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
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?
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
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
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
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.
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
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.
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
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
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
Quote from: nosaj 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
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.
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
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
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.
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
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.
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.
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