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 Troubleshooting Sending Units
                "What ya does if ya gets a Buzz"

With use, especially at higher temperatures, thermocouples eventually burn out or "open". This type of fault usually occurs at the junction of the two alloys that make up the thermocouple (in short, at the business end). Type K sending units have buzzers in them to warn you of this condition. [Type S sending units do not have buzzers in them.] Note, however, that the sending unit cannot detect exactly where the fault lies, just that it exists. For example, it cannot tell the difference between a thermocouple that is burned out and one with a broken lead or bad connection at the sending unit. Although the main purpose of the buzzers is to detect open thermocouples, there are a few other conditions that can also cause them to sound. In particular, the sending units have internal fuses to help protect them in case of accidental contact with a high voltage (say if you touch a live heating element with the thermocouple). A blown fuse will appear to the internal circuitry of the sending unit just like an open thermocouple connection described above. Other faults that can cause buzzing are a short circuit between the orange and green wires and leakage of electric current from heating coils to the thermocouple. This last one is quite obscure and usually occurs only at temperatures above 1800 F or so. If your sending unit buzzes, the first step is to determine whether the thermocouple is at fault. Disconnect the thermocouple from the sending unit and then connect a jumper wire between the red and yellow terminals of the sending unit. (A bare piece of copper wire about 1 inch long makes a good jumper for this purpose, but you can also use a bent paper clip, if that's all you have handy.) When the thermocouple is disconnected, the sending unit is guaranteed to buzz (unless the buzzer is defective, in which case how could you be troubleshooting a buzzing noise?) When the jumper wire is connected, the buzzing should stop. If this is indeed the case, then the problem is the thermocouple or its leads and connections.

If the buzzing continues even with the jumper wire, the next step is to check for bad fuses. You will have to remove the cover of the sending unit to see these. The cover merely snaps on, and it should be obvious how to remove it. Older units have removable glass fuses, whereas units made in the last 6 years have soldered-in fuses. These are solid colored and cannot be inspected visually (unless they've really been blasted, in which case they will look like charcoal or burnt toast.) Generally, a visible test is not accurate even with the older glass fuses, since the fuse wire is very thin and it's hard to discern its condition. The best way to test the fuses is to use an ohm- meter or some other form of continuity checker. Most voltmeters have a resistance scale that can be used for this purpose. (Note that neon type tester that electricians use to test receptacles etc. will not work at all for this test. Those devices measure voltage, not continuity.) If you find an opened fuse, then this is the problem. If the fuse is blown, there is a definite chance that the sending unit has been damaged by whatever blew the fuse. For this reason, even if you have the field replaceable glass fuses, we recommend that you return any sending unit with a blown fuse to Digitry for checkout. If you do replace a fuse and do not send in your unit, you should at least observe its operation long enough to convince yourself that reasonable temperatures are being transmitted by the sending unit. If the above described procedures do not point to the cause of the buzzing, then you are now living in the domain of the obscure and esoteric.

 
   

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