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Old 3rd January 2013, 02:59 PM   #31
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hi,
can anyone help me out with my MF P140. Im getting a very hot resistor,so hot it glows red. its rated 220hm probs 5watts. i'm not sure of the cause. I swapped the transistors over with the other channel but i get that same resistor getting hot,so i know that's not the cause. i will try to upload a pic. anyone else had hot resistor issues? what could be the cause? i would love a shematic or service manual if anyone has a copy.

thanks.
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Old 3rd January 2013, 03:15 PM   #32
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Old 3rd January 2013, 11:23 PM   #33
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Location: Coffs Harbour, on the east coast
There aren't any neat, service manual schematics available for early MF products. All we have is hand drawn versions that have to be guesstimated across several models such as the P series amplifiers. They do seem to be similar apart from rail voltages and number of output Mosfets. See earlier posts in this thread and you will get the idea. There are a few about like one for the A370 but they can only be indicative, as the posts in threads advise

Here is a discussion of the problems involved with many early MF Mosfet models. It is not pretty. Musical Fidelity P150 owners take note!

MF typically used power resistors in series with the load and I'm guessing this is what is hot, indicating a blown or shorted output stage, caps etc. When you locate the likely problem, the T03 Mosfets can be substituted with Profusion Exicon parts or Magnatec Alfet parts if needed. Expect to have to do a lot of replacements in there including the main electrolytics and the reassembly of the case is not simple - the cover is part of the heatsink.

This is not a simple model for repairs.
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Last edited by Ian Finch; 3rd January 2013 at 11:43 PM.
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Old 4th January 2013, 12:06 AM   #34
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Correction - I should have been clear that power resistors, likely the 220R that is hot, are in series with DC power to the amplifier rails indicating etc..
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Old 4th January 2013, 01:04 PM   #35
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ok thanks ian. The more i read about MF the more it starts to make sense. There's deffo DC at the speaker outputs as i used a tester speaker and the coil burned out. I did find those exicons on the bay, so would you recommend replacing the transistors anyway? Maybe that would stop the resistor burning out as it is linked directly of the casing of the K135. i noticed that the transformer is getting hot aswell, is this typical of MF early amps?

thanks.
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Old 4th January 2013, 11:03 PM   #36
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In your previous post, you concluded that the MOSFETS were OK because you swapped them and the resistor still became hot. Was that because the other channel from which you took the parts was originally working or were both channels dead?

Pressing on...
The threads here give a pretty unequivocal view on MF engineering and describe the high case temperatures well enough but that direct soldered resistor connection should not be repeated. Fit a small, 3-4 mm hole diameter eye terminal to the resistor lead and bolt this with the most suitable MOSFET mounting bolt to its case that way. Ensure that new mounting bushes (which you should also consider along with mica washers and correct grease) don't prevent reliable contact with the MOSFET case. The transformer does run hot on these - it is undersized but will heat even more due to the power burning up in the shorting resistor. It goes without saying that you don't place these amps in tight places or position anything near or on top of them. In this country, it took little time for failures of those P & B series amps to appear - few survive apart from those coming from deceased estates, with little use.

Failed MOSFETs are likely not the only casualties. You should check all resistors when the semis are removed and measurements should be less confused. Check supply voltages to the IC and its output which should be close to 0. Replace all heat damaged parts and, like the earlier posts urge, uprate the main electrolytic voltage rating to exceed the rail voltage by at least 10% - if required. However, don't waste money on boutique parts - MF certainly didn't! The MPSA93/43 (or similar) driver transistors are also doubtful after many years of stress.

Don't buy Exicons or any semiconductors on Ebay, particularly from HK traders. If there are difficulties buying EXICON brand direct from Profusion on the net, go to Magnatec and get the same parts branded ALFET. The prices are better when buying from the distributor this way, at least.
Farnell/Newark/Element14/CPC also market ALFET but the premium price they ask in my area makes them unviable. YMMV
Counterfeit Transistors

Suitable Magnatec part numbers are ALF08N16K and ALF08P16K.
Profusion EXICON TO3 parts are ECF10N16 and ECF10P16 . There are higher voltage options that could be used if availability is a problem. Don't be fooled by the apparent discrepancy in brand part numbers. One is derived from the peak maximum current rating and the other is continuous maximum current but they are actually the same chip, if you read the data sheets.
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