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Old 28th February 2005, 12:04 PM   #1
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Default musical fidelity repair

hi all,
ive been interested in hifi for years. i built a pre amp when i was 15. since then i have just bought hifi.
my pride and joy is broken and i cant afford to send it off to MS for repair.
Muscial Fidelity DrThomas, one channel works, blows fuses, 350 + vat for service
Musical Fidelity The PreAmp, one channel works, 250 + vat for service.

I would like to attempt to fix these myself. Starting with the 'PreAmp'. I have a multi-meter and can solder.

Could someone give me hints on where to start with this type of thing. I cant even remember how to figure out resistances and capacitance!!!

Will a good start be to replace all the capacitors? if so where would be a good place to source the components?
How do I tell what the capacitance should be? and is this all the information i need to replace a capacitor? the same applies to the resistors i guess.

Is this repair beyond a novice?

Thanks
Richard
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Old 28th February 2005, 12:15 PM   #2
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There are a number of people with MF info on their homepages.
An important reason for failure with MF amps is heat related breakdown of the output devices, due to deteriorated heat transfer from devices to heatsinks.
Early models used 3055/2955 devices, maybe testing the output devices is a good place to start.

With the preamp you might try checking contacts, source selector etc. MF used cheap switching material.
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Old 28th February 2005, 12:19 PM   #3
Netlist is offline Netlist  Belgium
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Hi Richard and welcome,
Sad to hear about your broken babies.

A good place to start reading about components and repair is this well written site.
It's basically about car audio but the principals of electronics stay the same. You'll find tons of good visualized info.

Let us now how things go.

/Hugo
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Old 28th February 2005, 12:37 PM   #4
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Mark Hennessy has info on his homepage.
You might ask him for advice on your models.

http://www.mhennessy.f9.co.uk

The Dr Thomas ran pretty hot and they should be close to 25 years old by now, checking the condition of the power caps is a good thing. I would swap them for new and better ones anyway.

The output stage of the Dr Thomas are Mosfets(from memory),
believe the types used were BUZ's, these can still be bought easilly.
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Old 28th February 2005, 12:55 PM   #5
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thanks all,
i will get myself a pair of safety gloves from maplin and try and trace the fault on the pre amp backwards to think.
i reckon on replacing all the caps first. will getting them from maplin be ok or are there 'audio' specific suppliers out there?
i guess i will have to get my brother to help me out with the colour codes etc on the caps etc, he has a electronics degree, saying that i bet he cant wire up a plug ;-)
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Old 28th February 2005, 01:16 PM   #6
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As long as Maplin has good caps, why pay audio grade prices.

Good brands are;

BH, BC, Sif Safco, Elna, Sprague, Mallory, Cornel Dubelier, Siemens.
Rifa or Sikorel, if you can afford them.
Philips, if they are not too old, they are not made anymore.

Preferably 105 C types, given the Dr Thomas mood, otherwise 85 C.

btw:
a good set of MF Dr Thomas and The Preamp is worth +300 PS.
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Old 28th February 2005, 08:18 PM   #7
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Maplin thoroughly suck for parts now, what they do have is largely overpriced and you will be lucky if their stores have it in stock anyway.

Try Rapid Electronics they have a decent parts range and also sell the audio MOSFETS.
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Old 1st March 2005, 10:09 AM   #8
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thanks Jaycee, that was going to be my next question, where to get good cheap parts from. i have tracked down some panasonic caps on RS which look ok. I think i will get stung for the big Elna ones i have hanging off a large sliver dougnut ;-)
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Old 1st March 2005, 09:30 PM   #9
jaycee is offline jaycee  United Kingdom
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Farnell and RS are also good although RS tend to be a bit expensive, and Farnell have a £20 minimum order charge. The Panasonic capacitors are good, though.
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Old 2nd March 2005, 01:40 PM   #10
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thanks again jaycee,
i had a play last night. took the thing apart, got the pcb off the case etc. traced the signal path from the output phono leads.
i left my notes at home but can recall most of it, will get them at lunch time and see if i can source the part i !!think!!! i need.

ok, first component is C10 a small cap. on the working side i get a constant pd of about 80mV (depending on volume control setting) on the broken side i get a really jumpy single from -200 to +200 mV. i am taking these readings on the DC setting on a multimeter.

i have drawn a little schematic of most of the other components i looked at last night.

next comes an opamp i think, cant recall the name something like LA355N but i will get the name at lunch time.

anyway, on the good side the output from pin 1 on the opamp is roughly the same as what is across C10, a nice steady voltage, moving alittle like you would expect music to? i dont have a signal generator, perhaps i should record a cd with just a constant beep, on the broken side it is really jumpy still.

i looked at all the pins on both channels on the opamps and all of the inputs, all the other pins (except pin2 which is linked to pin1 with a potential divider set of 2 resistors and ground i think) the pd's are about the same. i.e. the +/-15v and the signal off the alps pot. the only thing of note is that the pd of the signals is slightly different from R to L. R being slightly higher than L. the difference becomes more significant the higher the volume levels,,,i think. i wasnt smart enough to flip over the input source to double check it wasnt something funny with the cd i was playing or the crappy dvd player i was using as a source.

so, i am hoping that it is a blown opamp? they are tiny and i figure they cant be expensive. i think i might de-solder them both and swap them round and see if the problem switches??? at first i thought it was C10 which was knackered but im not so sure now.

could anyone advise on the signal strength coming off the alps pot difference from one channel to the other?? i see this as a problem if it is the pre amp doing it...

thanks all
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