Help needed with Musical Fidelity P270 II electrical problem - diyAudio
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Old 13th May 2009, 05:35 PM   #1
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Question Help needed with Musical Fidelity P270 II electrical problem

Hello, I need some help. I decided to replace all the electrolytic capacitors in my old -1990ish- Musical Fidelity P270 II power amp with ’better’ ones at least fresh ones. These were the ones on the main boards (dual mono design) not the big power supply ones.

I did one side, put the thing back together and tried it, everything OK. Did the other side… smoke! It was coming from a resistor -100R ceramic 4 watt- paralleled with a 68nf capacitor on an earth lead going to, or from?, one of the 4 large 10000uf’s arranged together. I realise this means little to anyone not familiar with this amp! I’ve checked everything I can think of. I don’t seem to have made any errors in my soldering etc. One thing I’ve found out is that the resistor doesn’t overheat unless the heatsink (the mono psb is on) makes contact to the rest of the amp’s body. An earth short? Where! It’s unlikely the new capacitors were faulty? (Panasonics & Nichicon) Before I start removing things any ideas at all where the fault could be!!?
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Old 14th May 2009, 07:26 AM   #2
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I seem to have flummoxed you!
here's a picture the ceramic resistor with the orange capacitor attached is the one overheating
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Old 14th May 2009, 07:29 AM   #3
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Does anyone even know what these two components are for? a safety device?
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Old 14th May 2009, 08:37 AM   #4
jaycee is offline jaycee  United Kingdom
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These form an earth loop disconnector circuit.

If they are smoking that means current is flowing. You have a power supply short to ground somewhere.
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Old 14th May 2009, 08:46 AM   #5
mjf is offline mjf  Austria
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hello.
have you tested the isolation between transistor cases and heatsink (with multimeter ) ?
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Old 14th May 2009, 11:25 AM   #6
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Quote:
Originally posted by mjf
hello.
have you tested the isolation between transistor cases and heatsink (with multimeter ) ?

the 'case' touches the heatsink with a thin plastic insulator sheet and a screw going throuh it, it must make contact?
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Old 14th May 2009, 01:41 PM   #7
AMV8 is offline AMV8  United Kingdom
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dynoport

On the P270,s that I own the ceramic resistor you mention on the P270 power supply is fitted on the positive, and negative, supply supply lines between the two power supply capacitors. I assume that it is intended to make a filter. On my P270,s the ceramic resistor is 1 ohm and not 100 ohm.

Don
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Old 14th May 2009, 03:12 PM   #8
jaycee is offline jaycee  United Kingdom
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There should be no conductivity between the transistor cases and the heatsink. If there is, then it means a problem with the mica insulator or some other connection.

There should be no conductivity from any power supply rail to the heatsink - the heatsink should only be connected to ground.

That resistor getting hot means there is current flowing from somewhere, to ground, via that resistor. You say that it is connected via a wire near the filter caps - do check you havent got any bridging, even just a tiny flake of solder or stray strand of wire will do it.
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Old 14th May 2009, 04:05 PM   #9
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the case is a TO-3p not one of those all metal ones
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Old 14th May 2009, 04:21 PM   #10
jaycee is offline jaycee  United Kingdom
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Same applies. The metal tabs should be completely isolated from the heatsink. If any of them conduct, there is a problem.
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