Amp was working fine, not on loud when it went pop! Cue smoke and smell!! I've taken the lid off and one of the transistors had the side blown off, i've downloaded the service manual(which annoyingly has the schematic missing!!) and the one with the side blown out was called a biasing transistor. I replaced that and fired up the amp, one of the power transistors got VERY hot(smoked!) and the amp blew the fuse in the plug. I read elsewhere on the 'net to pull the power transistors so i did, the amp now powers up but obviously does not work. Is it worth just replacing the power transistors or does this point to other faults? Thanks.
Blown amplifier
- Thread starter drgl
- Start date
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Yes it is likely something else is causing problems but first check the obvious - disconnect the speaker at the back of the amp.
You need a multimeter and you want to check transistors (or mono IC?) in the output stage and diodes etc. related to the amp for shorts (meter should read about 0.6V between e/b c/b nothing between c/e and same for diodes anode / cathode but not reversed - may need to remove a lead(s) for accurate result) but there are all sorts of other things like open circuit resistors - look for burn marks / small holes etc. and dodgy capacitors (unlikely here)- Also possible is a power supply fault but I would think both sides would be effected here.
Some components might break down under load and this is where things start getting complicated and needs proper tech. to investigate...
Look to Mauritron for a service manual or diagram. You might get lucky and find a free schematic if you look (ESA or somthing rings a bell) but not sure about being complete!
BTW I am not a a/v tech but I will try to help where I can!
Oh and you should be careful removing components in an amp as this could potentially make things worse and unbalance things resulting in more smoke - only disconnect to check the component in question then reconnect!
You need a multimeter and you want to check transistors (or mono IC?) in the output stage and diodes etc. related to the amp for shorts (meter should read about 0.6V between e/b c/b nothing between c/e and same for diodes anode / cathode but not reversed - may need to remove a lead(s) for accurate result) but there are all sorts of other things like open circuit resistors - look for burn marks / small holes etc. and dodgy capacitors (unlikely here)- Also possible is a power supply fault but I would think both sides would be effected here.
Some components might break down under load and this is where things start getting complicated and needs proper tech. to investigate...
Look to Mauritron for a service manual or diagram. You might get lucky and find a free schematic if you look (ESA or somthing rings a bell) but not sure about being complete!
BTW I am not a a/v tech but I will try to help where I can!
Oh and you should be careful removing components in an amp as this could potentially make things worse and unbalance things resulting in more smoke - only disconnect to check the component in question then reconnect!
Hi, thanks for the reply. The speakers are disconnected. I should have said, i also replaced all the large wire wound resistors(next to the power trans), on the side that has the blown trans someone had previously replaced these with the incorrect rating so they had joined two together, these seemed blackened slightly so i'm wondering if it was just that that let go which then took out the output trans & the biasing trans. I was planning to refit the output trans on the opposite side to see if it still gets hot, thinking been if it does then it is prob just a shorted output trans. I'll get the meter on all of them & report back. 
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OK, got chance to look over it more yesterday, i first found a short resistor & blown bias transistor, all replaced, i have checked ALL the output transistors and ALL are short. Was going to check all the driver transistors next before replacing the outputs & firing up!
Do you think it could just have been the resistor? They are 0.38 ohm wire wound, connected to the output transistors, i've replaced all 4 but only one was short and blackened! That along with the bias transistor with it's side blown off were the only visible signs i could see, i was surprised ALL the outputs were short TBH!!! I think i'll ehip the driver transistors out and check them(although somone else has suggested to just replace them but i do not know if i can source them yet).
Right, I actually got 10 mins with this amp today, I replaced all of the driver transistors & all 4 of the power transistors.
I fired it up & it powered up fine
I connected a source & speakers and powered up again, all working fine.
However, at this point I noticed the left hand power meter was reading even when the volume was off(& seemed to over read more than the right channel), I also noticed MASSIVE bass!!
Now I thought it may have been me so powered it off and stuck the lid on, I then powered it up again and it went straight off, I'm guessing it has blown the fuse in the plug again(the amp fuse is fine).
Any ideas? I'm guessing it has fried a load of transistors again, it's not so much fixing it because I need to more of the challenge!!
I fired it up & it powered up fine
I connected a source & speakers and powered up again, all working fine.
However, at this point I noticed the left hand power meter was reading even when the volume was off(& seemed to over read more than the right channel), I also noticed MASSIVE bass!!
Now I thought it may have been me so powered it off and stuck the lid on, I then powered it up again and it went straight off, I'm guessing it has blown the fuse in the plug again(the amp fuse is fine).
Any ideas? I'm guessing it has fried a load of transistors again, it's not so much fixing it because I need to more of the challenge!!
You are going to struggle without a circuit diagram tbh. You could go round measuring resistors in the output stage (disconect and compare colour bands to meter) and perhaps look more closely at electrolytics for signs of leakage or bulging at the tops although I would think unlikely. Could be something like another transistor or diode that is either leaky or tests ok until under load which can be real hard to find...
Amps are fairly critical in operation and takes a relatively small shift in bias current and bang goes a fuse and often the output stage.
A couple of forums with some experienced techs are electro-tech and UK Vintage radio (who always seem to be polite and helpful toward newbies) this may be a good start.
Anyway don't give up as it is all good experience!
Amps are fairly critical in operation and takes a relatively small shift in bias current and bang goes a fuse and often the output stage.
A couple of forums with some experienced techs are electro-tech and UK Vintage radio (who always seem to be polite and helpful toward newbies) this may be a good start.
Anyway don't give up as it is all good experience!
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