Fender Frontman 65R

[retgaz]

Hi.

I'm just wanting a bit of advice.
I've had a Fender Frontman 65R brought in for repair. The thermistor RT1 PTC100 is reading high and the amp is cutting out after about an hour of use.
I emailed Fender to get a part number or temperature rating for this part and received this response:

Due to the age of the amplifier, we do not have detailed enough records to provide part numbers and specifications for this amp. I have spoken to one of our amp techs and if you've bought a PTC100 Thermistor it should in theory work but we cannot guarantee it.

So could anybody tell me the temperature rating for this part please?
I've attached the schematic.
fender_fm_65r.pdf

Thanks

[mozz]

Digikey has a datasheet, if the maker is the same. Storage conditions seem funny, like it is only good for 24 months? If it's reading high you must have some specs?

[retgaz]

Hi mozz
The only specs I have for the existing device are those on the schematic, where it's shown as a PTC100.
Unless I'm remembering this wrong, PTC means positive temperature coefficient so the resistance of the thermistor increases with temperature, until it reaches a maximum temperature or switching temperature. After this point the resistance changes very little with further increases of temperature.
The 100 at the end of the PTC100 is the resistance at 25 degrees centigrade.

[nosaj]

What do you have at TP3 TP4 and TP5 an TP6 when it cuts out?
To test your PTC out of circuit should be easy enough with a Meter and a soldering iron to see if its opening or not. But a loss of your -16v rail would cause the same issue I believe. Let us know about the TP voltages before and after cutout.
nosaj

[J M Fahey]

Should be close to this:

https://www.mouser.com/datasheet/2/4...tl-1762894.pdf

but in any case "functional" testing rules, as suggested by Nosaj.

I guess it mutes power amp when heatsink (chassis) reaches 65-80 Celsius.
Forget those wonky Imperial units.

Doubt it´s "broken", but doing its job instead.

Find why your amp works way hotter than normal which is the actual problem.