Hi, I'm repairing one of these for someone. Aside from performing a recap, I've noticed darkened patches underneath certain resistors and transistors. Is this a sign of failure? Does anyone have a schematic?
One other thing - I'm going to use a 1:2 step-up isolation transformer to test the rail voltage of the PT, so that I can replace it with a US-wired version. How should I determine the VA of this transformer? It's around 3.6" in diameter and around 1.5-2" in height. I'll put up pics of the amp now.
Thanks!
One other thing - I'm going to use a 1:2 step-up isolation transformer to test the rail voltage of the PT, so that I can replace it with a US-wired version. How should I determine the VA of this transformer? It's around 3.6" in diameter and around 1.5-2" in height. I'll put up pics of the amp now.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Thanks!
b200 info
hi, you can see a very lot of informations on musical fidelity products (also b200) on this excellent site:
http://www.mhennessy1.f9.co.uk/mf_a1/technical.htm#later
walter
hi, you can see a very lot of informations on musical fidelity products (also b200) on this excellent site:
http://www.mhennessy1.f9.co.uk/mf_a1/technical.htm#later
walter
b200 info...... again
The best way for restoring a mf b200 (or a1 or a-100) is: change all electrolitics caps to 105°C. degree (computer grade), power resistors failure are often present, the transistors on power and driver section and the op amps.
You can change also zener diodes with high watt rated. The dark colour under components it's normal because they are normally very hot, but check it!!!!The selector switch and volume pot often fail because of high temperature, but before change it you can try with a good contact cleaner.
It's important put the conductive silicon between aluminium bar and heatsink.
The a100 have inside two fans the b200 not....
The output mosfet maybe 2sk135-2sj50 or buz 900-905 series.
the b200 it's a really good sounding amp, very dinamic and clear.
In your posted pics I see only two electrolitics caps on power supply, you loose the others two? Pay attenction to the voltage, in fact mf put a very low voltage capacitors inside (little difference max caps voltage-supply voltage), change all of them with biggger voltage type.
If you have any questions ask!
ciao from Italy
walter
The best way for restoring a mf b200 (or a1 or a-100) is: change all electrolitics caps to 105°C. degree (computer grade), power resistors failure are often present, the transistors on power and driver section and the op amps.
You can change also zener diodes with high watt rated. The dark colour under components it's normal because they are normally very hot, but check it!!!!The selector switch and volume pot often fail because of high temperature, but before change it you can try with a good contact cleaner.
It's important put the conductive silicon between aluminium bar and heatsink.
The a100 have inside two fans the b200 not....
The output mosfet maybe 2sk135-2sj50 or buz 900-905 series.
the b200 it's a really good sounding amp, very dinamic and clear.
In your posted pics I see only two electrolitics caps on power supply, you loose the others two? Pay attenction to the voltage, in fact mf put a very low voltage capacitors inside (little difference max caps voltage-supply voltage), change all of them with biggger voltage type.
If you have any questions ask!
ciao from Italy
walter
Re: b200 info...... again
Well I've already ordered 105°C electrolytic capacitors, is there anything else I need to do immediately before powering the amp up? I guess I could measure the power resistors but unfortunately one of them has loosened its traces from the PCB so I'm not eager to unsolder and resolder it...
mosfet1 said:
Well I've already ordered 105°C electrolytic capacitors, is there anything else I need to do immediately before powering the amp up? I guess I could measure the power resistors but unfortunately one of them has loosened its traces from the PCB so I'm not eager to unsolder and resolder it...
sixtek said:
Thanks, I'd seen that one but was a bit leery of the "errors" that it might or might not have. I'll refer to the schematic if the thing doesn't work after recapping...
Re: b200 info...... again
Nope, the amp came to me like that. I ordered four 6800uF 63V 105C caps to replace the two and fill the other two spots, you can't go wrong with a bit more oomph in the PSU, and 13600uF per rail is pretty reasonable anyway.
mosfet1 said:
Nope, the amp came to me like that. I ordered four 6800uF 63V 105C caps to replace the two and fill the other two spots, you can't go wrong with a bit more oomph in the PSU, and 13600uF per rail is pretty reasonable anyway.
Which reminds me: Should I consider bypassing the PSU capacitors with .47uF 630V film caps or should I not bother?
Time for an update. I replaced all the capacitors, used a stepup transformer, and ran the amp at a steady 230V. A little bit of heat came out, but nothing close to the sweltering furnace that this amp is supposed to be. My discman connected to the CD input, I got just the faintest volume coming from my test speakers 🙁. Adjusting volumes didn't really help, but it did make that tiny faint noise a little louder or softer. The heatsink never got hot. I measured 49-0-49 on the secondary, and 64 (!) V after rectification being fed to the 63V capacitors 🙁 I measured rail voltage on the TO-3 transistors, -64 on one and 64 on the other in the pair. I measured 4mV of DC offset on one channel, and 12mV on the other. What do you think is the problem? I'll start out by replacing the scorched parts..
b200
Oh..........
First: your supply voltage seems too high! The b200 it's 60 watts for channel , 60+60 volts are very high! Have you check the mosfets? Are they ok?
Have you check the lm 318 too? (if the op amp it's died nothing come on output stage), if you can, at a medium volume apply a signal after the lm318 and listen. Check also the resistors, in fact because of thermal effect it can breaks.
walter
Oh..........
First: your supply voltage seems too high! The b200 it's 60 watts for channel , 60+60 volts are very high! Have you check the mosfets? Are they ok?
Have you check the lm 318 too? (if the op amp it's died nothing come on output stage), if you can, at a medium volume apply a signal after the lm318 and listen. Check also the resistors, in fact because of thermal effect it can breaks.
walter
Well, the amp didn't fry at those volumes, and the dead (dry powder leaking out) PSU caps leads me to think that rail voltages this high were common. I ordered a 120VA 40-0-40 transformer from Plitron that should only put out around 56-57VDC rails so to avoid this problem. I didn't evaluate the sound quality over a long period of time due to not having the amp's top, but didn't notice significant distortion of any kind.
I replaced the stock 50K dual log pot with a Blue Velvet, and once this transformer arrives, I'll install it, and my work will be done. Thanks, everyone, for the help
I replaced the stock 50K dual log pot with a Blue Velvet, and once this transformer arrives, I'll install it, and my work will be done. Thanks, everyone, for the help
I'm also trying to restore a B200 and so far I have the same problem, 65V DC on the power supply caps and the sound is too bright. It's not getting hot at all and I suppose that the idle current could be too low? Does anyone know how much should be the idle current?
The excessive voltages on the caps are normal on this amplifier....
Although it may be thought that with these voltages the output should be maybe 120W per channel, the transformer is too small and the voltage drops considerably under load. Also the Rds of the output mosfets causes a fair bit of loss. They actually give 70W per channel.... or about 110W with just one channel driven!
Adding a second pair of mosfets to each channel (on the B200 and P140, P150 which use the same design and components) makes a big improvement to the sound and increases power to about 100W per channel.
Quiescent current should be 100mA and yes, they do run quite hot.
Although it may be thought that with these voltages the output should be maybe 120W per channel, the transformer is too small and the voltage drops considerably under load. Also the Rds of the output mosfets causes a fair bit of loss. They actually give 70W per channel.... or about 110W with just one channel driven!
Adding a second pair of mosfets to each channel (on the B200 and P140, P150 which use the same design and components) makes a big improvement to the sound and increases power to about 100W per channel.
Quiescent current should be 100mA and yes, they do run quite hot.
the quiescent current of 100mA per channel? is there a resistor for each channel to measure the voltage drop?
Ixigen,
You should find a pair of 0r22 resistors in each channel which go from the source of each mosfet to the power supply. They are labeled R25 and R26 on the factory schematic but may be different on each channel.
There is a 100K preset labeled VR1 on the schematic which adjusts the standing current.
You need to adjust the preset to obtain 22mV across either of the 0r22 resistors in each channel. It does not matter which of the two it is.
Be sure that the screws are tight on the mosfets (do not over tighten and cut through the sil-pads though!) and that there is a good even coating of fresh thermal compound between the internal heatsink and the lid.
The best way to get the lids off MF integrated amps actually looks quite brutal! Undo all the screws, place a magazine on top of the amp and then hit it very hard, once, with a large mallet! This breaks the seal between the lid and the internal heatsink due to the thermal compound.
Good luck.
You should find a pair of 0r22 resistors in each channel which go from the source of each mosfet to the power supply. They are labeled R25 and R26 on the factory schematic but may be different on each channel.
There is a 100K preset labeled VR1 on the schematic which adjusts the standing current.
You need to adjust the preset to obtain 22mV across either of the 0r22 resistors in each channel. It does not matter which of the two it is.
Be sure that the screws are tight on the mosfets (do not over tighten and cut through the sil-pads though!) and that there is a good even coating of fresh thermal compound between the internal heatsink and the lid.
The best way to get the lids off MF integrated amps actually looks quite brutal! Undo all the screws, place a magazine on top of the amp and then hit it very hard, once, with a large mallet! This breaks the seal between the lid and the internal heatsink due to the thermal compound.
Good luck.
Thanks for the info, I think my way to get the sink off is less brutal. I'm unscrewing the top screws just to get them a little above the heatsink, then I'm just pressing them for a few seconds, and the lid will come off easy.
Sometimes you can get the sink off as you say but often not! (especially with A1, A100 etc). The big mallet method is how we used to get the lids off at the MF factory.....
By the way, do you know where can be found some fans to replace the ones in A100? I also have to restore one A100 and one A100-X. Also do you happen to know what is the quiescent current for the Synthesis?
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