Amplifier Distortion, DC-Offset, and You!

Need to know how the power amp is set up. Start a new thread in the Fisher forum, or Vintage Solid State, and post a schematic of the power amp.

Kenwood DC Offset ... Please Help!

I have a Kenwood KM-206 amp, direct coupled amp throughout, with 2-5mv offset on one channel and .300v offset on the other. 3 volts at speaker terminals on defective channel. Voltages are drastically different from the good channel to the defective channel all the way back to the input stage. Checked all transistors, then swapped all transistors from one channel to the other. No change. I don't find a defective (leaky) capacitor or resistor out of tolerance. Anyone with experience on this or similar amps. Need to know where to look next.
Please give me some pointers if you can,
Dave

Technics SU-V4K, R- 1.1 mv L- 1.2 mv

Harman Kardon PM655 vxi:

Left DC Balance: 38mv
Right DC Balance: 130mv

adjusted to 0, +/- 10mv

Incidentally, sound went from ass to class! Or more succinctly. went from pretty good, to unexpectedly good.

Idling Current, Default is 30mv

Left: 91mv
Right: 72mv

Set to 30mv.

Hi, yesterday i changed the output transistors of muy pioneer sa 7800 and when i adjusted the DC and offset and i power on the amplifier sounds great but when this is time on begin to distort the left channel and the idling current of the left channel measure 0mv and the right channel measure 65 mv.can somebody help me? Thanks

Hi, I have just brought a Yamaha B-2 to life!

Actual DC balance reads as follows:
L channel: -9mV
R channel: +8mV

Should I worry that L channel has a negative DC balance while R’s a positive one?
Thanks!

Hi everybody
maybe a stupid question but what kind of multimeter is good to test an amplifier?
I mean,the most part of the non expensive ones can measure this range:

DC Voltage 200mV - 600V
AC Voltage 200mV - 600V
DC Current 200μ-10A
AC Current 2mA-10A
Resistance 200Ω－2000MΩ
Capacitance 20nF-200μF

so you see that dc voltage can be measured from 200mv to 600mv:quite useless for testing an amplifier dc voltage or the bias?!?
Please help me understand!

You are looking for minimal loading as well as ranges of operation, voltage loading is measured Ohms per volt.

Sent from my SM-N900V using Tapatalk

so,a cheap meter would be any good?
I mean specs for the minimal loading doesn't seem to be published for these and frankly i can't afford fancy stuff :-(

thes said:

Hi everybody
maybe a stupid question but what kind of multimeter is good to test an amplifier?
I mean,the most part of the non expensive ones can measure this range:

DC Voltage 200mV - 600V
AC Voltage 200mV - 600V
DC Current 200μ-10A
AC Current 2mA-10A
Resistance 200Ω－2000MΩ
Capacitance 20nF-200μF

so you see that dc voltage can be measured from 200mv to 600mv:quite useless for testing an amplifier dc voltage or the bias?!?
Please help me understand!

Click to expand...

What is being quoted are the full-scale ranges. The lowest FS range is 200mV, the highest is 600V. If you were looking to measure offset and bias voltage, then the 200mV range would be perfectly useful.

Whelp, I just did this offset procedure and found the following on my new-ish to me Sansui 9900z and my brand new multimeter:

Output Bias set to 2mV.

Left = -4mV
Right = -1mV

Trying to track down a weaker right channel problem; seem to need to have the balance to the right a little less than 2 to balance the sound output and indicators. Just thought I would share my values.

Thanks for the wealth of information!

My MCS 3248 measured about 36mV on right, and 38mV on left. I will try to find a service manual, and see about correcting that.

How new of a receiver would have this problem?

Hi everybody, please I need some help with bias adjustment on my sansui 8900 zdb. I have manual and there is procedure which is very simple, but there is NOTE that, before adjustment I should counterclockwise turn jVR2. Problem is that I can not find that, not ether on service manual. I found on F-3307 jVR1, jVR3, jVR4 (service manual page 14), but NO jVR2. I read all manual - no success. On page 19 i found VR2.

Please help. Regards!

Yamaha CR-440. Left = 22.8mV and right =11.0mV. Really surprised with this CL project.

Old Curtis Mathes Integrated Amp - Model HA230. I have no idea who actually made the guts for Curtis Mathes. Some have suggested it might have been NEC. I've not been able to find any manuals on line, and the company no longer exists.

Left = 24.4
Right = 26.1
About what I expected from what seems to be a rather "mid-fi" unit.

Another old integrated amp - a Kenwood KA1400G that I bought in 1975 as my first "decent" stereo equipment purchase. IIRC, it was $110 from a catalog company located in San Luis Obispo, CA. I had this worked over a couple years ago, but it's been doing 24/7/365 duty with a satellite radio receiver hooked up to it in the control room at work. It's back home now on the work bench, and the Curtis Mathes HA230 took it's place. I decided to bring it home because we thought the left channel was getting weak. I can't hear any audible difference here at home, so we must have been mistaken.

Left channel settled out at around 55.
The Right side settled out around 60.

The readings jumped around quite a bit, and I'll have to read more of this thread to see what that might mean. The numbers are not very good, so I might have to have it checked out again. Then again, maybe that's all I can expect out of a nearly 40 year old piece of equipment.

Interesting, and very long lived, thread.

I finally got a DMM to test my DC-Offset on my Technics SA-200. It's reading approximately 101mv on the left channel and 123mv on the right channel. YIKES.
This isn't normal, is it?

I checked my Nad C370 today. Left 93mv right 115 mv.
Consulted the manual which specified setting these to 30mv for each channel.
The adjustment pots are tucked away in a jungle of resistors so I wrapped my screwdriver with tape to all but the very tip.
I could have got them closer to zero but the manual stated 30mv so that's what I went with.
The amp seems to sound more dynamic now

I rechecked the service manual for my Nad and it said to set at 0mv +\- 30mv.
So I went back and adjusted the bias close as I could to zero mv on each channel.
I guess it pays to read the manual Properly

I have a Sanyo DCA-1700x 4 channel decoder with a serious volume imbalance though only on the Phono input. I measured about 80mv on the left and 240-ish on the right. I've got some things going on obviously...