This hitachi P.S. is driving me nuts!

You could ring test the trensformer....

--- sam | Sci.Electronics.Repair FAQ:

Important: Anyth> I've posted this before, but now I have some more info.

I would be more suspicious of what is happening on the primary side. The STK7308 could be dicky.. (looks pretty ancient)

You say you changed the primary side electro's but did you include C1508 (4.7uF/50V 105C) in the changes.

When loaded what is the voltage across the main input filter cap (C1508 ?)

The square wave looks like what you will find in a switching power supply as this one. If you are getting too much voltage drop, you probably either have an excessive load or the regulation circuits are not working properly. I see what looks like a driver regulator transistor with two zener diodes (TR1514). You could try checking those components. I also see feedback to what looks like a triac which would affect the voltage drop across the 10 ohm 3 watt resistor (R1501?). The 10 ohm resistor could be for initial "soft start"/mean value etc.

-- Dave M.

too

Yes I did change C1508 also, voltage accros main input filter caps ~160VDC and shows no ripple .

Ive checked into that also, replaced TR1514 and zener diodes, as for the triac it works properly , | even shorted the 10 ohm resistor while running to see if that will make any difference , it didn't.

How do I do that ? I mean whats the easiest way to do that , preferably without desoldering it?

Thanks.

It's still low on the probability scale but eliminating the transformer as a likely problem could prevent you from going down a bunch of dead ends.

See the Flyback Testing FAQ:

You will probably want to desolder it though.

--- sam | Sci.Electronics.Repair FAQ:

Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs.

Looking at the circuit diagram annotations I think you may have driven it slightly into over-current protection with your dummy load, and that could explain your voltage drop.

-12V should be 0.3A ==> 40R (you don't say what you used for this)

Dave

Well, -12V and 110V was left open, no load there. If you add the loads together I dont think I'm overloading it. Also dont forget when connected to the scope it overloads too. The rest of the scope works fine.

If you overload any ONE output then you are overloading the entire SMPS circuit. Standard SMPS operation dictates that each and every winding output must remain below load. Some SMPS need a load on every output as well.

SMPS supplies that are showing the MAXIMUM current output. Normally you cannot simply load the outputs all at once a turn on at maximum load as this is not how it would operate in a normal circuit.

I still think that you should use the correct dummy load values to test the PSU - I suspect that the values quoted on the schematic may not be steady state values, but limits, in which case your dummy load should use higher value Rs.

I think it is likely that a capacitor in one of the load circuits has gone (partly or completely) shorted. Measure the resistance to ground on the on the lines to the rest of the scope with the PSU disconnected - see if you can spot one that has an unusually low value.

It's also well worth checking the values of all the high wattage resistors (R1501, R1502, R1512 and R1522). Also check C1511 and C1513.

The second waveform you linked to - with the diodes reattached and with the resistive loads reapplied - does that include the filtering circuitry in the schematic, too? (C1561-3, L1562)?

If so, maybe I'm misinterpreting what I'm seeing in the scope picture, but that semi-square wave with the ringing on the leading edge has an amplitude of over 17V P-P. That seems excessively high to me for a rail that's supposed to be delivering +5V DC (especially given it's got close to 1000 mfd filtering it).

Are you sure that there's not some kind of weird grounding problem happening here?

Sorry , let me explain better. Both waveforms are taken at the same point : Primary windings of the transformer, probe ground was connected to negative leg of C1509,(used isolation transformer of course) tip of the probe to leg no 5 of the transformer. Probe was set to 10x1 , so you have to multiply what you see by 10 ie

Now as far as not loading one rail while slightly overloading other, while it may be true in most of cases, I dont think it applies here, if you study the schematic you'll see that theres no feedback whatsoever from any of the secondary windings. Theres a separate feedback winding, so loadig one or all of the secondaries should have the same effect (within reason).

And yet you are still overloading some of the windings which will mess up the overall efficiency factor of the entire SMPS.

What I should do, just load one of any outputs:

for 12V positive output, I use 30 ohms 10W overrated but this is converative to keep dummy load from overheating too much. This will load down about .4A.

Or load 55V output I load it with 220 ohms 15W for .25A load.

By the way, I had a STK IC go soft on me, undervolting (sagging under load causing TV to shut down on overcurrent and replace IC fixed this.

So in your situation replace ALL small caps in hot side (STK7308) and a new STK7308.

The TR1514 is voltage regulator transistor to set specific voltage for STK7308, Make sure this circuit is working correctly.

Also MEASURE all resistors in hot side. This sometimes fail. This power supply is a hot side feedback regulation by monitoring the flux density of the transmformer.

The type of waveforms under load is normal looking for this type of STK IC with this kind of ringing on rising edge. I get this all the time with STK 730.

Cheers, Wizard

I really apreciate what you said, thanks. As for TR1514 circuit , could you give me any hints how to check it properly? Ie what to meassure (I've allready changed both zeners and the TR itself). If you want me to post any specific measurements/waveforms just say what ? I've checked the zeners with the scope and they both clamping at the correct volages. As far as replacing STK cip , I'm having hard time finding one locally, may have to order one online, it'll take time. Thanks again Wizzard.

Hey, I've got good news! (its not geico) I just replaced C1507 22/50V cap. Guess what? Seems to be working now , at least with my dummy load I can bring the voltages back to normal. It's late so I wont be hooking it up to the scope today, but I think I've got it. I'll report tommorow night. Thanks guys!!

I hate to say " told you so ", but when you first posted on this back on the

Seriously though, glad you've got it going.

Arfa

Another "told you so". When you have a hot power supply (touch the heatsink right after unplugged and turned off.) Then you know whatever to check or replace all small caps.

Good thing came out of this, you got it fixed, finish changing the small caps first before closing scope up.

:)

Cheers, Wizard