Help needed with Musical Fidelity P270 II electrical problem

dynoPort · 13th May 2009, 05:35 PM

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!!?

dynoPort · 14th May 2009, 07:26 AM

I seem to have flummoxed you!
here's a picture the ceramic resistor with the orange capacitor attached is the one overheating

dynoPort · 14th May 2009, 07:29 AM

Does anyone even know what these two components are for? a safety device?

jaycee · 14th May 2009, 08:37 AM

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.

mjf · 14th May 2009, 08:46 AM

hello.
have you tested the isolation between transistor cases and heatsink (with multimeter ) ?

dynoPort · 14th May 2009, 11:25 AM

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?

AMV8 · 14th May 2009, 01:41 PM

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

jaycee · 14th May 2009, 03:12 PM

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.

dynoPort · 14th May 2009, 04:05 PM

the case is a TO-3p not one of those all metal ones

jaycee · 14th May 2009, 04:21 PM

Same applies. The metal tabs should be completely isolated from the heatsink. If any of them conduct, there is a problem.

13th May 2009, 05:35 PM	#1
dynoPort diyAudio Member Join Date: Oct 2004 Location: Cymru/Wales	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!!?

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Musical Fidelity P270-2 output devices?	kenneth	Solid State	28	10th January 2012 01:55 PM
Musical fidelity P270 and A370 fets	AMV8	Solid State	14	16th September 2011 03:40 AM
Musical Fidelity P270	Lenin	Everything Else	9	5th February 2010 01:44 PM
Musical fidelity p270	ingemar	Solid State	3	13th June 2006 05:12 PM
Musical Fidelity P270	Lenin	Solid State	3	24th March 2002 12:27 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

14th May 2009, 08:37 AM	#4
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	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.

14th May 2009, 08:46 AM	#5
mjf diyAudio Member Join Date: Jan 2009 Location: upper austria/near linz	hello. have you tested the isolation between transistor cases and heatsink (with multimeter ) ?

14th May 2009, 01:41 PM	#7
AMV8 diyAudio Member Join Date: Jan 2005 Location: near london	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

14th May 2009, 03:12 PM	#8
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	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.

14th May 2009, 04:05 PM	#9
dynoPort diyAudio Member Join Date: Oct 2004 Location: Cymru/Wales	the case is a TO-3p not one of those all metal ones

14th May 2009, 04:21 PM	#10
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	Same applies. The metal tabs should be completely isolated from the heatsink. If any of them conduct, there is a problem.