I would guess his readings are 40mV to 10mV. My Fluke meter will not read below 1mV (.001V) Still not bad.
Amplifier Distortion, DC-Offset, and You!
- Thread starter EchoWars
- Start date
on my cheapy, it's set at 200mV so the decimal is 00.0mV. This will give you mV rather than tenths or hundredths of a mV. My Fluke 87 RMS is auto ranging but ends up with the same reading.
Please don't misconstrue my comments, you are looking for any reading above ~50mV to the left of the decimal. So, an example of 25.5mV would be correct whereas 0.255mV would be insignificant and probably could not be adjusted out due to the sensitivity of the trim pot. Your pots may not be too sensitive, but most are pretty sensitive. Also, make sure the unit has warmed up for at least 30min or more so it is stable before any adjustments are made. Good Luck!
Please don't misconstrue my comments, you are looking for any reading above ~50mV to the left of the decimal. So, an example of 25.5mV would be correct whereas 0.255mV would be insignificant and probably could not be adjusted out due to the sensitivity of the trim pot. Your pots may not be too sensitive, but most are pretty sensitive. Also, make sure the unit has warmed up for at least 30min or more so it is stable before any adjustments are made. Good Luck!
So I just checked with my meter at 200. Showed at 00.0 on left and fluctuate between 00.0 and 00.2 on the right. Adjusted it to 00.0. Checked back at 2 and it was fluctuated between .050 and .012, a little higher than I had set it at before. How concered should I be about the fluctuations?
Those are good numbers and I'd leave it alone. Depending on the quality of the DMM, some variation might be residual created by the meter itself. Vintage equipment is more susceptible to erroneous readings due to the age of all the other components. Don't chase your tail trying to set it perfect.
Down the road, you might consider restoring it. If not experienced enough, might take it to a tech that knows how to work on it and I'd bet he would check the bias then reset the DC offset. For now, if it sounds good, leave the settings and start listening to your system.
Regarding the fluctuations..... sleep well and don't worry about it!
Down the road, you might consider restoring it. If not experienced enough, might take it to a tech that knows how to work on it and I'd bet he would check the bias then reset the DC offset. For now, if it sounds good, leave the settings and start listening to your system.
Regarding the fluctuations..... sleep well and don't worry about it!
Bias can/will affect offset, but:
- The fact that it won't hold steady suggests a component that is not working correctly, rather than a bias issue.
- Setting bias is not difficult, but be aware that a mistake, or a trimmer that goes open while adjusting it, can cause some serious damage. If you decide to adjust bias, first step is just to check it and see if it's way off. Second step is to assure the trimmers are working correctly (with power off!), and perhaps cleaning them...or ideally, just replacing them.
All that said, I would not spend much time worrying about your readings...they are not a significant issue.
Last edited:
That's one of those"Do I plan to keep it" decisions. and can I find someone competent. In the SM it says to use a milliammeter for bias. How is this different than using a DMM?
A multimeter has multiple functions...it can be a voltmeter, ammerter, and ohmeter, and may have other functions as well.
If you decide to pursue this, be very careful as the Sansui measurement method (removing fuse and measuring across the fuse holder) lends itself to "oops" moments. Use mini-grabbers, or even mini-alligator clips, instead of needle probes.
Power off the receiver. Pull fuse. Attach meter leads. Power on the receiver. Wait ten minutes. Measure. Power off. Remove meter leads. Repeat for other channel.
If you need to adjust bias, only do so after confirming trimmers are working correctly. This is done with power off, using ohmeter function.
If you are not planning to keep the receiver, and it is working fine, no need to do anything (other than sell it).
If you decide to pursue this, be very careful as the Sansui measurement method (removing fuse and measuring across the fuse holder) lends itself to "oops" moments. Use mini-grabbers, or even mini-alligator clips, instead of needle probes.
Power off the receiver. Pull fuse. Attach meter leads. Power on the receiver. Wait ten minutes. Measure. Power off. Remove meter leads. Repeat for other channel.
If you need to adjust bias, only do so after confirming trimmers are working correctly. This is done with power off, using ohmeter function.
If you are not planning to keep the receiver, and it is working fine, no need to do anything (other than sell it).
Yeah, I think I'll leave it be. Funny the 2 outer dial lamps pop in and out. I'll replace them all, if I can find a good 7V lamp source, that's what they spec in the SM. 2 of the function lamps are out.
www.dgwojo.com
Send an email to Dave at dwojo69@msn.com. Let him know exactly what you need and he'll get it out to you. Reasonable prices and great customer service.
Start a separate thread. This is getting pretty far from the topic.
Maybe in the Sansui forum.
paulram
Super Member
Not perfect but it is still under the 50mV threshold. You might see if the service manual is available and look for "offset adjustment". If you are lucky there will be trim pots for each channel that should bring it back in line. If it is not adjustable, leave it and keep an eye on it ever so often.
See this thread:
DC offset on amp with no pots : Carver TFM-15CB
I have two Carver TFM-15cb amps. I'd like to reduce the DC but there are no trim pots on the board to adjust nor is there any specific information in the service manual (besides simply saying to check the DC offset...) I'm mechanically capable enough to replace components and I can follow...
audiokarma.org
inzite
New Member
just got my fluke 17b+ and took readings on my "new" to me au-8500. L: 10mv+/- 2mv, R: -13mv +/- 2mv. Unit interior appears stock. Have a 1 second slight faint static, rumble noise on right channel immediately following relay switch action upon cold power ups though. Sounds fantastic otherwise.
I measured the DC offset in my Kenwood KR-5600 and one channel was around 15mV while the other was around 40mV. There's no pot to adjust, so I replaced some input transistors with closely-matched pairs and now the DC offset is around 7mV in each channel. I was surprised it worked that well.
"Keep in mind adjusting trimpots that have not been moved in decades may not be a good idea. Like a scratchy volume pot this may be similar. Messing with the bias trimpot could cause it to go wide open an take out many transistors, diodes, etc. It might be a good idea to use a meter to see how the trimpots respond (with no power of course). Might be best to just replace them".
THIS!! Had it happen to me with a big Adcom amp one time (forgot which one - many years ago). Had those really cheasy crap trimpots in it that shouldn't be in anything, let alone a high powered audio amp. Bang/poof, lots of magic smoke, dirty underwear. Never again. I see those types of trimmers, and they come out before I even think about tweaking them!
