Hello,
I just bought a dead 2nd hand Musical Fidleity A100 integrated amplifier. I decided to start this thread as I need help from experienced users of this great little amp. Besides, since MF A-series has such a following cult, I also think this thread will be of interest for a lot of fellow audiophiles.
I bough this amp as a non-working unit. The seller said the power light comes on but there's no sound. Upon opening the cover, I found out that this amp has been tampered with and some repair or mods were attemped
. There's a lot going on here so I thought I will summarize in topics:
01. One of the power supply capacitors was replaced by two capacitors connected in parallel, 15,000 uF and 6,800 uF. No big deal here.
02. The 2 power supply resistors that should be close to the big caps are missing. I believe they should be 0.47 ohm each. The spot in the PCB where they were supposed to be is heavily burnt
.
03. Two of the output transistors are not factory originals. They are Toshiba 2SD424 (3G) and Toshiba 2SB554 (1H). The others are labeled as A2N and A2P and the rest of the numbers are rubbed off, all that I can read is the end of the number which is "714".
04. R114 and R113 are darkened, not sure if totally burnt. The spot in the PCB is also darkened too. It is hard to read their values, the second band is black but it could have been changed by the heat. It is possible that they are 470 ohm. Oddly enough, R14 and R13, which should be the same resistors, are 680 ohm resistors.
05. Now that's the most strange thing: R3, R4, R27 and R29 have been lifted from the PCB on both channels. Any idea why this mod was done?
I haven't powered up the amp yet in fear of causing any damage. However, the seller already did that prior to list the amp so I think my worries don't make sense.
My plan is to replace all 4 output transistors, all caps, power resistors and zener diodes. I was wondering if I should test/replace BD139 and BD140 and, in this case, which type should I use.
Any advice will be greatly appreciated. Thank you.
I just bought a dead 2nd hand Musical Fidleity A100 integrated amplifier. I decided to start this thread as I need help from experienced users of this great little amp. Besides, since MF A-series has such a following cult, I also think this thread will be of interest for a lot of fellow audiophiles.
I bough this amp as a non-working unit. The seller said the power light comes on but there's no sound. Upon opening the cover, I found out that this amp has been tampered with and some repair or mods were attemped
. There's a lot going on here so I thought I will summarize in topics:01. One of the power supply capacitors was replaced by two capacitors connected in parallel, 15,000 uF and 6,800 uF. No big deal here.
02. The 2 power supply resistors that should be close to the big caps are missing. I believe they should be 0.47 ohm each. The spot in the PCB where they were supposed to be is heavily burnt
.03. Two of the output transistors are not factory originals. They are Toshiba 2SD424 (3G) and Toshiba 2SB554 (1H). The others are labeled as A2N and A2P and the rest of the numbers are rubbed off, all that I can read is the end of the number which is "714".
04. R114 and R113 are darkened, not sure if totally burnt. The spot in the PCB is also darkened too. It is hard to read their values, the second band is black but it could have been changed by the heat. It is possible that they are 470 ohm. Oddly enough, R14 and R13, which should be the same resistors, are 680 ohm resistors.
05. Now that's the most strange thing: R3, R4, R27 and R29 have been lifted from the PCB on both channels. Any idea why this mod was done?
I haven't powered up the amp yet in fear of causing any damage. However, the seller already did that prior to list the amp so I think my worries don't make sense.
My plan is to replace all 4 output transistors, all caps, power resistors and zener diodes. I was wondering if I should test/replace BD139 and BD140 and, in this case, which type should I use.
Any advice will be greatly appreciated. Thank you.
i would change the output devices, drivers and replace all the electrolytics.wont cost u much
a lot of the resistors discolour, as in typical mf style, the thing ran too hot, lots of underrated parts in use.
(nice sounding amp though)
and put the cct back as it should be
sounds like someone has been messing about fault finding by lifting resistors, not modding
this guy has some great general info on a1/100
http://www.mhennessy1.f9.co.uk/mf_a1/others.htm#a100
the a2n/p is just mf 's own labelling to stop you knowing what they are
ie 3055/2955 or similar.
they charge £30 pair even now thats why. £3 each from RS !
oh there is a thread somewhere here by a guy called jez with more info
he's a helpful guy
hope that helps
a lot of the resistors discolour, as in typical mf style, the thing ran too hot, lots of underrated parts in use.
(nice sounding amp though)
and put the cct back as it should be
sounds like someone has been messing about fault finding by lifting resistors, not modding
this guy has some great general info on a1/100
http://www.mhennessy1.f9.co.uk/mf_a1/others.htm#a100
the a2n/p is just mf 's own labelling to stop you knowing what they are
ie 3055/2955 or similar.
they charge £30 pair even now thats why. £3 each from RS !
oh there is a thread somewhere here by a guy called jez with more info
he's a helpful guy
hope that helps
Better to use MJ15003/4, MJ15024/5 or even MJ21193/4 as replacements, rather than 2955/3055s
BD139/140 manufacturer isnt too important - go with a good brand like OnSemi or ST.
Resistors lifted from board are probably to improve airflow and heat dissipation. Not a bad idea in these amps.
Mark's page linked above is a goldmine of info about MF's amps - i've never heard one but the engineering quality alone puts me right off even considering it!
BD139/140 manufacturer isnt too important - go with a good brand like OnSemi or ST.
Resistors lifted from board are probably to improve airflow and heat dissipation. Not a bad idea in these amps.
Mark's page linked above is a goldmine of info about MF's amps - i've never heard one but the engineering quality alone puts me right off even considering it!
oh!
sounds like someone was trying to repair and didnt have a clue.
You definitely need to get a schematic. There should be one on this forum somewhere. If it looks like it's been tampered with, replace it. How badly is the board burnt in places? If it's burnt so bad that it's charred, that could be a problem as charred fibreglass can be conductive
sounds like someone was trying to repair and didnt have a clue.
You definitely need to get a schematic. There should be one on this forum somewhere. If it looks like it's been tampered with, replace it. How badly is the board burnt in places? If it's burnt so bad that it's charred, that could be a problem as charred fibreglass can be conductive
At a guess - R3 and R4 removed to bypass the preamp. Maybe the original guy thought the preamp had a fault and he disconnected it to inject a signal. See Mark's "replacement preamp" for a better way - and yes, replace the preamp because it stinks.
R27 & R29 - fried when the output transistors blew perhaps?
At a guess the power resistors near the caps went when the output stage went. They are used to make a CRC filter out of the power supply. I suspect it doesn't affect much and bridging them out won't hurt. If you want to put them back, I'd put some 0.47R 5W wirewounds.
R114 and R113 should indeed be 470 ohms. the reason is because the regulator for the preamp connects to this channel's power rails.
Basically have a good read of Mark's website about the A1. The A100 is the same thing, beefed up a bit.
Personally, If I came into posession of one of these things, i'd probably gut it and put a more sensible Class AB amp inside it - possibly even a pair of LM3886's. It'd be more reliable, probably sounds just as good, and wouldn't cause global warming
R27 & R29 - fried when the output transistors blew perhaps?
At a guess the power resistors near the caps went when the output stage went. They are used to make a CRC filter out of the power supply. I suspect it doesn't affect much and bridging them out won't hurt. If you want to put them back, I'd put some 0.47R 5W wirewounds.
R114 and R113 should indeed be 470 ohms. the reason is because the regulator for the preamp connects to this channel's power rails.
Basically have a good read of Mark's website about the A1. The A100 is the same thing, beefed up a bit.
Personally, If I came into posession of one of these things, i'd probably gut it and put a more sensible Class AB amp inside it - possibly even a pair of LM3886's. It'd be more reliable, probably sounds just as good, and wouldn't cause global warming
jaycee said:
Hey Jaycee, thanks for your comments.
I found a copy of the schematic right here in DIY, in one of the older threads.
The PCB is heavily burned on the place where the 0.47 ohm resistors were, but not that bad. It is very darkened but the PCB is still good.
The second worst spot is under one of the 470 ohm resistor. Strangely enough, it is on the same side as the original outputs are.
I already odered new output devices, 2 pairs of BD139/140, new power resistors, new caps and new diodes. The only parts I am not replacing are the small transistors.
Do you have any idea how the fans are connected? Are they AC or DC powered? They are currently connected underneath the PCB board and to the positive of one of the smoothing caps. It looks like it was also tampered with. I will try to post some pictures later.
I will change all the parts I mentioned, check the solder joints, wiring, etc.
If there's any bad part that I did not replace, is there a chance the I blow the outputs again? I mean, can a bad transistors cause an output device to burn? I don't have a variac so basically I would just hook it up after the repairs.
The fans are PAPST, probably a discontinued model. The label says 12V, 50-60hz. I was puzzled by the 50-60hz information. There are 3 wires coming out from each fan, brown, yellow and blue, any clues as how these should be wired?
If there's any bad part that I did not replace, is there a chance the I blow the outputs again? I mean, can a bad transistors cause an output device to burn? I don't have a variac so basically I would just hook it up after the repairs.
The fans are PAPST, probably a discontinued model. The label says 12V, 50-60hz. I was puzzled by the 50-60hz information. There are 3 wires coming out from each fan, brown, yellow and blue, any clues as how these should be wired?
i have a semi functional a100 here
been meaning to fix it for a while but not got round to it
its currently in pieces so i can help you a bit no problem. im a bit busy today but maybe tomorrow?
drop me a pm i will send you my main email address (dont use the one on here much)
the fans work and so does the power amp from what i remember
been meaning to fix it for a while but not got round to it
its currently in pieces so i can help you a bit no problem. im a bit busy today but maybe tomorrow?
drop me a pm i will send you my main email address (dont use the one on here much)
the fans work and so does the power amp from what i remember
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