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Exerpts from the CD-ROM:

THE COMPLETE MICROWAVE OVEN SERVICE HANDBOOK: Operation, Maintenance, Troubleshooting and Repair
Copyright © 1989-2013 J. Carlton Gallawa . All Rights Reserved Worldwide

(Excerpt)

13.3 SOLID STATE COMMERCIAL TIMERS

When symptoms indicate a possible problem in the timer there are but a few ways in which to test that likelihood. The symptoms themselves can give ample evidence of a defective control board. Whereas all of the following symptoms apply to electronic commercial-type timers, many of the indications are equally symptomatic of most domestic-type electronic control panels.

Amana Control Baord for HDC Models

13.3.1 Symptoms That Can Denote a Defective Electronic Timer

SYMPTOM:

Any type of irregularities or inconsistencies in the display, such as: 1) missing, dimming, flashing, flickering (a slight fluorescent flicker is normal), or overly bright numbers or segments; or 2) the occasional appearance of illogical or mysterious Chinese-like characters (this may also be the result of improper or inadequate grounding).

SYMPTOM:

Any type of programming problems, for example: 1) Pressing one number or function and the panel responding with another; or, some pads program and others do not. These symptoms may be caused by either the touch panel or the control panel or both. When these panels are available separately, a determination of fault must be made. Section 13.5.1 and Appendix I will be of great assistance toward making a logical conclusion. 2) Limited or no programming functions. An exception to this symptom is the fact that some models made by Sharp will not program unless the stop switch is in the closed position. Therefore the oven door must be closed and the stop switch properly adjusted before the control panel will accept programming. 3) The control panel "locks up"; maybe one or more numbers will be displayed, but the panel will not reset or program.

SYMPTOM:

The control panel "kicking out" of a cook cycle prematurely, then possibly "locking up." The oven may have to be unplugged then plugged back in before the panel will accept programming. Possible causes: poor or improper grounding; or, excessive amounts of stray RF leakage, either from the latch side of the door, or from the magnetron within the component compartment. If removing the outer case alleviates the symptom, suspect leakage from the RF gasket or the internal structure of the magnetron.

SYMPTOM:

The control panel counts down too quickly or too slowly. Inaccurate timing may also be the result of an oven (of the type that can be converted from 50 to 60 cycles and vice-versa) being set for the wrong voltage or frequency for the area in which it is being used.

SYMPTOM:

The control panel will not start, or fails to count down when started; or, the panel counts down but neglects to activate the cook relay, triac, stirrer or blower motor, or other panel-controlled component. This may also be the result of a misadjusted or defective interlock switch, faulty wiring or related connections, or an open thermal protector or fuse.

NOTE: The triac or triac module (part of many control units) may be checked in several ways, all of which are outlined in Sections 13-12 through 13.16.

SYMPTOM:

Either all indicator lights are lit, or no display at all, as though the panel is "dead." (In the case of no display or a "dead" panel, ensure that the low-voltage control transformer is supplying the appropriate voltages. Or if the low-voltage transformer is part of the control board, check for the appropriate input (primary) voltage to the panel before judging it to be defective.)

SYMPTOM:

Either a continuous beep or buzz; or none at all.

SYMPTOM:

The timer buttons remain depressed, thus holding the oven in a continuous cook mode. Litton and some other models use individual pushbuttons that are soldered on to the printed circuit board. These buttons may stick in the depressed position due to inadequate clearance through the cutouts in the timer faceplate. In these cases, the circuit board position may be adjusted to equally center the buttons in their openings, ensuring that the cutouts and buttons are free of grease and residue. In more extreme cases, the size of the hole can be increased by filing the sides of the obstructing cutouts with a small, fine file (the circuit board must be removed first). Then clean any metal filings from the casting and re-assemble the timer, making sure that the timer board is centered so that the buttons operate freely.

Self-Diagnosis: As digital circuitry becomes more sophisticated, more manufacturers are providing a self-diagnostic test sequence that can be programmed into the control unit. The control panel responds, in most cases, with a code that indicates the likely problem area. For the respective test and related codes for any of the various models that offer this feature, consult the appropriate service manual or contact the manufacturer.

13.4 DOMESTIC CONTROL CIRCUITS—CHECKS AND FAILURES

If new technology is reflected anywhere, it is in the control circuitry of the sophisticated microwave ovens designed for home use. It seems, paradoxically, that as technology advances, the field-availability of the corresponding technical data declines. Indeed, the service literature testing procedures for many control units amount to nothing more than programming instructions, although the value of these should not be minimized, because not knowing how to operate the oven you are about to repair can be quite perplexing to the anxious onlooker who owns the unit.

Isolating a fault in a domestic control panel is accomplished, for the most part, by observing symptoms, just as with the preceding commercial units. In fact, all of the symptoms, causes and corrections just listed for commercial control panels apply equally to their domestic counterparts.

The service literature supplied by many manufacturers provides input and output data for the respective control unit. In these cases, a fairly certain diagnosis can be made using that data. A few non-U.S. manufacturers provide detailed schematic information for their commercial and domestic control panels. In these cases, the servicer has the option of either attempting to repair the unit at the board level, or simply replacing the entire panel. Either way, the option is nice.

Many control panels are designed to supply the drive voltage for components such as the blower and stirrer motors, an external triac or relay, the transformer primary winding, and so forth. The panel’s output in each case can be verified in the following manner: 1) Unplug the oven, remove the outer cover and DISCHARGE THE HIGH-VOLTAGE CAPACITOR(S). 2) Attach a meter (using insulated alligator clips) capable of the measuring the intended voltage to the input terminals of the component in question. (When no technical data is available, the appropriate drive voltage for a given component can usually be ascertained from the case of the component itself). 3) Operate the oven and observe the reading on the meter. 4) If the proper drive voltage for that component is shown, the panel is operating normally in that respect. 5) If an abnormal reading is obtained, either the panel or the wiring in between is at fault.

In conjunction with the preceding symptoms, some additional, rather uniquely domestic-type control panels symptoms are as follows:

SYMPTOM:

The time-of-day clock does not hold or keep the correct time.

SYMPTOM:

Temperature probe, sensor cook or other special features do not function properly. Do not assume the control panel is at fault, though, until the temperature probe, humidity sensor, or other related circuitry and wiring are checked. Also, tactfully determine if the operator is using the feature properly.

SYMPTOM:

The oven cooks constantly, regardless of the power level selected. Check the control panel’s triac-drive output as described in Section 13.12.2. If a constant voltage is measured when no cook operation has been initiated, or a constant voltage is shown during defrost and other low-power operations, the control panel is defective and must be replaced.

SYMPTOM

The program erratically shifts or jumps to different functions or memories, or resets itself by erasing the display—except possibly for the colon.

SYMPTOM:

If while counting down, the display jumps or skips time, suspect the panel if the grounding is proper and the polarity correct.

SYMPTOM:

The control module (Amana) appears to be "dead." Indeed, it may well be, but first check for an open thermal fuse on the magnetron or cavity, or both.

13.5 MYLAR (MEMBRANE) TOUCH PANEL OR THE CONTROL PANEL—WHICH?

For those who like games of chance, the process of determining whether programming problems are being caused by the control panel or the touch panel can be quite challenging—to your skill as well as your patience. Making the right choice becomes especially venturesome if the symptoms are intermittent; it becomes an educated guess with a 50-50 chance of being correct. However, for those who are not so inclined, the following symptoms, tests and visual indicators will help in separating the cause from the effect, thereby enabling the most logical deduction.

13.5.1 Membrane Touch Panel Evaluations and Considerations

Examine the ribbon cable for evidence of contaminants or "bleeding" between the lines on the ribbon. While a certain amount of tarnishing is normal and does not in itself mean the touch panel should be replaced, the appearance of black spots or fine web-like lines between the silver conductor traces are good signs that the touch panel should be replaced.

Touch Panel OperationInspect the area of the ribbon tail that slips into the board connector for cleanliness, for sections where the silver has worn off, or for evidence of scratching or hairline cracks. If cleaning is necessary, do so using an alcohol or freon-based contact cleaner. Do not use silicone-based cleaners or lubricants. Gently rubbing the terminals with a soft pencil eraser will also clean them, but, a word of caution: test the effect of the eraser on a non-essential area first! Erasers have been known to clean the silver right off of the ribbon. In many cases, the problem can be resolved simply by evenly trimming off about 1/16-inch from the tail of the ribbon, leaving fresh new terminals to assure good electrical contact with the circuit board connector.

A close examination of the front of the membrane touch panel may reveal dents or deep scratches that may be creating a short in certain touch pads. Also, brightening or dimming of the panel-display colons while pressing certain pads is a reasonably good indication that a signal is getting through to the control panel. In that case, try replacing control panel.

Some additional symptoms that indicate a defective key or touch panel are: 1) Only certain pads or groups of pads will respond while others will not. 2) Touching a pad produces a string of characters on the display. 3) One character remains constantly on the display, and disconnecting the ribbon connector from the control board removes it. 4) Some or all pads must be pressed hard or pressed several times to produce a response. 5) The number displayed is different from the one pressed (i.e., press the "5" pad and a "9" appears on the display).

A categorized collection of used touch panels (or keypads) is a good idea for testing purposes. Unless they are totally inoperative, do not throw away used touch panels. They take up little room and can be used very effectively. For example, suppose you replace a touch panel because the "2" and "4" pads do not work. Label the defective panel appropriately, and save it. One day, a similar oven comes along, with a different symptom: the "start" pad will not respond. Temporarily replace the latter touch panel (with the unresponsive "start" pad) with the used touch panel having the functional "start" pad, and test the unit. If, with the substitute test touch panel installed, the oven still will not "start", the problem is likely not in the original touch panel. On the other hand, if the test panel "start" pad works, replace the original touch panel. A categorized collection of the touch panels—commercial and domestic—will take much of the gamble out of control panel-touch panel dilemma.

Besides the substitution of a known good panel, the next most certain method of isolating this type of problem is to make continuity checks through each of the touch unit circuits with an ohmmeter. However, without specific data on the matrix configuration of the unit to be tested, this is virtually impossible. Therefore, Appendix I contains many of the common matrix diagrams, listed by make and model number, and instructions on how they are tested.

An alternate method of checking touch panels when no matrix diagram is available is performed as follows: 1) Unplug the oven, remove the cover and DISCHARGE THE HIGH-VOLTAGE CAPACITOR(S). 2) Disconnect one or both leads from the primary side of the high-voltage transformer so no high voltage will be generated during the test. 3) Remove the flex-tail ribbon from the control board connector.

USE EXTREME CAUTION WHEN WORKING AROUND "LIVE" CIRCUITS.

4) Plug in the oven and, cautiously, use a jumper to make momentary contact between random points on the control unit connector. This, in many cases, simulates the touch pad (key unit) contacts. 5) If the control panel appears to respond in a relatively normal way, the touch panel is likely the problem. However, assuming this test is compatible with the unit under test, a control panel that fails to respond is probably defective. NOTE: For lack of access, some models may require that the control assembly be removed and placed beside the oven with all harness connections joined.

Many replacement touch panels are shipped with a protective plastic film. If the face of the panel has a bubbled or a hazy appearance, it is probably due to this transparent film. Be sure to remove the plastic film before installing the touch panel.

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Copyright © 1989-2013 J. Carlton Gallawa . All Rights Reserved Worldwide