Great. Now you need to lift, or carefully unsolder, that thin metal heatsink and the insulation under it to reveal the controller chip that i'm pretty sure is lurking under there....

 

Great. Now you need to lift, or carefully unsolder, that thin metal heatsink and the insulation under it to reveal the controller chip that i'm pretty sure is lurking under there....

Yes, there is a 6-pin IC2 under there, alongside two 3-pin Q transistors I guess. They have a code etched in but it's really hard to read. Photo below. What else do we need to know about the controller?

 

see if you can get some good hi-res photos of this area from directly above. Oblique photos can sometimes reveal the writing on the chip... turn your camera on, zoom into the chip and move the camera around until you can see the writing. Strong direct lighting helps.

 

see if you can get some good hi-res photos of this area from directly above. Oblique photos can sometimes reveal the writing on the chip... turn your camera on, zoom into the chip and move the camera around until you can see the writing. Strong direct lighting helps.

I was afraid you were going to say that! Had to lift the metal cover almost fully vertical, but still couldn't make out the writing on the IC chip because of that white dirt on top of it. So put some isopropyl alcohol on a q-tip to clean it off, now they read very clearly:

What does that tell us? What's next?

 

Check R3A from bottom side. Looks funny

 

That's good picture but it would be more helpful if it was slightly wider field of view to get context. eg (without heatsink)

 

That's good picture but it would be more helpful if it was slightly wider field of view to get context. eg (without heatsink)

Fine, but it would help if I understood what you're looking for and why. I thought you wanted the codes printed on the chips, so I focused on them. Are we now studying the circuit paths instead?

 

 

Yes, exactly... you wanted a learning exercise - so here's how I do it...

First get some good photos. Try to get them perpendicular to board keeping skew to a minimum. Then using a suitable app position them side by scale and scale them, flipping the bottom of the board to the same orientation as the top, like this:



Makes it so much easier to trace whats going where...

Then you can start to trace out the circuit as below - not completed yet - but enough to start thinking about what to test.



So the first thing to check is what potential the heatsink is going to be at when powered. We'll do this without power, making sure the big cap is discharged. Forgive me the baby steps - I have no idea of the depth of your experience and as you want this to be a tutorial I'm assuming very little at this stage.

I'm also assuming here you have a reasonable quality multi-meter? What is it? If its not a good brand certified to CAT III 600v I'd not recommend testing with it.

With the multi-meter on the ohms or continuity range measure between the end of ZD1 nearest the edge of the board (black probe), and the heatsink - it should be 0 ohms, which means the heatsink is our ground reference on the high voltage side only.
That doesn't mean its 0v, it could still be 200v relative to the AC input, so fingers off!

Keeping the black probe on ZD1 put the red probe on the + lead of the big capacitor. It should show a relatively low resistance which increases to a much higher value quite quickly, maybe several 100k, in a few seconds. What is that value? Don't forget to return the multi-meter to the DC Volts setting after that test.

Now we will perform our first voltage test. With the multi-meter set to DC Volts and on the 200v or higher range (if not auto- ranging) we are going to measure the voltage across ZD1. Black probe to the end near the board edge, red to the end with the black stripe. Ideally, for safety, you should use a test-lead with a gripper on the end for the black probe so you only need one hand.

Power the unit on, unloaded, and measure the voltage across ZD1. It should be approx 21 - 23v. Now plug it into the printer. Does the voltage change at all?

That's a good start - there some weird circuitry around the feedback for voltage regulation but as I've not identified the actual controller yet I don't know quite why. Most of the common controllers don't need it. Also the pin-out doesn't appear to follow one of the common layouts which suggests its a far-east special.

 

Updated image from your last photo, corrected for perspective and size (using GIMP2.0)

 

Yes, exactly... you wanted a learning exercise - so here's how I do it...

I know from the markings that the IC controller is a SG6848x1.

Your diagram is pretty impressive, and while I'm not very literate in the symbols used on complex electronics diagrams, yours does seem to mirror the typical circuit given on the controller's data sheet (Mouser has it, see attached). Certainly, it may be easier for us if I just look closely at the board and identify any other circuit components or paths that are needed, than it would be if I try to get good pictures of everything?

I have two meters. One is a Radio Shack pocket digital meter and clearly marked as CAT II 500V 200mA, which I guess isn't safe to use for this project under your advice. The other one is an older Sperry, and while it doesn't have any CAT designation (I guess that didn't exist 25 years ago?), but it does have higher ranges on it so I presume this should meet today's CAT III requirements?



Anyway, I'll do the tests that you asked for later today. But, as for plugging it into the printer, I'd prefer to find a different load to use for these tests on this device. I guess 10-watt or more power resistors (ceramic?) would be the best choice? Unless you or someone else can think of a very common object that would be suitable to connect to a 30VDC 15-watt power supply?

 

Hold off on those tests for now, ZD1 isn't where I thought it was...

A couple of 30ohm 10W, or a 56ohm 20W would do... with a heatsink... they won't be the wattage rating you think without one.

Good call on the SG6848 - where did you find that as it didn't come up on my sources (yet)?

 

Good call on the SG6848 - where did you find that as it didn't come up on my sources (yet)?

It wasn't straightforward to figure out that AAHxx = SG6848x1. Especially since the leading 'A' is faded away on the actual controller (AAHBW). But there is an eBay listing with both designations in the heading that allowed me to find the actual component model, so Google proved it can sometimes be more than some professional sources!

 

OK, in the absence of any other equipment I'd use the Sperry, starting on the 1000v range and dropping down to 200v if necessary.

Looking at the board again, its clear the heatsink is at the ground reference potential for the high voltage side, so that should be the connection point for your black probe for all voltage measurements.

The voltage you need to measure is at the end of R2b where the track widens, near the 'R' of the designation for R17. Measure both on and off load.

 

I would check what looks like a thermal fuse (1A/250V?) that is taped to the yellow tape on the transformer. I've had one of those go due to "stress", and not thermal issues. It does not show up anywhere in your schematics, but it is usually on the primary side of a transformer.
I can be intermittent, and overheat under load.

 

I would check what looks like a thermal fuse (1A/250V?) that is taped to the yellow tape on the transformer. I've had one of those go due to "stress", and not thermal issues. It does not show up anywhere in your schematics, but it is usually on the primary side of a transformer.
I can be intermittent, and overheat under load.

I was wondering about that. Guess it would be smart to monitor it, perhaps bypass, to see if it's that something that simple!

 

I was wondering about that. Guess it would be smart to monitor it, perhaps bypass, to see if it's that something that simple!

Just wire in a regular 1A fuse of some sort, to bypass the existing one, to see if that fixes anything.

 

The voltage you need to measure is at the end of R2b where the track widens, near the 'R' of the designation for R17. Measure both on and off load.

Pretty sure the Sperry is the 1990's equivalent of at least today's 600V CAT III (IEC 61010-1 which created the meter CAT ratings wasn't published until then). I'm taking all readings on the 1000VDC setting, as I don't think we need better than whole number voltage values at this point.

Upon power-up with no load and the power supply seeming to be working, current comes out of the bridge rectifier at 168VDC. It then passes through the first 754 resistor and drops to 66VDC. It then passes through another 754 resistor and drops to 15VDC at the spot on the board that you asked for. The final output of the power supply is 29.8VDC, as it should be.

After duty load has been applied by connecting to the printer and the power supply shuts down, there is indeed a change in these voltages, which I did not expect so early in the circuit. While the bridge rectifier is still putting out 168VDC, after the first 754 resistor, voltage is now 62VDC. After the second 754 resistor, voltage is now 4VDC. The final output of the power supply is now also 4VDC, insufficient to power the printer.

If these two 754's are supposed to be 754k ohm resistors, they don't measure quite right. With no power, they seem to have 850k ohm with current flowing in one direction and 650k ohm if I test in the opposite direction. But I'm aware that may not be an accurate observation because these are still installed into circuit, and there could be side effects of the other components on the board (like capacitors) inadvertently charging up using the meter's test current.

The connection point you asked about is also an interesting location on the circuit. It's the spot where the 50V 10 micro-farad capacitor has its positive side connected, where the diode immediately next to the transformer (buried under the white goop) takes its current, and also a lead point powering the components under the metal shielding.

I'm not sure if the results we just saw indicate the shutdown is actually a bad resistor, or if a short elsewhere that gets triggered by the load, or a safety measure is being triggered somehow to protect the circuit from damage. How is the voltage being dropped so significantly unless a lot of current is being re-routed through a low resistance emergency exit somewhere?

Let me know your thoughts and what to test next.

 

Check R3A from bottom side. Looks funny

Tsk...Tsk..

Pls do check this Resistor

Is there a burnt hole in tht 100K SMD.........I see 104 on the marking somewhat

If I am not mistake tht R could be part of the Vcc PWM IC Supply or ??

 

Is there a burnt hole in tht 100K SMD.........I see 104 on the marking somewhat

If I am not mistake tht R could be part of the Vcc PWM IC Supply or ??

Both 104's read 100k ohm just fine, and there is no hole but rather they don't photograph very well. There is some brown over one end of one of them, but that probably is due to years of heat from the transformer ground pin between them just above. I have another working transformer, and I will note that it is warm on the exterior even when printer is sleeping.

The line from these two resistors goes to the outer diode next to the transformer, and from there, yeah, combines within the MOSFET drain and goes into the transformer.