Philips AG4126/7 blown thermistor

[Dodgy Geezer]

Hi,

I have an old portable Philips Diamond record player, with a blown component on the PCB. It's R24, which I believe is an NTC thermistor. I'm not that good at fault finding, so I was just going to replace it and see what happens, but before that I have a couple of questions for anyone who might be so kind as to help:

1 - Do thermistors come with different values?
2 - If so, what might the original AG4126 value have been?
3 - Does this blown thermistor indicate a fault elsewhere? I append a schematic (copied from elsewhere on this site - thanks to the uploader) for information

Hope someone can help,

DG

[Techman]

The first question I would ask is how do you know it's blown?

Post a picture of it for members to give their opinions.

If it's blown there's likely to be other faults.

[ms660]

Various Philips schematics suggest that it might be a BC/a130e...value on the schematics given as 130 ohms, rated at 1 watt, I can't be sure if that's the "cold" resistance or the "hot" resistance.

Apparently it's a disc type so it might be a case of the usual soldered wire connection(s) come adrift from the actual disc.

Lawrence.

[Dodgy Geezer]

Very kind of you to respond so quickly!

I am pretty sure the thermistor has lost its magic smoke - it is in two pieces. I shall look for a BC/a130e. In the meantime here is a picture of it... it is the component connected to the green insulated wire....

[roffe]

A Dutch thread discussing this NTC and with a datasheet on it.
https://www.circuitsonline.net/forum/view/138716

[Techman]

That has certainly 'blown', there's no doubt about that!

I would advise checking all the components between the two transformers, but the first ones would be the two output transistors and the preset pot. Other than the obvious 'blown' part, all three of the other parts could well be faulty in some way. Check the end to end resistance of the pot and also the continuity of its slider to the track, but take careful note of its position on the track before disturbing it. Check the junctions of the output transistors in all directions with an ohm meter, the transistors will have to be disconnected from the circuit to make these tests.

[paulsherwin]

I agree, you may well have an output transistor fault. There are lots of reports of AC128s developing internal shorts now.

[Dodgy Geezer]

Great! The AC128 does not seem to have any indication of the what the output leads are - no marks or tags at all....

['LIVEWIRE?']

They are arranged in E-B-C- order when viewed from underneath. The main thing to check for is whether any of the leads are in any way connected to the metal case of the transistor. There should be an open - circuit reading between the can and each lead even on the highest resistance range - if not, the transistor is faulty.

[Dodgy Geezer]

Open circuit between all three leads and can on both.

Multimeter on 200k ohms setting connected with +ve to (what I hope is) base - reading of about 50 climbing to 160 on both emitter and collector over period of about 1 minute. Connected with -ve to base - solid reading of 5-6 at both C and E. Both transistors behave similarly.

The pot is a 2k pot, set to about 500 ohm. Slider and one end are connected, so it's a simple variable resistance. Looks fine...

[Station X]

Does your meter have a diode test facility? If so you could try this test:-

https://www.vintage-radio.net/forum/...ad.php?t=42194

[Dodgy Geezer]

I've taken off the remains of the thermistor. It runs parallel to R19, which looks like 68 ohms. Interestingly, the connection with R19 and the +ve line looked dryish (hard to tell if you have already put a soldering iron on...). If it was dry, that might explain the thermistor failure with no other component issue?

The best match I can find for a 130 ohm NTC thermistor is this - do you think it will be ok?

NTC-120D-15-Inrush-Current-Limiter-Power-Thermistor-150-ohm-1-6Amp

[Dodgy Geezer]

Does your meter have a diode test facility? If so you could try this test:-

Hmm... cheap Chinese - it has a diode and transistor checker, but who knows how well they work? However, doing a diode check, both transistors showed open circuit with +ve on the base, and a variable figure with negative. When I use the transistor tester, both show 18-19 on the screen - what that means I do not know. Is it the gain?

[paulsherwin]

Those numbers are plausible for an AC128. The numbers are the hfe (gain).

The thermistor adjusts the bias on the output pair to compensate for ambient temperature changes and stop thermal runaway. The amp should work after a fashion with the thermistor removed, so if it's completely silent then there's another fault somewhere.

[poppydog]

How would a thermistor blow? I have had to deal with 1 or 2 thermistor problems of late (including 1 at present) and have never come across one that looks like its blown, I have had a couple that have come apart due to age and damp conditions.
Yours looks like the 2 halves have just come apart, you could try gently holding the 2 halves together and just putting a tiny dab of quick setting epoxy over the tops of the 2 halves to glue them back together, then measuring its resistance to see if its roughly in spec. Trying to find a 130r thermistor is no mean feat.

poppydog

[paulsherwin]

It's possible that it's just fallen apart, though a severe overcurrunt might cause it to fail like that. The circuit should sort-of-work without it though.

[Luxman1050]

I would agree with poppydog try and repair first as I have found these very rarely blow. If you can repair just measure ohms first think should be around 220ohm cold then it should drop as things warm up as all it does is provide a soft start.

[Dodgy Geezer]

The thermistor is in several pieces, and there is a white deposit from an explosive disassembly on the surrounding components, as you can see from the picture.

I have heard about grinding down disc capacitors to achieve lower values, but putting this back together does not seem very practical...

Would a 150k 1.6A one work, as specified above?

[Techman]

The thermistor certainly looks like it's suffered heat damage in the photo, but that could be deceiving, as said, the circuit should still work so some degree without it.

Readings in ohms with the transistors completely removed from the circuit should be rock steady and not starting at one reading and climbing to another. Readings are best taken with an analogue meter, such as an AVO, but most people don't have one of those to hand.

Base to emitter and base to collector should be taken in both directions and the readings reported - they'll both read as diodes if the transistor is good. However, there's also the collector to emitter reading that's crucial and I don't think has been reported yet - it should read as completely open circuit. All readings taken with components completely un-soldered and disconnected from the circuit with the meter on the lowest ohms range.

Make sure you take pictures and make notes of where connections are made before removal of components, otherwise you'll get in a real mess.

[Dodgy Geezer]

Collector to emitter diode check for both transistors both open circuit both ways. Base to E/C reads as diode. Seems as if the AC128s are fine. So I wonder what caused the thermistor to blow, and if it's worth just replacing it...