Musical Fidelity B200

[mosfet1]

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

[sorenj07]

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

[ixigen]

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?

[jez]

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.

[ixigen]

the quiescent current of 100mA per channel? is there a resistor for each channel to measure the voltage drop?

[jez]

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.

[ixigen]

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.

[jez]

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.....

[ixigen]

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?

[nigelwright7557]

Originally Posted by jez

Ixigen,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.Good luck.

You need to do this without a speaker connected or any dc offset will make one output transistor conduct more than the other.