Musical Fidelity B200

[sorenj07]

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!

[mosfet1]

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...ical.htm#later

walter

[mosfet1]

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

[sixtek]

Schematic: http://www.mhennessy1.f9.co.uk/mf_a1/mf_b200.jpg

[sorenj07]

Originally posted by mosfet1
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.

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...

[sorenj07]

Originally posted by sixtek
Schematic: http://www.mhennessy1.f9.co.uk/mf_a1/mf_b200.jpg

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...

[sorenj07]

Originally posted by mosfet1
In your posted pics I see only two electrolitics caps on power supply, you loose the others two?

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.

[sorenj07]

Which reminds me: Should I consider bypassing the PSU capacitors with .47uF 630V film caps or should I not bother?

[sorenj07]

a little update: got the replacement 105C caps, I'll probably post before/after pics

[sorenj07]

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..