What the heck is this circuit board component?

[EmoCowMoo]

I'm restoring a portable Masterworks M-8608 8-track cartridge player. I couldn't ever find any schematics on this unit unfortunately. There's these little 2-leg purple disk things on the main circuit board and I'm trying to figure out what the heck they are. I'd replace them if they're bad, but I don't know what they are or what they're supposed to read when working. There's no markings on them.

Removing them from the CB, my LCR meter reads each one as a resistor, starting at around 52 ohms and then the readings all slowly climb past 65+ ohms. My other tester unit immediately shows an RL filter type of device (Resistor in series with an inductor), then it changes to a resistor with a reading of 52 and climbing. I attached pictures of all of this stuff.

What is this guy??? Does anyone have a schematic of this unit or another Masterworks model that contains these or something similar, for reference?

 

[avionic]

Varistor or thermistor perhaps .. heat the component while monitoring the resistance.

 

[jdurbin1]

Or a polyswitch, looks like there's a pair of them on what appears to be an amplifier board. Guessing that's some form of on-board protection circuit, though why that thing needs them is beyond me.

If they go open with heat, polyswitch...if the resistance changes, thermistor.

John

 

[EmoCowMoo]

It appears it's a thermistor. I held a lighter under it while measuring it and it kept dropping down to around 25 ohms. I've only ever heard of them in the industry field w/ temperature sensors. Yeah, why the heck would they need something like this amplifier circuit?

They're each tied to a leg on a separate germanium transistor (4 total). Do germanium transistors heat up more so than normal transistors or are influenced by heat? Wouldn't my signal change in amplitude then with a change in temp (sound decreasing as you play a tape)?

 

[Lavane]

They are used in some tube gear to give a bit of a soft start, maybe drop a couple volts if needed, etc. Then there are the ones that go open if they get to hot and are attached to output transistor heat sinks as protection. Poly switches are similar and used in some speakers to protect the tweeters and sometimes midrange drivers. Maybe they are being used to delay full voltage until the rest of the circuit energizes and becomes stable?

 

[arts]

Thermistor. Varies the output stage bias according to heat level.

 

[chazix]

This ^^^

 

[gadget73]

yep, bias control. As transistors get hotter they flow more current. Guessing this is wired in such a way that it causes the bias voltage to drop in order to reduce current. Sometimes you'll see diodes used in the same way.

 

[Binkman]

sometimes seen mounted to the heat sink.

 

[jdurbin1]

Huh, interesting... learned something new here.

John

 

[JimboBigtum]

When mag tape heads get warm, they change parameters. As I recall, you could run that thing off batteries and I think I remember somebody carried one in his car back seat, before FM became affordable. Anyway, the thermistor may be there to adjust the tape head bias so you could take this thing to the beach. It was always cooler to have your own tunes, not have to listen to WLS.

 

[chazix]

the thermistor may be there to adjust the tape head bias

Interesting idea, but I think the pix make it clear that the application is in the audio power amp. Not to mention that "tape head bias" (in my lexicon, anyway) is something that has only to do with recording, and I think the only concern here is playback.