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Amplifier Distortion, DC-Offset, and You!

Finally got my hands on multimeter:

Original readings:
45mV on the right
32mV on the left

Played around for 30 minutes, now both chan's are averaging at 0mv! :banana:
Boy those pots are anooying :sigh:

I'll join in on the cheers for this wonderful thread. If only it wasn't so late at night here I'd be blasting the new Opeth CD just to see if it's sounding any sweeter :smoke:
 
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OK serves me right for not checking more carefully befor messing with these trim pots, and it turns out of ciurse they didn't change things much from my DC offset reading of L-60mV R-49mV.

I played around with them for a while and then thought, they are probably bias adjustment or something. I took a pic to ask if anyone knew and what do I see when I get it in photoshop? It was the bias adj.! damn. :no:


I tried to set them back where they started but I didn't take a pic before I messed with it. Could someone please point me in the right direction for setting the bias correctly. BTW I don't have a manual for this amp, I have tried to source one but I can't even find info to suggest it really ever existed (Pioneer A-777).

Also I suppose my only option regarding DC offset is to replace the input transistors? I have included pics which also tell me this amp could do with a clean inside.

thanks for your help.

 
Yep...appears that there is no adjustment for offset...

Get the bias back to a reasonable value. The emitter resistors at the bottom make a convienent place to check. It appears that the leads for these resistors extend through the tops of the resistors, so for each channel, connect your voltmeter to the center of one resistor, and the end. Adjust the bias pot to read about 10mV, and that wil give you about 45mA of current flow, which will be in the ballpark (assuming this is a Class B amp).
 
My measurements.....

Just measured my receivers:

Akai AA-1135: Left = 7.2mV, Right = 10.8mv
Marantz SR-7300 (Home Cinema Amp): Left = 0.3mV, Right = 0.3mV
Marantz SR6000G: Left = 56.5mV, Right = 69.2mV (not bothered about this one as I'll be selling it soon anyway!....)
Marantz SR6000DC (my pride and joy): Left = 13.2mv, Right = 103.5mV

:wtf: My pride and joy's got an offset of 103.5mv! Is this dangerous? Should I get it fixed? Can a novice like me fix it myself or should I play it safe and hand it in to my local repair shop? I'm sure I've got the full schematics for it somewhere, but to someone who hasn't fixed a piece of hifi yet the diagrams looked pretty intimidating!!!

Could this offset be the reason why the right channel is playing louder than the left when the balance control is set centrally?

Richard
 
100mV isn't dangerous, but it's too high for sure.

Look in the manual for the amp adjustment procedures and see if there's a section on setting the offset and bias (sometimes called 'idling current' and 'zero voltage adjust', respectively).
 
EchoWars said:
100mV isn't dangerous, but it's too high for sure.

Look in the manual for the amp adjustment procedures and see if there's a section on setting the offset and bias (sometimes called 'idling current' and 'zero voltage adjust', respectively).
Click to expand...

There's a section in the manual called 'Audio Alignment Procedures'.

It says -
"Set RG01 to minimum position. Adjust RN09 (L-Ch) and RN10 (R-Ch) so that voltmeter is reading 3mv or less. Connect 8ohm load to speaker terminals. Adjust input signal so that the output voltage at speaker terminal is 23.7V. Adjust RK01 (L-Ch) and RK02 (R-Ch) so that voltage at JK07 or JK08 is 2.25V. Check to ensure that LED reading is 70W."

I'd think I'd be safer handing it in as the above quote is like trying to read Japanese!........
 
I have no idea what they are describing, so I'd leave it alone.

The unit uses STK outputs, so in all likelyhood there is no adjustment for bias or offset.

Could this offset be the reason why the right channel is playing louder than the left when the balance control is set centrally?
Click to expand...
No..that's either a dirty switch (or switches) or a flaky volume pot.
 
Hi, my Carver m1.5t is capacitively coupled. Can the dc offset still be checked at the output terminals ? Does it have to be checked at the amp board ?
 
Hi, after looking at my service manual, I discovered that my amp is capacitively coupled at the inputs, not the outputs. My readings are -.2 right +1.7 left.

Thanks for the quick reply and VERY informative thread.
 
Adjust the bias pot to read about 10mV, and that wil give you about 45mA of current flow, which will be in the ballpark (assuming this is a Class B amp).
Click to expand...

Thanks so much Echowars, you even pointed the way with my pics, I love this forum.
 
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440Hz: Yep, I think that your A-777 is a domestic A-717. As soon as I saw your pics I picked it off right away because I have an A-717. — Lorne in Sendai, Japan
 
I will start another thread ...

440hz: This thread that was started by EW has become a reference for thousands of people, so I don't want to steer it away from its intended purpose. Just give me a day or two and I will start another thread on this forum (vintage) regarding the A-717/777 in order to answer your question and call on other AK members to hopefully pool information and resources concerning this machine. Cheers and best regards — Lorne
 
Am i doing right?

My meter is an old analog type but i am not shure i´m using the right settings? I set the meter to "200m" i have 2 holes to connect the pin and i´m using the fused "max 600v -~200ma max" and meashured 29,7 l and 25,4 on the right. I turned the knob until i got a value i gues is the right and ended up on the "200m" position.
Thanks
Espen
 
EchoWars said:
Looks like about 25 and 30mV.
Click to expand...

Hi,
i made another meashure of an old Onkyo ss amplifier and the readings was as follow: 00,1 and 00,2 . The MC´s readings were 25,5 and 29,5. Do you know what´s happening?
Thanks
Espen
 
I don't see a problem. If you want to confirm, go shopping for a digital meter with a 200mVDC voltage range (or some voltage range that is appreciably less than 1V).
 
Hi-
I have two questions about testing DC-offset that I wasn’t able to find answers to within this thread:
1) Two of my receivers (the modest Sansui R-30 and the mighty Onkyo TX-8500) do not have Aux inputs on the input selector switch per se. They have FM, AM, and phono choices on the switch, and tape monitor buttons. Is it a valid test of DC-offset to set the input selector switch to FM (or AM) and engage the tape monitor button?
2) Is it a valid test to check DC-offset on a power amplifier with no source connected to the unit?
I’ve already tested several receivers and integrated amps and have gotten various results (I’ll give the rundown once I’ve tested the them all). …and I want to add my thanks to EchoWars for starting and keeping up with this thread, now 50 pages long!
 
My new eBay score, Kenwood KA-907:

left channel: 11.7 mV
right channel: 1.9 mv

Looks OK for now, but it is wating to be fully restored anyway...
 
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