denon pma 250 Trouble

srinath · 26th July 2010, 06:02 PM

OK I have a denon pma 250 that seems to be a pre-main amplifier whatever that is.
It was only working in 1 channel. Left.

I found a bad power transistor in it. It has a c3853 and a1489 which translate to NTE390/391 pair in each channel or so I think.
One c3853 tested bad.

I removed it and after looking at everything in it mostly a visual check, I swapped the other transistor with the one in the other channel just to see if it does switch sides. Yes the right channel started working.

Then I bought the nte390 chip and replaced it from the location that other one was in. Now it only has 1 channel as well. But its the left channel.

But wait it get more tricky. I pull the left channel fuse, then the right one works. Put the left fuse back in and remove the right and left works. So its not that one is dead. So both fuses = left, one fuse = whatever that fuse connects to.
This thing has no balance its a rather bare bones amp. Its also an extremely simple amp. No tuner no nothing. Simple ...
Any one have somehting like this happen.

The other thing that may be related is that it puts out a lot of volume with very low volume setting. Like I need 1/4 volume on my 26 watt marantz to get the sound I get with 1/10th volume on this on the same speakers. That is odd with a 30 watt amplifier. Maybe it was bridged - and if so, how to check, and how to unbridge it.

Thanks y'all.
Cool.
Srinath.

sakis · 27th July 2010, 08:46 AM

nte replacements are crappy get original parts ....nte is close ...is not exactly what you want ....also you have some other type of failure that you need to trace down

i will look for a schematic and get back to you i think tomorow

regards sakis

sakis · 27th July 2010, 10:06 AM

hmmmm you are in the wrong track ...my friend ... from the things you write above it seems that you dont have what it takes to repair the amplifier

probably there is no left fuse and right fuse but positive fuse and negative fuse ...removing one or the other will result amplifier out of balance

as said above NTE replacements do not work properly you need to get proper tranistors ... then again its most likely that beyond output tranistors ( and not chips or potatoes as you call them ) fail but also other smaller devices will follow so replacing one output transistor will not do the trick ....

you may read this vintage amplifier repair/upgrade manual and if you find it understandable you should proceed for the repair ...if not look for expert help

regards sakis

srinath · 27th July 2010, 01:46 PM

Thanks I will read that manual. This amp is very simple. I would actually not even care if I dont fix it. I am learning more than anything. I have fixed much more complicated amps, but I had schematics and other things to help me. Also I suspect somehow it has been bridged. It has a lot of power, much more than it should for the 30 watts it is supposed to have.
Thanks.
Srinath.

srinath · 31st July 2010, 11:17 PM

Fixed ...

2 problems, both of which related to the srinath ...

First the srinath tried to pull the NTE390 transistor ... so he unscrewed it from the heat sink, then turned it over and desoldered 2 legs and ... the transistor fell out. Yes, desolder 2 legs and it falls out ... yup ... not 3, just 2 ... yea collector and emmitter, that's all you need ...
OK so now the srinath solders the legs all in with 35lb of solder per leg, all 3 in there now ... then powers up ... and its working ... wait ... yes it is, but the volume knob is a 2 dial knob, only the 2 knobs are so tight fitting and same radius ... and near impossible to spot that its 2, and they spin separate from each other ... so fnally it works ... and it has balance ... well ... duh.

OK its got tons of static mainly related to having treble. De oxing and the output caps gonna be pulled and swapped ... but can someone point me to anything else.

Thanks guys for all the help.

Cool.
Srinath

sakis · 1st August 2010, 07:59 AM

there is no output caps in this amplifier ... only power supply caps ...
then you need to verify that Dc ofset and idle is the same for both chanels ...if it is keep in mind that nte replacements will fail if stressed hard ... or simply compaired with the original

small electrolytics can cause more damage than the big ones if leacky ...

that is about it ( if repair for you means that music comes out of the speakers you may as well stop there .... personally i would treat this amplifier with something better than that )

kind regards sakis

srinath · 1st August 2010, 01:02 PM

Hey man, you have a schematic for it ? you seem to know more about this amp than I do.

Anyway I am not even close to done fixing it. This amp is going to work like it did when new, and I am certain it didn't sound like this when new.

Its my learning and experimenting platform, its simple enough and yet offers all the things I need to learn. Thanks for your help, stay tuned.

Cool.
Srinath.

srinath · 1st August 2010, 10:46 PM

OK I worked on it a little more.

The thing is giving out a volume dependent whine, like a high pitched shreak that is just smooth and ear piercing that gets louder when you open the volume more. It actually starts earlier and is louder with higher treble. makes me think its there all the time but gets audible only past 1/2 treble with ~1/4 volume of course it gets audible with 1/4 treble pnly past 1/2 volume and at full treble it starts lower on the volume.

Then I see TP1, 2, 3, 4 - what are these supposed to measure.

I dont see a dc offset adjustment in it, so I presume its non adjustable, and it gets out of spec when components are dead.

Thanks.
Srinath.

srinath · 4th August 2010, 02:34 AM

The amp sounds better, a lot better with the fuse for the rear section removed, so left speaker alone works, and sounds great. Is it possible that a leaking cap ruined that channel and since its bad even now, its sounding bad, and I need to isolate it and fix it.
Thanks.
Srinath.

Quote:

Originally Posted by sakis

there is no output caps in this amplifier ... only power supply caps ...
then you need to verify that Dc ofset and idle is the same for both chanels ...if it is keep in mind that nte replacements will fail if stressed hard ... or simply compaired with the original

small electrolytics can cause more damage than the big ones if leacky ...

that is about it ( if repair for you means that music comes out of the speakers you may as well stop there .... personally i would treat this amplifier with something better than that )

kind regards sakis

mantaray · 14th November 2011, 12:25 PM

need schematic for denon pma250
i have problem with this amplifier - loud "pop" from speakers when turn it off.

26th July 2010, 06:02 PM	#1
srinath diyAudio Member Join Date: Feb 2010	denon pma 250 Trouble OK I have a denon pma 250 that seems to be a pre-main amplifier whatever that is. It was only working in 1 channel. Left. I found a bad power transistor in it. It has a c3853 and a1489 which translate to NTE390/391 pair in each channel or so I think. One c3853 tested bad. I removed it and after looking at everything in it mostly a visual check, I swapped the other transistor with the one in the other channel just to see if it does switch sides. Yes the right channel started working. Then I bought the nte390 chip and replaced it from the location that other one was in. Now it only has 1 channel as well. But its the left channel. But wait it get more tricky. I pull the left channel fuse, then the right one works. Put the left fuse back in and remove the right and left works. So its not that one is dead. So both fuses = left, one fuse = whatever that fuse connects to. This thing has no balance its a rather bare bones amp. Its also an extremely simple amp. No tuner no nothing. Simple ... Any one have somehting like this happen. The other thing that may be related is that it puts out a lot of volume with very low volume setting. Like I need 1/4 volume on my 26 watt marantz to get the sound I get with 1/10th volume on this on the same speakers. That is odd with a 30 watt amplifier. Maybe it was bridged - and if so, how to check, and how to unbridge it. Thanks y'all. Cool. Srinath.

27th July 2010, 08:46 AM	#2
sakis diyAudio Member Join Date: Jan 2006 Location: nea makri athens greece	nte replacements are crappy get original parts ....nte is close ...is not exactly what you want ....also you have some other type of failure that you need to trace down i will look for a schematic and get back to you i think tomorow regards sakis __________________ SERVICE ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr

27th July 2010, 10:06 AM	#3
sakis diyAudio Member Join Date: Jan 2006 Location: nea makri athens greece	hmmmm you are in the wrong track ...my friend ... from the things you write above it seems that you dont have what it takes to repair the amplifier probably there is no left fuse and right fuse but positive fuse and negative fuse ...removing one or the other will result amplifier out of balance as said above NTE replacements do not work properly you need to get proper tranistors ... then again its most likely that beyond output tranistors ( and not chips or potatoes as you call them ) fail but also other smaller devices will follow so replacing one output transistor will not do the trick .... you may read this vintage amplifier repair/upgrade manual and if you find it understandable you should proceed for the repair ...if not look for expert help regards sakis __________________ SERVICE ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr

31st July 2010, 11:17 PM	#5
srinath diyAudio Member Join Date: Feb 2010	Fixed. Fixed ... 2 problems, both of which related to the srinath ... First the srinath tried to pull the NTE390 transistor ... so he unscrewed it from the heat sink, then turned it over and desoldered 2 legs and ... the transistor fell out. Yes, desolder 2 legs and it falls out ... yup ... not 3, just 2 ... yea collector and emmitter, that's all you need ... OK so now the srinath solders the legs all in with 35lb of solder per leg, all 3 in there now ... then powers up ... and its working ... wait ... yes it is, but the volume knob is a 2 dial knob, only the 2 knobs are so tight fitting and same radius ... and near impossible to spot that its 2, and they spin separate from each other ... so fnally it works ... and it has balance ... well ... duh. OK its got tons of static mainly related to having treble. De oxing and the output caps gonna be pulled and swapped ... but can someone point me to anything else. Thanks guys for all the help. Cool. Srinath

1st August 2010, 07:59 AM	#6
sakis diyAudio Member Join Date: Jan 2006 Location: nea makri athens greece	there is no output caps in this amplifier ... only power supply caps ... then you need to verify that Dc ofset and idle is the same for both chanels ...if it is keep in mind that nte replacements will fail if stressed hard ... or simply compaired with the original small electrolytics can cause more damage than the big ones if leacky ... that is about it ( if repair for you means that music comes out of the speakers you may as well stop there .... personally i would treat this amplifier with something better than that ) kind regards sakis __________________ SERVICE ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr


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27th July 2010, 01:46 PM	#4
srinath diyAudio Member Join Date: Feb 2010	Thanks I will read that manual. This amp is very simple. I would actually not even care if I dont fix it. I am learning more than anything. I have fixed much more complicated amps, but I had schematics and other things to help me. Also I suspect somehow it has been bridged. It has a lot of power, much more than it should for the 30 watts it is supposed to have. Thanks. Srinath.

1st August 2010, 01:02 PM	#7
srinath diyAudio Member Join Date: Feb 2010	Hey man, you have a schematic for it ? you seem to know more about this amp than I do. Anyway I am not even close to done fixing it. This amp is going to work like it did when new, and I am certain it didn't sound like this when new. Its my learning and experimenting platform, its simple enough and yet offers all the things I need to learn. Thanks for your help, stay tuned. Cool. Srinath.

1st August 2010, 10:46 PM	#8
srinath diyAudio Member Join Date: Feb 2010	OK I worked on it a little more. The thing is giving out a volume dependent whine, like a high pitched shreak that is just smooth and ear piercing that gets louder when you open the volume more. It actually starts earlier and is louder with higher treble. makes me think its there all the time but gets audible only past 1/2 treble with ~1/4 volume of course it gets audible with 1/4 treble pnly past 1/2 volume and at full treble it starts lower on the volume. Then I see TP1, 2, 3, 4 - what are these supposed to measure. I dont see a dc offset adjustment in it, so I presume its non adjustable, and it gets out of spec when components are dead. Thanks. Srinath.

14th November 2011, 12:25 PM	#10
mantaray diyAudio Member Join Date: Nov 2011	need schematic for denon pma250 i have problem with this amplifier - loud "pop" from speakers when turn it off.

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