power transistor assembly(how to?)

picchiatello · 20th March 2005, 02:25 PM

Hi,
I have to replace the power transistor on my amp and I've a question.
I've seen that between the rear part of transistor and the aluminium cooling side there is a plastic film. Is it to isolate electrically the transistor from the cooling aluminium ?
Then if so I must check that the metallic case of transistor is electrically isolated from cooling alum before put the amp in main?
Please answer me.
Regards

Giaime · 20th March 2005, 02:36 PM

Hello! Yes, that is to isolate the tab of the transistor from the heatsink. Notice that it isn't plastic, it is either mica or silicon, which gives a better thermal resistance (I mean you want that the heat goes to the heatsink without problems). If your amp (and you will help me telling what amp is) has these things, you should assure that the transistor tab is effectively isolated from the heatsink (but wait, on some amps the continuity test between tr's tab and heatsink shows conductivity even with the insulators; those are strange amps, the best way to test is to isolate every pin of the transistors from the circuit, and THEN do the test). You'll have to put thermal compound (silicone grease) on both faces of the insulator, to ensure contact with either the transistor and the heatsink. Don't put too much of it!

shusha · 20th March 2005, 04:10 PM

You should also pay attention to little plastic insulator (sorry I can find word fot that) which comes underneath the screw. Otherwise you will have direct contact from transistor to the heatsink through the screw.

picchiatello · 20th March 2005, 05:00 PM

Hi,

and thanks for your replies.
My amp is a Denon pma1060.
I've just replaced the power transistors (2sa1491/2sc3855 SANKEN) with 2sa1491/2sc3856 (2sc3855 not avaiable).
All works fine for 2 minutes and then.... the fuse was out and the 3856 were broken!!.
Probably I get too few attention in soldering or something else, so I'll repeate with 2sc1695/2sc468 doing everithing slowly.
I'm still looking for a cheap supplier of sanken transistor, any suggestion? ( I've contacted Profusionplc and pacificsemi and I'm waiting for the price of delivery)
Regards

Giaime · 20th March 2005, 06:12 PM

Hello! I hope that before turning the amp on you checked ALL the transistors (drivers, current limiting, others) and even resistors, caps, diodes, etc... Many things can go wrong (and GO wrong) when you burn an output transistor. Pay attention even to the other things mounted on the heatsink near the output transistors (there are usually 2 transistors or diodes, there are for thermal compensation, i.e. when the amp gets hot they limit the power). And like our fellow shusha said, don't let the screw make short-circuit to the heasink! (But I have an amp that was designed for that...)

janneman · 20th March 2005, 06:27 PM

If you replace the output transistors, there's a 50-50 chance that the bias current for the new ones is too high. In that case, it will play for some time, until it gets too warm, and poof! Just what you reported.
Did you check the bias setting after replacing the transistors?

Jan Didden

whalefat · 20th March 2005, 07:13 PM

Do not buy from pacific semi!
They are a local store here and the guy sells fake transistors.

2SK1058/2SJ162 - fake
2SC3281/2SA1302 - fake

I wouldn't be surprised if everything he sold were fakes.
My experience with them was 2years ago, first and last.
After I found out they were fakes, I got a refund from him by stating all the tests that pointed to a fake transistor and contacting the Better Business Bureau.

picchiatello · 20th March 2005, 09:04 PM

Thank a lot for your answer!
Regarding the bias current, I've to say that I don't know how to check it, and atthis point I belive that I have few hope to repair my amp by myself!
I've not a big experience in audio and electronics, I just am, a guy with a lot of curiosity and patience.
Anyway, if you can suggest me how to measure with my digital multimeter....

Regards

Giaime · 21st March 2005, 11:37 AM

Well... you know, I can tell you but... there are high (not so, but dangerous) voltages involved, so if you have little experience it would be better to take it to a friend who knows what to do and learn from him: that's how I started.

Look for a service manual for your amp: usually inside them there's a detailed explanation about biasing that particular amp...

Or if you have a schematic, post it here and we'll see...

picchiatello · 26th March 2005, 06:52 PM

Hi all,

and thank you for your replies.
Unfortunatelly I've not found any document on about my amp denon pma-1060.
On the pcb, near the transistors there are 2+2 trimmer (the only one). Are they fo bias adjust?
Any suggestion befor re-burn everything? (Just replaced n.4 2sa1695 + n.4 2sc4468 of sanken)

To Giaime : can I write in PM and send you a detailed pic of pcb ?

Thank you

20th March 2005, 02:25 PM	#1
picchiatello diyAudio Member Join Date: Mar 2005 Location: Naples, ITALY	power transistor assembly(how to?) Hi, I have to replace the power transistor on my amp and I've a question. I've seen that between the rear part of transistor and the aluminium cooling side there is a plastic film. Is it to isolate electrically the transistor from the cooling aluminium ? Then if so I must check that the metallic case of transistor is electrically isolated from cooling alum before put the amp in main? Please answer me. Regards

20th March 2005, 06:27 PM	#6
janneman diyAudio Member Join Date: May 2002 Location: Where Germany, The Netherlands and Belgium meet Blog Entries: 6	If you replace the output transistors, there's a 50-50 chance that the bias current for the new ones is too high. In that case, it will play for some time, until it gets too warm, and poof! Just what you reported. Did you check the bias setting after replacing the transistors? Jan Didden __________________ /Another new issue: Linear Audio Volume 3!

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20th March 2005, 02:36 PM	#2
Giaime diyAudio Member Join Date: Oct 2004	Hello! Yes, that is to isolate the tab of the transistor from the heatsink. Notice that it isn't plastic, it is either mica or silicon, which gives a better thermal resistance (I mean you want that the heat goes to the heatsink without problems). If your amp (and you will help me telling what amp is) has these things, you should assure that the transistor tab is effectively isolated from the heatsink (but wait, on some amps the continuity test between tr's tab and heatsink shows conductivity even with the insulators; those are strange amps, the best way to test is to isolate every pin of the transistors from the circuit, and THEN do the test). You'll have to put thermal compound (silicone grease) on both faces of the insulator, to ensure contact with either the transistor and the heatsink. Don't put too much of it!

20th March 2005, 04:10 PM	#3
shusha diyAudio Member Join Date: Sep 2004 Location: Velika Gorica	You should also pay attention to little plastic insulator (sorry I can find word fot that) which comes underneath the screw. Otherwise you will have direct contact from transistor to the heatsink through the screw.

20th March 2005, 05:00 PM	#4
picchiatello diyAudio Member Join Date: Mar 2005 Location: Naples, ITALY	Hi, and thanks for your replies. My amp is a Denon pma1060. I've just replaced the power transistors (2sa1491/2sc3855 SANKEN) with 2sa1491/2sc3856 (2sc3855 not avaiable). All works fine for 2 minutes and then.... the fuse was out and the 3856 were broken!!. Probably I get too few attention in soldering or something else, so I'll repeate with 2sc1695/2sc468 doing everithing slowly. I'm still looking for a cheap supplier of sanken transistor, any suggestion? ( I've contacted Profusionplc and pacificsemi and I'm waiting for the price of delivery) Regards

20th March 2005, 06:12 PM	#5
Giaime diyAudio Member Join Date: Oct 2004	Hello! I hope that before turning the amp on you checked ALL the transistors (drivers, current limiting, others) and even resistors, caps, diodes, etc... Many things can go wrong (and GO wrong) when you burn an output transistor. Pay attention even to the other things mounted on the heatsink near the output transistors (there are usually 2 transistors or diodes, there are for thermal compensation, i.e. when the amp gets hot they limit the power). And like our fellow shusha said, don't let the screw make short-circuit to the heasink! (But I have an amp that was designed for that...)

20th March 2005, 07:13 PM	#7
whalefat diyAudio Member Join Date: Mar 2005 Location: Canada	Do not buy from pacific semi! They are a local store here and the guy sells fake transistors. 2SK1058/2SJ162 - fake 2SC3281/2SA1302 - fake I wouldn't be surprised if everything he sold were fakes. My experience with them was 2years ago, first and last. After I found out they were fakes, I got a refund from him by stating all the tests that pointed to a fake transistor and contacting the Better Business Bureau.

20th March 2005, 09:04 PM	#8
picchiatello diyAudio Member Join Date: Mar 2005 Location: Naples, ITALY	Thank a lot for your answer! Regarding the bias current, I've to say that I don't know how to check it, and atthis point I belive that I have few hope to repair my amp by myself! I've not a big experience in audio and electronics, I just am, a guy with a lot of curiosity and patience. Anyway, if you can suggest me how to measure with my digital multimeter.... Regards

21st March 2005, 11:37 AM	#9
Giaime diyAudio Member Join Date: Oct 2004	Well... you know, I can tell you but... there are high (not so, but dangerous) voltages involved, so if you have little experience it would be better to take it to a friend who knows what to do and learn from him: that's how I started. Look for a service manual for your amp: usually inside them there's a detailed explanation about biasing that particular amp... Or if you have a schematic, post it here and we'll see...

26th March 2005, 06:52 PM	#10
picchiatello diyAudio Member Join Date: Mar 2005 Location: Naples, ITALY	Hi all, and thank you for your replies. Unfortunatelly I've not found any document on about my amp denon pma-1060. On the pcb, near the transistors there are 2+2 trimmer (the only one). Are they fo bias adjust? Any suggestion befor re-burn everything? (Just replaced n.4 2sa1695 + n.4 2sc4468 of sanken) To Giaime : can I write in PM and send you a detailed pic of pcb ? Thank you

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