Pioneer SPEC 2

[lorenzok38]

Hi,
I am working in repair a Pioneer SPEC 2 amplifier, this amp have missing in both channels the P.T.C. thermistor (on my schematic is the Th3 P.T.C. thermistor, part number AEX-044), I don't find this part.
Anybody know which is the value ??.
Which the replacement can be ??
Could it be replaced with another thermistor's ??.
Thanks in any advice.

[sandyK]

I don't know of this particular device, but if you go to www.jaycar.com.au and search for "PTC fuses" you will be able to get data on several "Polyswitches", and perhaps be able to select a suitable replacement, which may be available locally.
SandyK

[xmitterengineer]

Originally posted by lorenzok38
Hi,
I am working in repair a Pioneer SPEC 2 amplifier, this amp have missing in both channels the P.T.C. thermistor (on my schematic is the Th3 P.T.C. thermistor, part number AEX-044), I don't find this part.
Anybody know which is the value ??.
Which the replacement can be ??
Could it be replaced with another thermistor's ??.
Thanks in any advice.

Is this thermistor in the bias network? Mounted to the heatsink maybe?
I repaired a few of these amps a long while ago. IIRC the thermistor you refer to has rather fragile leads and is mounted to the output heatsink.
A wild guess would be to sub this device with a low value resistor (220 ohm)? or maybe a 1K pot. Sub a 100w light bulb for the main fuse to limit current and see what happens. It was standard procedure to use the light bulb method on amps that had unknown troubles or on the 1st try after rebuilding.
If everything else is right you can use a pot to calculate the charicteristics required for proper operation.
What failure mode did the thermistor experience?
Good liuck sir.

Rickey.

[EchoWars]

I believe you may be under a misconception that a PTC works like a NTC...in that you have a specific value which will change in a more or less linear fashion according to temperature. Thermal protection PTC's don't work that way...

This, from GE Sensing:

Commercial PTC thermistors fall into two major categories. The first category consists of thermally sensitive silicon resistors, sometimes referred to as “silistors”. These devices exhibit a fairly uniform positive temperature coefficient (about +0.77% /°C) through most of their operational range, but can also exhibit a negative temperature coefficient region at temperatures in excess of 150°C. These devices are most often used for temperature compensation of silicon semiconducting devices in the range of -60°C to +150°C.

The other major category, and the one that we shall concentrate on in this section, are referred to as switching PTC thermistors. These devices are polycrystalline ceramic materials that are normally highly resistive but are made semiconductive by the addition of dopants. They are most often manufactured using compositions of barium, lead and strontium titanates with additives such as yttrium, manganese, tantalum and silica. These devices have a resistance-temperature characteristic that exhibits a very small negative temperature coefficient until the device reaches a critical temperature, that is referred to as its “Curie”, switch or transition temperature. As this critical temperature is approached, the devices begin to exhibit a rising, positive temperature coefficient of resistance as well as a large increase in resistance. The resistance change can be as much as several orders of magnitude within a temperature span of a few degrees.

This is the type of device generally used as a thermal protection PTC...its resistance remains fairly constant as temperature rises above ambient, perhaps even falling a bit. But as the Curie point is reached (wonder if the famous Madame has something to do with the name?), the resistance begins to rise dramatically.

Pioneer puts the two PTC devices in series, and uses them to trigger the Q11 PNP transistor on the Meter Amp Assembly. A simple deal. The upshot of all this is that the actual ambient value of the PTC doesn't matter too much...pick your trip temperature, and you're ready to go.

Digikey sells some GE PTC devices that range in trip temperatures from 30°C to 110°C. The 80°C unit ought to be about right.

Oh...and I have a Spec 2 on the bench right now...one of the PTC's measures about 30 ohms resistance at ambient, and the other about 50 ohms.


Rickey, there is also a couple of multijunction diodes (for the bias) mounted on top of the heatsink...they aren't PTC's, but by all means they should be treated gently.

[sandyK]

Not being familiar with this equipment, I jumped to the conclusion that the missing devices may have been in series with the O/P , but strapped out. I didn't imagine that 2 biasing thermistors would be missing. Yes, I am quite familiar with both polyswitches and NTC/PTC thermistors.
Years ago, I did have an amplifier with polyswitches, but removed them and fitted wire links.
My apologies for jumpimg to the wrong conclusion .
SandyK

[lorenzok38]

Hi Rickey,
Thank you for your answer
I'll prove their advice.
When I bought the amp didn't bring the thermistors, they got lost in a workshop.
LK

[EchoWars]

You can thank Rickey all you like, but he is wrong. The bias diodes don't have a bloody thing to do with the circuit you're working on.

I give up...

SandyK...it took me a while to look up and verify the info I posted (which I can now see was a huge waste of my time)...when I started composing, there were no replies. My post was directed strictly to lorenzok38. Apologies are quite unnecessary.

[xmitterengineer]

Originally posted by EchoWars
You can thank Rickey all you like, but he is wrong. The bias diodes don't have a bloody thing to do with the circuit you're working on.

I give up...

SandyK...it took me a while to look up and verify the info I posted (which I can now see was a huge waste of my time)...when I started composing, there were no replies. My post was directed strictly to lorenzok38. Apologies are quite unnecessary.

It's been 20 years since I worked on one of these.

And of course my memory isn't perfect.

Sorry if I have led anyone astray.

Rickey

[Netlist]

Originally posted by lorenzok38
(on my schematic is the Th3 P.T.C. thermistor, part number AEX-044)

You can find it here and it seems to be replaceable by AEX-004:
https://www.ued.net/ued/addItems.do?itemCode=PNRAEX-044

/Hugo

[EchoWars]

Originally posted by xmitterengineer


It's been 20 years since I worked on one of these.

And of course my memory isn't perfect.

Sorry if I have led anyone astray.

Rickey

No big deal...you thought he was referring to the bias diodes, but he named the part schematic designation, and even Pioneer's part number. Plus, within the last year I've rebuilt about a dozen of these, so I know them well. I wanted to help, so I gave him a ton of info, and he came back and thanked you for information that had nothing to do with his issues. I thought it a bit odd...

Such is life on the net...