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
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
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!
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.
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
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
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...)
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
Did you check the bias setting after replacing the transistors?
Jan Didden
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.
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.
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
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
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...
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...
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
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
Hi all,
I can't resist anymore, so I put a 100w bulb light in series with the main and switch the power ON.
I have tested at a low volume but still now (20 minutes) everythings seems to be ok and the transistors are quite cold.
One question: when I pump up the volume the bulb light became red (dull glow): is it normal ? Can I pump more ?
Thank you for your answers
I can't resist anymore, so I put a 100w bulb light in series with the main and switch the power ON.
I have tested at a low volume but still now (20 minutes) everythings seems to be ok and the transistors are quite cold.
One question: when I pump up the volume the bulb light became red (dull glow): is it normal ? Can I pump more ?
Thank you for your answers
Hello! I can't understand from your post if the amp works or not. Does it emit sound? If yes, I think you can safely pump a little up the volume (the glow in the bulb is normal, you are increasing the amp's consumption so in the lamp will pass more current). But I think you can plug it directly to the mains if everything seems ok.
Full volume is always dangerous with the bulb in series: with a different than expected rails voltage, some things couldn't work (like some zeners in the power amp section) or could work strange (expect oscillations at very low rails voltages - don't ask me why I don't know).
Good luck paisà!
Full volume is always dangerous with the bulb in series: with a different than expected rails voltage, some things couldn't work (like some zeners in the power amp section) or could work strange (expect oscillations at very low rails voltages - don't ask me why I don't know).
Good luck paisà!
Hello, Giaime
YES the amp works on both the channels.
A red light comes out from bulb when I pump up volume, but it is completely off at minimun volume.
So it'is ok ?
Tomorrow I'll do an "harder test".
Good night
YES the amp works on both the channels.
A red light comes out from bulb when I pump up volume, but it is completely off at minimun volume.
So it'is ok ?
Tomorrow I'll do an "harder test".
Good night
I think it's ok... but it is rare that an amp which burned its outputs didn't burn it's drivers... or you're just lucky. Be sure to double check everything: do a little test on each transistor you can find in the unit. It's easy to do with a DMM with "diode test" function. Search the net to find how.
Good luck!
Good luck!
Fake Sanken power transistors
I had bought some bad power transistor from Pacific Semi years ago, and since then, I had spoken to other electronic hobby guys that also found ALL their transistor are FAKE. I had spoken to the owner, and he just does not care, as long as he could sell the fake transistors and make a few $$$$$.
I had bought some bad power transistor from Pacific Semi years ago, and since then, I had spoken to other electronic hobby guys that also found ALL their transistor are FAKE. I had spoken to the owner, and he just does not care, as long as he could sell the fake transistors and make a few $$$$$.
Output transistors rarely go for no reason.
Shorts on the output can blow them.
Faults in the bias circuit can also blow them.
You need to dig around and find out why they blew.
I had a amp blow like that and the reason they went is because the speakers had gone short circuit and put too big a load on the amp.
Shorts on the output can blow them.
Faults in the bias circuit can also blow them.
You need to dig around and find out why they blew.
I had a amp blow like that and the reason they went is because the speakers had gone short circuit and put too big a load on the amp.
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