pm3355 PSU old fashion flyback refuse to regulate - Page 1
Forum section: electronics / testgear

author: iso14000
author_rank: Contributor
timestamp: July 21, 2015, 10:58:09 am
content: Hi, tout le monde.Please apologise my very bad english (I'm French).This is my first topic, I was previously active on french forum but know I'm stuck on this problem and I don't find somebody with the same problem.A friend leave me this oscilloscope to fix it. He tried to do it by himself but fail to do so. Because I use to design PSU I'm aware of few things around there.So please find attached the schematics of this nice but tricky PSU :

author: iso14000
author_rank: Contributor
timestamp: July 21, 2015, 11:13:49 am
content: As you can see, it's a fly back topology, the lack of inout filter is compensate by the slope controled circuitery around the BUZ80 (which cannot saturate according the polarisation circuit).know look at the limitation system : if primary current raise too high the SCR (old component BRY39) is triggered by R6017 and R6018 shunt resistor voltage.this part seems working well (hopfully , because BRY39 is nowaday almost unavaible).But when I power up this supply (by the main of high voltage laboratory supply) the supply deliver a very bad sound, consume almost 10W without loads and BUZ80 become very hot.In fact the secondary circuitery activate the overvoltage protection (at the top right) which trigger V6112, this SCR shorts circuits +48v and -48V outputs together maintaining the supply in a "security" state.So the clue is that normal regulation isn't active (no feed back)I first belevied that optocoupler was defective.... and test in individualy .... it's finally OK.so the last track is it's polarisation and I plugged a small 5V floating supply on C6011 and surprise! the PSU is know in "silent mode" all secondary voltage are OKis anybody have a clue of what's wrong on this PSU.regards

author: Andy Watson
author_rank: Super Contributor
timestamp: July 21, 2015, 12:15:49 pm
content: Given the age of this device I would suspect that the electrolytic capacitors are drying-out. Do you have a means of measuring C6011? - might be easier to test by simple replacement. I would also check the main 68uF capacitors C6007, C6008..

author: iso14000
author_rank: Contributor
timestamp: July 21, 2015, 12:52:22 pm
content: hi Andy.All caps were renewed before any power up, C6011 also, that's why I'm a little puzzled....btw do you have any idea about the small cap 47nF and the way the npn V6009 are used?during startup R6007 and R6003 proved trickle current for the first cycle, but after that the only way to get current seems to be 47nF....thanks for your interest, did you already seen this kind of supplyregards

author: mij59
author_rank: Frequent Contributor
timestamp: July 21, 2015, 01:49:01 pm
content: The transistor V6009 limits the gate voltage of mosfet V6014 to about 14.3 V.Did you check R6005, V6008 and V6009 ?

author: iso14000
author_rank: Contributor
timestamp: July 21, 2015, 01:58:39 pm
content: oh god! yes.... I forgot my basic knowledges, thanks! the npn is a emitter follower... shame on me!I have already test V6008 and V6007 but not V6009... I will check this tomorow.R6005 is OK....regards

author: iso14000
author_rank: Contributor
timestamp: July 22, 2015, 11:54:25 am
content: OKI have tested V6009 and it's OK.I have also snap some voltagein all picture bellow we can see voltage at R6017//R6018 so it gives a good idea of the current into the primary winding.for the first picture the red trace is voltage at T1-2 winding

author: iso14000
author_rank: Contributor
timestamp: July 22, 2015, 11:57:10 am
content: on this picture, red trace is voltage on C6011as you can see, it goes down to 1V,

author: iso14000
author_rank: Contributor
timestamp: July 22, 2015, 12:04:37 pm
content: on this last picture, the red trace is the output pin 7 of N6001 which drive the opto's led.As you can see it's constantly said : hey! the output is too high according the little hysteresis ... but primary does't matter, until the -6.4V crowbar circuitery activate the V6112 SCR making +48V shortcut to -48Vso that's wrong.....?

author: mij59
author_rank: Frequent Contributor
timestamp: July 22, 2015, 12:16:31 pm
content: Quote from: iso14000 on July 22, 2015, 11:57:10 amThe voltage on C6011 is very low, clould you scope the voltage on the anode of V6008 ?

author: Andy Watson
author_rank: Super Contributor
timestamp: July 22, 2015, 12:36:19 pm
content: Quote from: iso14000 on July 22, 2015, 11:57:10 amWhere is the ground (0V) relative to the red trace? I would expect to be able to see charging pulses from each pulse of the green trace.https://www.eevblog.com/forum/index.php?action=dlattach;topic=51060.0;attach=162096;image

author: iso14000
author_rank: Contributor
timestamp: July 22, 2015, 12:49:06 pm
content: hi both!so stupid I'm . I push too much the cursor to "low" into the menu quality of my picture compressor! and crop too much the raw image.here is the last picture availlable from the scope. I didn't move the zero from one to another snapshot, sorry, now I have removed the setup from my desk because I'm not suppose to work on this "top-secret" project in my officeif you look carfully on the left of pm3355_1 picture you can "guess" something : it's the zerosorry

author: Andy Watson
author_rank: Super Contributor
timestamp: July 22, 2015, 12:57:10 pm
content: I was referring to the second (?) image of the voltage on C6011 (but the forum software would not let me link directly to the image). It would be interesting to look at the voltage on both the anode and cathode of V6008. At the moment it looks like C6011 is not being charged properly - this would fit with your earlier test in which you supplied 5V (?) to charge C6011. I think the pulse from the feedback of T1 (pins 1 and 2) should charge C6011 - looks like this is not happening.Edit: V6006 should have been V6008.

author: mij59
author_rank: Frequent Contributor
timestamp: July 22, 2015, 01:04:52 pm
content: Although V6008 measured good it cloud be faulty, replace it with a fast recovery diode.

author: iso14000
author_rank: Contributor
timestamp: July 22, 2015, 01:56:31 pm
content: OK for a raw replacement of V6008... tomorow only.If I well anderstood, the T1 1-2 winding is working like a simple transformer effect, when current is raising into the primary winding there is a positive voltage on T 2-1 winding.So the only way to charge C6011 is to have some pulses, if I compute well R*C=464*33µF = 15ms and the caps should be recharge after 5*15ms=75ms, it's longlonger than 30µs ramp up 1st part of cycle.but during flyback, and because V6008 and V6008 configuration, the energy to turn the Mosfet on cannot come from C6011, so it must comes from the remaining components at the left side of non collector : the small 47nf cap and/or R6007 and R6008 (used for startup I guess)...are you agree with that?thanks for your interest

author: mij59
author_rank: Frequent Contributor
timestamp: July 22, 2015, 02:13:51 pm
content: Quote from: iso14000 on July 22, 2015, 01:56:31 pmDuring flyback the mosfet is turned off, the gate source of the mosfet is shorted by the thyrisor.

author: iso14000
author_rank: Contributor
timestamp: January 08, 2016, 12:58:40 pm
content: hime again.Good news! I finally fixed the scope yesterday... it was very special, the low voltage isolated side of the PSU is under permanent monitoring of a crowbar circuit build around V6134 V6136 V6137 zener diode V6133.If 5V rise up of about 6V1 V6137 is activated and fire a thyristor that short circuit +48V and -48V transformer output together . Same thing if -6.4V isn't low enough after a small amount of time (typicaly a few seconds).This thyristor dispate good behavior under DMM was defective and a new one make the PSU reborn!(the thyristor is located at middle right of the schematic : V6112)This PSU is very strange but really interesting because of the dv/dt under control during switching on and off I keep into my database.Because I ran out of BUW42 I have modified the circuit to accept a modern Mosfet such as SPP11N60 from Infineon, in fact it's almost a drop in mod. According to schematic below you can see that removing the bipolar transistor is necessary and only one cut and one wire are requiered...For those who want to view the full (and long) research aroud this fixing : they can read the french forum .... here :http://abcelectronique.com/forum/showthread.php?t=90629bye!bye