This board seems to follow some of the schematics found on the web, its very similar to the attached, at least on the primary side. So photo 1 shows along the top R > L, 110v in, fuse, input choke, bridge rectifier, 220v secondary input(?) or jumper for 110/220 operation, smoothing capacitors. Q1, Q2 are the main switching transistors, Q3,Q4 are the base/gate drivers via T2. What are Q1,Q2 part #s?

Can you do a high res photo of the back of the board as well...

View attachment 235238

I found an open 100k close to the main filtering capacitor. Changed the VCC capacitor of tl494 to 220mf, output voltage went to 11v. But still some noise and not regulated: a small load drops the output to 5v and at the primary bridge drop too from 310Vdc to 294Vdc.

 

Hello there can you confirm this is not a game of light and shadows no need for another photograph
Is the trace completely intact
View attachment 235291

Hello Delta Prime. Yes, nice catch, but those traces are good.

 

For pictures of the etch side of the board I find it better to place the board on a scanner. This avoids a distorted image and glare from the light source. I have added a flipped version of the track side to make it easier to match up with the component side.

I too noticed the apparent corrosion on the tracks and agree that this should be checked out before spending a long time in tracing out the schematic. What is the part number of the 8 pin IC U2 ? After checking the possible corroded track problem the next step will be for you to trace out the schematic. (I don't have enough time at the moment to do that for you and it is easier for you to do as you have the board in front of you than than for others just working from pictures.)

Les.

 

For pictures of the etch side of the board I find it better to place the board on a scanner. This avoids a distorted image and glare from the light source. I have added a flipped version of the track side to make it easier to match up with the component side.
View attachment 235312
I too noticed the apparent corrosion on the tracks and agree that this should be checked out before spending a long time in tracing out the schematic. What is the part number of the 8 pin IC U2 ? After checking the possible corroded track problem the next step will be for you to trace out the schematic. (I don't have enough time at the moment to do that for you and it is easier for you to do as you have the board in front of you than than for others just working from pictures.)

Les.

Hello Les, thanks for your comments. U2, as mentionned before, is the op-amp HA17358. I have no idea on how to trace the schematic, is it easy to do ? i have by the way no scanner.
As the voltage drops from 310VDC to 295VDC with a small load even with new caps, i suspect the TL494 to be the culprit...

 

Tracing the schematic is relatively easy but very time consuming. You start at a component connection and follow the track and draw all of the components that connect to that track. For example pin 12 on the TL494 goes to the + of an electrolytic capacitor (whose designation I can't see) to one end of R35, to one end of R16, to one end of R17 to jumper link. You would then follow the track from the other end of the jumper to identify all the other components it connected to. You would then do the same for the other end of all the component that connect to the original point. You may stop before you draw the complete schematic if you identify something that could explain the problem. I picked pin 12 of the TL494 as a starting point as we suspected that the was a problem with the voltage on that pin.

Les.

 

Tracing the schematic is relatively easy but very time consuming. You start at a component connection and follow the track and draw all of the components that connect to that track. For example pin 12 on the TL494 goes to the + of an electrolytic capacitor (whose designation I can't see) to one end of R35, to one end of R16, to one end of R17 to jumper link. You would then follow the track from the other end of the jumper to identify all the other components it connected to. You would then do the same for the other end of all the component that connect to the original point. You may stop before you draw the complete schematic if you identify something that could explain the problem. I picked pin 12 of the TL494 as a starting point as we suspected that the was a problem with the voltage on that pin.

Les.

Hello Les,

well noted, i am on it, not easy fo follow when components are in parallel, those engineers must have had a very good time designing this... as soon as i finish it, i will send it. And... thanks for the challenge ! )

 

Hello Les,

well noted, i am on it, not easy fo follow when components are in parallel, those engineers must have had a very good time designing this... as soon as i finish it, i will send it. And... thanks for the challenge ! )

The trick is to have hi-res images of both sides of the PCB and reverse the underside one so it lines up with the top... print them out to the same scale and transfer components from top sheet to bottom sheet.... to make it easy to trace routes.

 

The trick is to have hi-res images of both sides of the PCB and reverse the underside one so it lines up with the top... print them out to the same scale and transfer components from top sheet to bottom sheet.... to make it easy to trace routes.

Wow, i did not do it like that, i did it manually... here are the pictures. I did my best, i hope you guys could read it.

 

You have managed to trace out the schematic much quicker than I expected. I think I have spotted a few errors. (But I would expect to make a few errors on the first pass of drawing the schematic.) Overall I think you have done a good job of tracing it out. It looks to be a similar basic design to the one Irving provided in post #18. I have not seen this system of providing isolation using two transformers. It will take me some time to understand exactly how it works. (Particularly how it starts up.) One thing that looks unusual (At first sight.) is the LED seems to be used as a voltage reference in the feedback loop. It looks it together with the preset resistor close it that it sets the output voltage.
I think the junction of C20 and L2 should go to the other side of the diode.
I have never seen a resistor (R2) connected in series with the main reservoir capacitors. I can't see it well enough on the picture to read it's value. It might be worth reading its value from the colour code and measuring it's value to make sure it has not gone high resistance.

Les.

 

You have managed to trace out the schematic much quicker than I expected. I think I have spotted a few errors. (But I would expect to make a few errors on the first pass of drawing the schematic.) Overall I think you have done a good job of tracing it out. It looks to be a similar basic design to the one Irving provided in post #18. I have not seen this system of providing isolation using two transformers. It will take me some time to understand exactly how it works. (Particularly how it starts up.) One thing that looks unusual (At first sight.) is the LED seems to be used as a voltage reference in the feedback loop. It looks it together with the preset resistor close it that it sets the output voltage.
I think the junction of C20 and L2 should go to the other side of the diode.
I have never seen a resistor (R2) connected in series with the main reservoir capacitors. I can't see it well enough on the picture to read it's value. It might be worth reading its value from the colour code and measuring it's value to make sure it has not gone high resistance.

Les.

Hello Les, i should be able to reply to your questions right after this week end where i have to work. Thanks.

 

You have managed to trace out the schematic much quicker than I expected. I think I have spotted a few errors. (But I would expect to make a few errors on the first pass of drawing the schematic.) Overall I think you have done a good job of tracing it out. It looks to be a similar basic design to the one Irving provided in post #18. I have not seen this system of providing isolation using two transformers. It will take me some time to understand exactly how it works. (Particularly how it starts up.) One thing that looks unusual (At first sight.) is the LED seems to be used as a voltage reference in the feedback loop. It looks it together with the preset resistor close it that it sets the output voltage.
I think the junction of C20 and L2 should go to the other side of the diode.
I have never seen a resistor (R2) connected in series with the main reservoir capacitors. I can't see it well enough on the picture to read it's value. It might be worth reading its value from the colour code and measuring it's value to make sure it has not gone high resistance.

Les.

Hello Les. Here are my answers to your comments:
* you are right l2 & c20 go to the bar side of the diode.
* you are also right, r2 (100k) links the negative side of the main capacitor and not the positive as per my schematic. I found r1 open and replaced it.
Thanks for your guidance

 

I have been re arranging the schematic to make it easier to understand. Here are a few things I am not sure of.
Where the wire to R23 crosses the wire to the + end of C16 does it connect at the crossing ?
There does not seem to be anything connected to pin 8 of the 8 pin op amp. I suspect it should be connected to C29 +. Could you check ?
I think there should be more than just R15 and R16 connected to the base of Q2.
What is the component between R51 and R62 ?
Are pins 3 and 5 on the TL494 connected together and do they connect to R60 ?
What is the part number of the two dual diode components the rectify the output ?
Are you sure that R2 is 100K ? I would expect it to be a very low value.

This is as far as I have got drawing out the schematic with Eagle PCB. ) Let me know if you if you would like the eagle file.

Les

 

I have been re arranging the schematic to make it easier to understand. Here are a few things I am not sure of.
Where the wire to R23 crosses the wire to the + end of C16 does it connect at the crossing ?
There does not seem to be anything connected to pin 8 of the 8 pin op amp. I suspect it should be connected to C29 +. Could you check ?
I think there should be more than just R15 and R16 connected to the base of Q2.
What is the component between R51 and R62 ?
Are pins 3 and 5 on the TL494 connected together and do they connect to R60 ?
What is the part number of the two dual diode components the rectify the output ?
Are you sure that R2 is 100K ? I would expect it to be a very low value.

View attachment 235931This is as far as I have got drawing out the schematic with Eagle PCB. ) Let me know if you if you would like the eagle file.

Les

Hello Les. Yes it looks way better than mine. But why do i see resistors alone on the top and not connected.
To answer your questions:
* r23, the + of c16 and r46 are all linked.
*pin 8 or the op amp is linked to r60 then to d21 (the diode connected to r60) and also linked to r57 that leads to d20. Sorry, i forgot to draw that pin path.
*base of Q2 is linked to r14, r59 and r12, as shown.
*between r51 and r62 is an ntc.
*pin 2 and 3 of tl494 are not directely connected. Between them are c17 c15 and r22. They are far and not connected to r60.
*output rectifiers are stps40h100cw.
*r2 is brown black black orange and brown. I believe 100k.
Thanks.

 

The way Eagle works is it just increments the component number when you add it. To try and keep the original component numbers . I added a large number of resistors and capacitors so I could just select the right component number as I was drawing a section of the circuit.
Tracing out a schematic from the board always give something like you have drawn. Re drawing it is required to form logical blocks of circuit. It is at this stage that you find things that don't look which is the stage we are at.
R2 is unlikely to be such a high value. From the picture I could not see the end band next to the orange. I was guessing it might be brown and taking that as the first band so I thought it was 1, 3, 0, (0 decimal multiplyer .) and (1 tolerance). So I was guessing at 130 ohms which I think is still too high.
EDIT. In post #32 when I asked " I think there should be more than just R15 and R16 connected to the base of Q2. " I should have said Q3 (Not Q2)
I have noticed that you have show no connection to the cathode of one of the dual diodes. Have you included the paths provided by the metal links J7, J10, J26 and J27 ?
Also I have not found any power supply to pin 8 of the dual op amp. I suspect it might come from the positive of C29.

Les.

 

I have been looking to try to confirm if R2 is where you have shown it. I don't think it is where you have shown it but I can't trace exactly where it is connected. I also think you have swapped the designations for C5 and C6.

Les.

 

The way Eagle works is it just increments the component number when you add it. To try and keep the original component numbers . I added a large number of resistors and capacitors so I could just select the right component number as I was drawing a section of the circuit.
Tracing out a schematic from the board always give something like you have drawn. Re drawing it is required to form logical blocks of circuit. It is at this stage that you find things that don't look which is the stage we are at.

Thats one of the reasons I don't like Eagle and prefer Kicad - that retains the component # as R?, C? unless you give it a number and then you annotate afterwards - with the option of not touching stuff already annotated.

If you have an idea of how the components are grouped, ie here we have HV supply, PWM chip. low-level driver stage, power output stage, feedback etc. - gleaned from data sheet or reference design - then its straightforward to drop components in the right 'box' and generally speeds up the process. It also makes it easier to see when its gone wrong, or when a component is clearly misplaced!

 

Hi Irving ,
yesterday I found I could change component designations. Just right click in the component and select properties.
In the past I have used Eagle to draw schematics of things I was building so it did not matter about component designations.
I am using the schematic you posted in post #18 to try to understand how this type of power supply works. I have not seen one using just transformers to provide isolation. (I have only seen ones using opto isolation in the feedback loop.) Although the TSs unit only has two transformers and your schematic has three I think in your T3 in yours is sensing overcurrent. I am guessing that at startup the circuit self oscillates without the YL494 and then the oscillator in the TL494 takes over. (I have not yet read the TL494 datasheet fully.) One thing I have though about recently is if the power supply has a minimum load rating. I need to ask the TS that question. I have attached the Eagle file so you can add any correction that you spot.

Les.

 

@LesJones The first transformer, T2 in the original schematic and also in this PSU provides the base drive to the output transistors Q1/Q2 while isolating the predrivers Q3/Q4 from the fact that Q1/Q2 are respectively sitting +/- 70v relative to ground, driving the primary of T1 (T4 in original cct) which connects between Q1/Q2 and the nominal midpoint of the 2 big smoothing caps, which is roughly the incoming ground point.

There will be an isolated feedback to the TL494, either from the output or a winding on T1.

In the original, T3 provided power to the TL494, though I'm not entirely clear how as it seems to be a negative supply. In our circuit here I suspect again its from a winding on T1, but I've not looked too closely.

 

@LesJones The first transformer, T2 in the original schematic and also in this PSU provides the base drive to the output transistors Q1/Q2 while isolating the predrivers Q3/Q4 from the fact that Q1/Q2 are respectively sitting +/- 70v relative to ground, driving the primary of T1 (T4 in original cct) which connects between Q1/Q2 and the nominal midpoint of the 2 big smoothing caps, which is roughly the incoming ground point.

There will be an isolated feedback to the TL494, either from the output or a winding on T1.

In the original, T3 provided power to the TL494, though I'm not entirely clear how as it seems to be a negative supply. In our circuit here I suspect again its from a winding on T1, but I've not looked too closely.

Hello Les and Irving, i changed the tl494 by a new one and it solved all the issues. I am now ready to give you oscilloscope or other measurements if.needed to better understand the circuit.

 

As replacing the TL494 has fixed the problem we no longer need to trace out the schematic of your power supply. The idea was to create a schematic so we could understand exactly how it was designed to work and from that work out which parts of the circuit may cause the symptoms you describe. From that stage we could suggest tests for you to do to prove or disprove any theories.
This is normal faultfinding practice to identify the part that is causing the fault and replace it. You are lucky that you guessed which component was the cause. The most common fault with switch mode power supplies is a component failing short circuit and causing other components to fail. With that sort of fault you have to be sure that you identify all of the failed components as missing one very often causes all the replaced components to fail again when powered back on.

Les