Welcome, Guest. Please login or register.

June 15, 2021, 05:22:57 AM

Login with username, password and session length

Recent Posts

 

Author Topic: Wharfdale PM700 SMPS intermittant turn on  (Read 4215 times)

nosaj

  • Elite
  • ****
  • Posts: 102
  • Chip Points: 9
    • View Profile
Wharfdale PM700 SMPS intermittant turn on
« on: June 12, 2016, 06:50:16 PM »
Wharfdale PM700 SMPS intermittant turn on
I've picked up the wharfdale PM700 PA again in hopes of making it stable. Sometimes it will turn on and just play fine other times you have to pop the on/off switch several times to get it running.

Monitoring the -+ 63 v rails and the 15v rails when it doesn't come on they produce on a few volts. The largest caps have been replaced. The little fan always comes on. I have this power supply schematic that utilizes the same smps controller chip and a lot of it looks very similiar to what I have.
From what little my brain has gathered SMPS supplies need a kick start to get them oscillating to produce the voltage kinda of like a startrun capacitor on a AC motor. But not sure where I should be checking.
Any helps would be greatly appreciated.

If the voltage regulators are iffy not putting out a clean 15v would that cause the SMPS to be intermittant?

Thanks,
Nosaj

Enzo

  • Legendary
  • ******
  • Posts: 2015
  • Chip Points: 220
    • View Profile
Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #1 on: June 13, 2016, 12:10:34 AM »
SMPS only needs a kick start if it powers itself, like a car needs a battery to start but once running will power itself from the alternator.

Look at your schematic, the control IC has a continuous power supply, the same one that runs the fan.  Below the main fuse, see transformer T4?  It is wired across the mains (and drawn wrong) to provide a constant low voltage supply.  I will guess about 20v.  That is then regulated by U2 down to +15, which in turn goes through U3 to make +5 for the IC.  There is a line back from the secondary side through D7, but that shouldn;t be needed to start.  Do you get +5 on the power pins of the IC?  Do you get +15 and +5 supplies?

See Q10 near the fan?  Is the voltage in cap C966 charging up to put enough voltage across R54 to turn on Q10?

nosaj

  • Elite
  • ****
  • Posts: 102
  • Chip Points: 9
    • View Profile
Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #2 on: June 15, 2016, 09:00:13 PM »
Changing c966 made the difference I also shotgunned the rest of the smaller capxcon cap I didn't get last time.  So far turns on everytime, will check in a few days to be sure.

Thanks so much ,
nosaj

nosaj

  • Elite
  • ****
  • Posts: 102
  • Chip Points: 9
    • View Profile
Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #3 on: November 26, 2016, 11:56:56 AM »
This one reared its head as u3 failed putting out only about 5v. I replaced it and 15v rail is good now.  What I'm missing now is the 300v rail.
q11 is getting 56vacI'm going to check voltage at c44 and D1 to see what I'm getting there.

D1-D4 what kind of circuit is that?  A rectifier or somekind of voltage doubler, trippler, etc?

Thanks,
nosaj

nosaj

  • Elite
  • ****
  • Posts: 102
  • Chip Points: 9
    • View Profile
Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #4 on: November 26, 2016, 12:57:32 PM »
Going back to R5 and R41   I'm getting about 58-60vac  it varied each time I turn it on.  Guess I'll go back further to c40.

nosaj

Enzo

  • Legendary
  • ******
  • Posts: 2015
  • Chip Points: 220
    • View Profile
Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #5 on: November 26, 2016, 01:22:22 PM »
Is this a different unit?  Or is this the original one still fucked up?

U3 is a 7805, so it only is supposed to put out 5v.  In fact, 15v is the INPUT to U3.

STOP

You are in danger.  Are you using an isolation transformer?  Are you measuring this 300v to ground?  Step back and look at the circuit.  The 300v comes from the bridge of D1-4. and the stacked caps.  Note the 300v "ground" is not the same ground as other parts.  You need 300v with respect to R37.

C40 looks to me across the mains as it enters.  Those inrush thermistors are OK if the thing works at all.

nosaj

  • Elite
  • ****
  • Posts: 102
  • Chip Points: 9
    • View Profile
Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #6 on: November 26, 2016, 01:39:29 PM »
This is still the original unit.  My mistake in stating U3 which is a 5v as you stated.( I have not actually touched U3) It was U4 the 7815.  I apologize for not proof reading my post.

I am using a B&K 1655  Isolated power supply with a variac.

I included a picture showing what I understand your saying when you say
You need 300v with respect to R37.

Is that correct?

Thanks,
nosaj

Enzo

  • Legendary
  • ******
  • Posts: 2015
  • Chip Points: 220
    • View Profile
Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #7 on: November 26, 2016, 04:00:24 PM »
Yes.

Or you could measure across each filter, or the pair of them.  All the same thing.  Wherever those two points in the circuit are accessible.

I wouldn't be surprised if the 300v is actually higher than that, like maybe 340.  It is rectified 120vAC doubled.

nosaj

  • Elite
  • ****
  • Posts: 102
  • Chip Points: 9
    • View Profile
Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #8 on: November 26, 2016, 10:30:28 PM »
304v is what I'm getting.  So all the rails look good now I have to hit the output section, because 1k .5 v input  into a 8ohm load I only get about 10v on the output  which I believe is around 12.5watts  definately not even close to what it should be doing.

Input was into CH1  I realize now I should try the aux and other inputs  to narrow some down.  In the power amp input  about what voltage should I be inputting there( I have an hp200 signal generator I think it will put out at least 20v)  For preamps I normally use about half a volt.


Thanks,

Nosaj
« Last Edit: November 26, 2016, 10:37:39 PM by nosaj »

nosaj

  • Elite
  • ****
  • Posts: 102
  • Chip Points: 9
    • View Profile
Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #9 on: November 27, 2016, 07:47:40 PM »
No matter what input  I input into  I cannot max more than about 12volts.  If any of the outputs were shorted It would't produce sound correct?

What I will try to do tomorrow is to check voltages on the output transistors.

Thanks,
nosaj

nosaj

  • Elite
  • ****
  • Posts: 102
  • Chip Points: 9
    • View Profile
Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #10 on: December 01, 2016, 10:46:12 PM »
Still haven't gotten a chance to check voltages, but was still curious that since I get sound.  Would it more helpful to follow gain through the stages
Starting with Q1 through Q21

Thanks,
nosaj

phatt

  • Legendary
  • ******
  • Posts: 2201
  • Chip Points: 271
    • View Profile
Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #11 on: December 02, 2016, 02:02:33 AM »
304v is what I'm getting.  So all the rails look good now I have to hit the output section, because 1k .5 v input  into a 8ohm load I only get about 10v on the output  which I believe is around 12.5watts  definately not even close to what it should be doing.

Input was into CH1  I realize now I should try the aux and other inputs  to narrow some down.  In the power amp input  about what voltage should I be inputting there( I have an hp200 signal generator I think it will put out at least 20v)  For preamps I normally use about half a volt.


Thanks,

Nosaj

I think in general a power amp stage only needs 1~2 Volts to give full power,, though some may need a bit more but surely not much.
Maybe check the preamp output into another poweramp and establish if the issue is in the preamp or power amp.
Phil.

nosaj

  • Elite
  • ****
  • Posts: 102
  • Chip Points: 9
    • View Profile
Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #12 on: December 02, 2016, 06:37:08 PM »
When I main out into another amps line in. It drives it normally. So I'd say power amp issue. But what would cause it to not push more than 12 watts?

Thanks,
Nosaj

phatt

  • Legendary
  • ******
  • Posts: 2201
  • Chip Points: 271
    • View Profile
Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #13 on: December 03, 2016, 05:44:41 AM »
Well obviously there is something wrong in the poweramp circuit.

I Just Dloaded the schematic from your 1st post,,,
URRR  xP
That is the circuit for a powered speaker but you seem to be talking about a powered mixer,, can you clarify what you actually have in front of you? 8|

*IF* the posted schematic is the same then look for  SW101 as it would effect the sensitivity of the input and that would alter the final output level.
In powered speakers they often have a Line/Mic switch to allow for low /high inputs. I'm guessing that is SW101 while SW100 is a phase flip.
Phil

phatt

  • Legendary
  • ******
  • Posts: 2201
  • Chip Points: 271
    • View Profile
Re: Wharfdale PM700 SMPS intermittant turn on
« Reply #14 on: December 03, 2016, 06:19:39 AM »
Arrh  huh,, I just read the other thread so obviously the same circuit less the input parts
*Top left dashed lines* not used for mixer.

Signal input is on pin 1 of *P6* ,, Q1 & Q2 are likely part of a mute circuit controlled by the lower section.
I'd lift Q1 & Q2,, see what happens.

AFAIK these are only there to mute the Thump when you power on / power off and likely not needed.

If the mute circuit is failing then that may cause issues.  Looks like the control for that comes from Pin 1 at *P7*
Might be be some kind of overload circuit that drives that mute setup,,, better mind here will know more,,,
Phil.