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Corning PC-620 120V 1113W Repair

Tha fios agaibh

Aug 11, 2014
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Also check the thermocouple/thermistor feedback to make sure its calling for heat.

I'd do an ohm check of that cable with the red and yellow wire going back to the controller board.
 

73's de Edd

Aug 21, 2015
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Sir Sparky . . . . .

Are we having fun yet ?

I'm using your good second set of photos, showing both sides of the board paired up . . .which I usually have to prep up myself.

Now I'm only having to do its mark up.

Here are the connections and the power flow path for that board.

Initially, over to right center is the AC input power.

Looks like the fuse is in series in the HOT AC line and is continually putting power to the primary of the units power transformer.
The hefty sized HOT power buss passes further on to the left, and ends at the MT1 terminal of the Triac which controls the units heater element..
Also, a lighter buss off of the HOT AC line passes up and to the right via the YELLOW dotted path, until it connects to the MT1 terminal of the Triac which controls the stir motor.

THIS PATH IS COLOR CODED in RED

Now if the top left Optical Isolator which has 2 RED STARS marking its input terminals is receiving a pulse train from the control board, its secondary section will the let a path be made from the Triac gate to its MT1 terminal, and conduction of AC will be made to the MT2 terminal an AC flow will be fed to the Stir Motor terminals.

THIS PATH IS COLOR CODED in FUCHSIA .

The other situation is for the right of the two optical isolators, which shows 2 Fuchsia Stars marking its input terminals, to be receiving a pulse train from the control board for heating level parameters.
Its secondary section will then let a path be made from the Triacs gate to its MT1 terminals, and conduction of AC will be made to the MT2 terminal and AC supply will be fed to the HEATER terminals,.

One exception on the HEATER path is that the power relay has to be closed, by having 9VDC across its coil. That will then let the heater path to the AC cold buss then be completed.

The relay coil is fed by the relay driver transistor and that transistor gets its activation voltage thru a 1K resistor that is fed from the cabling to the other control PCB at the YELLOW circled terminal.

BTW your black banded unit(s) are Zero ohm "resistors" aka a jumper wire . . . . .but these units are high class . . . . . and are wearing their tuxedos.

Check and see if the stirrer function works initially, if so, then we will move to the HEATER portion.

Mark Up . . . . . .

Hd7dDNb.jpg





73's de Edd


.
 
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EastCoastToast

Dec 21, 2015
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Sir Sparky . . . . .

Are we having fun yet ?

I'm using your good second set of photos, showing both sides of the board paired up . . .which I usually have to prep up myself.

Now I'm only having to do its mark up.

Here are the connections and the power flow path for that board.

Initially, over to right center is the AC input power.

Looks like the fuse is in series in the HOT AC line and is continually putting power to the primary of the units power transformer.
The hefty sized HOT power buss passes further on to the left, and ends at the MT1 terminal of the Triac which controls the units heater element..
Also, a lighter buss off of the HOT AC line passes up and to the right via the YELLOW dotted path, until it connects to the MT1 terminal of the Triac which controls the stir motor.

THIS PATH IS COLOR CODED in RED

Now if the top left Optical Isolator which has 2 RED STARS marking its input terminals is receiving a pulse train from the control board, its secondary section will the let a path be made from the Triac gate to its MT1 terminal, and conduction of AC will be made to the MT2 terminal an AC flow will be fed to the Stir Motor terminals.

THIS PATH IS COLOR CODED in FUCHSIA .

The other situation is for the right of the two optical isolators, which shows 2 Fuchsia Stars marking its input terminals, to be receiving a pulse train from the control board for heating level parameters.
Its secondary section will then let a path be made from the Triacs gate to its MT1 terminals, and conduction of AC will be made to the MT2 terminal and AC supply will be fed to the HEATER terminals,.

One exception on the HEATER path is that the power relay has to be closed, by having 9VDC across its coil. That will then let the heater path to the AC cold buss then be completed.

The relay coil is fed by the relay driver transistor and that transistor gets its activation voltage thru a 1K resistor that is fed from the cabling to the other control PCB at the YELLOW circled terminal.

BTW your black banded unit(s) are Zero ohm "resistors" aka a jumper wire . . . . .but these units are high class . . . . . and are wearing their tuxedos.

Check and see if the stirrer function works initially, if so, then we will move to the HEATER portion.

Mark Up . . . . . .

Hd7dDNb.jpg





73's de Edd


.

Wow! Thanks for the effort you put into that. There's so little info out there on these hot plates maybe we can turn this into a definitive guide when this one is up and running.

So I've came back with a fresh mind after the holidays and haven't thoroughly read the posts yet. I have checked the thermistor/thermocouple. If it's a thermistor it is a strange one because at room temp it has infinite resistance. At room temp it should be at a common value 10K ohms, 5K ohms. I've made a spreadsheet to plot the coefficients for a thermistor according to the stein-hart equation.

1/T= a+b(R1/R0)+c(R3/R0)

The insulating wire looks to have been badly burned so maybe there is a short there causing a malfunction? From the burn backwards the thermistor/thermocouple plugs into the LOW voltage control board and appears to have an appropriate resistance value.

Turns out its a thermocouple not a thermistor!
 

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EastCoastToast

Dec 21, 2015
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Did you get your 2.5v thru the optoisolator and on the gate?

Haven't done this yet bu with the posting 73's de Edd made I'll be perfectly capable.

Did find the goldmine though apparently these hotplates have a tendency to "runaway" and continue heating causing lab fires which UC Davis has addressed in a concise pdf that may be of use.
 

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73's de Edd

Aug 21, 2015
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Sir East Coast Toaster . . . . . . . .



Whhhhhoooooooooops . . . . . . looks like I addressed you with another posters name, last time around.
UNLESS your hand slips while taking a measurement . . . . .then you might fully fit those parameters along with your also being a VERIFIABLE "Toaster"

I have passed the addee of the Rowlett address on Main St. several times thru the years but had no knowledge of their endeavors, ACT, thus the need to even look over their way.

https://www.google.com/maps/@32.9024707,-96.5710061,3a,75y,151.91h,71.02t/data=!3m6!1e1!3m4!1sAtDyXtI7otYgp5fFQvs0Rw!2e0!7i13312!8i6656

Looks like, as of 2014, the big cheese is still driving his Baby "Rincon Con-tri-rintal" and the other employees must be parking at the back.
This unit could have been using two other "local" manufacturers, with Teccor /Littelfuse supplying the units Teccor power triacs, and located at nearby Irving, Texas and the PCB
could have come from SVtronics in nearby "Arlen Texas" . . . .home of Hank Hill , Peggy, Bobby and Lu Ann.

Looking at the fiberglass wire loomed thermal sensor, it looks more like a thermocouple *** wire " bead "instead of the smallest open thermistor die that I have ever seen .
*** (Good Lawds . . . . . be vewwy-vewwy carefuls . .a la E.Fuddster, . . . .as spel cheks done tellin' me that it 'posed to bees " thermonuclear " . . .shudder . . .shudder !

Now if it is a J or K type of thermocouple junction it should be having a low resistance reading, just for testing purposes of the bead junctions ohmmic integrity, as its ACTUAL manner of operation, is being its GENERATION of a varying DC output voltage , in accordance to the temperature being sensed.

See this reference, for relating to common wire resistances versus lengths.

http://www.conaxtechnologies.com/tech/newpdf/WireSize.pdf

In the up to 1 "tousand" degrees that this heater unit is capable of being subjected to, the thermocouple would be the preferred temp sensing medium, as compared to using a thermistor.

Also in the use of clip leads in your ohmmic testing, a hard oxide, relating to the heating in the creation of its end bead, may not let a confirmed connection be established with your clips.
Unplug the unit at the controller board and make measurement at those terminals, to see if a low ~2-5 ohms resistance is found. If not, then clip ohmmeter to one connectors lead and test at its end
connection to the thermal bead by additionally scraping the wire to get a clean bare connection exposed.Then do its other lead to the bead.
If reading open circuit, that's an oddity, since those units are usually quite hardy, unless mechanically abused.

Pass me the Teccor Triacs part numbers so I can ascertain thie specs and see if other than the tack soldering in of a 220 ohm resistor from Heater Triacs MT1 terminal to its gate would be needed.
That would "switch" on your heater element IF the power relay is turned on by its relay driver transistor.
Also confirm, if ~9 VDC is present on that transistors collector, if so, the other switching aspect of the heating element could be tested by shorting that tansistors C-E to turn on the relay,
along with the Triac already being shunted, should make the heater element active. Then you pull both "test aids" and back track to see if there is base drive coming to the relay driver transistor OR if any
combination of knob and switch kerblitzing causes the relay drivers transistors base drive to come forth.
If the thermocouple proves to be open, the subbing in of a 2 ohm resistor across its plug in at the controller, might make the unit operable, for a further " test aid" / evaluation but without viable temp control.

BTW does the stirrer motor activate ?

Also, over near the corner of the power PCB , I thought that there was a small bat handle switch there, but it's labelled as J2. So my guess is that the JACK is for an optional long leaded rod sensor
to dip into the heated Pyrex beaker for additional temp sampling.
Your photo clips off part of the plug in to J2.
I think that there might be a stubby plug in, present now, that has a mini temp sensor on its end, that is just measuring room temperature, considering that one is not using the extended probe plugged into that jack.

Thaassssit for now . . . . . .


73's de Edd


.



.
 
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EastCoastToast

Dec 21, 2015
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So I got it working!

What I think happened was that the black oxide layer on the thermocouple was bypassing the thermocouple by completing the circuit before it, which would return the resistance of the wire and it wouldn't call for heat. Alternatively, an open circuit could've been created with the carbon deposits left on. When I scraped off the wire I was able to get a reading of 1 ohm whereas before no resistance was present indicating an open circuit. Currently I'm burning off the contaminants on the ceramic top. I have another hotplate for repair as well so I'll check the thermocouple to see if the same oxide is present and post the results.

I think the relay on the new hotplate is bad because I don't hear any switching at all, whereas on the repaired hotplate an audible clicking is present when the relay is engaged.
 

EastCoastToast

Dec 21, 2015
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So I got it working!

What I think happened was that the black oxide layer on the thermocouple was bypassing the thermocouple by completing the circuit before it, which would return the resistance of the wire and it wouldn't call for heat. Alternatively, an open circuit could've been created with the carbon deposits left on. When I scraped off the wire I was able to get a reading of 1 ohm whereas before no resistance was present indicating an open circuit. Currently I'm burning off the contaminants on the ceramic top. I have another hotplate for repair as well so I'll check the thermocouple to see if the same oxide is present and post the results.

I think the relay on the new hotplate is bad because I don't hear any switching at all, whereas on the repaired hotplate an audible clicking is present when the relay is engaged.

Here are some photos!

Oh yeah relay on the 5x7 is perfect unit heats under one condition...
 

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EastCoastToast

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So a few things the first hotplate repaired was 10"x10" second is a 5"X7" hotplate.

I noticed now that the triac is overheating and discoloring the board on the 10"x10". This is a common problem, maybe when I dremeled off the screw that portion acted as a heat sink to dissipate heat?

On the 5x7 the potentiometer for the heat function doesn't spin the axle to reduce the resistance, but when pressed down, forcing the potentiometer to rotate clockwise on the PCB the unit begins heating!
 

73's de Edd

Aug 21, 2015
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Sir Eastern Coasr Toaster . . . . . .



If the 10x10 unit is the first unit concerned with, there are 2 triacs, with the one at the end of the board being bolted to the CAST aluminum /?/ Pot metal housing, which should almost be an infinite heat sinking if silicone grease or heat sink compound was used between the thermo-mechanical interfacing of the triac and cast housing.
BUT with your wording of PCB, it must be the boards central triac, and that is where I would add on a vertical heat sink, or fab up your own custom unit of approx 1/16 in --- 1/8 in aluminum plate , within an available spacing of the upper area., with heat sink paste thermal transfer interlacing also .

On your other unit, find that pots value and replace it with one of these series of "forever" pots . . .built like a battleship on its resistance element and its wiper.
You may have to find it as a new old stock item on e-bay . . . . .
This is an Allen Bradley . . .fired element . . . . .composition pot . . . . . .


cb1031.jpg





Thasssssit


73's de Edd

.
 

EastCoastToast

Dec 21, 2015
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Sir Eastern Coasr Toaster . . . . . .



If the 10x10 unit is the first unit concerned with, there are 2 triacs, with the one at the end of the board being bolted to the CAST aluminum /?/ Pot metal housing, which should almost be an infinite heat sinking if silicone grease or heat sink compound was used between the thermo-mechanical interfacing of the triac and cast housing.
BUT with your wording of PCB, it must be the boards central triac, and that is where I would add on a vertical heat sink, or fab up your own custom unit of approx 1/16 in --- 1/8 in aluminum plate , within an available spacing of the upper area., with heat sink paste thermal transfer interlacing also .

On your other unit, find that pots value and replace it with one of these series of "forever" pots . . .built like a battleship on its resistance element and its wiper.
You may have to find it as a new old stock item on e-bay . . . . .
This is an Allen Bradley . . .fired element . . . . .composition pot . . . . . .


cb1031.jpg





Thasssssit


73's de Edd

.

The triac overheating is the one mounted on the aluminum housing which I dremeled off (earlier in the thread)...therefore no longer an effective heatsink.

Potentiometer is a specific one (mounting is unique and shaft diameter is important) which is 50k ohms. It's an X type potentiometer and the flatted axle allows for a knob to be mounted. Along with it having 5 pins.
 

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73's de Edd

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Sir East Coast Toast . . . . . .

Ahhhhh yes . . . . . I've certainly seen that CTS unit and switch . .with pcb mount bracket and solder terminal tabs . . . . in many years past.
The 37th week of 1997 . . . . . to be specific . . . .for that unit of yours.
If my problem to solve, I would be unsoldering all the connections, lifting the unit, and straightening out the 4 bent over clamping tabs.
That then lets one take the unit apart, to see what is the fault on the bifurcated rotor wiper contacts, to be making loose contact with the resistance element proper.
It could be evaluated then, and possibly mechanically corrected on the spot, or further evaluate, to see if a donor part could be acquired from another mechanically sound CTS unit. . . . .found on the internet /E-bay.

On your Teccor triac, looks like you ate off from the center of the mounting hole tab outwardly, with your Dremel cutoff wheel. . . .and it LOOKS like the tinned copper tab of the Triac had about 50% of it that didn't stretch on, all of the way over, to lay on the heat sink stud, originally, from the factory..
Some Teccor units even came with a quasi clipped off heat sink tab, as a TO-251 V-pak housing.
You should be able to get a replacement, full TO-220 tabbed unit, via Digi Key or Mouser.

Your photo is not letting me see the Teccor prefix portion of its part number , but the Q ??015L5 is telling me that it is a 15 amp unit, and we know of it being at least above a 200V rating.


73's de Edd

.
 

EastCoastToast

Dec 21, 2015
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Sir East Coast Toast . . . . . .

Ahhhhh yes . . . . . I've certainly seen that CTS unit and switch . .with pcb mount bracket and solder terminal tabs . . . . in many years past.
The 37th week of 1997 . . . . . to be specific . . . .for that unit of yours.
If my problem to solve, I would be unsoldering all the connections, lifting the unit, and straightening out the 4 bent over clamping tabs.
That then lets one take the unit apart, to see what is the fault on the bifurcated rotor wiper contacts, to be making loose contact with the resistance element proper.
It could be evaluated then, and possibly mechanically corrected on the spot, or further evaluate, to see if a donor part could be acquired from another mechanically sound CTS unit. . . . .found on the internet /E-bay.

On your Teccor triac, looks like you ate off from the center of the mounting hole tab outwardly, with your Dremel cutoff wheel. . . .and it LOOKS like the tinned copper tab of the Triac had about 50% of it that didn't stretch on, all of the way over, to lay on the heat sink stud, originally, from the factory..
Some Teccor units even came with a quasi clipped off heat sink tab, as a TO-251 V-pak housing.
You should be able to get a replacement, full TO-220 tabbed unit, via Digi Key or Mouser.

Your photo is not letting me see the Teccor prefix portion of its part number , but the Q ??015L5 is telling me that it is a 15 amp unit, and we know of it being at least above a 200V rating.


73's de Edd

.
Here's a better view.
 

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EastCoastToast

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Tha fios agaibh

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Thanks for your help as well! Also what would you do about the pot and the triac?
If you can't find a replacement, you can try and get a similar one and modify the shaft to suit.
As far as the triac, redrilling and tapping the mounting hole will take some time. Why not spend another 5 minutes and just replace it?
 

EastCoastToast

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If you can't find a replacement, you can try and get a similar one and modify the shaft to suit.
As far as the triac, redrilling and tapping the mounting hole will take some time. Why not spend another 5 minutes and just replace it?

There was never a mounting hole they used a thermal glue/clay to screw into...I think I''l replace it with the one I have, will the one I have work in this application?
 

Tha fios agaibh

Aug 11, 2014
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There was never a mounting hole they used a thermal glue/clay to screw into...I think I''l replace it with the one I have, will the one I have work in this application?
Just drill down deeper until you have about 3/8" of metal to tap into. Or alternatively drill a hole next to it.
I don't know off hand about a match, you'll have to compare the specs.
 

EastCoastToast

Dec 21, 2015
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Pretty sure I found the problem with the pot....it might be the pcb paths
 

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