You mean the dc offset?
I turned the trimmers as low as possible, i got 60-70 mV on each channel.
Hope it will change after the recap...
I dunno, i mean.. I think the power consumption is quite high, so it could be even normal.
I turned the trimmers as low as possible, i got 60-70 mV on each channel.
Hope it will change after the recap...
I dunno, i mean.. I think the power consumption is quite high, so it could be even normal.
DC offset will not improve with a recap.
The DC offset is determined by the 2SA798 transistors that you've already replaced. These must be replaced by tightly matched KSA992 / 2SA992 / 2SA970 transistors. The pair should be very closely hFE rated. Check this with a hFE meter. It also needs to be high enough - really, as high as possible. 400 is a minimum value. This is the only way to get the DC offset down.
If you do not set the idling current, it's likely that the power stage will blow up after a while, or when playing loud. Do you have a service manual? It should specify how to do this. It's also called idle current or bias, in some cases.
Here's how to do it:
The DC offset is determined by the 2SA798 transistors that you've already replaced. These must be replaced by tightly matched KSA992 / 2SA992 / 2SA970 transistors. The pair should be very closely hFE rated. Check this with a hFE meter. It also needs to be high enough - really, as high as possible. 400 is a minimum value. This is the only way to get the DC offset down.
If you do not set the idling current, it's likely that the power stage will blow up after a while, or when playing loud. Do you have a service manual? It should specify how to do this. It's also called idle current or bias, in some cases.
Here's how to do it:
Thanks mate.
I can't understand though why the 1845 would generate on output more DC than a 992..
I will set the idle current tomorrow. Thanks guys.
I can't understand though why the 1845 would generate on output more DC than a 992..
I will set the idle current tomorrow. Thanks guys.
Hey guys,
I just did this operation. I got a strange behaviour though on the right channel, which is the same one i replaced the transistors. The value doesn't stay fixed, it increases slowly. That is, i might find the right point of 13 mV, but then it increases every second of 0.02 mV or so. Is that normal? Is it kinda dangerous for the amp?
I will replace the 1845 with ksa992, even though it's quite difficult to find the in my country.
I just did this operation. I got a strange behaviour though on the right channel, which is the same one i replaced the transistors. The value doesn't stay fixed, it increases slowly. That is, i might find the right point of 13 mV, but then it increases every second of 0.02 mV or so. Is that normal? Is it kinda dangerous for the amp?
I will replace the 1845 with ksa992, even though it's quite difficult to find the in my country.
Never replace 2SC1845/KSC1845 with KSA992, it's NPN vs PNP.
What I mean is you need to replace the 2SA798 dual transistor with two tightly matched KSA992 transistors. If you used 2SC1845 here, it's a miracle the amp still works!
The idling current will increase by itself. Just set it to whatever value it should be, after the mentioned amount of time, and then let it go. That's how you do it. 🙂
What I mean is you need to replace the 2SA798 dual transistor with two tightly matched KSA992 transistors. If you used 2SC1845 here, it's a miracle the amp still works!
The idling current will increase by itself. Just set it to whatever value it should be, after the mentioned amount of time, and then let it go. That's how you do it. 🙂
Tony Tecson you are my saviour!
I am a newbie to this tech stuff so I did what any goose would do, I poked and prodded my temperamental au4900 until a diode caught my attention while trying to blow my speakers. After reading many discussions on different sites about the replacement of diodes i cam across Tony's recommendation and gave it a shot. After giving the suspect diode legs a scrub with a mascara brush and some alcohol I couldn't believe my ears, it was like a brand new amp!! Lucky for the amp because I doubt my soldering skills would have brought it back to life..
Thanks Tony😉
I am a newbie to this tech stuff so I did what any goose would do, I poked and prodded my temperamental au4900 until a diode caught my attention while trying to blow my speakers. After reading many discussions on different sites about the replacement of diodes i cam across Tony's recommendation and gave it a shot. After giving the suspect diode legs a scrub with a mascara brush and some alcohol I couldn't believe my ears, it was like a brand new amp!! Lucky for the amp because I doubt my soldering skills would have brought it back to life..
Thanks Tony😉
I just finished repairing one of these mischievous kids that was in my possession (inherited) a few years ago without working. It was hard to identify the problem as it has happened to many, but I think that in 90% of cases the problem is in the differential torque of the input to the power plate, the TR 2SA798.
As you already know, it is a TR that is no longer manufactured, so you must find two individual TRs and couple them by HFE.
A replacement option is KSA992FB, but there was not enough in my country here and the price was outrageous.
So I bought 40 units of BC557, of Chinese origin ...., we already know that the specs can differ a lot from the originals ....
I managed to pair two 360 Hfe games, but one channel had less gain, I tried again with another pair achieving 350 Hfe, same result.
And since the PCB was already spoiling from both soldering and desoldering, I thought of ignoring the advice to "always change TRs on both channels" and replace the original 2SA798 that was not noisy.
The result is 100% successful. 🙂
Thanks to the contribution of the DiyAudio member - welcome - who uploaded the service manual for the adjustment of the DC Bias, that was very useful to me since I do not have an oscilloscope.
It is gratifying to find well-meaning people here who provide good information.
As you already know, it is a TR that is no longer manufactured, so you must find two individual TRs and couple them by HFE.
A replacement option is KSA992FB, but there was not enough in my country here and the price was outrageous.
So I bought 40 units of BC557, of Chinese origin ...., we already know that the specs can differ a lot from the originals ....
I managed to pair two 360 Hfe games, but one channel had less gain, I tried again with another pair achieving 350 Hfe, same result.
And since the PCB was already spoiling from both soldering and desoldering, I thought of ignoring the advice to "always change TRs on both channels" and replace the original 2SA798 that was not noisy.
The result is 100% successful. 🙂
Thanks to the contribution of the DiyAudio member - welcome - who uploaded the service manual for the adjustment of the DC Bias, that was very useful to me since I do not have an oscilloscope.
It is gratifying to find well-meaning people here who provide good information.
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I just finished repairing one of these mischievous kids that was in my possession (inherited) a few years ago without working. It was hard to identify the problem as it has happened to many, but I think that in 90% of cases the problem is in the differential torque of the input to the power plate, the TR 2SA798.
As you already know, it is a TR that is no longer manufactured, so you must find two individual TRs and couple them by HFE.
A replacement option is KSA992FB, but there was not enough in my country here and the price was outrageous.
So I bought 40 units of BC557, of Chinese origin ...., we already know that the specs can differ a lot from the originals ....
I managed to pair two 360 Hfe games, but one channel had less gain, I tried again with another pair achieving 350 Hfe, same result.
And since the PCB was already spoiling from both soldering and desoldering, I thought of ignoring the advice to "always change TRs on both channels" and replace the original 2SA798 that was not noisy.
The result is 100% successful. 🙂
Thanks to the contribution of the DiyAudio member - welcome - who uploaded the service manual for the adjustment of the DC Bias, that was very useful to me since I do not have an oscilloscope.
It is gratifying to find well-meaning people here who provide good information.
Just out of curiosity tell us what else did you repair on the amp
east electronics, my apologies for not responding, I haven't seen this thread since your post. No, I didn't do anything else, just regulate the quiescent current.
So now I have some news for you, the amplifier was working perfectly (that's what my daughter tells me, to whom I gave it because for her the amplifier has a sentimental value because it was her grandfather's amplifier) until it recently presented the same fault again .
So I will go for the replacement of the other differential pair that I manufactured with 2x BC557 paired to replace the original 2SA798, and that I keep as a precaution against possible loss, adhered to the inside of the metal cover to have it immediately available, you know, save Components is easy, finding them is hard. I hope it's still there.
Paragraph aside, as my Optoma projector stopped working, and it is not full HD, first I thought about buying a new one, then I stopped watching movies in DVD format, and there was a Yamaha AVR receiver with little use and in immaculate condition in the attic, so my daughter did not run out of music and I have the same problem again ..... 😉
I'll keep you posted when I deal with him.
P.S:
This is the typical fault of the 2900, 3900, and 4900 series of Sansuí from 40 years ago, the first TR of the power plate is a differential pair 2SA798 that came defective from the factory, if someone has the same fault, that is, explosions, crackles in both channels, - in a totally random way, which can appear and disappear without any previous warning - in 99% of the cases the fault is due to these damn TR.
I hope that it is not up to me to be in the remaining 1% in this new failure, I will keep you informed.
Worked for my Mitsubishi DA-U200 as well! This amp also has the 2SA798 transistor. And they had black legs. I was not looking forward to make my own dual transistors so I tried @TonyTecson 's cleaning method. Et voila! Since I cleaned them I've had no more "explosions" or other mishap. - fingers crossed! -
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here is what you say ....
Its like a 55 year old man goes to the doctor saying i got pain here ,i got pain there , i got high blod pressure , been smoking all my life , some issues with my prostat
and the doctor replies "go home and take a bath "
No way you are getting away with it .....
Imagine two tiny pieces of metal glued together with spot welding ..... in simple words this is what happens inside a transistor .
so you go home all your life turn on the amplifier cold and after a while amp is warm and you listen to music ....metal expands Late at night you go to sleep power down the amp it gets cold now metal shrinks
after 50 years of expanding and shrinking you have metal fatigue, spot welding gets loose Cool it down with freezer spray and it might wotk it shrinks and conducts again
warm it up and its going to make the same trouble again
there is no way aroud it Replace them
so you go home all your life turn on the amplifier cold and after a while amp is warm and you listen to music ....metal expands Late at night you go to sleep power down the amp it gets cold now metal shrinks
after 50 years of expanding and shrinking you have metal fatigue, spot welding gets loose Cool it down with freezer spray and it might wotk it shrinks and conducts again
warm it up and its going to make the same trouble again
there is no way aroud it Replace them
I agree with east electronics, brushing your teeth will not be a definitive remedy for cavities.
I have not yet replaced the TR 2SA798 of the other faulty channel. I just took the cover off and the handcrafted diff is there waiting for me. I'm on vacation right now away from music and enjoying nature. I will inform when the change is made.
I have not yet replaced the TR 2SA798 of the other faulty channel. I just took the cover off and the handcrafted diff is there waiting for me. I'm on vacation right now away from music and enjoying nature. I will inform when the change is made.
Update: I still haven't replaced the 2SA798 transistors, but the "cleaning session" seems to have prolonged their lives. The amp still plays noiselessly since I rubbed the transistors with isopropanol.