Unknown component PTC or NTC?

[emastyle]

Hello, I'm trying to repair an old amplifier, I haven't got the schematic.
There is a PTC (or NTC) wich is in short circuit (see the image attachement).
It is black with green top.
Anyone has an idea wich component is? (PTC or NTC I believe) and how I could know its value?
Thank you for your help!
Emanuele

 

[mister_rf]

I think that’ s a NTC thermistor used to limit the amount of inrush current in a switching power supply or other devices when the power is first turned on. Normally that’s a low value thermistor, the device could be faulty and it could also be working perfectly, how did you measured it?

 

[emastyle]

The component is mounted on a little BF amplifier and is mounted in contact with the aluminium heat sink of two 2n3055).
I measured it with an ohmmeter that gets 0 ohms.
Is it normal?
Does the color cod the same as resitors? (green - black -> 50ohm)

 

[mister_rf]

As you have already mentioned the thermistor is located near the power transistors so maybe that’s NTC type. As temperature increases, the resistance of the thermistor decreases. Commonly used in audio amplifiers to prevent thermal runaway (33-100 ohms). Removing the thermistor can result in a total burnout of the output or power supply stage.
First try to test the thermistor.
You need an multimeter in resistance mode. Connect the multimeter terminals to the thermistor leads. Now try to heat the thermistor by moving your heated soldering iron tip to it. Check the multimeter reading smoothly increases or decreases depending on whether the thermistor under test is PTC or NTC. But this will happen only for a healthy thermistor… :-D

 

[murali_dece]

connect a 1k or 10k pot in place of thermistor and vary the resistance value. at some point circuit becomes healthy, now you can select a new thermistor which provides that resistance value at room temperature.

 

[tpetar]

I think its PTC, like protection from overheating of amplifier.

 

[emastyle]

I measured it as you suggest. It is 350ohms, heating it the resistance descrease (NTC).
Thanks at all!

 

[D.A.(Tony)Stewart]

I remember using these to control the temperature of my waterbed back in 1975.

https://www.digikey.ca/product-detail/en/MF11-0035005/317-1274-ND/1191049

 

[mister_rf]

In this case there’s no need for a replacement … ;-)

 

[D.A.(Tony)Stewart]

@Emanuele If this old amplifier is circa >30yrs old. I would assume the Alum. electrolytics are expired and replace them all. Digikey has stock and same day delivery. I did the same to fix my old Technics Amp.. Worked like a charm.

 

[emastyle]

@Emanuele If this old amplifier is circa >30yrs old. I would assume the Alum. electrolytics are expired and replace them all. Digikey has stock and same day delivery. I did the same to fix my old Technics Amp.. Worked like a charm.

...yes, I replaced all electrolytics (some was evidently burnt!). The amplifier now seems work but I hear smoothly 50hz again .... The ampli is >30 yeard old... and it is very simple.

Bye bye

 

[D.A.(Tony)Stewart]

did you choose low ESR types? hum= ripple measure it..

 

[mister_rf]

Sometimes need to place a ceramic capacitor (2.2nF-10nF) across each diode in the rectifier bridge section in order to to remove the electrical noise...

 

[emastyle]

Hello, I replaced all capacitors and all resistors not good on my ampli. I added somes cap on rectifier for 50hz. Now the ampli seems to work but I can hear 50hz again and just I power the circuit the 2n3055 become very hot (after 10seconds).
I have not replaced the prefinal (BC141 and BC 161) yet.
I think I have to adjust the bias(?) 3k3 trimmer, but how?
I haven't the schematic but only this 3 photos. Any suggestion?
Thank you.
Emanuele

 

[mister_rf]

Ideally, in a class-AB push-pull amplifier, the goal is to set the output stage current just the right amount of overlap between output devices to attain the transfer function that is closest to linear.
For this amp try measuring the total current draw of an amp (that is actually a pretty imprecise way of setting bias, but for this circuit it’s not so important :-D ). I think the bias current is 20mA and this can be measured by cutting the PCB circuit (as in the attached picture) and inserting a multimeter to the power supply line. Set the instrument for DC milliamps and adjust the bias pot to measure about 20mA.

 

[emastyle]

Ideally, in a class-AB push-pull amplifier, the goal is to set the output stage current just the right amount of overlap between output devices to attain the transfer function that is closest to linear.
For this amp try measuring the total current draw of an amp (that is actually a pretty imprecise way of setting bias, but for this circuit it’s not so important :-D ). I think the bias current is 20mA and this can be measured by cutting the PCB circuit (as in the attached picture) and inserting a multimeter to the power supply line. Set the instrument for DC milliamps and adjust the bias pot to measure about 20mA.

Hei, thank you for your clear suggestion... I'll try as I can.
But is it normal the two 2n3055 are so hot without input signal?

 

[mister_rf]

No, that’s not normal, the output transistors currents are too big now, and that’s the cause of excessive heating.