Checking DC Offset On Old Vintage Stereo Amplifiers And Receivers - Audio Repair And Restoration

This video shows how to check the DC Offset on older vintage audio equipment. When purchasing a vintage stereo receiver or vintage amplifier you should always check the DC Offset. This can help you to determine on older equipment the receiver or the amplifiers operating condition. Checking the DC Offset doesn't tell you everything about the condition of a piece of vintage stereo equipment but it's pretty easy to do and only requires a digital multi-meter(DVM). Bringing a digital multi-meter to a vintage audio equipment ad that you've seen on Craigslist is a must.
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[@davewallace8219]

thank you...many people are getting back in to vintage audio...most all these amps...are very old and have problems....this sounds like a 1st step...to ensure you don't have....excessive voltage going to your speakers....again thank you.....

[@g.fortin3228]

Love the knowledge you share here.

[@vintageaudioaddict]

Thank you for watching.

[@midmodaudio6576]

Great video !

[@merlenbach1232]

what is volume contol set at during the test? I also noticed that the right channel had a - DC mV reading and the left channel had a +DC mV reading, is that considered normal?

[@kevinhollenbeck7503]

I wish he would answer this question.

[@kennethiman2691]

Very helpful.

[@vintageaudioaddict]

Glad to hear!

[@wadehicks9270]

If I have an amplifier with 200 millivolts of offset on both of its channels what’s the best course of action to get that below 30 millivolts ? Thanks for any assistance.

[@cat-lw6kq]

Second hand store used to let me plug the units in. But they do allow returns.

[@robertwalker82]

Good video! If the dc offset is good, does that mean the output transistors and/or electrolytic caps on the drive board are okay? Thanks.

[@robertdragon7604]

Hi great videos you put out. I got a question. I have a Pioneer SX-727. One ch is solid with stable DC offset. The other ch is flakey DC offset all over the place. Got any ideas were I should start looking? Caps on amp board? Protection circuit?

[@justingoers]

Thanks!

[@vintageaudioaddict]

Your welcome and thank you for watching!

[@jeffreydieselshank8421]

I checked my Kenwood VR-6040 just to see. A R & L started at 2.4 and then settled into 0.1 both channels. B the L Channel went up to 10.2 and then would go down to 2.4, but it did not reach the 0.1. The R Channel responded like the A speaker outputs beginning at 2.4 and settled into 0.1. Should I worry about the difference of the B L channel? Thanks.

[@dfsilversurfer]

can this test be applied to a amp for a subwoofer .

[@goblazers4435]

After measuring my 2270- Main L & Remote L both read ~13mv when selected and 0mv when not selected. On the right side; Main R reads 52mv when selected and unselected, while Remote R reads 52mv when selected and 0 mv when not selected. From what you said, I'm less concerned about the number, but more concerned that the Main R is giving a reading when not selected. Where would you start to troubleshoot? Thanks!

[@kukko83]

Some good information here. I'm no expert but I disagree with your advice, on starting the measurements with less than 1mV scale. I'd put it on to the highest setting to begin with, and drop down from there. After all, you're looking to see if there's a problem with the amp. When you have 38V at the outputs, like I did recently, you know you have a problem!

[@vintageaudioaddict]

Thanks a lot for sharing and for watching. Yes, 38 volts is an issue!

[@tjones1524]

Cool video! Can you elaborate? What is the DC offset telling you?

[@rl7487]

They never replied to you. There is not supposed to be any DC current in the output since the output is AC current. DC current will burn the speaker. But In tiny millivolts it’s not harmful, yet .

[@pulpanochamaster]

Thanks, question, when i run the procedure my voltimeter goes to -98 mv ... If i turn off that channel... Then it starts going to +50 and keep going up. Then if i turn on the channel again, it goes to -90 again. Both left and right channel. And i. a, B or C set of speakers, i have to say, last week it burned my left speaker. Thanks for the advise.

[@audiorick841]

Thanks for the video, can I use this method on a SET tube amp?

[@vintageaudioaddict]

In tube amplifiers there is no DC on the speakers as they are transformer isolated from any DC by the output transformers. There is no DC offset to adjust unless it is output-transformerless, but there are not many of those.

[@audiorick841]

@@vintageaudioaddict Thank you for the clarification, I can confirm the amp I have has output transformers but I was lacking the knowledge to know what it meant for DC offset. Thanks for the new knowledge!

[@DavideRizzotti]

Hello. Thank you very much. I have an old amplifier that, if I measure bias with speakers installed, show less than 1mv DC, but if I measure without load the bias is 3.2 volts DC. I don't understand if this is a problem or not? If with speakers the bias is almost absent shouldn't be risks for the speakers, or not? Can you explain why the test is done without speakers/load? Thank you very much.

[@russfelger]

The speakers are absorbing the load. You need to see what the speakers are seeing, so you test with the mm as the load. 3,2 vdc is very high.

[@jovanj.9400]

Hi, I have a question, is it normal for dc voltage to raise over time? a month ago I adjusted dc voltage about 5 mV on both channels, now Im having -70mV on L and -60mV on R. Sugested voltage is +-10 mV for my reciever, it is sansui 8080. Thanks

[@jhjove]

Could weak differential pair, thermal issues, leaky capacitor... Have any upgrades been completed on this chassis?

[@jovanj.9400]

@@jhjove No, everything is original, I just cleaned the pots and switches. It sounds fine but that dc voltage bothers me.

[@jhjove]

@@jovanj.9400 yes, anything over 50mV and increasing is of concern, more distortion+more problematic for the speaker VC. This chassis is at least 45 years old, I have to believe there are over stressed and now weak components. At the very least the power supply caps should be changed. That said when you do your adjustments for Bias and DC offset were these after the unit was on for 10-30 minutes (controls flat, no source, volume minimum)?

[@jovanj.9400]

@@jhjoveYes, folowed instructions by service manual, volume on lowest, no sorce, didn't adjusted bias and offset on same time when powered on, I waited at least 5 minutes for curent to be stable. Only thing that i did adjustments with speakers hooked because i dont have a dummy load for testing...that dc voltage on output didnt raise same day to 70mV, it is was raisin gradually day after day...do you suggest that I try to adjust bias and offset again?

[@jhjove]

@@jovanj.9400 I think the circuit, with the age of the components, is going be wide ranging for current and voltage measurements/adjustments. And it could be a simple solder connection gone bad in the 45 years. If you're gonna use it you'd best do a maintenance job on it.

[@erictiscoff4296]

Volume set to zero ?

[@jaman878]

The reading should be Independent to where the volume is set. You aren’t looking at absolute voltages here, just the differential voltage (offset) between the 2 channels.

[@jhjove]

Nice info but remember this only works for direct coupled outputs. Capacitor coupled outputs will block DC. If there is an issue with the amplifier you'll have to do other tests.

[@vintageaudioaddict]

Good comment. Thanks!

[@chwk2003]

@@vintageaudioaddictHow can one know if he's testing a direct coupled output or a capacitor coupled output? Is there an easy "rule of thumb"?

[@tutrong9182]

I have a Sony TA-F60 with the right channel issue : the voltage at the C of Q213 and Q214 is always -8,0V, so the protection relay do not work. I turn the RT201 to maximum but it can not reach 0V. All orther voltages are ok. Could you please advice me some solutions? Sorry for my bad English Thank you in advance. TuTrong from Viet Nam. I have the service manual of TA-F60. If you need I will send you via email.