player has a 2 prong cable with identical widths.

The other problem (maybe it's just usual with some amps) there is a thump through the right speaker when the amp is first turned on, after the relay has done its job and a static type of noise when choosing between speaker A and speaker B.
Any thoughts?
 

BobK

Jan 5, 2010
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Get a ground loop isolator to plug between the CD player and the amp.

Something like this

Bob
 

73's de Edd

Aug 21, 2015
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Sir dire . . . . .

That was being my final option, to be suggested, if the earlier "freebie" options didn't pan out.

Here is what is being inside of a unit . . . . .
http://freemansgarage.com/blog/?p=760

I do know that THIS guy, moi, has some old modems dating back to the days of 150 / 300 baud speeds, and that was when you actually watched the separate individual lines of a page actually being created . . . and . appearing.
One specific, plug in board I have, actually has (8) 600 Ω transformers mounted on it.

PLUS
The other effects that you mentioned might be attributable to the

Thump . . . . .from being a slight DC presence on the amp in the speaker output line of that channel, yet, not bad enought to activate the units shut down protective circuit

Switching noise . . . . . . Could be from that same aspect , of having some DC presence on one channel, transitioning to the other channel or just dirty switch contacts, if it hasn't been moved frequently, lately.

Testing :
Disconnect the speakers and have the volume at ZERO and power up and test across the vacated speaker output terminals of the amp with a DVM to see if any DC voltage is being read out on either channel.
IF so, slightly load one channel at a time by placing an ~ 100 ohm resistor across its output while re measureig the voltage across it.

When I look at the design of this units power output stages, I am seeing 4 fistfulls of power FETS (16X) . . .2SJ162 and 2SK1058's BUT with them them not having a very sophisticated protective sampling circuitry being incorporated..

73's de Edd
.....
 

Dire

Apr 30, 2018
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Thank you everyone for your replies. I have tried 2 different pairs of interconnect but that didn't fix the problem. Haven't had time to try the other suggestions, yet. Will post if I can figure out how to fix the problems.
 
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