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Author Topic: Wharfdale PM700 SMPS intermittant turn on  (Read 4343 times)

nosaj

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Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #15 on: December 03, 2016, 11:39:21 AM »
Q1 and Q2 removed  I'm getting about 32 volts into a 4.2 ohm load  so looking at around 250watts  Which should be roughly right seeing as it says a 300 watt amp(though we all know amp wattages are like politicians).

Q1 and Q2 are j111  (a npn chopper transistor)  What would be a downside to permantly leaving these out?

Thanks,
nosaj

phatt

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Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #16 on: December 03, 2016, 08:39:13 PM »
That's hard to know?? xP
As that control pin runs the Fan as well so I'm not able to help,,xP
Hopefully the teck chaps will see this and know how that all works.

If the amp does not thump really bad then it's safe *up to a point*.
Remember that a lot of this gear now tends to rely on the fan for cooling and heat sinks are rather small
 (in some cases pathetic) so it may struggle at high power if the Fan circuit is not working.

Q1 & Q2 might be working but the circuitry below that is also involved in turning them on and off.
Also the schematic does not show what drives pin1 of P7,, likely some kind of heat sense system,, maybe  on the heat sink?

Anyway at least you know it works. :tu:
Sorry I can't help more,, Phil. 

Enzo

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Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #17 on: December 03, 2016, 08:42:15 PM »
They are JFETs, not NPN.

They are a pair of muting transistors, when they are on they put a low impedance shunt across the power amp input signal.  They need a voltage at the gate to turn them OFF.  So if no voltage is present at their gate leads, they are just doing their job.
No, it won;t hurt anything if you leave them out, but you will lose their function.  Turn-on thumps might return, noises at power up. 

Unfiltered negative DC comes in pin 1 of P7.  Is C16 charging up?  R12?  GAtes of Q1,2?

Also, thermal sensor R4 below, controls the fan, but also the mute.  It does that through Q23,24.  If Q24 turns on, it mutes the amp.

Eidt to add:  of you remove Q1,2, it won;t affect the fan, but you might not like the noises.  However just removing Q1,2 does not adress the problem, and if that problem DOES affect the fan, you can be in trouble.  Get to the bottom of it, no bandaid fixes.

nosaj

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Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #18 on: December 04, 2016, 08:34:41 PM »
I will check those as soon as I can.  Been a lot of Rain here this weekend So theres a river between Shed and here.

Will reply back soon.

Thanks so much,
nosaj