Tektronix TDS 540 with power supply problems...

[joseravat]

Good evening.
I bought on an eBay auction a Tektronix TDS 540 without working.
It arrived today, and for the age it is on a very good condition.
I open it and I felt very happy, because there are no leakage capacitors and any kind of corrosion 
Anyway I'm going to change all the SMD capacitors and also the electrolytic capacitors of the power supply.
And I think I found the problem: one thermistor out...
I'm going to change Q9, VR4 and VR5 and the thermistor RT4 that is Burned.
But now begins my problem!
I read on an old post the reference of Q9, VR3 and VR4 (and I already bought them) but I didn't find nothing about the Thermistor RT4...
Does any of you knows the reference of this Thermistor?
There are so many temperatures and resistance to the thermistors that it will be difficult to chose the right one.
I attach the schematics of my power supply and a  photo of the burned Thermistor.
Hope someone can help me.

[tautech]

The thermistor should be listed in the BOM with the schematics.
Have a hunt here for the service or user manual with the BOM:
http://w140.com/tekwiki/wiki/TDS540

[radioguy123]

There is no BOM for that version of the power supply.
I would like to find the thermistor too.

[shakalnokturn]

I'd assume RT4 to be identical to RT5 as they are in series.
If either is dead turn to the other for markings, if there are no markings use it to get the value / coefficient then find a substitute, matching physical size should be enough for the power rating.

[radioguy123]

I am clueless testing a thermistor. They both read about 30 ohms. But one obviously goes open when hit with current.
I would presume that both are damaged. The only marking there is, is the letters SL.

[rf+tech]

Just measured both RT4 and RT5 in a TDS640, which appears to use the same standby supply.

One measures 40.2 Ohms and the other 45.5 Ohms, at room temperature. Applying heat to both devices causes their resistance to decrease, confirming these devices are NTC.

Something like Epcos B57234S0400M000, 40 Ohms ±20% would be a first approximation for a replacement. They're even the right color 

https://www.digikey.com/product-detail/en/epcos-tdk/B57234S0400M000/495-76268-ND/3500301

ETA: Just looked again at the standby supply schematic and noted the +t° which would indicate a Positive TC device. Why my scope's thermistors show a *negative* response to higher temperature, is counter to the notation. Heatsink temperature 45°C, 34.7 Ohms and 37.5 Ohms. A single data-point, FWIW.

RF+ Tech

[radioguy123]

rf+tech
Yes I noticed that +t0 also.
It's going to be tough to figure out the the correct value for these devices by measuring.
Both of mine measure near 30 ohm at rt. But I know one of them is damaged.
So I would suspect that that both are damaged to some degree.
And of course the static measurements I did are not accurate because of that damaged thermistor.
It just goes open when current is pulled through it

[radioguy123]

Both of the thermistors measure fine out of circuit. About 30 ohms cold.

I put some current through both of them and the resistance goes down when warm.

But one of these when in the circuit had nearly full bulk voltage across it. +159volts.
The other read just a few volts.

So one of these thermistors is going open under load.