Musical Fidelity P270 - strange power off noise - diyAudio
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Old 28th February 2009, 04:46 PM   #1
Nrik is offline Nrik  Denmark
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Default Musical Fidelity P270 - strange power off noise

Hi there

I have just fixed one of these, and it works fine and sound good and all.

But one channel (not the one I fixed) makes a strange power of sound. The outputs are not protected so you hear the emptying of the capacitors.
When one switches off the power, it takes some 5-10 seconds where the music still goes on, then there is silence and then after 10 more seconds the left channels gives a medium-low volume 'beeeaaap' oscillation-thing below 1kHz and then some "DC-blaffer".

It is probably nothing to worry about, and it doesn't seem to put the speakers in danger, but does anyone have a tip on what I could do to get rid of this (without installing power-off relays) ?
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Old 28th February 2009, 10:02 PM   #2
AMV8 is offline AMV8  United Kingdom
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Hi

From memory I believe that the P270 has a power on/off circuit that should kick in when power is switched off. I would guess that it is not working properly on the connection to one side - each channel has a separate power supply.

I havea few P270 and I can confirm that the power on/off circuit does sometimes cause a problem. Test the capacitors in that circuit which is situated on the rear panel.

Don
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Old 28th February 2009, 10:50 PM   #3
Nrik is offline Nrik  Denmark
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okay thanks.
will check.
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Old 1st March 2009, 12:08 AM   #4
Nrik is offline Nrik  Denmark
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Don: Do you mean the small PCB on the (single) power resistor on the backpanel? It looks more like a softstart relay to me.

The picture is not from my unit, and mine doesn't even have a capacitor at that spot. If I should add one, what value should it be, and do you have like a small schematic? - because I cannot see where to attach the leads.

Sorry to bother.


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Old 1st March 2009, 07:34 AM   #5
AMV8 is offline AMV8  United Kingdom
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Nrik

Yes, that is the correct location.

The problem with detailed diagnoses is that Musical Fidelity made a few small changes to that circuit and also to the main pcb. I think you need to look at that small pcb and try to find out if it is only a soft start circuit or if it also acts as a relay to disconnect power.

Another possibility that you could also look at is the condition of the 5 ohm (?) resistor and 68n capacitor between the negative output of the channel and earth. Are they in good condition and properly connected? I think that a poor connection there could also cause your problem.

The main problem that the amplifier normally suffers from is that the 1.5k and 1.8k ohm resistors that reduce the power on each power supply rail from 63v to 12 v ( to provide a 12 v supply to the input ic ) often/normally overheat and burn out. On later models these were replaced by a pass transistor which works without problem. You could also check that possible problem. I would not have thought that would lead to your problem but then again you never know what the effects of wrong supply voltages to the input ic will be.

Don
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Old 1st March 2009, 07:10 PM   #6
Nrik is offline Nrik  Denmark
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Don
Thanks for your help so far.
Today I got a bit further.

The small PCB on the backside is in fact a soft-start circuit.
The zobel filter 5 ohm are actually to 10 ohms in parallel, and they are measuring fine, and the 68nF looks fine.

Yes one of the 1.5k was exactly what was wrong in the first place. The component itself was fine, but the connection on the PCB had failed, obviously because of excessive heat.

I am now suspecting one of the 22pF to be defective, just by the way it looks. Could that cause this strange behavior?

Basicly I would like to dismantle the whole channel and replace several of the passive components that have suffered heavy dissipation. But the PCB itself looks fragile several places so I'd rather not.


Another thing is, that there is quite a decent amount of 50HZ hum on the outputs, even without anything plugged in to the inputs. Any hints on that one?
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Old 1st March 2009, 11:30 PM   #7
Nrik is offline Nrik  Denmark
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I see now compared to the pictures on this page: http://amp8.com/tr-amp/foreign/musical/p270.htm
...that the previous owner changed the groundpath in my P270 so that the input ground and the output ground runs together.

So now I know how to remove the hum.

Plus I decided to install output relays, yeah I know: it wasnt part of the original desing, but better safe than sorry.
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Old 2nd March 2009, 01:14 PM   #8
AMV8 is offline AMV8  United Kingdom
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Nrik

Good to hear that you seem to have everything sorted out.

I am also pleased that you found the 1.5k ohm resistor in time and before any real harm was caused. The original 1.5k and 1.8 k ohm resistors are 2watt or 5 watt. I had to change these to 20 watt resistors before the heat became acceptable. I also raised them 5 mm from the pcb to allow cooling.

On my later P270 upgrades I revised the circuit by using a pass transistor to replace the 1.5k and 1.8 k ohm resistors.

I would not expect the 22pf capacitor to be a problem unless its connection to the pcb is loose.

You seem to be ready for music now.

Don
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Old 13th February 2010, 04:20 PM   #9
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Quote:
Originally Posted by AMV8 View Post
...The original 1.5k and 1.8 k ohm resistors are 2watt or 5 watt. I had to change these to 20 watt resistors before the heat became acceptable. I also raised them 5 mm from the pcb to allow cooling.

On my later P270 upgrades I revised the circuit by using a pass transistor to replace the 1.5k and 1.8 k ohm resistors.

I would not expect the 22pf capacitor to be a problem unless its connection to the pcb is loose....

Don

Hello AMV8

I'm a newbie from Hanoi, Vietnam.

Recently I bougth the P270-2 (Reference?) in attach photos. But unfortunably, I have the problem, not only the same as the problem of Nrick, but also even when switching it off with big 'beeeaaap' oscillation-thing below 1kHz and then some "DC-blaffer" for all of two channels.

Please help me to find out and indicate capacitor(s) and/or resistor(s) in these photo. I will ask to replace by some one in Hanoi

Thank you very much and hope to read from you soon!
Best regards,

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Old 13th February 2010, 05:15 PM   #10
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I'm sorry to make a mistaping:

"I have the problem, not only the same as the problem of Nrick, but also even when switching it ON with big 'beeeaaap' oscillation-thing below 1kHz and then some "DC-blaffer" for all of two channels"

Please help me!
Thank you very much!
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