Then replace the wires to the TO3 also and please replace the emitter resistors of the power transistors too. They can be broken, it will have the same effect as you describe.
Remember this amp get's incredibly hot and components really wear out in these. You can turn down the idle current a bit so that the amp will live longer. This was described in a thread.
It's already open and the parts are not expensive so why not do it right in one repair ?!?! When the total cost *might* be 30 or 40 Euro it still is a better solution to do it right than to do it half and have the amp in for repair a second time in a few months including the big risk of blown loudspeakers( that cost quite a bit more than 30 or 40 Euro )...
As I stated earlier ( maybe in another thread ) I did not want to replace the volume potentiometer and input switch in a MF amp for it would take too much of my time and hey, they were still OK. It came back some time later for crackling switch and potentiometer. A good example of cost and time-cutting the wrong way
When done right you'll have some more credit with your father in law=> being the ideal son in law 😉
Remember this amp get's incredibly hot and components really wear out in these. You can turn down the idle current a bit so that the amp will live longer. This was described in a thread.
It's already open and the parts are not expensive so why not do it right in one repair ?!?! When the total cost *might* be 30 or 40 Euro it still is a better solution to do it right than to do it half and have the amp in for repair a second time in a few months including the big risk of blown loudspeakers( that cost quite a bit more than 30 or 40 Euro )...
As I stated earlier ( maybe in another thread ) I did not want to replace the volume potentiometer and input switch in a MF amp for it would take too much of my time and hey, they were still OK. It came back some time later for crackling switch and potentiometer. A good example of cost and time-cutting the wrong way
When done right you'll have some more credit with your father in law=> being the ideal son in law 😉
These Are Not Called MF's For Nothing...............
A8,
My experience of these (and plenty of other) amplifiers is duff/aged solder joints causing such intermittent troubles 99.9% of the time.
If I recall correctly these amplifiers during manufacture had the joints soldered and then the component leads cut, and the solder flux left on the boards - correct me if I am wrong !!.
In most gear this is asking for long term trouble, in a red hot running amp like these MF's this is ensuring long term trouble.
With an intermittent fault like this, the first cure is to clean the pcb, blanket resolder the complete pcb, and clean again - this should ensure that you have first class soldering.
While you are at it, it would be a very good idea (but not mandatory usually) to replace all the caps - you can most likely omit replacing the main supply caps.
Modern high quality SMPS caps are usually an improvement in amplifiers, although I read lately that Bipolar electros exhibit lower distortions - it would be worth the experiment to replace all the amplifier circuit caps with bipolars.
The reason for the noisey pots is electrolysis of the grease/wiper contact - thorough washing and rotating with a clean spray solvent should get deposits out, then a good contact oil spray should help keep it quiet.
I don't really buy the dud solder joints on the output transistor cans - if you have any doubts just use a suitably hot iron to redo these connections.
Output and driver transistors usually fail dead short (rarely open) circuit, and are very rarely intermittent (old MPSA93 is an exception), so I expect the problems you have are due to an intermittent joint somewhere.
If I owned one of these I would mount a small (30mm ?) fan at one end of the casing pulling hot air out - air entry holes needed at the opposite end.
I would also place a couple of larger (120mm) fans spaced 8mm or so above the top panel drawing air up and away from the heatsink and powered from a variable external supply.
In my memory these are a sonically quite ok amplifier (coloured but ok colour) more especially at low and medium levels, but nothing to particularly write home about.
Maybe this helps, Eric.
BTW, watch out for all that damm heatsink paste.
A8,
My experience of these (and plenty of other) amplifiers is duff/aged solder joints causing such intermittent troubles 99.9% of the time.
If I recall correctly these amplifiers during manufacture had the joints soldered and then the component leads cut, and the solder flux left on the boards - correct me if I am wrong !!.
In most gear this is asking for long term trouble, in a red hot running amp like these MF's this is ensuring long term trouble.
With an intermittent fault like this, the first cure is to clean the pcb, blanket resolder the complete pcb, and clean again - this should ensure that you have first class soldering.
While you are at it, it would be a very good idea (but not mandatory usually) to replace all the caps - you can most likely omit replacing the main supply caps.
Modern high quality SMPS caps are usually an improvement in amplifiers, although I read lately that Bipolar electros exhibit lower distortions - it would be worth the experiment to replace all the amplifier circuit caps with bipolars.
The reason for the noisey pots is electrolysis of the grease/wiper contact - thorough washing and rotating with a clean spray solvent should get deposits out, then a good contact oil spray should help keep it quiet.
I don't really buy the dud solder joints on the output transistor cans - if you have any doubts just use a suitably hot iron to redo these connections.
Output and driver transistors usually fail dead short (rarely open) circuit, and are very rarely intermittent (old MPSA93 is an exception), so I expect the problems you have are due to an intermittent joint somewhere.
If I owned one of these I would mount a small (30mm ?) fan at one end of the casing pulling hot air out - air entry holes needed at the opposite end.
I would also place a couple of larger (120mm) fans spaced 8mm or so above the top panel drawing air up and away from the heatsink and powered from a variable external supply.
In my memory these are a sonically quite ok amplifier (coloured but ok colour) more especially at low and medium levels, but nothing to particularly write home about.
Maybe this helps, Eric.
BTW, watch out for all that damm heatsink paste.
He told us in a post that he had replaced all caps. BTW you are very wrong concerning the main supply caps. I had my share with exploding 10000 uF caps in A100 ! Of course one blew while I was measuring it and such things can cost you an eye. It was shorted inside because of drying out.
In MF products like this one it is mandatory because of their age and because they are standard grade ( 85 degrees ) caps in a 90 degrees environment. I once measured them all and IIRC 5 of them were halved in value.
Besides that it isn't worth the discussion to spend some tens of Euro's on caps even when they're not suspicious....
These amps have fans in them which in original state produce too much noise already.
Not wishing to sound like mr Grumpy, (but I will anyway.😡 ) I've always been a bit baffled by the 'mini cult' status of these amps.
They are a major pain to own and work on, the design is just bizzare and claims of class-a operation are stetching the truth somewhat. With an Iq of 800mA the A1 can manage just 3.5 Watts in class-a.
If someone offered me a faulty one for free I'd probably turn it down.
They are a major pain to own and work on, the design is just bizzare and claims of class-a operation are stetching the truth somewhat. With an Iq of 800mA the A1 can manage just 3.5 Watts in class-a.
If someone offered me a faulty one for free I'd probably turn it down.
"Not wishing to sound like mr Grumpy, (but I will anyway. ) I've always been a bit baffled by the 'mini cult' status of these amps. "
Yeah, me too - anybody who has had to repair at least one of these MF'ers will have a different view.
It is 14 years at least since I heard one of these, and the sound did not particularly grab me then.
IIRC they had quite nice/acceptable lows, mids and highs, but get squashy (but not objectionally so) real quick.
IOW they had that 'nice' English sound that goes reasonably well with 8" 2 ways in a small room, but will never be a power house machine.
Eric.
Yeah, me too - anybody who has had to repair at least one of these MF'ers will have a different view.
It is 14 years at least since I heard one of these, and the sound did not particularly grab me then.
IIRC they had quite nice/acceptable lows, mids and highs, but get squashy (but not objectionally so) real quick.
IOW they had that 'nice' English sound that goes reasonably well with 8" 2 ways in a small room, but will never be a power house machine.
Eric.
On the plus side, even a faulty MF(er)🙂 will fetch good money on ebay. This route will save a lot of pain
They just sound OK, part of the cultstatus is the heat they produce and the reviews at that time that all described the dissipation and the strange heatsink that's part of the case. It also was one of the first class A amps that was affordable. Another important factor is that there were/are no equivalents then and today at that price A poor man's class A amp so to speak.
Are you sure about the Iq of 800 mA ? They do get incredibly hot for an amp with a total of only 1.6 A Iq.... You describe A1, could it be that A1/A1X is different from A100/MA100 ???
off-topic: I've never seen A1X open. Does somebody know what differences there are apart from switch and volume potentiometer ( that were supposed to last longer than the old types ) ???
A poor man's class A amp so to speak.Are you sure about the Iq of 800 mA ? They do get incredibly hot for an amp with a total of only 1.6 A Iq.... You describe A1, could it be that A1/A1X is different from A100/MA100 ???
off-topic: I've never seen A1X open. Does somebody know what differences there are apart from switch and volume potentiometer ( that were supposed to last longer than the old types ) ???
My 0.02€
Replace Zeners 1 & 2 with 1W versions,
all electrolytes and resolder thoroughly as mentioned above.
/Hugo 🙂
Replace Zeners 1 & 2 with 1W versions,
all electrolytes and resolder thoroughly as mentioned above.
/Hugo 🙂
The Iq for the A1 is around 600-800mA, there's no means of adjustment which is probably why it's quoted over such a wide range. 800mA gives a dissipation of about 75W and since the casework seems to have been designed to retain as much heat as possible....
Netlist said:
Good advice. Resoldering solder joints in gear older than 10 years is standard practice with me. Certainly with these kind of amps.
A8, before I forget: Please see this site for excellent info on MF amps:
http://www.mhennessy.f9.co.uk/mf_a1/
There is also a schematic on that site that resembles your amp nearly 100 %:
http://www.mhennessy.f9.co.uk/mf_a1/a1_schematic.jpg
It is of course possible to replace C6 and C7 with MKT caps with 5 mm pitch.
I've re-soldered the pcb twice now (that was my first effort before starting this thread) I've changed the caps after advice here and got film types in the signal path since long before.
I've also changed the output resistors R30 and 31.
I've also re-done and moved the transistor case soldering.
The zeners measure fine while the amp is faulty.
The only solder joints I have not actually done are those inside the U shaped thing with the white crap on it, I have however repeatedly moved the cables in there while running the amp to provoke a possible broken wire or solder joint but nothing would change.
BTW I measure 0.553 (goes up to 0.9V just when the amp leaves its faulty state)volt over R30 and 31, the combined R30/31 is 0.44 ohms...so I have around 1.25 amps idle current per channel on this A100.
Thx JeanPaul but I know of him already.
I've also changed the output resistors R30 and 31.
I've also re-done and moved the transistor case soldering.
The zeners measure fine while the amp is faulty.
The only solder joints I have not actually done are those inside the U shaped thing with the white crap on it, I have however repeatedly moved the cables in there while running the amp to provoke a possible broken wire or solder joint but nothing would change.
BTW I measure 0.553 (goes up to 0.9V just when the amp leaves its faulty state)volt over R30 and 31, the combined R30/31 is 0.44 ohms...so I have around 1.25 amps idle current per channel on this A100.
Thx JeanPaul but I know of him already.
A 8 said:
"a small click". Could it be that there is a relay somewhere? like a delayed start or something like that?
In such cases, I usually take a small isolated screwdriver, and start tapping very gentle on components till the error or good-state reproduces.
I also push and pull components mildly and twist the pcb a bit. All carefully noticing what happens and with moderate movements.
It can take two minutes and sometimes half an hour or more to locate the problem. Once the error can be made repeatable, the hardest part of the process is done.
It is tedious, but rather effective.
So what you did in post #16 is basically what I mean to explain.
A broken trace on the pcb comes to mind as well.
That “small click” is indeed strange. Where do you hear it? Inside the amp or in your speakers?
Oh, and don’t exclude the Zener tip too easily. 😉
/Hugo 🙂
I also push and pull components mildly and twist the pcb a bit. All carefully noticing what happens and with moderate movements.
It can take two minutes and sometimes half an hour or more to locate the problem. Once the error can be made repeatable, the hardest part of the process is done.
It is tedious, but rather effective.
So what you did in post #16 is basically what I mean to explain.
A broken trace on the pcb comes to mind as well.
That “small click” is indeed strange. Where do you hear it? Inside the amp or in your speakers?
Oh, and don’t exclude the Zener tip too easily. 😉
/Hugo 🙂
The click is interesting but I don't think these things contain any relays.
It might indicate partial falilure of an output transistor: thermal expansion causing failure and electrical/mechanical click?
It might indicate partial falilure of an output transistor: thermal expansion causing failure and electrical/mechanical click?
idle current
I built several power amps of A1-type from scratch. I used bigger cooling blocks, bigger transformers (one for each channel) and better parts (mostly MKP of MKT caps, straight into signal path of bypassing the electrolytics).
The idle current is self-setting at around 650-700 mA indifferent of the type of power transistor you use (3055/2955, 15003/4, I even used Japanese exotic pairs from Hiraga amps) at +/- 25 Volts. The A100 (which I did not built) uses around 1,25-1,5 Amps at 32 Volts (bigger reservoir caps but also using a single pair output transistors per channel). Also the output DC offset is self-setting, without any matching always within 20 mV and very stable over time).
The sound of the amp is very good not to say exceptional for a transistor amp. The MA50 amp (same circuit) was found the best sounding amp ever built in Britain at the time (proviso is that the speaker must be an easy 8 ohm load, true). This MA50 sounded better than the most expensive A370 power amp of MF (I can not confirm this because I never listened to them side by side).
The A1/100 etc sounds valve-like, has some colouring to the warm side of neutral and lacks some degree of ultimate transparancy when compared to the very best (tubes, Hiraga etc).
Rudy
I built several power amps of A1-type from scratch. I used bigger cooling blocks, bigger transformers (one for each channel) and better parts (mostly MKP of MKT caps, straight into signal path of bypassing the electrolytics).
The idle current is self-setting at around 650-700 mA indifferent of the type of power transistor you use (3055/2955, 15003/4, I even used Japanese exotic pairs from Hiraga amps) at +/- 25 Volts. The A100 (which I did not built) uses around 1,25-1,5 Amps at 32 Volts (bigger reservoir caps but also using a single pair output transistors per channel). Also the output DC offset is self-setting, without any matching always within 20 mV and very stable over time).
The sound of the amp is very good not to say exceptional for a transistor amp. The MA50 amp (same circuit) was found the best sounding amp ever built in Britain at the time (proviso is that the speaker must be an easy 8 ohm load, true). This MA50 sounded better than the most expensive A370 power amp of MF (I can not confirm this because I never listened to them side by side).
The A1/100 etc sounds valve-like, has some colouring to the warm side of neutral and lacks some degree of ultimate transparancy when compared to the very best (tubes, Hiraga etc).
Rudy
forgot
Jean Paul,
you say you have the circuit of the MA100. I am interested in this. Would you be so kind to share it in this forum? I know there are hardly differences but I want to know the exact differences with the original A1.
Rudy
Jean Paul,
you say you have the circuit of the MA100. I am interested in this. Would you be so kind to share it in this forum? I know there are hardly differences but I want to know the exact differences with the original A1.
Rudy
It is as follows.....when faulty only one of the transistors draw current, when the click occurs (in the speaker) the other starts to draw momentarily and draws up to 2.5ah and then settles around 1.25Ah.
The click probably comes from a dc change on the output.
Using a screwdriver to tap the output transistors will pull the amp out of its faulty stage but will not bring it back.
It will also leave its faulty stage on its own when it gets warm enough.
Semi broken traces is something that I've not checked for.
The click probably comes from a dc change on the output.
Using a screwdriver to tap the output transistors will pull the amp out of its faulty stage but will not bring it back.
It will also leave its faulty stage on its own when it gets warm enough.
Semi broken traces is something that I've not checked for.
A 8,
There’s nothing really broken in the amp otherwise it wouldn’t play.
Some component is intermittent. It can be anything that makes the output transistor turn on or off. If voltage is present on the collector while the transistor is off, its not the power supply.
Check base-emitter voltage.
Check what happens with Tr7, Tr8 and surrounding resistors.
Trying to make the error repeatable is the key factor here.
A can of cold-spray could be handy. Although I never had problem with it don’t freeze too much not to harm transistors. Work only with limited zones and wait long enough in between freeze-shots.
Pcb traces are candidates as well for some cooling.
/Hugo 🙂
There’s nothing really broken in the amp otherwise it wouldn’t play.
Some component is intermittent. It can be anything that makes the output transistor turn on or off. If voltage is present on the collector while the transistor is off, its not the power supply.
Check base-emitter voltage.
Check what happens with Tr7, Tr8 and surrounding resistors.
Trying to make the error repeatable is the key factor here.
A can of cold-spray could be handy. Although I never had problem with it don’t freeze too much not to harm transistors. Work only with limited zones and wait long enough in between freeze-shots.
Pcb traces are candidates as well for some cooling.
/Hugo 🙂
A 8 said:
sounds like mechanically stressed transistors. Maybe you can retorque / remount the transistors but to be safe, I would get new ones.
My first post on diyaudio......
I used to work for musical fidelity and repairing these amps was a dailychore for me!. I can't add much to the good advice that has already been posted on this subject other than to replace R12 and R5 (22k) which I seem to remember were prone to fail for no apparent reason. The zeners and associated area are another source of regular trouble as is general failures due to the massive heat generated by these amps. To reduce the bias and cool the thing down a bit, increase the value of R6 and R11.
Jez.
I used to work for musical fidelity and repairing these amps was a dailychore for me!. I can't add much to the good advice that has already been posted on this subject other than to replace R12 and R5 (22k) which I seem to remember were prone to fail for no apparent reason. The zeners and associated area are another source of regular trouble as is general failures due to the massive heat generated by these amps. To reduce the bias and cool the thing down a bit, increase the value of R6 and R11.
Jez.
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