Hi, I have a musical fidelity A100 which runs like a wok. To my regret, I happened to forget to turn it off last night, and when I was back from work tonight it was as hot as a volcano.
I quickly turned it off and scrubbed it with semi-wet cloth to cool it down.
After an hour or so, the amp was powered up for check. Power was ok, but no sound. I'm sure it's not the thermal fuse, as the thermal fuse failure doesn't turn it on at all.
Any idea is appreciated. Thanks,
I quickly turned it off and scrubbed it with semi-wet cloth to cool it down.
After an hour or so, the amp was powered up for check. Power was ok, but no sound. I'm sure it's not the thermal fuse, as the thermal fuse failure doesn't turn it on at all.
Any idea is appreciated. Thanks,
Thanks for your reply, Hjelm
I did not check the voltage due to the funny design of this amplifier. The enclosure itself is the heatsink. So when I open the case to see the circuit, I can't power it up and measure the voltage. (Perhaps, I need very good fan to start with)
I hooked my other amp up and the speakers and all others are ok.
Electrolytics for this amps are rated 85degrees where it runs in over 90degrees environment. Everyone suggested me to replace them with 105degrees. "It ain't broke, do not change it " philosophy prevented me from doing that. Perhaps it's now a good reason to replace them...
I did not check the voltage due to the funny design of this amplifier. The enclosure itself is the heatsink. So when I open the case to see the circuit, I can't power it up and measure the voltage. (Perhaps, I need very good fan to start with)
I hooked my other amp up and the speakers and all others are ok.
Electrolytics for this amps are rated 85degrees where it runs in over 90degrees environment. Everyone suggested me to replace them with 105degrees. "It ain't broke, do not change it " philosophy prevented me from doing that. Perhaps it's now a good reason to replace them...
I do not have the inside look of this amp, but there is an excellent site shows everything of A1, which is a little brother of A100.
http://www.mhennessy.f9.co.uk/mf_a1/
Yes, it is class-A claiming amplifier--excessive heat is then justified
http://www.mhennessy.f9.co.uk/mf_a1/
Yes, it is class-A claiming amplifier--excessive heat is then justified
Did you look at this thread?
http://www.diyaudio.com/forums/showthread.php?s=&threadid=20017&highlight=
There's some good info there.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=20017&highlight=
There's some good info there.
Netlist said:
Yes, I did, thanks.
Everything I know about this amp has been obtained from this thread.
They however, don't give me a straightforward answer towards my problem..
In summary, I think what has been discussed in the thread is:
1. Check any physical defects, intermittent soldering etc.
2. replace all electro caps
3. replace all transistors
4. replace some resistors
Of course, it's very good practice to do so. But I just like to know where will mostly likely to be the problem, before I spend excessive labour, time and money.
Thanks for quick replies!!
Well you can measure, almost, all of the suggested components while the amplifier is off.
The amplifier is stone dead according to your description so the broken device should be possible to find.
Do you have a ohm meter then all caps should look like shortcuts, or increase in resistance as you measure them.
A diod tester should tell if the zeners are broken.
Measure if possible from leg to leg on all components so possible faulty traces are found.
Start from the input and back.
Good luck, it is fun to search for errors if you have the time!
The amplifier is stone dead according to your description so the broken device should be possible to find.
Do you have a ohm meter then all caps should look like shortcuts, or increase in resistance as you measure them.
A diod tester should tell if the zeners are broken.
Measure if possible from leg to leg on all components so possible faulty traces are found.
Start from the input and back.
Good luck, it is fun to search for errors if you have the time!
Running the amp with the lid off....
I works for a few minutes, use a temp meter(I used a passive one for cooking) on the u-shaped metal thing and switch off when it hits 80 degrees celcius.
You will then be able to measure some of the fundamentals like ps voltage and any other supply voltage as per the schematics.
I works for a few minutes, use a temp meter(I used a passive one for cooking) on the u-shaped metal thing and switch off when it hits 80 degrees celcius.
You will then be able to measure some of the fundamentals like ps voltage and any other supply voltage as per the schematics.
Re: Running the amp with the lid off....
Sorry, but not sure what Frank's theory is..
On the very first day I bought this amp secondhand, I opened the top panel and ran it and straight away, it went dead. Perhaps with powerful fan, I would try..Otherwise..I'm not a very brave man
I had some trouble to take the pcb out of the enclosure. A thin spanner seemed necessary to release the volume knob before taking the board out.
BTW, can I replace electro caps with polyester or MKT? Electrocaps sold by local component shops are cheap stuffs made in china, and even if it is claimed 105degrees, I doubt the quality.
It was in fact, first time to take a thorough look at my amp since I bought it. To my surprise, transistors in left/right channels are not the same types. In the left (or the right) channel, they are the standard MJI5003 and MJI5004, while the other is equiped with A2n8705 and probably A2n8706 (not sure, since the last digit was covered with solder). Perhaps, the ex-owner has replaced them, I suppose.
I just remembered that before the amp broken, when the volume was set 0, the right channel still remains while the left goes silent. I knew some components were mismatched. Heterogenous transistors are definite reason for this, aren't they?
BobEllis said:
Sorry, but not sure what Frank's theory is..
A 8 said:
On the very first day I bought this amp secondhand, I opened the top panel and ran it and straight away, it went dead. Perhaps with powerful fan, I would try..Otherwise..I'm not a very brave man
I had some trouble to take the pcb out of the enclosure. A thin spanner seemed necessary to release the volume knob before taking the board out.
BTW, can I replace electro caps with polyester or MKT? Electrocaps sold by local component shops are cheap stuffs made in china, and even if it is claimed 105degrees, I doubt the quality.
It was in fact, first time to take a thorough look at my amp since I bought it. To my surprise, transistors in left/right channels are not the same types. In the left (or the right) channel, they are the standard MJI5003 and MJI5004, while the other is equiped with A2n8705 and probably A2n8706 (not sure, since the last digit was covered with solder). Perhaps, the ex-owner has replaced them, I suppose.
I just remembered that before the amp broken, when the volume was set 0, the right channel still remains while the left goes silent. I knew some components were mismatched. Heterogenous transistors are definite reason for this, aren't they?
"Franks's Theory," at least as I read it, is that either you have lost a rail or that an output transistor has failed in a short circuit. This would put rail voltage on the output and cause the opposite transistor to run extra hot (since it now is dissipating twice the power)
Are you saying that the right channel still works and only the left is dead? This would change the troubleshooting picture.
Are you saying that the right channel still works and only the left is dead? This would change the troubleshooting picture.
Re: Re: Running the amp with the lid off....
The transistors that start A2 are the origional devices - Musical Fidelity relabled 2N3055/2N2955's to make them appear more exotic. The 4 digit numbers are date codes (weeks 5 and 6 of 1987). You should find that one of them says A2N (for NPN) and A2P (for PNP).
The MJ devices should be perfectly good replacements, but I've never tried, so can't comment...
It sounds like something is very wrong. Have you got another amplifier so that you can test your speakers - I really hope they didn't suffer, but the A100 doesn't incorporate any speaker protection... Make sure you use an old, unwanted speaker for any testing!
The different transistors won't explain the difference in volume at 0 - that is probably a faulty volume potentiometer...
Good luck,
Mark
otaku said:
The transistors that start A2 are the origional devices - Musical Fidelity relabled 2N3055/2N2955's to make them appear more exotic. The 4 digit numbers are date codes (weeks 5 and 6 of 1987). You should find that one of them says A2N (for NPN) and A2P (for PNP).
The MJ devices should be perfectly good replacements, but I've never tried, so can't comment...
It sounds like something is very wrong. Have you got another amplifier so that you can test your speakers - I really hope they didn't suffer, but the A100 doesn't incorporate any speaker protection... Make sure you use an old, unwanted speaker for any testing!
The different transistors won't explain the difference in volume at 0 - that is probably a faulty volume potentiometer...
Good luck,
Mark
Re: Re: Re: Running the amp with the lid off....
). Couldn't find 510pF as given in your design. So I just used 470pF monolithic cap. Would it be ok?
I guess I should replace the transistors for better consistency. A2 in the left and MJ in the right seems not very pleasant..
Thanks, my speakers are alive. I have a backup amp and works ok.
Thanks for your advice and your design!! I enjoyed making it!!
Hi, Mark. What a coincidence! Today I made the preamp replacement you designed (Had to wait long until the amp breaks downmhennessy said:
). Couldn't find 510pF as given in your design. So I just used 470pF monolithic cap. Would it be ok?I guess I should replace the transistors for better consistency. A2 in the left and MJ in the right seems not very pleasant..
Thanks, my speakers are alive. I have a backup amp and works ok.
That's what I first thought but wondered the effect of different transistors after I discovered them..Anyway that's why I made your preamp replacement to make it even better! Just one electro cap is missing, and I'll finish that tomorrow..but when the amp is still stone dead, how am I going to test?
Thanks for your advice and your design!! I enjoyed making it!!Hi Otaku,
Pleased to hear your speakers are ok. And I hope the replacement preamp helps with some of your problems. As others have suggested, to get the amp working, I'd replace all of the electrolitic Cs, and the output devices. Be careful when fitting them, and replace the silicon insulating washers as well. If is still doesn't work, we might have to start applying logic, but lets hope that isn't necessary!
The MJ devices that were used as replacements ought to be ok, as they are more powerful than the origional 2N3055/2955 devices, but as I said, I've no direct experience here...
One piece of advice - it might be worth fixing the amp before you attempt to fit the replacement preamp - otherwise there are too many variables to think about. And yes, 470pF is fine...
Good luck,
Mark
Pleased to hear your speakers are ok. And I hope the replacement preamp helps with some of your problems. As others have suggested, to get the amp working, I'd replace all of the electrolitic Cs, and the output devices. Be careful when fitting them, and replace the silicon insulating washers as well. If is still doesn't work, we might have to start applying logic, but lets hope that isn't necessary!
The MJ devices that were used as replacements ought to be ok, as they are more powerful than the origional 2N3055/2955 devices, but as I said, I've no direct experience here...
One piece of advice - it might be worth fixing the amp before you attempt to fit the replacement preamp - otherwise there are too many variables to think about. And yes, 470pF is fine...
Good luck,
Mark
Hi Mark,mhennessy said:
I know you have no direct experience, but theoretical answer will do.
While 2n3055 is easily obtainable, there's no 2n2955 available in my town. MJ device is much more expensive, so I don't like to get MJ for 2n3055 that is easy to get and cheaper.
I wonder if MJ-2n combination each channel is ok.
diss Vcb Ic Hfe Hfe Bias
2N: 115w|100v|15A|20/70|4A
MJ: 250w|140v|20A|25/75|8A
MJ2995:150w|100v|15A|5 |10A
What about MJ2995 as a replacement for PNP device? It's as cheap as 2N.. Relatively low Hfe makes it not perfect replacement though...
One piece of advice - it might be worth fixing the amp before you attempt to fit the replacement preamp - otherwise there are too many variables to think about. And yes, 470pF is fine...
Good luck,
Mark
Of course! Thanks for that!
otaku said:
I think I probably meant MJ2955 - certainly, if you download the datasheet for the 2n3055 from On-Semi, they suggest that the MJ2955 is the compliment for the 2N3055...
Looking at the datasheet, I'm not really convinced that the 3055/2955 pair is really beefy enough for this amp. I'd be inclined to try upgrading to the MJ15003/4 - should be more reliable in the long run if the amp is happy to use them...
Don't forget that you can buy these direct from OnSemi.com - they might even send you samples