Hi y'all
While setting my bias current i blew up the two output transistors of the right channel of my denon pma 2000 mk IV :-(. Yes, I know, it's a stupid mistake...
I tested both transistors - Toshiba 2SK2967 mosf n channel - and they're shorted between source and gate/drain.
I'm looking for replacements, but I'm not really well informed on what to look for.
- And, while the amp is completely on the table, are there other modifications that someone knows about that can improve sound? I know the preamp sections isn't the best...
I'm looking forward to your feedback!
Kind regards
Will
While setting my bias current i blew up the two output transistors of the right channel of my denon pma 2000 mk IV :-(. Yes, I know, it's a stupid mistake...
I tested both transistors - Toshiba 2SK2967 mosf n channel - and they're shorted between source and gate/drain.
I'm looking for replacements, but I'm not really well informed on what to look for.
- should I go for the same type or are there already better replacements? Are those from China on Ebay really the same, or are they copy's?
- Are there other interesting platforms to buy on than Ebay? I know mouser, but these particular toshiba's are not available... I live in Belgium.
- Do I preferably change all 4 transistors, so the left channel also?
- And, while the amp is completely on the table, are there other modifications that someone knows about that can improve sound? I know the preamp sections isn't the best...
I'm looking forward to your feedback!
Kind regards
Will
Last edited:
Hi Will, have you asked a Denon dealer if they can supply new correct type transistors? Your amp is not very old, so there could still be original parts available.
I don't think it is a good idea to try and improve it with other transistors. It may just as well lead to the opposite result... 😉
I don't think it is a good idea to try and improve it with other transistors. It may just as well lead to the opposite result... 😉
https://www.denon.com/nl-nl/support/service-network
For the parts: address in Belgium, so you're oke;-)
I would strongly recommend to first do the repairs, then maybe tweak. If you are going to tweak, administration costs and transport are likely going to add up that much for the parts you are going to need minimum, that buying a few more fets won't hurt financially and can turn out to be a good investment if something goes wrong while doing the tweaks.
For the parts: address in Belgium, so you're oke;-)
I would strongly recommend to first do the repairs, then maybe tweak. If you are going to tweak, administration costs and transport are likely going to add up that much for the parts you are going to need minimum, that buying a few more fets won't hurt financially and can turn out to be a good investment if something goes wrong while doing the tweaks.
Out of curiosity, I just had a look at the schematics, and the Preamp section certainly looks nice enough:
- The circuit looks quite classic (and typical for Denon I think), and they have used good transistors. Don't see anything bad here! 🙂
Of course, it may be possible to do minor tweaks, but then you first have to study and understand the circuit really well.
Denon engineers are (were) not stupid, so generally not so simple to improve on their work.
Without deeper analysis, I might try and replace the two DC blocking feedback capacitors (C427 and C429) with two nice new bipolar caps - but with same uF values (!) - Just because I know from people smarter than me, that these capacitors are a critical point in the NFB circuit, and Bipolars are better in this position.
- But Denon is also aware of this, as they used back to back normal caps, to make a "fake bipolar" here - so perhaps no improvement to be had..
And the power amp section also looks nice enough, although I don't know why they had to use 3 differential pairs - seems overly complex for an amp with just one set of outputs:
I guess the most unusual feature (for a modern amp), is that it's a "quasi amp" with dual N channel mosfets. So there is some extra circuitry for phase inversion.
Regarding the SK2967 mosfets: IF you can't get new originals, then I think FQA40N25 should be a good replacement (and perhaps a better transistor in some ways) - and then I would personally change them on both channels 🙂
I guess the most unusual feature (for a modern amp), is that it's a "quasi amp" with dual N channel mosfets. So there is some extra circuitry for phase inversion.
Regarding the SK2967 mosfets: IF you can't get new originals, then I think FQA40N25 should be a good replacement (and perhaps a better transistor in some ways) - and then I would personally change them on both channels 🙂
Hi Will, C427 and C429 ARE a bipolar cap together.....just like C441 and C443 and everywhere else in the device. That are no fake bipolar caps as a "real" bipolar cap is simply 2 x back to back polar caps inside. So things have been done above average. Only age of the caps could be the reason to replace them. Just leave it like it is and contemplate if it would have been better not to touch it at all.
I was taught to replace power transistors in both channel in used amplifiers to avoid channel differences so make that 4 x Toshiba 2SK2967. Please check the driver transistors though to avoid sudden BOOM of the new Toshiba 2SK2967.
I was taught to replace power transistors in both channel in used amplifiers to avoid channel differences so make that 4 x Toshiba 2SK2967. Please check the driver transistors though to avoid sudden BOOM of the new Toshiba 2SK2967.
Last edited:
the "almost" only way to buy 2sk2967s and find a Denon with a single failed channel and get the working channel Mosfets back.
It is sometimes found on Ebay in Germany but it is becoming very rare.
I worked in the Hifi sector in Brussels about twenty years ago (in Uccle at Fort Jaco and in St Gilles).
It is sometimes found on Ebay in Germany but it is becoming very rare.
I worked in the Hifi sector in Brussels about twenty years ago (in Uccle at Fort Jaco and in St Gilles).
Toshiba TK70J20D are used in the 2500NE and 110. They are oversized but I guess the choice was made with some reason. You could get them through Mouser.
Hi everyone.
So I've finally received my new transistors. Thanks for the advice, I bought them with a Denon dealer nearby that ordered them for me.
I checked a lot of components in the power amp section and I found three of them faulty:
R557 (near Testpoint TP502)
R553 (just in front of endtransistor TR521)
And of course the transistors TR523 & 521 itself.
So these components have been replaced and, I don't seem to find any other faulty components in the power amp section, but it's not good. I have sound, but really distorted.
I tried adjusting the bias, but when measuring on the TP501 test points, I have AC power!! (millivolts) while the manual says it should be DC milivolts...
All red line voltages are okay, I don't see where to look further, maybe someone can point me in the right direction?
I posted the complete schematics so you don't have to search for it yourself.
Hope to hear ya soon.
So I've finally received my new transistors. Thanks for the advice, I bought them with a Denon dealer nearby that ordered them for me.
I checked a lot of components in the power amp section and I found three of them faulty:
R557 (near Testpoint TP502)
R553 (just in front of endtransistor TR521)
And of course the transistors TR523 & 521 itself.
So these components have been replaced and, I don't seem to find any other faulty components in the power amp section, but it's not good. I have sound, but really distorted.
I tried adjusting the bias, but when measuring on the TP501 test points, I have AC power!! (millivolts) while the manual says it should be DC milivolts...
All red line voltages are okay, I don't see where to look further, maybe someone can point me in the right direction?
I posted the complete schematics so you don't have to search for it yourself.
Hope to hear ya soon.
Last edited:
Update: I started desoldering components near the faulty measurements and for now I did find another faulty one.
Zenerdiode ZD509.. It's closed all the time, it acts as a resistor of 6 ohm instead...
If I understand the schematics a little, this closed zener diode would result in bypassing the endtransistors and testing points completely... Therefore no DC can be detected.
Zenerdiode ZD509.. It's closed all the time, it acts as a resistor of 6 ohm instead...
If I understand the schematics a little, this closed zener diode would result in bypassing the endtransistors and testing points completely... Therefore no DC can be detected.
Update: Just soldered the zener diode back in place and I have DC voltage at my testing points!!
Zener just has a little lower zener voltage point (8.2 v instead of 8.3 volt). But i think it wil work!
Zener just has a little lower zener voltage point (8.2 v instead of 8.3 volt). But i think it wil work!