Samlex SMPS Question

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bwilliams60

Joined Nov 18, 2012
1,233
I have a Samlex SEC1235 on my bench and need a little help interpreting some results. I have a DBT on it and when I turn it on, the 100 watt bulb comes on and immediately goes out. Across my points B&C I have 120 VAC. On a known good unit (KGU) with same setup I have 48.9VAC
When I check my voltage between the +ve rectifier and -ve rectifier, I find 340VDC. When I switch off the unit, it does not dissipate very quickly where the good unit does.
I have replaced Q1, Q2 and all of the components in that section after the rectifier. They all test good at this time. I have also replaced the switching IC but I can't put my finger on what I am missing.
A little push in the right direction please.
I have a KGU running with the DBT so I know it works. I just don't get why the bulb doesn't stay partially lit like the KGU and why it does not dissipate the caps after shutdown. Caps are new as well.
 

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Thread Starter

bwilliams60

Joined Nov 18, 2012
1,233
After some more searching, I have found that there is no voltage on my IC which means that I am not getting anything out of my transformer to provide that voltage.
 

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Thread Starter

bwilliams60

Joined Nov 18, 2012
1,233
Attached to Post #2. Not quite understanding how the switching action occurs on the hot side to pulse that transformer.
 
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Thread Starter

bwilliams60

Joined Nov 18, 2012
1,233
Dim Bulb Tester and Known good unit. I tested bridge rectifier when I had it out but will have to have another look at that. D10 and D11 tested okay as well and are hard to get at as they are located under the transformer. I suspect the primary side but not sure why my voltage is so high. The dim bulb tester on my good unit will go about half brightness when I put it in place of the fuse. On the bad unit it comes on momentarily and then goes out. I would suspect there should be some sort of circuit through a resistance to ground for what it is doing on the good unit? I do not see that here.
 

AlbertHall

Joined Jun 4, 2014
8,846
To start up this supply requires that Q1, Q2 self oscillate using T2 to generate enough voltage for the IC. Then that takes over the base drive for the transistors.
In your case that initial oscillation is not happening. That could be because of Q1, Q2 or the components around them, or possibly a shorted diode or capacitor on the secondary of T2 providing enough damping to stop the oscillation.
 

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,233
@AlbertHall, thank you for your answer, it makes sense to me almost. Can you explain a little bit more about the self oscillating feature and how that works? The circuit calls for 2SC3320 but I used C4140 as I cannot find the previous anywhere. Perhaps this is the problem but I thought I had used these before.
 

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AlbertHall

Joined Jun 4, 2014
8,846
R6 and R10 supply some bias for Q1 and Q2. When these conduct that current flows through the feedback winding on T1 and then T2 primary. The feedback winding produces voltage for D1 and D2 to reinforce (positive feedback) to Q1 and Q2 bases - hence oscillation with Q1 and Q2 conducting alternately.
 

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,233
@AlbertHall. thank you for explaining that. I don't know if you had the opportunity to look at the datasheets for these two transistors but does 4140 look like a good substitute for the 3320? I think it is fairly well matched and the ones I put in were tested and new. That is why I am troubled with the result and where to start looking. I have basically gone over the whole hot side but I guess I have missed something and need to do it again.
 
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bwilliams60

Joined Nov 18, 2012
1,233
@AlbertHall , I made some measurements on board and here are my findings. On a good unit, D10 and D11 in forward bias are 0.845. Reverse bias is 0.451. The bad unit measures 0.820 and 0.455 respectively.
One thing I did note was that when I measured forward and reverse on D1 and D2 on the good unit, I got 0.494 (F) and 0.551 (R). On the bad unit, I got 0.494 (F) and 1.164 (R). Removing R11 from Q2 circuit dropped the RB down.
Does this mean anything? It is the only thing I have found wrong so far in comparison to my good unit.
 

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,233
When I removed R11, there was no voltage in the reverse direction at all. The good board showed slightly higher voltage than FB at 0.551.
 

AlbertHall

Joined Jun 4, 2014
8,846
When I removed R11, there was no voltage in the reverse direction at all.
I presume this is using a multimeter on the diode test range.
If so a reading of zero volts would indicate a short circuit but that couldn't be vaused by removing a resistor (except a sloder splash made during removing the resistor).
I suspect that 'no voltage' means no voltage indication - an overrange indication on the meter.

Please confirm some of this.
 

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,233
@AlbertHall , you presume correct. I was in diode mode and you also suspected correctly that I have no voltage or an overrange indication on my meter. There is no short. I suspect I am getting a reverse reading on the good one because the transistor is still in play on the board. I am scratching my head at this one as I don't seem to find anything wrong with that section of the board. I have not had one go bad on the secondary side other than the 3 2200uF caps which have been changed.
 

AlbertHall

Joined Jun 4, 2014
8,846
OK, grasping at straws now, check the transformer windings in case one is open circuit. This is, in my experience, a very rare fault but it does happen.
I guess it is a double-sided board with plated through holes. How good are you at removing components from such boards? It is quite easy to pull out the plating from the holes.
 

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,233
I am pretty good at removing components so I am going to start working my way through this and see where I land. It may be time consuming but always a good lesson. I will keep you posted. Might be a couple of days before I get back to it. Cheers and thanks again for your help.
 
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