Soundcraftsmen Amp Negative Volt Reading DC-Offset - diyAudio
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Old 19th November 2008, 10:59 PM   #1
wingzam is offline wingzam  United States
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Default Soundcraftsmen Amp Negative Volt Reading DC-Offset

I have an RA7501 Soundcraftsmen amp that I did a DC offset measurement on.The A channel read -36.0mv and the B channel read -213mv.
I just learned of this test at Audiokarma which gave the procedure as:Speakers disconnected.
Input set to an unused position.
Volume control at minimum.
Balance in center.
Tone controls defeated or mid position.
Take measurement across Pos and Neg speaker
terminals.
0-15mv=Good
16-50mv=Acceptable
50-85mv=Something amiss
100mv-?=definately not good/crippling.
It goes on to describe the fix as changing out the amp input differential pairs.

The high mv reading on B channel is concerning but the negative mv reading has me a bit confused (at least).I don't know what to think.
While I was at it I checked out my Pioneer SA9500II and got great readings that were in the pos. scale.
Thinking that the amp may have been cross wired as a result of a possible previous repair SNAFU I hooked up the speakers bass ackwards,turned it on and the protection circuit kicked in.Hooked it up normally and was rolling again.
Is anyone familiar with this negative DC offset reading situation?Also can anyone sell or send me a set of schematics for this unit?Thanks alot..John
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Old 20th November 2008, 04:14 PM   #2
rob3262 is offline rob3262  United States
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How old is this amp?
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Old 20th November 2008, 04:19 PM   #3
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This is an old amp, late 70's early 80's at best. im sure the caps are toast and could stand to be replaced. and its possible there is an offset adjustment that has drifted with age.

Zc
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Old 20th November 2008, 04:30 PM   #4
rob3262 is offline rob3262  United States
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I have an MA5002 - all the caps needed replacement. As Zero said...
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Old 20th November 2008, 05:00 PM   #5
Bigred is offline Bigred  Canada
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With regards to your confusion on the - (neg) DC offset readings there is no cause for concern. If you think about an audio signal it is AC which swings both + (pos) and - (neg). The offset then can certainly be on either side of 0.
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Old 20th November 2008, 06:46 PM   #6
wingzam is offline wingzam  United States
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Thanks for the replies!This has got to be about a 1980 model.Talking to the guys at Polk Forum they said the caps in this are a bit weak to start with so it does'nt perform well with the SDA-1B's that i'm driving.The Pioneer integrated amp is a great little unit.The DC offset reading is great and it sounds excellent with smaller speakers but the SDA's suck the juice right out of it.Alot of need for maintnance on this Soundcraftsmen so i'll run another post to see what other advice I can get on it.Help much appreciated!
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Old 20th November 2008, 10:38 PM   #7
rob3262 is offline rob3262  United States
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copies of copies of copies of service manual and schematic are available on the net. I paid $25 for documentation but much of it is hard to read.
The manual calls for use of a distortion analyzer while adjusting bias trim R17

I'll see about scanning the schematic

FWIW, the 5002A ROCKS (when it's working right )
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Old 21st November 2008, 01:45 AM   #8
wingzam is offline wingzam  United States
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That would be great!Any info I can get would help.I'm new to actually working on/upgrading stereo equipment but have found the hobby to be an enjoyable pastime.Can anyone give me a source for replacement caps,etc..?
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Old 21st November 2008, 05:01 AM   #9
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Digikey or Mouser would be the best choice for the small electrolytics. The large power supply caps get a lot more complicated, and many suppliers may have to be checked to get ones that will fit and have the proper connection method.
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Old 3rd December 2008, 03:53 PM   #10
rob3262 is offline rob3262  United States
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John, email me your e-address for pdf scan of schematic
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