Question about Audio Players & Recorders

1 Answer

I have rebuilt one the amp boards of a Pioneer spec-4 amplifier. The bias is good but I'm getting to much drift(+ to -) on the DC offset and it is to high. The new trim pot does work but the Dc offset voltage is just to unstable.I have replace all the questionable transistors including the outputs (mje15093/94 ). Do i have to have all match pairs (hFe) of transistors for the amp to be stable ? And what else can check . All the supply voltages to the board are good and stable. What do you think

Posted by on

  • calman46 Sep 28, 2009

    Hi , I'm going to replace those varistors D2,D3,D4,D5 I think their leaking too .The transistors are new replacements but have the same or better specs than the(blown) originals . The protection circuit is working and I have the bias turned down a little so I should be safe. I'll recheck the resistors .I figured and open resistor would give no Dc offset at all. Thanks

  • calman46 Oct 02, 2009

    I'll keep you updated , thanks



1 Answer

  • Level 3:

    An expert who has achieved level 3 by getting 1000 points


    An expert that got 20 achievements.


    An expert that got 10 achievements.


    An expert that got 5 achievements.

  • Master
  • 5,807 Answers


you may need to replace the transistors connected to it.

same with the other transistor(s) on VAS circuits.

should have at least 60 mA , 80 mA max

more than 80mA will damaged your output transistor.

you may need also to check on the resistors

Posted on Sep 28, 2009

  • 5 more comments 
  • Rommel Cana
    Rommel Cana Sep 28, 2009

    try setting your bias to lower setting like 10 ohms or max is 100 ohms

    here is your guide

    there might also be leaky diode ( 3 diode in series)

    if still having problems , if the transistors on top and bottom is OK , you may need to temporary

    shunt 18 ohms resistor on either of the two 3 diode in series

    note: you might see only 1 pc of the 3 diode in series, the one i encircled

  • Rommel Cana
    Rommel Cana Sep 28, 2009

    if the drifting is too high, ie more than 4V

    then you might need to extract the output transistors to be safe.

    continue tracing, setting the initial 60 mA

    till the output is zero, only then you connect the output transistors

  • Rommel Cana
    Rommel Cana Sep 28, 2009

    your amp will work with small speakers just for testing before connecting the output


    please note , that it will make the driver transistor hot,

    but it wont be destroyed.

    if it is destroyed you have other problem, finished with amp working to a little speaker

    then if it is stable you can connect the output.

    when measuring output voltage on the amp, don't connect be very sure of the output of the amp, and not damaged your small speaker

  • Rommel Cana
    Rommel Cana Sep 28, 2009

    good luck , and keep me posted, remember 60 mA no more than 80 mA

  • Rommel Cana
    Rommel Cana Sep 28, 2009

    the collector on the upper should have at least 1,1 V

    the collector on the lower should have at least -1.1v

  • Rommel Cana
    Rommel Cana Sep 30, 2009

    a good amplifier never produce more than 0.7V on its output

  • Rommel Cana
    Rommel Cana Oct 01, 2009

    try also to check the differential inputs, sometimes it also causes the problem.

    the transistors and the resistors connected to it, assuming that the final output transistors

    are not connected.



1 Suggested Answer

  • 2 Answers

SOURCE: I have freestanding Series 8 dishwasher. Lately during the filling cycle water hammer is occurring. How can this be resolved

a 6ya expert can help you resolve that issue over the phone in a minute or two.
best thing about this new service is that you are never placed on hold and get to talk to real repairmen in the US.
the service is completely free and covers almost anything you can think of (from cars to computers, handyman, and even drones).
click here to download the app (for users in the US for now) and get all the help you need.

Posted on Jan 02, 2017


Add Your Answer

Uploading: 0%


Complete. Click "Add" to insert your video. Add



Related Questions:


Fixed amp won't turn on and click noise.

The amplifier would sometimes not turn on, when it did there was a loud click from one channel of the amplifier.


Inside the amplifier there is a protection board that plugs into the amplifier, the design uses 3 x 25V 47uF capacitors charged up by 22K and 220K resistors.
As the amplifier ages with time, a parasitic resistance across each capacitor gets less and less, this means that the capacitor can not charge fully up and reach thresholds, in the **** design of a delay circuit. The parasitic capacitance is affected by the room temperature making the fault intermittent.

The pop sound is caused by DC offset, it was 25mV, this was nulled using a trimmer on the amp pcb to stop the click-pop sound, this offset could of damaged a loudspeaker.

on Jan 14, 2016 | Marantz PM7200 2-Channel Amplifier

1 Answer

Kenwood with dc offset

DC offset is most often caused by the bias level for the output transistors changing for some reason. This could just be that the control is dirty and needs to be cleaned and re-adjusted, or the biasing network has a problem, most likely the diode or transistor that is being used to set the reference. The diode or transistor is often mounted to the heatsink for thermal feedback, which allows the bias to change depending on how hot the output transistors are getting, There is no mention as to how high the DC offset is. The usual range for DC offset adjustment is less than +/- 1 volt. If your offset is higher than this, there usually is a problem with the bias circuit that needs to be repaired. If it is MUCH higher, then one of you output transistors or driver transistors is most likely defective.

Jun 16, 2014 | Audio Players & Recorders

1 Answer

My audio/video amp will not turn on.

Check the DC off set on all of your channels. There is a good chance the one is drifting. If you find a chanel where the DC Offset is drifting (+) check for a leaky cap C113 on that channels negetive feedback circuit.

May 10, 2012 | Denon Audio Players & Recorders

1 Answer

DC Offsets on output - protection don't allowd to switch on

you don't want to do this( bypass the protection0
that d.c. output will burn the coils in your speakers with out making a sound!! you have a biasing problem
or like you said a d.c. offset problem it is best to work on this problem with out a load on the amp, this will allow you to trace back where the trouble is starting at. caps will cause this sort of problem but leaky transistors can also do this, even diodes.
this will require an experienced trouble-shooter.

Apr 26, 2010 | Audio Players & Recorders

2 Answers

Amp : sansui au11000 blown transistors on driver board

may i ask you did you put silicone grease in your transistor and apply a good mica insulator in all of your transistor---------look for any burnt resistor or explosive capacitor -----check for scew tie your supply voltage to earth by wrong.

Aug 03, 2008 | NAD C320 2-Channel Amplifier

1 Answer

I have this pioneer receiver model VSXD608 in which when the volume goes below 20db the unit goes in protection and after some time it tries to switch on back but goes to protection again and keeps on...

Check to see that the electrolytic capacitors that are in the output stage circuits and driver stage circuits are not high in ESR.

Check to see that the power supply is in specs. The main electrolytic caps may not be up to specs. To test the power supply it is best to run a load test on it. You will need the proper setup for this.

The supply current to the output stage may be high, and thus causing the protection to turn on. You will need to have the service manual for details.

It is also possible that the speakers are too low impedance, or their crossovers are going defective and causing a miss-match for their loading impedance.

Look with a scope to see that there are no high frequency oscillations in the output of the amp going to the speakers. A common cause of this type of fault are worn de-coupler caps in the bias circuits for the various stages along the signal path.

Check to see that there is no DC out to the speakers with any position of the volume control with no signal going in. The speakers must be disconnected for this test. Do the same with a sine-wave signal from the test generator. 1 kHz is good to use for this test. Use a proper rated 8 ohm dummy load for your amplifier when applying the test signal. Do not put steady strong tone in to your speakers. You will have to refer to the service manual to know the allowable DC offset at the output stage to the speaker terminals. Most consumer amplifiers must be less than 50 mv. The higher end amplifiers are down to less than 10 mv.

It is also good to do a distortion test to see if there is too high a distortion that is causing the protection to trip.

You can also use the dummy load and the audio generator along with the scope to do power handling tests. This gets involved to put over this type of email.

These are only some basic examples of what to check. You will need the service manual, a scope, ESR meter, DVM, audio generator, and distortion analyzer to go through all the tests to find out where the fault is.

Jerry G.

Jul 07, 2008 | Pioneer Audio Players & Recorders

2 Answers

Marantz PM-74D

OOPS-forgot to mention if your bias turns out ok after replacing any blown outputs, turn it off and resolder the collectors (center) pins of all 4 output transistors and turn it back on-should work but you'll need to recheck bias and dc offset. They appear to both be adjustable on the board sticking up on the top side-it's right in the middle and has 4 trim pots on it......

May 17, 2008 | Audio Players & Recorders

1 Answer

Pioneer amplifier 1980's

bias can be out in shape of volume written bias-or it can be in printed circuit board as potentiometer labeled by bias word.

Nov 08, 2007 | Audio Players & Recorders

3 Answers

Problem with my Peavy CS 1200x power amp

1200x with over heating output...does not care if it has a load unless it is too low,(Scott is wrong) have a leaky output, or a triac firing and clamping down the voltage to the speakers..the foldback shutdown may also be leaky, but these usually cause it blow fuses, and these get damaged by bad outputs or running the amp into shorts..since this amp has some really high voltages in it I suggest you take it to a shop before you KILL YOURSELF!

Jun 20, 2006 | Peavey CS 1400 Amplifier

Not finding what you are looking for?
Audio Players & Recorders Logo

Related Topics:

53 people viewed this question

Ask a Question

Usually answered in minutes!

Top Audio Players & Recorders Experts


Level 3 Expert

102366 Answers


Level 3 Expert

5113 Answers

The Knight
The Knight

Level 3 Expert

74198 Answers

Are you an Audio Player and Recorder Expert? Answer questions, earn points and help others

Answer questions

Manuals & User Guides