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-   -   musical fidelity MA50 (https://www.diyaudio.com/forums/solid-state/23531-musical-fidelity-ma50.html)

dtkky 2nd December 2003 05:13 AM

musical fidelity MA50
 
Hi everyone,

Anybody familiar with these monoblocks? One of mine keeps blowing the fuse on turn on, so I'm trying to diagnose where the problem is. A schematic of some sort would help but I can't seem to find one. Any help would be much appreciated.

David Tan

bowronsp 17th May 2004 04:29 AM

MA50
 
Hi David - I have 4 of these amplifiers biamped to a pair of missions. i dont know a lot about them but i think they are a top amplifier. there is a guy named mark hennesy who contributes a lot re these MF amps - he seems to think they are the same as the A100, but the stereo amp is connected to make it a mono amp

how long have you had your MA50's?

Steve

dtkky 17th May 2004 10:50 AM

Hi steve,
I've had them for a good 12 years now. And they were bought second hand. Thay are nice amps but run hot. I would change out the power supply caps but finding the right ones can be a problem as the space is tight. Another mod may be to change the power transformer to a bigger one but again space is tight.
I'm still looking for the schematic as some resistors have turned brown, and I don't know what values they are. So if you have any leads, let me know.

David

mhennessy 17th May 2004 11:14 AM

Hi David,

I've never examined one personally, but Steve is right according to what I've been told. And, the A100 is pretty much the same as the A1, shown here: http://www.mhennessy.f9.co.uk/mf_a1/

If you haven't already tried, it's worth searching these forums for past threads - a lot of the common problems with A1's and A100's have been discussed before, and apply equally to your MA50's. For example, http://www.diyaudio.com/forums/showt...407&highlight=

Finally, your name rings a bell. Have we spoken before about these before? Forgive me, but I get a lot of emails about these amps ;)

Best regards,

Mark ;)

dtkky 17th May 2004 01:40 PM

Hi Mark,
Thanks for the links. Yeah, i must have posted before requesting for an MA50 schematic. I know they all share the same circuit design, but just need to know the exact resistor values as the ones that need replacing have turned brown and the colour codes are gone. Some zener diodes need replacing too and I don't know what to use as a replacement.

BTW, your website is excellent

David

mhennessy 17th May 2004 02:30 PM

Quote:

Originally posted by dtkky
Hi Mark,
Thanks for the links. Yeah, i must have posted before requesting for an MA50 schematic. I know they all share the same circuit design, but just need to know the exact resistor values as the ones that need replacing have turned brown and the colour codes are gone. Some zener diodes need replacing too and I don't know what to use as a replacement.

BTW, your website is excellent

David

Thank you ;)

It might be an obvious thing to say, but does the other amp have the same problem? You could check component values there.

Assuming you mean the zeners in the power amp stage, they're 12V. Might be wise to fit bigger replacements - think the originals are only 1/2 watt - go for at least 1.3W BZX85C. Also mount them spaced slightly away from the board to allow for cooling.

Which resistors are affected?

Cheers,

Mark ;)

dtkky 17th May 2004 04:22 PM

Thanks for the tip. The other amp works fine but the same resistors are also brown. I'd thought I changed them out before anything happens. I've isolated the problem on my non-working amp to the power trans (shorted). The power supply caps look dodgy but finding the 105 degC caps at the right value ( I don't know if they are original, but they are 6800uF) at that size is difficult.
David

bowronsp 17th May 2004 10:58 PM

MA50 again
 
:xeye:

lol - hmm - My knowledge of electronics is obviously a lot less than yours - i just plug them in and hope for the best!

they do get very hot though - i direct a fan on them when i am using them in summer, although they are supposed to run at about 70 degrees c according to MF i believe. i bought two of them and a P3A preamp several years ago second hand, and a while later i saw another two advertised [also second hand] so i grabbed them as well - not for any particular reason i suppose other than i liked the sound and i though having two for each channel would give a bit more whack - which it does - they go quite loud. I also bought an MK 12 inch sub as the missions dont have a huge amount of low frequesncy in them - they seem to fall away quickly after about 80hz, so the sub gives it a bit more in the bottom end. i have had only one probelm with a power transistor that blew up, but i got that fixed at Len Wallace Hi Fi in LAne Cove in Sydney - that was a faor while ago now. i want to try and get as much info as i can about them in case they need repair again since they are unusual and might be a bit tricky to get fixed.

Steve

rmgvs 1st June 2004 03:34 PM

2 pairs?
 
Hallo DTTKY,

can you confirm that the MA50's use 2 pairs of output transistors (so 4 pieces per channel)? And can you see from the inside whether both halves of the printed circuit board are stuffed with components or that only one half is used?

The pcb of the MA50 is the same as that from the A1. I wonder which part of the pcb is used.

Rudy

dtkky 1st June 2004 04:23 PM

Hi Rudy,
I'll try to take some pics for you, but please be patient as I'm quite tied up at the moment.

David


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