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Amplifier Distortion, DC-Offset, and You!

Checking and setting the offset with the protection relay open is going to be a big PITA.

Behind the driver board, on the large board that the driver plugs into, are eight white resistors...I believe the four on the right are for the right channel, and the four on the left are for the left. Set the meter up to read DC VOLTS!! Connect the black lead to the chassis somewhere. Connect the red to either side of any of the four resistors on the right. It needs to be a low reading (less than 700mV, or 0.7V). Ideally, it should be less than 15mV. If it looks OK, go to one of the resistors on the left and do the same. If it is way off on one channel or the other, adjust it to a low enough value so that the protection will release by adjusting VR1 and VR2 (the inner two pots of the four at the top of the driver board).

If this was simple, then people would not get paid to do it for you.

RE: Your adjustment...did you put the meter back to read volts after you set it up to read amps? This requires moving the red lead to a different port on the front of the meter.
 
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If this was simple, then people would not get paid to do it for you.

RE: Your adjustment...did you put the meter back to read volts after you set it up to read amps? This requires moving the red lead to a different port on the front of the meter.

You ain't kidding!! And of course i know how to use a meter but I understand your concern
 
Bob, relax dude. You've got one unit that works, right? Consider this one a learning opportunity! You might as well have a little fun with it, take it as far as you can, learn a few things, and then if you really get stuck you can always send or take it in later and then maybe get some feedback on how EW fixed it.

As long as you don't fry yourself in the process :yikes: , it's all good!

Jeez, I wish I'd finally get the time to get to my own "learning opportunites"!
 
Bobby4244 said:
You ain't kidding!! And of course i know how to use a meter but I understand your concern
Click to expand...
A lot of guys don't even know what a meter is, but jump in here and start twiddling with internal pots, and then email me when they blow something up...so I gotta ask the obvious. Plus, you wouldn't have been the first to connect the meter...set to read amps...to the speaker terminals (to anyone who is crystal clear on this last thing, a meter is a dead short when set to read amperage, and it is a bad idea to connect it to the speaker terminals in this state....!!!)
 
EchoWars said:
Plus, you wouldn't have been the first to connect the meter...set to read amps...to the speaker terminals (to anyone who is crystal clear on this last thing, a meter is a dead short when set to read amperage, and it is a bad idea to connect it to the speaker terminals in this state....!!!)
Click to expand...

It is a bad idea because you end up blowing the fuse in the meter for the AMP section, and for an HP meter that 44/100 amp fuse is $11.00 or so.
(Speaking from experience sad to say)
 
My Fluke has a 10A fuse for the high range amps scale. With offset high enough, I could blow an amplifier with that.
 
Thats true but the low amps is a 44/100 amp fuse. I hardly ever use the high amps scale on the meter.
 
I don't understand why i measure a negative voltage on my right channel. Its the same on all clamps and the amp was never opened. on my second amp (connected to the pre amp out) Its switching between positive and negative. its probably playing 25 years that way. Do i have to buy a new amp ? :headscrat
 
Soran said:
I don't understand why i measure a negative voltage on my right channel. Its the same on all clamps and the amp was never opened. on my second amp (connected to the pre amp out) Its switching between positive and negative. its probably playing 25 years that way. Do i have to buy a new amp ? :headscrat
Click to expand...
Only a few posts back....;)
 
but how can i read a negative voltage when i don't use pnp transistors ? all my transistors are new. but ok i'l take it doesn't matter you read a positive or a negative dc noise.
 
You are correct...it doesn't matter. We want a value as close to '0V' as possible, and deviation from that can be in either direction '+' or '-'.
 
Just measured DC offset...

SX-650, L - 50mv, R 34mv
SX-1050, L - 10mv, R 98mv

Can anyone tell me whether either (or both) receivers have trimpots to adjust and if so where / how?

Thanks,
Dave.
 
I'm not sure about the 650, but the 1050 absolutely does. Unfortunately, I don't have a manual for it, and can't tell you which ones are the ones to be concerned with...
 
Adjustments for SX-1050

I don't have a 1050 but I do have a 950. The adjustments for it are as follows: (on the Power Amp board)
1. Turn VR3 and VR4 (bias current) fully counter-clockwise before turning on power.
2. Remove jumper plugs connecting POWER IN and PRE OUT jacks.
3. Connect 5.1 Kohm resistor to POWER IN jacks.
4. Set power amp for no load.
5. Set POWER switch to ON.
6. Adjust VR1 for 0V between terminals No. 10 and No. 9.
7. Adjust VR2 for 0V between terminals No. 25 and NO. 24.
8. Adjust VR3 for 20 mV between terminals No. 12 (+) and No. 13 (-).
9. Adjust VR4 for 20 mV between terminals No. 27 (+) and No. 28 (-).
Note - VR1 and VR2 set the offset current
10. Allow set to warm up for at least 10 minutes, then readjust.

I hope this helps - but remember, this is for a 950 not a 1050. EchoWars might be able to guide you at this point.
 
I did the DC offset and Bias on my brothers 1050 and thats not the way to do it. I would get the service manual for it :yes:

Ron
 
Well, consider it the $20 tweak you can actually hear...unlike the umteen massively expensive BS out there that pass as 'tweaks' that don't tweak anything except your wallet.

Here ya go...
 
Pioneer SX-450, acquired today...
Left Channel ~ -20mV
Right Channel ~ -12mV

Seems pretty good, but is there any easy way to improve these numbers?
Also, the number on my multimeter just keeps drifting around - why? Is it cause it's a cheaper Radio Shack Auto-Range Digital Multimeter?
 
Uhm... I just noticed that I get a value when the leads aren't touching anything. Battery in the multimeter going bad?
 
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