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Pioneer amplifier 1980's

I need information on how to bias the output transistors on a pioneer amplifier 1980's on

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

Posted on Mar 11, 2008


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How to calibrate the westrak amplifier before connecting the output power to read in multitester and how's the first step?

First off, I don't know what calibration you are referring to. If you are talking about a bias adjustment, you must have the output transistors connected, assuming that this amp has a bias adjustment.

Jul 22, 2015 | Audio Players & Recorders

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

I have a behringer 2500. both channels are distorted but not out. what would need to be replaced.

It seems to me that you have a bad transistor, T14 or T13 to be precise. T14 most certainly is in short circuit or its companion T13 (T14 is a small signal NPN transistor whereas T13 is a small signal PNP transistor). I just fixed one with the same symptoms as yours. You should be able to measure T14 collector voltage against ground (metal chassis) and the reading must be very close to +15VDC. Also, measure T14 emitter voltage to ground and the reading must be very close to -15VDC. If any of those voltages is compromised, you will not be powering the amplifier's operational amplifiers (U4, U5, U6) right. Check T14 and T13 with your multimeter and replace them as needed. Also, keep in mind that after replacing any of those transistors you will be required to readjust the BIAS current for your amplifier's drivers, which by the way, is a very easy task to accomplish. You know your BIAS current is misadjusted when your output transistors get really hot even under no signal conditions or when your amplifier distorts at very low volume levels or on high frequencies amplifying. To adjust the BIAS current for channel 1's driver transistors, wait until the amplifier has cooled down, locate and adjust VR3 until you read 80 mV across R2. To adjust BIAS current for channel 2's driver transistors, locate VR6 and adjust it until you read 80mV across R106. You need to wait until the amplifier cools down to set BIAS current right, If the output transistors' heat sinks get hot before reaching proper BIAS current adjustments, wait until they cool down to average room temperature and retake BIAS current adjustment. Once both channels BIAS currents have been readjusted, apply a thin coat of thick nail polish to the sides of VR3 and VR6 to keep vibration from misadjusting BIAS currents again... T14 goes down under severe BIAS currents shifting and it just makes me wonder as to why BEHRINGER does not apply a setting substance to VR3 and VR6 so they do not shift under sever vibration conditions, after all, almost any major amplifier manufacturer in the world does it. Good luck!

Nov 10, 2012 | Behringer EUROPOWER EP2500 Amplifier

1 Answer

Denon AVR2106 One output channel shorted and blown. 2 power transistors shorted, emitter resistors and base resistors blown Have replaced all, including bias transistor I think bias is still wrong (no...

I would also replace 2sa992 (2pcs) and 2sc1845 on preamp board- sometimes they look ok when you test them by regular meter. Check 1.2 kOm and 4.7ohm resistors at preamp pcb. Check diodes on output pcb (could be smd type, connected from base of output transistors to emitter i think)

Sep 07, 2011 | Denon Audio Players & Recorders

1 Answer

I have qsc amp model noisa500it 2nd channel is not working i replased the fuse and 2sc5200 and 2sa1943 everthing get shorted

It is likely that additional components were damaged when the output transistors (2SC5200, 2SA1943) were shorted. Check the driver transistors which preceed the outputs in the circuit as well as resistors in the area. Also, there should be a bias transistor mounted on the heat sink that needs to be checked as well.

Keep us posted,

Mar 02, 2010 | QSC Audio Players & Recorders

1 Answer

I have blown the protection circuit on my yamaha rx 600d amp anyone with a clue whats needed to fix?

Hi friend,when something goes wrong in the output of the amplifier the protection circuit take place to prevent further damage could cause by DC power such as speakers or other related circuits...I think in your situation either one or both output amp stage have blown,then the protection cct turned on (not blown protection circuit!?). If so you have to check all the output transistors for short cct, if shorted you must also check all the DC bias cct nearby such as small transistors,resistors...If you are not familar with amplifier it better to find a tech as all amplifiers need a very precide repair technic , If the DC BIAS were wrong , when you turn on the power it will blow straight away! All times and cost will be waisted!
Tan Ta

Nov 23, 2009 | Yamaha Audio Players & Recorders

1 Answer

Marantz SR19 no sound?

When replacing output transistors on amplifiers it is generally a good practice to replace the output capacitors as well as the biasing resistors. Just because one transistor is bad does not mean that that is the whole problem. Look for what caused the problem. Bad speakers,bad wires,defective traces on circuit board,poor soldering. Also when mounting the transistor make sure it is insulated and grease it with a dab of silicone heat sink compound. Whenever I service a board I examine all components for tolerance as well. A resistor marked at 1% tolerance,example 1k ohm + or - 1% should read no less than 990 ohms and no more than 1010 ohms. Anything else your asking for trouble. Another point is that some amplifiers use matching pairs. Pairs complement each other in an output stage and must be replaced in pairs. I repaired a Sansui Amplifier a while back and it looked as if a power output resistor was burned. Comparing the two channels I figured the matching value from the other side and replaced all of them and the system has worked flawlessly for years now. I am a retired electrical engineer and have worked in a few areas of electronics for over 35 years. Hope this information helps you.   

May 06, 2009 | Teac A-1D 2-Channel Amplifier

1 Answer

I baught an amplifier kit from maplin. its a 200w power amplifier. the problem is now when i turn it the two of the resistor gets hot and they are 5W 0.47ohm resistor. can you please help me with this...


Have you checked and set the bias current for the output stage. These 2 resistors will get hot if the bias is too high. Is it the firsy time you have fired the amp up? Operating in this condition may cause the output transistors to fail. If the output transistors are damaged,you would also experince heating of these resistors. Get back to me please and let me know if you have set bias


Nov 08, 2008 | 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


It sounds like it could be biasing. Measure the DC voltage across the emitter resistors. If it's more than 0.001volts (1mV), reduce it to 1mV. The output transistors MUST be clamped to the sink when setting the bias current. Recheck the biasing after ~5 minutes of idling. The bias transistors (Q7, Q27) need to be in contact with the heatsink.

These amplifiers also have a problem with the shield ground fuse opening. This can lead to oscillation which can mimic biasing problems. You would need to use an oscilloscope to see the oscillation. You can check the shield ground. Disconnect the RCAs from the amp and measure the resistance from the shield of the RCA jacks to the center tap on the secondary side of the transformer. It should be ~0 ohms.

Dec 01, 2007 | Kicker ZR600 Car Audio Amplifier

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