Thermistors?

Harry Klippton

Harry Klippton

Not Interested
I have encountered what I think is a thermistor and I have some questions. The component in question can be seen below. It's red and labeled RT1:
1000007120.jpg

It appears on the schematic here:
1000007135.jpg

And since the circuit spans several pages of the schematic, I've redrawn it here:

1000007136.jpg


I'm assuming that its purpose in this circuit is for some kind of current limiting to protect the motor rather than thermal protection or something. The circuit is not behaving as expected. How would I know if it was out of spec? How can I test it? Is this a thermistor?

Searches for SDT-09 don't return any results for any components that look like this, nor do any searches for the part number listed in the service manual. I measured it in circuit and got 120 ohm. I don't think this component is actually causing a problem but I'm curious about it, not having encountered it before.

The tape transport runs a bit slow. R47, R48, and R1 are in spec. I also disassembled the motor and it is not physically damaged. I replaced the internal motor control chip to no change. I also swapped in an entirely different motor to no change. I disassembled the transport to look for sources of physical resistance to which I found none, and lubricated all moving parts for good measure. The motor is receiving 12 volts as specified in the schematic. The pulley is not slipping on the motor shaft and there is a new capstan belt

To recap, is RT1 a thermistor? How can I test that it's working properly? What's it doing there?
 
Normally when I've seen thermistors in use they are either physically touching a component that is intended to get warm, or are in series in a high current line where the thermistor itself will heat up due to the high current.

CRT televisions use them in series with the degaussing coil to demagnetize the CRT at power-on. The thermistor allows the coil to get a high initial inrush current then gradually ramps down as the thermistor warms up.

I'm not entirely sure what it'd be doing in this circuit with those relatively high surrounding resistance values.

Measure the resistance while warming it up, you should see a pretty significant swing in one direction or the other. (Depending on whether it's NTC or PTC)
 
Google says it is indeed a thermistor, it's obsolete, its part number is SDT09 and it measures 90 ohms (at some baseline temp), it's common in tape players (recorders?) as it compensates for changes in temp (that affect tape sensitivity?). Thermistors are either NTC or PTC, which means negative or positive temp coeficient, so the resistance either goes up or down along with rise in temp




here's how to test

120 ohms may be its standard readiing at room temp meaning its likely ok. I couldn't find a data sheet so idk but it's not shorted or broken and the value is close to its rated value so I'd look for the problem elsewhere
 
I desoldered the A terminal to the motor and clipped in a 1k resistor. That got me into the right speed range so I'm gonna swap R47 for some value I have on hand in the 2k range. Thanks for the thermistor info y'all.

Nostradoomus said:
Really, @Harry Klippton? You’re gone for months and just suddenly appear at the beginning of the month of our Lord, Rocktober? My word.
Click to expand...
Yeah I'm Hot Rockin'
 
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