These fusible resistors are rated at 500mW.
At 500mW there are 123mA going through them.
If I got that right, fusible resistors have to run relative close to the nominal power to meet the wanted fusing characteristic.
But I assume that this should be the absolute maximum current.
For long term stability it will probably be something below, but I don't know the power requirement of the input stages.
@ 100mA at the output there should be ~108mA through the restistor.
A 1W resistor will go open very late comparatively.
My first thoughts was to take the heat and use a 500mW nonflamable resistor.
Another thought was to replace the 33 ohm resistors with a fuse.
The voltage drop is only about 4V @ expected maximum load and Q708/Q709 should be able to handle the slightly higher current and dissipation easily.
Theses are KSA1220A/KSC2690A TO-220 (Vcbo/Vceo=160V, IC=1,2A, Pc=20W Tc@25°C) devices with their own heatsinks.
During testing on the bench they were only 36°C warm after one hour playing music.
Or are these resistor needed to add some pre load?
At 500mW there are 123mA going through them.
If I got that right, fusible resistors have to run relative close to the nominal power to meet the wanted fusing characteristic.
But I assume that this should be the absolute maximum current.
For long term stability it will probably be something below, but I don't know the power requirement of the input stages.
@ 100mA at the output there should be ~108mA through the restistor.
A 1W resistor will go open very late comparatively.
My first thoughts was to take the heat and use a 500mW nonflamable resistor.
Another thought was to replace the 33 ohm resistors with a fuse.
The voltage drop is only about 4V @ expected maximum load and Q708/Q709 should be able to handle the slightly higher current and dissipation easily.
Theses are KSA1220A/KSC2690A TO-220 (Vcbo/Vceo=160V, IC=1,2A, Pc=20W Tc@25°C) devices with their own heatsinks.
During testing on the bench they were only 36°C warm after one hour playing music.
Or are these resistor needed to add some pre load?
I wouldn't worry to much over these with regard to what to fit, but I would still leave them in place. They may play a small part in reducing HF noise/ripple due to adding significant impedance to the rail.
They needn't be very close rated power wise, you just want something that will fail in the event of a short circuit on the stabilised rail. I'd fit a 0.5 watt part and then measure the actual volt drops across it in use and then decide whether to keep it or go up a notch to the next power rating.
They needn't be very close rated power wise, you just want something that will fail in the event of a short circuit on the stabilised rail. I'd fit a 0.5 watt part and then measure the actual volt drops across it in use and then decide whether to keep it or go up a notch to the next power rating.
Another question about the driver Transistors.
First I've ordered 2SA1837/2SC4793 UTC devices, because there are no more original Toshiba parts available.
Then I saw fortuitously some at my local dealer and and bought the few remaining parts.
UTC parts have high hFE of ~320 and NPN/PNP are very close.
Toshiba parts have low hFE NPN ~200 and PNP ~150 and a NPN/PNP difference of about 50 hFE.
Which should I prefer?
When i exchange the fusible resistors, i could swap them too.
First I've ordered 2SA1837/2SC4793 UTC devices, because there are no more original Toshiba parts available.
Then I saw fortuitously some at my local dealer and and bought the few remaining parts.
UTC parts have high hFE of ~320 and NPN/PNP are very close.
Toshiba parts have low hFE NPN ~200 and PNP ~150 and a NPN/PNP difference of about 50 hFE.
Which should I prefer?
When i exchange the fusible resistors, i could swap them too.
If you have the original Toshiba parts (and they are genuine) then I would probably stick with those.
Although higher gain parts might seem preferable, the fact they are modern second sourced replacements mean they probably differ in other ways as well such as Ft and junction capacitance.
It almost certainly wouldn't make a difference, but as you have the originals then stick to those.
Although higher gain parts might seem preferable, the fact they are modern second sourced replacements mean they probably differ in other ways as well such as Ft and junction capacitance.
It almost certainly wouldn't make a difference, but as you have the originals then stick to those.
How did it go?
I have myself a C272 of the newest batch from 2009. They’ve made some changes regarding transistors. Q710, 711 is 2240 and q712 is a970. On left and right boards there is now also 1220 and 2690 (input stage) like down in the +68v regulator. The 2240 pair and the 970 goes very hot still..
The isc sensitivity is a tricky one.. need to measure under load 15 vs 150w... hmm.
I have myself a C272 of the newest batch from 2009. They’ve made some changes regarding transistors. Q710, 711 is 2240 and q712 is a970. On left and right boards there is now also 1220 and 2690 (input stage) like down in the +68v regulator. The 2240 pair and the 970 goes very hot still..
The isc sensitivity is a tricky one.. need to measure under load 15 vs 150w... hmm.
I have this same updated model and my amp went dead. I have a thread addressing it here:
NAD C272 Amp Fix
I would love some help in diagnosing the problems with it and getting it back up and running.
Many thanks for any and all help.