Broken Laminator (Thermal regulation issue?)

[MyHeadHz]

First and foremost, I'm a noob.  My knowledge is limited, so please excuse my mistakes!

Equipment available: an emtech multimeter, basic hakko soldering iron, and general screwdrivers, random resistors, diodes and whatnot.  I can get what i need if I need something else.  Not oscilloscope, though I hope one won't be necessary

How the thing works... the heating element is on or off.  Internally, it has a thermometer on one of the two (top/bottom) heating elements.  When it gets to the desired temperature, the elements turn off.  If it loses a degree of temp or two, they turn back on.  There is a digital readout (not pictured) along with basic keypad attached to the PCB via a ribbon cable.  The drive motor is controlled by the keypad and is manually on/off with a few speeds-- independent of any temperature settings.

I completely disassembled and cleaned it all.  The internal thermometer works and displays an accurate temperature on the screen, but whatever i set the temp cutoffs to is ignored.  If it's turned on, it is just on all the time.  I can still use it... I just have to wait for the correct temp, then I have about 1 min before it gets too hot.  Wash, rinse, repeat. :/

Possible ghetto bypass would be to put an in-line switch and manually switch the heating element off and on to maintain desired temp.  That would work, but I would like to fix it properly if possible.

I don't see anything obviously failed on the pcb.  No expanded caps, nothing melted or burned that I can see. Again, that's as far as I can tell.  I'm a noob, so take that with a grain of salt. 

I suspect the problem is with the omron thing (picture 4).  It looks like it is probably what physically connects/disconnects the power to the heating elements, but I have no idea how I would test it.  Other that that hunch, I have no idea what to do next as far as troubleshooting and testing. 

Let me know if you want a picture of something else or a different angle.

Picture Time!

[Ian.M]

The Omron 'thing' is a 5A 250V AC DPDT relay with a 12V coil.  The contacts are visible through its case at the righthand end.  The discolouration of the rear contact set indicates there may be a problem - It could be stuck 'On' if two of the contacts have welded themseves together.   

Caution: it appears to use a capacitive dropper PSU so the whole control board and any displays and sensors must be treated as mains 'Live'.  Disconnect from the supply before testing!
You *may* be able to unclip the clear plastic cover from the base using two fine tipped flat blade jewellers screwdrivers to release it from the retaining clips.  If so, you can operate the armature manually by pressing down on it till it touches the coil pole piece, while observing the contacts carefully.  If there is a welded contact pair, try prying them apart with a sharp Xacto knife, and if you separate them successfully, dress them with a folded over strip of 400 grit wet&dry paper to clean up the burnt surfaces, and if they are the N/O contacts, hold down the armature to maintain contact pressure on the wet&dry paper.  Then clean by pulling a strip of cartridge paper wetted with alcohol through the contacts.   Reassemble and test.

Its not likely to be a permanent cure so if the relay is the problem you should order a replacement Omron DPDT G2R-2 12V relay.

[MyHeadHz]

(I made this post before I saw the above reply, but I will leave this anyway.  I will try the relay suggestions and update!  Thanks!)

OK, so I realized I never actually looked at the bottom of that PCB...

I did, and there was obviously some overheating from something.  The I took a closer look at the transistor... and yeah, the legs are colored from heat.  See pictures below.

So, I searched and it seems to be an adjustable voltage regulator.  2 Questions.  Is this the problem, or the symptom?  2.  How do I get another one like it (if I need to)?

terminals 1 and 3 are open, as are 2 and 3.  I get about 390 ohms between 1 and 2.  I don't know if any of that is good or bad though   But since it isn't shorted, maybe it is OK?

For what it's worth,, there was no thermal compound on it, so I will add some when I reassemble.

[Ian.M]

The display works so the LM317 voltage regulator must be O.K.  Add a little heatsink grease and re-mount it.  Edit: As there are two other regulators, the display working doesn't tell you much about the condition of the LM317

If you cant get the relay open, and you can safely set up to test the voltage across the relay coil with the laminator running, you should see it switch at the setpoint temperature.  You'll want the meter on a  DC volts range and are expecting about 12V when the relay is active.   I don't know whether it activates the relay to turn the element on or to turn it off - you'd have to trace the contact connections to figure that out.   If the relay drive is on all the time, there's probably a blown driver transistor - trace back from the relay coil till you come to a transistor and check for a short from C to E.

[MyHeadHz]

I took off the cover as suggested and the relay was not stuck.  That discoloration was actually red paint/lacquer on that side.  The metal lever that operates the terminals was free to move smoothly, and I think it would work as long as it gets proper power.

I am suspicious of Diode A.  It has a resistance both ways of 270 ohms, and .17 v drop across it.  Others on the board are in megaohm range.  Granted, that's in circuit and may mean nothing at all.

In the attached picture, you see the dodgy diode.  That diode, the 7805CT, 7815CT, LM317, and the bridge rectifier were all manually soldered and trimmed.  Probably from a previous repair (or repairs) while under warranty.  Maybe the bridge rectifier died before and took out the other stuff with it?




(On the bottom-right is Diode A, coming from the relay coil- same image as before)

Let me know if any of that matters....  I'm going to reassemble and check the relay and function.

[Ian.M]

When you attempt to measure Diode A, you are simply measuring the relay coil resistance.

When you connect power does the relay pull in immediately?  You should be able to see if the armature moves through the cover.

[MyHeadHz]

Well, it turns out that the relay actually just reverses polarity of the drive motor for reverse feed (clearing jams, etc).  And it works well

I am going to take some temp logs and then update.

[Ian.M]

OK.  That's eliminated the relay and associated circuit.  Sorry about the wild goose chase.

Next suspect is whatever is driven by the white optocoupler.  It will be the device on the heatsink marked 120V, which is probably a TRIAC. Odds are its failed S/C.  A good photo of that part of the board from the top side would be helpfull.

[MyHeadHz]

It was hard to get a pic with the part numbers all viewable. 

the traic: http://www.onsemi.com/pub_link/Collateral/BTA16-600BW3-D.PDF
I tried measuring the triac but whenever i removed the gate, the terminals would go back up to open circuit.  I'm not really sure what's going on there.   That tutorial said it should remain closed.

My guesses for the dual-pin connectors: AC-in, transformer-in, temperature-sensor, upper heater, lower heater.




I did some testing with the different temp settings.  Turning the heater off while the machine was on does work.  However, any temperature setting other than off just seems to lock them on.  The temperature sensor didn't seem as accurate this time.  It underestimated by as much as 100F on the way up, but was closer on the way down.  The JM317 and the capacitors beside the heatsink all stabilized at about 150-160F.

[Ian.M]

A TRIAC will only stay conducting after it has been triggered if it is passiing a minimum load current.  Odds are your test jig wasn't putting enough curret through it.  However as it switched on and off and isn't shorted its likely the TRIAC is good.

Most of the easy things to check have been eliminated.  It now looks like you  have a problem with the control system that drives the white optocoupler.   If the MCU is repeatedly crashing, it could account for the misbehaviour.  Bad caps on the poiwer rails could cause it to crash, but without an ESR meter you cant test them.  You'll need to decide if its worth attempting to 'shotgun' it by changing all the small electrolytics under 100uF, starting with the ones next to that heatsink that was running hot.

[MyHeadHz]

Why the ones under 100uF?  Because the larger ones do not appear blown? 

Also, how much do caps like that cost?  There are something like 15-20 of the small caps, and about 3 or 4 over 100uF.  What would it cost to replace them (approx)?

Also, what is important to find an equiv cap?  Just temp and farad rating, or are voltage and brand important?

[Ian.M]

All else being equal, larger electrolytics tend to last better. They are also more expensive, so if you are 'shotgunning' a suspected bad caps problem without an ESR meter it makes sense to start with the small ones.  100uF is an arbitrary cutoff that should see you replacing most of the small caps.   

Things you need to match:
*  form factor, most critically, distance between leads as there is often space for a taller or slightly greater diameter cap.
* Capacitance
* Voltage rating - same or greater, but its not advisable to go for a far higher rating as the ESR may be higher.
* Temperature rating - same or greater, usually 85 deg C or 105 deg C.  An 85 deg C cap will work as a temp replacement for a 105 deg C cap but usually wont last.

You also have to consider whether the circuit needs low ESR and/or high ripple current rated caps, but that's usually only in switching regulators and power supplies.  I doubt there are any on your board.

Pricing is another matter - it depends on your local distributors, and whether or not you keep stock of the most commonly used values, though I should note that the hobbyist bulk packs of assorted electrolytics, are often not of very good quality - usually third tier, but occasionally bottom-feeding crap.