In this video I discuss the reasons for bias, adjustment of bias and offset and demonstrate the procedures on a Sansui AU-717 integrated amplifier. #amplifierrepair #audio
As an absolute beginner who's watched countless videos, I can say that yours have more useful knowledge/wisdom per minute than anything else on the 'net. Thank you for doing these.
Tomas, you were fortunate. Some service manuals have cryptic, incomplete or just plain wrong procedures outlined. Thanks for posting and the kind words!
Thanks, Ray. I appreciate your videos (which I found on Audiokarma!) Very informative and helpful to a noob like me. I was adjusting the bias on my Sansui Ba-5000 and it calls for 180 mA and has no offset adjustments.
As a beginner vintage audio enthusiast this is exactly the level of introduction to electronics I have been searching for. I have been reading a very long (and old) DC offset thread on Audiokarma and this video really compliments that, bringing it all together with the audio and video hands on demonstrated here.
Thanks Mohamad. Many are familiar with the terms bias and offset but don't really understand what they are. This was my attempt to shed some light on that.
I don't know why you'd need to know this, but it's easy enough to extrapolate. In a properly functioning amplifier under no signal conditions, the output transistor's collectors will be at the supply voltage, either positive or negative. Their emitters will be at zero volts. The driver transistors collector's will also be at the supply voltage, and the emitters will be at .6 volts, also either positive or negative depending on whether we're looking at the NPN or PNP transistors.
Always wanted to know about bias and offset. Never found a source and you just set me straight. Thank you so much. Totally makes sense, but is it a form of compression? Excuse the ignorance.
Never thought about it, but bias actually prevents compression. By getting out signal into the linear portion of the amplifier's response curve it exhibits a 1:1 gain ratio.
Marginally if at all. DC offset is all about setting up the balance between the two halves of the circuit, i.e. the NPN/PNP halves. Bias is setting the idle current, kind of like setting the idle on a carburetor.
First of all many thanks for sharing your knowledge. I'm a beginner and I'm trying to fix a marantz pm350, I powered on for a few seconds to check the bias and on one channel it went instantly over 500mV, immediately turned it off. I'm looking for obvious shorts and blown components with a dmm, the outputs aren't shorted and resistors match on both channels, do you have any other suggestions for possible causes? Thanks again
Hi, great video, but a quick question, do you always have the need of multiple multimeters? For correcting the offset and bias? Fact is, I only have one. Thanks.
hi ray, I have a sansui au-217 and the test points are not across both emmiter resistors. One test point is between the emmiter resistors. So to calculate the bias current, just use one resistor. calls for 7mv +0.5mv. emmiter resistors are .33 ohms. 7mv divided by .33 is 21 ma. is that correct and do you need a load when adjust the bias. Thanks your channel is top notch !
With the offset control that touchy, wouldnt it be a logical to assume the control should be replaced? Having had them drift when bumped, or simply become erratic over time. At the factory when built, clearly they would not have been that touchy right? Tank you fir speaking in such a way that i can grasp some if this stuff. The xover distortion specifically. The manual for the Marantz 240 amp says to measure that and adjust bias fir the least ammount. While they changed thier mind a few times with updates, ive always wanted to measure it or view this in some way at the same time i set bias to get a better understanding of what your describing. Thanks for your time in all this!
You're right, changing the bias and offset trimmers is the way to go. Those are new, it's a really touchy adjustment in some amps. Appreciate the kind words!
@@raygianelli3612 oh new pots? I see. This was why you mentioned multiturn and these being single. Ive had 30 restorations done so far, as i own about 100 pieces. I dont do the work, but love to watch videos and learn some of the terminology. Most recent was the 240 i mentioned. Im always looking for others to send my gear to if your interested! Next is the Sansui au 919 and have the "black flag capacitors " removed (as well as others) Is there by chance a service you offer? Thank you much, Charles
I have an old Yamaha A-700 amp where a pair of the 4 transistors failed. I am in the middle of repairing it. It took me forever to find rare and overly priced used original Toshiba 2sa1333 & 2sc3370 transistors so I am worried about damaging them again. Some of the capacitors were well past their best before. Could lower capacitance lead to blown transistors? Could it cause too much bias in the transistors?
I'd say no. Electrolytic caps tend to degrade, and performance with it. They do occasionally short, which, depending where they are in the circuit could cause failure. But if you're worried about the new outputs blowing, make sure to test everything feeding them. Bad drivers. predrivers are far more likely to cause catastrophic failures that take out output transistors.
Sure thing. Offset is referring to how far the output is from balancing between the positive and negative voltage supplies. Bias is a small amount of current that is used to keep the output transistors from turning off completely to prevent distortion,. The term used for a transistor that begins conducting is called forward bias.
Hi Ray, great video. I have a question regarding setting up bias after the change of output mosfets on my Sony ta-fb930r amp. When measuring as described in the service manual, I dont get any readings, even when I adjust the trim pots. The instruction in the manual is a bit confusing. The test points are across only one of the resistors, not across both, like on your video. Can you give me any points please. Let me know if you need a link to the service manual.
Sorry rookie mistake, i had my meater on AC rather than DC🤫 So when measuring as described in the service manual I get 0.2mV on left chanel and 1.2mV on right chanel. And no changes when I turn the trim pots. Cheers
It doesn't matter if you measure across one resistor or both. In the Sony, if you measured across both resistors they'd have you set it to 20 mV instead of 10. Did you replace the MOSFET's with the exact same ones? If not, that could be a problem. In addition, I believe these are obsolete, so the chances of getting counterfeits is greater. Check to see if the voltages coming from the driver's emitters changes with the bias pot.
Thanks for your reply. I replaced the mosfets with IRFP240PBF and IRFP9240PBF. I will check the voltages across the drivers emitters tonight, what should I expect over there? By the way the Amp sounds great to me, I was just worried of to not cook the new mosfets.🤣
I have tested voltages across the emitters resistors of the power drivers is at min bias 2.64V and at max bias is 5.28V at both channels. They were set at 2.8V so thats where I have left it.
Hi Ray, I have a Pionner VSX 523 that power down but it self when volume get 50 (not high at all) . I asume that it goes to pretention mode. It could be a BIAS or DC offset? DO you know if taht modes it could be adjusted? or I need to buy a new board?
My old amplifier doesn't have trimmers for reducing the DC offset. Left channel 0.3 mv , right channel 50 mv. I have never change the main capacitors, will this fix the problem? Thanks.
Changing the main filters won't affect the offset. Replacing the input differential pair with a matched pair of transistors at the should. .3 mV is really low. Is the amp a servo design?
In my HK PM655Vxi, after replacing the burnt power transistor (Q441) (bias measured = 780mV), I also found a burnt resistor (R481), which I also replaced. Then I checked all the resistors - they are ok (I compared them with the right channel, which is good). I checked and I compared all Ueb voltages in the transistors, they are identical to those in the good channel. I still cannot obtain the required bias voltage - the lowest is 230mV and it can only be increased. On one of the forums I found advice to replace the R457 resistor (470 Ohms) with 330 Ohms, but I'm not sure if it can be done. Please advise something.
@@raygianelli3612 Thanks for advice - with the VR403 pot set to minimum, I measured the voltages on the Q429 transistor (bias regulator) and got Ub=-0.37V, Uc=-1.34V, Ue=-1.02V, they can only be increased. Analogous voltages in a well-functioning channel are Ub=-0.56V, Uc=-1.22V Ue=-1.20V. I checked on the table (after dismantling from PCB) transistors Q429 (bias control), Q437 (attached to Q429), Q421+Q425, Q423+Q427, Q435 - they are also good. I also checked the driver transistors - Q437 and Q439 - they were also good. I have no idea what could be damaged there.
@@danekogolniak9680 Have you checked the value of the bias adjustment pot? Also, compare voltages from the outputs back and see if anything looks suspicious.
@@raygianelli3612 Pot reg. bias from 0-500 ohms changes correctly. I checked all voltages in the power amplifier and compared them with a well-functioning channel. Only the voltages on transistor Q429 differ slightly from the voltages on Q430 (a well-functioning channel). First of all, there is a visible difference in the voltages Uc and Ue - it should be 1.2V and it is not, even though the Q429 tyransistor is good - measured with a multimeter in the diode mode on the table after removing it from the PCB.
Hello, the factory data is 5mv. If I change it to 10 or 15 mv, will there be any harm or will I see any benefit? My device is rotel rb 1090 power amplifier
Higher bias will cause your output stages to run hotter. There may be a small (inaudible) drop in THD. I tend to run the bias at, or slightly lower than the factory recommended settings. Heat is bad for semiconductors.
No, excessive bias current just adds excessive heat, and can shorten the life of your output transistors. Amplifiers that are biased for Class A operation need enormous heat sinks.
one power transistor (black) is burned out, the other transistor (green) is good. After replacing the black transistor, I measured the voltage between the emitters, it is 0.754V too high and cannot be reduced, kindly tell me if it is because I only replaced the burnt transistor. Should I replace both power transistors in the damaged channel? I'm repairing HK655Vxi.
@@raygianelli3612 I checked back and I found varistor mv12ym (chain of diodes) which is burnt out. I can not find any replacement. Maybe you know any substitute. Amplifier PM 855Vxi, element D409, kindly please help me.
@@danekogolniak9680 I can find nothing with that model number. I'm guessing it's a Harman Kardon PM655VXI. You can use 2 1N4148 diodes in series to replace the MV12YM.
At 6:39 “… some people like to use multi-turn pots either 10 or 25 turns so that is not an issue. I use the single turn pots, you can get it, you just have to be patient”. At 7:40 “Our offset is really good right now (meter shows perfect 0.000 mv) and I hate to touch it… because you will see just how touchy these adjustments are” [proceeds to just touch it (not turn it)] “it’s already over 300 millivolts” (factory spec is within 5 millivolts of zero). 😂 At 8:50 Finishes fiddling with offset “We got negative 2.6, 2.7 millivolts” (within factory spec). 😁 At 11:05 offset has been constantly drifting in the background over past several minutes and is now to negative 41 millivolts and is rapidly moving further away from factory spec of plus/minus 5 millivolts of zero. 😲😬
It's not stupid to have a high biased amp if it was designed to have a high class A bias. It is actually a good thing, because class A completely eliminated cross over distortion, thus sounds better. My JC1 mono blocks are biased to 25 Watts of class A and they sound amazing. It's only stupid to set the bias higher than factory specs, because in most cases the amp will only run hotter with no quality gain. If you set your customers amps lower than the manufacture tested to be optimum, then you are short changing your customers in sound quality. Some people have a "bias" to amps that run warmer than others and such a bias should never enter the equation when servicing amps.
If Marantz likes to see ~75 milliamps and you know how ridiculously hot that runs, and you (and they) know heat is a bad thing, why do they have that specification? And what is gained or lost by reducing that level of bias current?
They may have wanted it to run in Class A for the first few watts. IMO the heat sinks won't support that, and watching the distortion as the bias was dropped to sane levels led me to advocate for setting these amps to a far lower level.
What I need is not to be too cheap to buy multiturn pots. :) But you're right, it would make life easier. Still, between the coarse and fine adjustments it's not too bad.
sorry ray, i haf a amp ( realistic 32 9003 ), ant i tinks it need 2 bin tune in the bias. the richte site, is ferry hot. hotter than the leftsite so. the amp is from 1995 ??? ant som caps bin don. so.. now i am on the point to do this. batt dont now how. your way i onderstand it. batt mi amp, i dont now how 2 do it.( thers 1 pot on it ant turns, bat the messer on the multimeter. ?? i dont now the spot ) gr fr holland en sorry for mi shitty englis. thanks for the vid.
Jouw Engels is beter dan mijn Nederlands, dus laten we eens kijken hoe Google Translate werkt. Ik kan geen schema vinden voor die realistische 32 9003. Heb je een exemplaar?
I doubt that these 4 wire cables are any good because they measure the voltage at the same point where the current is inserted. If you use 4 wire resistance measurement you need to make sure, that you also insert the current at differnt points to the DuT. Otherwise you also measure the contact resistance in series with the resistance of the DuT. Especially if the device has a very low resistance, your results can be very wrong.
@@raygianelli3612 correct. But four terminal sensing doesn't work if you only have 2 terminal at the DuT. It is important to sense the voltage without the voltage drop, which is caused by the transition resistance and the measuring current. If you want to sense a resistance in the mOhm or even µOhm range you need to transfer larger currents to your DuT. You cannot measure the correct resistance at the same point where you introduce the current. Four terminals means four terminals at the instrument and also at the DuT.
The engineers don’t know what tubes you are putting in or degradation of those tubes or the other materials in your amp.. things change over time.. even materials used by the oem mfg of the tubes could change and effect the voltage running through.
As an absolute beginner who's watched countless videos, I can say that yours have more useful knowledge/wisdom per minute than anything else on the 'net. Thank you for doing these.
Thanks so much! As I like to say, the only way technicians can reproduce is to share our knowledge.
True!!!
Hola, Muchas gracias por la tremenda explicación brindada. Es un gusto poder encontrarse con estos videos tan valiosos en la plataforma, Saludos!
Thank you for the kind words, Michel! Many wonderful people took time to teach me these things, and now the time has come to pass them on.
Very interesting teacher never explained better.🙌
Thank you so much! .It's gratifying to know that the goal of sharing this knowledge worked.
Hi Ray, your skills, , along with your teaching ability, are excellent. Thank you for sharing both with us. ,
Thanks for the kind words, Todd!
I just adjusted both on my Kenwood KA8100. The service manual was very helpful doing this. Thanks for explaining in detail.
Tomas, you were fortunate. Some service manuals have cryptic, incomplete or just plain wrong procedures outlined.
Thanks for posting and the kind words!
Thanks, Ray.
I appreciate your videos (which I found on Audiokarma!) Very informative and helpful to a noob like me.
I was adjusting the bias on my Sansui Ba-5000 and it calls for 180 mA and has no offset adjustments.
Thank you Ray for clearification. I especially like the ohms law demo at the end.
As a beginner vintage audio enthusiast this is exactly the level of introduction to electronics I have been searching for. I have been reading a very long (and old) DC offset thread on Audiokarma and this video really compliments that, bringing it all together with the audio and video hands on demonstrated here.
Thank you!. This kind of comment really makes my day, since the whole purpose of this channel is to share the knowledge that was shared with me.
Short and simple,excellent tutorial ray tk
Great vid this stuff is critical with vintage audio. You the man.
Thanks Ray , very informative which helped me fix the offset issue on my Sansui AU-717 left channel that was distorting after 8-12 mins
Thanks Jack. Love hearing a success story!
Excellent. I am going to watch this several times. It is really helping me pull together some of the things that I already knew. Thank you.
I totally get that. Half of learning is filling in the blanks.
Thank you very much, my right AMP was running very hot. After my adjustments both AMP are nice and cool.
Thanks Ray for another excellent tutorial!
Hello ray,
Thank you for the video very good explanation you should do these more often
I certainly will when time permits!
Thank you for explaining this. I now understand the concept
You've made my day! The entire purpose of this channel is to share what has been taught to me. It's gratifying beyond words to know it's working.
Thanks for the wake up yea you have been there and done that👍👍
Very well explained, thank you!
I appreciate the comment, it's very gratifying to hear this!
Nice style & professional video 😊
Thanks Steven! I prioritize substance over style, but no reason you can't have both.
Thank you for great informative & useful video 💯🌻
Thanks Mohamad. Many are familiar with the terms bias and offset but don't really understand what they are. This was my attempt to shed some light on that.
Excellent - THANKS!
Great great video teacher
Thanks for the kind words, they are appreciated!
Great video!
Very interesting video
Nice video, informative. I would like to know also about the output and driver transistors Collector-Emitter Voltage readings,
I don't know why you'd need to know this, but it's easy enough to extrapolate. In a properly functioning amplifier under no signal conditions, the output transistor's collectors will be at the supply voltage, either positive or negative. Their emitters will be at zero volts. The driver transistors collector's will also be at the supply voltage, and the emitters will be at .6 volts, also either positive or negative depending on whether we're looking at the NPN or PNP transistors.
@@raygianelli3612 How about the collector quiescent current and collector emitter qiescent voltage of the outputs and drivers at max. input signal?
Always wanted to know about bias and offset. Never found a source and you just set me straight. Thank you so much. Totally makes sense, but is it a form of compression? Excuse the ignorance.
Never thought about it, but bias actually prevents compression. By getting out signal into the linear portion of the amplifier's response curve it exhibits a 1:1 gain ratio.
SIR , THANK-YOU VERY MUCH..just a guick question..won,t adjustment to the the bias change the dc off set ?
Marginally if at all. DC offset is all about setting up the balance between the two halves of the circuit, i.e. the NPN/PNP halves. Bias is setting the idle current, kind of like setting the idle on a carburetor.
First of all many thanks for sharing your knowledge. I'm a beginner and I'm trying to fix a marantz pm350, I powered on for a few seconds to check the bias and on one channel it went instantly over 500mV, immediately turned it off. I'm looking for obvious shorts and blown components with a dmm, the outputs aren't shorted and resistors match on both channels, do you have any other suggestions for possible causes? Thanks again
Join Audiokarma.org. I'm a member there. Post in the Marantz subforum and you'll get the assistance you need.
Would someone please show how to do these measurements on a Sansui BA3000.
Do you use a resistor on the speaker terminals when checking bias?
No, we measure the voltage drop across the output transistor's emitter resistors, and use that value to calculate the bias current using Ohm's Law
Hi, great video, but a quick question, do you always have the need of multiple multimeters? For correcting the offset and bias? Fact is, I only have one. Thanks.
No, one is sufficient. I'm fortunate to have many multimeters, so I utilize them. It does makes it easier.
hi ray, I have a sansui au-217 and the test points are not across both emmiter resistors. One test point is between the emmiter resistors. So to calculate the bias current, just use one resistor. calls for 7mv +0.5mv. emmiter resistors are .33 ohms. 7mv divided by .33 is 21 ma. is that correct and do you need a load when adjust the bias. Thanks your channel is top notch !
Hi Fredrick. Welcome aboard!
Your calculations are spot on. No load needed for bias adjustment.
With the offset control that touchy, wouldnt it be a logical to assume the control should be replaced? Having had them drift when bumped, or simply become erratic over time.
At the factory when built, clearly they would not have been that touchy right?
Tank you fir speaking in such a way that i can grasp some if this stuff.
The xover distortion specifically. The manual for the Marantz 240 amp says to measure that and adjust bias fir the least ammount. While they changed thier mind a few times with updates, ive always wanted to measure it or view this in some way at the same time i set bias to get a better understanding of what your describing.
Thanks for your time in all this!
You're right, changing the bias and offset trimmers is the way to go. Those are new, it's a really touchy adjustment in some amps. Appreciate the kind words!
@@raygianelli3612 oh new pots? I see. This was why you mentioned multiturn and these being single.
Ive had 30 restorations done so far, as i own about 100 pieces. I dont do the work, but love to watch videos and learn some of the terminology. Most recent was the 240 i mentioned.
Im always looking for others to send my gear to if your interested! Next is the Sansui au 919 and have the "black flag capacitors " removed (as well as others)
Is there by chance a service you offer?
Thank you much,
Charles
I have an old Yamaha A-700 amp where a pair of the 4 transistors failed. I am in the middle of repairing it. It took me forever to find rare and overly priced used original Toshiba 2sa1333 & 2sc3370 transistors so I am worried about damaging them again.
Some of the capacitors were well past their best before. Could lower capacitance lead to blown transistors? Could it cause too much bias in the transistors?
I'd say no. Electrolytic caps tend to degrade, and performance with it. They do occasionally short, which, depending where they are in the circuit could cause failure. But if you're worried about the new outputs blowing, make sure to test everything feeding them. Bad drivers. predrivers are far more likely to cause catastrophic failures that take out output transistors.
thanks for the informative video. I'm having a hard time with the terminology, can you explain why it is called "offset" and "bias"?
Sure thing. Offset is referring to how far the output is from balancing between the positive and negative voltage supplies.
Bias is a small amount of current that is used to keep the output transistors from turning off completely to prevent distortion,. The term used for a transistor that begins conducting is called forward bias.
@@raygianelli3612 Thank you
Hi! If you don't have any service manual of amplifier how can you set the proper idling current or bias for amplifier? Thank you.
General rule of thumb is about 30mA. You can calculate your bias voltage by multiplying one (or both) emitter resistor values times .03
@@raygianelli3612 Ok. Thanks again.
Hi Ray, great video. I have a question regarding setting up bias after the change of output mosfets on my Sony ta-fb930r amp.
When measuring as described in the service manual, I dont get any readings, even when I adjust the trim pots. The instruction in the manual is a bit confusing. The test points are across only one of the resistors, not across both, like on your video.
Can you give me any points please. Let me know if you need a link to the service manual.
Sorry rookie mistake, i had my meater on AC rather than DC🤫
So when measuring as described in the service manual I get 0.2mV on left chanel and 1.2mV on right chanel. And no changes when I turn the trim pots. Cheers
It doesn't matter if you measure across one resistor or both. In the Sony, if you measured across both resistors they'd have you set it to 20 mV instead of 10.
Did you replace the MOSFET's with the exact same ones? If not, that could be a problem. In addition, I believe these are obsolete, so the chances of getting counterfeits is greater.
Check to see if the voltages coming from the driver's emitters changes with the bias pot.
Thanks for your reply. I replaced the mosfets with IRFP240PBF and IRFP9240PBF.
I will check the voltages across the drivers emitters tonight, what should I expect over there?
By the way the Amp sounds great to me, I was just worried of to not cook the new mosfets.🤣
@@TheAviator2 Just check the voltages at the emitter of each driver and see if the bias adjustment affects them.
I have tested voltages across the emitters resistors of the power drivers is at
min bias 2.64V and at
max bias is 5.28V at both channels. They were set at 2.8V so thats where I have left it.
Hi rey, I wanted to ask where you download the schematics for the amplifiers, thank you 🤝🙏
Usually from hifiengine.com You need to sign up, but it's free.
@@raygianelli3612 Thank you, I appreciate it 😇😊😉
Hi Ray, I have a Pionner VSX 523 that power down but it self when volume get 50 (not high at all) . I asume that it goes to pretention mode. It could be a BIAS or DC offset? DO you know if taht modes it could be adjusted? or I need to buy a new board?
Does the HDMI LED blink when this happens, or do you see TEMP flashing in the fluorescent display?
My old amplifier doesn't have trimmers for reducing the DC offset.
Left channel 0.3 mv , right channel 50 mv.
I have never change the main capacitors, will this fix the problem?
Thanks.
Changing the main filters won't affect the offset. Replacing the input differential pair with a matched pair of transistors at the should.
.3 mV is really low. Is the amp a servo design?
In my HK PM655Vxi, after replacing the burnt power transistor (Q441) (bias measured = 780mV), I also found a burnt resistor (R481), which I also replaced. Then I checked all the resistors - they are ok (I compared them with the right channel, which is good). I checked and I compared all Ueb voltages in the transistors, they are identical to those in the good channel. I still cannot obtain the required bias voltage - the lowest is 230mV and it can only be increased. On one of the forums I found advice to replace the R457 resistor (470 Ohms) with 330 Ohms, but I'm not sure if it can be done. Please advise something.
Measure the voltages around the bias adjustment. That may point to the issue.
@@raygianelli3612 Thanks for advice - with the VR403 pot set to minimum, I measured the voltages on the Q429 transistor (bias regulator) and got Ub=-0.37V, Uc=-1.34V, Ue=-1.02V, they can only be increased. Analogous voltages in a well-functioning channel are Ub=-0.56V, Uc=-1.22V Ue=-1.20V. I checked on the table (after dismantling from PCB) transistors Q429 (bias control), Q437 (attached to Q429), Q421+Q425, Q423+Q427, Q435 - they are also good. I also checked the driver transistors - Q437 and Q439 - they were also good. I have no idea what could be damaged there.
@@danekogolniak9680 Have you checked the value of the bias adjustment pot?
Also, compare voltages from the outputs back and see if anything looks suspicious.
@@raygianelli3612 Pot reg. bias from 0-500 ohms changes correctly. I checked all voltages in the power amplifier and compared them with a well-functioning channel. Only the voltages on transistor Q429 differ slightly from the voltages on Q430 (a well-functioning channel). First of all, there is a visible difference in the voltages Uc and Ue - it should be 1.2V and it is not, even though the Q429 tyransistor is good - measured with a multimeter in the diode mode on the table after removing it from the PCB.
Dc offsetting on temperature. Amém. Wow.
Hello, the factory data is 5mv. If I change it to 10 or 15 mv, will there be any harm or will I see any benefit? My device is rotel rb 1090 power amplifier
Higher bias will cause your output stages to run hotter. There may be a small (inaudible) drop in THD. I tend to run the bias at, or slightly lower than the factory recommended settings. Heat is bad for semiconductors.
Can you double the bias current? for better sounding
No, excessive bias current just adds excessive heat, and can shorten the life of your output transistors. Amplifiers that are biased for Class A operation need enormous heat sinks.
@@raygianelli3612 Yes, add or replace heatsinks or force convection, check xsistors max abs ratings
Hi, i have an harman 730 with distorted right channel. Right bias is 0. What might be the problem? Thanks
This is where an oscilloscope and sine wave generator shine. This would show where the trouble would likely be.
@@raygianelli3612 would adjusting the bias recommended?
@@VW--mo3zt I'd bet there wouldn't be any change, either in symptoms or zero bias. But it can't hurt to try.
@@raygianelli3612 thank you
one power transistor (black) is burned out, the other transistor (green) is good. After replacing the black transistor, I measured the voltage between the emitters, it is 0.754V too high and cannot be reduced, kindly tell me if it is because I only replaced the burnt transistor. Should I replace both power transistors in the damaged channel? I'm repairing HK655Vxi.
Often when an output transistor shorts it takes other components with it. Check back from the output, drivers and resistors.
@@raygianelli3612 I checked back and I found varistor mv12ym (chain of diodes) which is burnt out. I can not find any replacement. Maybe you know any substitute. Amplifier PM 855Vxi, element D409, kindly please help me.
@@danekogolniak9680 I can find nothing with that model number. I'm guessing it's a Harman Kardon PM655VXI. You can use 2 1N4148 diodes in series to replace the MV12YM.
@@raygianelli3612 thanks very much for your quick reply, yes, that was my mistake, this was for model PM655Vxi. Your advice is always very helpful.
@@raygianelli3612 After connecting the 1N4148 diodes in series, should they somehow be fastened to the main heat sink holder?
At 6:39 “… some people like to use multi-turn pots either 10 or 25 turns so that is not an issue. I use the single turn pots, you can get it, you just have to be patient”.
At 7:40 “Our offset is really good right now (meter shows perfect 0.000 mv) and I hate to touch it… because you will see just how touchy these adjustments are” [proceeds to just touch it (not turn it)] “it’s already over 300 millivolts” (factory spec is within 5 millivolts of zero). 😂
At 8:50 Finishes fiddling with offset “We got negative 2.6, 2.7 millivolts” (within factory spec). 😁
At 11:05 offset has been constantly drifting in the background over past several minutes and is now to negative 41 millivolts and is rapidly moving further away from factory spec of plus/minus 5 millivolts of zero.
😲😬
It's not stupid to have a high biased amp if it was designed to have a high class A bias. It is actually a good thing, because class A completely eliminated cross over distortion, thus sounds better. My JC1 mono blocks are biased to 25 Watts of class A and they sound amazing.
It's only stupid to set the bias higher than factory specs, because in most cases the amp will only run hotter with no quality gain. If you set your customers amps lower than the manufacture tested to be optimum, then you are short changing your customers in sound quality.
Some people have a "bias" to amps that run warmer than others and such a bias should never enter the equation when servicing amps.
If Marantz likes to see ~75 milliamps and you know how ridiculously hot that runs, and you (and they) know heat is a bad thing, why do they have that specification? And what is gained or lost by reducing that level of bias current?
They may have wanted it to run in Class A for the first few watts. IMO the heat sinks won't support that, and watching the distortion as the bias was dropped to sane levels led me to advocate for setting these amps to a far lower level.
you need a multiturn pot there .
What I need is not to be too cheap to buy multiturn pots. :) But you're right, it would make life easier. Still, between the coarse and fine adjustments it's not too bad.
This dude bench presses Marshall cabinets and does biceps curls with Fender Twins daily.
Yeah, but only Twin Reverb's with JBL speakers, not those wimpy ones with CTS. 😝
😂
Hy
sorry ray, i haf a amp ( realistic 32 9003 ), ant i tinks it need 2 bin tune in the bias.
the richte site, is ferry hot. hotter than the leftsite so. the amp is from 1995 ??? ant som caps bin don. so..
now i am on the point to do this. batt dont now how.
your way i onderstand it. batt mi amp, i dont now how 2 do it.( thers 1 pot on it ant turns, bat the messer on the multimeter. ?? i dont now the spot )
gr fr holland en sorry for mi shitty englis. thanks for the vid.
Jouw Engels is beter dan mijn Nederlands, dus laten we eens kijken hoe Google Translate werkt. Ik kan geen schema vinden voor die realistische 32 9003. Heb je een exemplaar?
I doubt that these 4 wire cables are any good because they measure the voltage at the same point where the current is inserted. If you use 4 wire resistance measurement you need to make sure, that you also insert the current at differnt points to the DuT. Otherwise you also measure the contact resistance in series with the resistance of the DuT. Especially if the device has a very low resistance, your results can be very wrong.
en.wikipedia.org/wiki/Four-terminal_sensing
@@raygianelli3612 correct. But four terminal sensing doesn't work if you only have 2 terminal at the DuT. It is important to sense the voltage without the voltage drop, which is caused by the transition resistance and the measuring current.
If you want to sense a resistance in the mOhm or even µOhm range you need to transfer larger currents to your DuT. You cannot measure the correct resistance at the same point where you introduce the current. Four terminals means four terminals at the instrument and also at the DuT.
so were gonna modify something designed by professional engineers suppose we can do
better than they did? bias an amp? yeah, follow thier schematics!
And what exactly was modified?
The engineers don’t know what tubes you are putting in or degradation of those tubes or the other materials in your amp.. things change over time.. even materials used by the oem mfg of the tubes could change and effect the voltage running through.