Soundcraftsmen Amp Negative Volt Reading DC-Offset

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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
 
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!
 
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 :( )
 
Rob,my account is STILL under moderation so instead of e-mailing you directly I have to reply thru this post.My e-mail address is (address removed by mod) added a Rotel rdc-950 cd player,Pioneer burner to the system as well as upgading the cables and wires.Picked up Jimi Hendrix Crash Landing and Midnight Lightning...now am chomping at the bit to work on the amp so you're right on time!How did things go with your amp?
 
Wingzam, you've got mail.

My A5002 is on hold while I venture into Leach amps (Jens Rasmussen clone)

Last effort was to diagnose a buzz that permeates throughout the amp, including signal and chassis grounds :confused: Being from the 80's, I'm sure that the large electrolytics need to be replaced. So... if you find appropriate size/rated PS Caps, let me know.

I don't have a PCB layout either :(
 
wingzam said:
Rob,my account is STILL under moderation so instead of e-mailing you directly I have to reply thru this post.My e-mail address is (address removed by mod) added a Rotel rdc-950 cd player,Pioneer burner to the system as well as upgading the cables and wires.Picked up Jimi Hendrix Crash Landing and Midnight Lightning...now am chomping at the bit to work on the amp so you're right on time!How did things go with your amp?

I have a Rotel RCD-971 player running into a QSC RMX2450 on the bedroom system. With upgraded wires.
Sure wish I had Crash Landing to play thru it...
 
Ya,if I had more $$ I would venture off to a different amp myself.This electronics learning curve is a real bitch..not to mention gathering all the info to get the job done.Still,Polk Audio forums aside,Soundcraftsmen amps appear to be fairly highly regarded so i'd like to see what I can get out of it on my own.
I got this info from Goat67 at Audio karma whose advice appears to be highly regarded.On the big main power caps - Mouser P/N's:
Original part at schematic location C32 & C35=
30v 22,000uf.Upgrade to 40v 40,000uf P/N 75-36DY403F040BC2A.
Original part at schematic location C33 & C34=60v 11,000uf.Upgrade to 75v 18,000uf P/N 75-36DY183F075BC2A.

Here on diyAudio some time ago a member said he had replaced the 11,000uf caps with Aerovox M series 41,000uf caps and the 22,000uf caps with Aerovox M series 39,000uf caps.You'll notice that the 11's got bigger caps than the 22's(did'nt list voltages).Don't know if this was a mistake in his post.
There are two companies that have schematics for our amps:vintageaudiomanuals.com & musicparts.com.It's a little confusing figuring out which one to order from.Music parts(go to their site) seems to have a more complete package of info which is sent as a PDF file with a page size of 8x11,whereas Vintage audio sends out high quality 11x17 schematics and says "our schematics include a wealth of service info,includes troubleshooting tips,modifications,parts substitution,amplifier testing and voltage conversion procedures."...
So I don't know which set to buy.They're both $20.
Additionally Vintage audio schematics cover all the class H amps (probably including yours)whereas Music parts is specific to the model.
I got your e-mail attachment,but have to crash now so will download it tomorrow.Hopefully we can piece this together and get these things rockin!
 
wingzam said:
I got this info from Goat67 at Audio karma whose advice appears to be highly regarded.On the big main power caps - Mouser P/N's:
Original part at schematic location C32 & C35=
30v 22,000uf.Upgrade to 40v 40,000uf P/N 75-36DY403F040BC2A.
Original part at schematic location C33 & C34=60v 11,000uf.Upgrade to 75v 18,000uf P/N 75-36DY183F075BC2A.
The MA5002 has 62v & 93v rails, I'm sure the Mouser #'s won't work for me.

Here on diyAudio some time ago a member said he had replaced the 11,000uf caps with Aerovox M series 41,000uf caps and the 22,000uf caps with Aerovox M series 39,000uf caps.You'll notice that the 11's got bigger caps than the 22's(did'nt list voltages).Don't know if this was a mistake in his post.
If bigger caps will fit either position, I would shoehorn them in! I'll expect to pay over $100 for new electrolytics from what I've seen.
 
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