Hello! I am repairing an Adcom GFA5800 amp which has a thermistor mounted to each heat sink to drive the fan. The problem is that one of the leads broke off flush with the part. This side of the amp calls for a 40C part. How would I go about finding a sub for this part? I looked on DIGI-KEY but didn’t see a spec for 40C. Any help would be much appreciated!
Man, I've been doing some searching, and finding a PTC thermistor at 40C with around 140 ohms at 25C is very hard to find, not to mention mounting options. What would be the best approach for a substitute? Could the thermistor be replaced with a normally closed thermo switch? Attached is the schematic the part in question is TH1.
You might be able to adapt a different value thermistor. The fan circuit is an LM317 regulator that is using the thermistor as part of the voltage divider that determines its output voltage. That voltage divider could go up or down in total resistance, as long as the regulator is happy. (Check the LM317 datasheet on minimum impedance values for the divider.)
The math is easy, but here's a calculator to do it.
https://circuitdigest.com/calculators/lm317-resistor-voltage-calculator
You'll have noticed that one channel as a 40C thermistor and the other is 60C. They're doing this to keep the noise down. Pulling air through the one channel's heatsink tunnel gets the air moving and actually does pull a little air through the other channel. If it gets even hotter, the second channel's fan kicks in, and they will both start moving faster if they need to.
It's not an awesome arrangement, one channel gets hotter than the other and may die sooner because of it.
I don't remember who posted it, but someone either here on AK did a fan replacement and found a nice, quiet fan that worked well with the 5800. I would be inclined to change both regulators to 40C and put in the quiet fans, and they will just run slowly most of the time. They're nearly inaudible.
The math is easy, but here's a calculator to do it.
https://circuitdigest.com/calculators/lm317-resistor-voltage-calculator
You'll have noticed that one channel as a 40C thermistor and the other is 60C. They're doing this to keep the noise down. Pulling air through the one channel's heatsink tunnel gets the air moving and actually does pull a little air through the other channel. If it gets even hotter, the second channel's fan kicks in, and they will both start moving faster if they need to.
It's not an awesome arrangement, one channel gets hotter than the other and may die sooner because of it.
I don't remember who posted it, but someone either here on AK did a fan replacement and found a nice, quiet fan that worked well with the 5800. I would be inclined to change both regulators to 40C and put in the quiet fans, and they will just run slowly most of the time. They're nearly inaudible.
Thank you, that is very helpful, I learned something! My biggest issue is trying to find a 40C thermistor. Either I am searching and using the wrong parameters or they are really hard to find. The lowest temp I’m finding is 60C.
I removed both the 40 and 60 degree thermistors from my gfa5800 and placed them on the heated bed of my 3D printer. Waited about 5 minutes between each temp increment. Here are the results.
temp....40......60
25..........121......99 ohms
30..........140......102
35..........158......106
40..........188......114
45..........238......125
50..........348......142
55..........527......163
60..........980......205
70..........2.7K.....392
80..........9.6K.....920
90..........36K......2.3K
temp....40......60
25..........121......99 ohms
30..........140......102
35..........158......106
40..........188......114
45..........238......125
50..........348......142
55..........527......163
60..........980......205
70..........2.7K.....392
80..........9.6K.....920
90..........36K......2.3K
Regarding the table above, be aware that the LM317 reaches full 12v output when the thermistor gets above around 1300 ohms, so any ohm values above that don't increase the fan speed.
The portion of the schematic you posted contains an error. Resistors R105/R205 are shown as 825 ohms, but in reality are 82.5 ohms. If you look at the actual installed resistors, they have a five band color code--- Gray 8, red 2, green 5, gold x0.1 multiplier, and brown 1% tolerance. So they are 82.5 ohms after all. The parts list in the manual shows the correct 82.5 ohms. I find it very odd that such a specific non-standard value resistor is specified for this position, as it isn't of critical importance for the circuit.
Basically, the r105/205 82.5 ohm resistor are in series with the thermistor, so it is their combined value that defines one half of the voltage divider, with R104/204 at 158 ohms defining the other half of the divider. Using the calculator linked above by Phloodpants, you would enter 158 ohms for the R1 field and 82.5 + thermistor ohms in the R2 field. Looking at the gfa5800 manual schematic and the calculator schematic you would think the field values should be swapped, but the math only works the way i just described, so perhaps theres another error in the schematic.
The portion of the schematic you posted contains an error. Resistors R105/R205 are shown as 825 ohms, but in reality are 82.5 ohms. If you look at the actual installed resistors, they have a five band color code--- Gray 8, red 2, green 5, gold x0.1 multiplier, and brown 1% tolerance. So they are 82.5 ohms after all. The parts list in the manual shows the correct 82.5 ohms. I find it very odd that such a specific non-standard value resistor is specified for this position, as it isn't of critical importance for the circuit.
Basically, the r105/205 82.5 ohm resistor are in series with the thermistor, so it is their combined value that defines one half of the voltage divider, with R104/204 at 158 ohms defining the other half of the divider. Using the calculator linked above by Phloodpants, you would enter 158 ohms for the R1 field and 82.5 + thermistor ohms in the R2 field. Looking at the gfa5800 manual schematic and the calculator schematic you would think the field values should be swapped, but the math only works the way i just described, so perhaps theres another error in the schematic.
Honestly, the easiest solution is to just wire the second fan in parallel with the one on the 40 degree side. The lm317 is capable of handling 1.5A, and both fans together at full speed only draw about 300mA. If both channels have the bias set properly and the amp is being used for ordinary stereo sound, both channels should be generating the same amount of heat anyway.
For the amp I worked on, I wired the fans in parallel to the 40 degree side and bumped up the 82.5 ohm resistor to 120 ohms, which gives a slightly more aggressive fan turn on and ramp up profile. This amp is ridiculously hot and could use more cooling than provided in stock configuration. I also replaced the fans. The old fans were getting kinda noisy and they didn't have equal turn-on thresholds when tested with a bench power supply. I bought ordinary 3 wire 120mm PC fans and removed the extra wire. One critical dimension is the the 25mm thickness, as the fans are sandwiched between the channel heatsinks and a large steel structural brace that spans the width of the amp.
For the amp I worked on, I wired the fans in parallel to the 40 degree side and bumped up the 82.5 ohm resistor to 120 ohms, which gives a slightly more aggressive fan turn on and ramp up profile. This amp is ridiculously hot and could use more cooling than provided in stock configuration. I also replaced the fans. The old fans were getting kinda noisy and they didn't have equal turn-on thresholds when tested with a bench power supply. I bought ordinary 3 wire 120mm PC fans and removed the extra wire. One critical dimension is the the 25mm thickness, as the fans are sandwiched between the channel heatsinks and a large steel structural brace that spans the width of the amp.
Thanks for the great information. Your amp looks almost identical to mine, down to the caps you used 🙂 You are correct about the errors in the schematic. I think I found another error somewhere but I don't recall where. When I measured my thermistors, I got somewhat different results, where at the higher temps I got 100k - 2M ohms. Also, please correct me if I'm wrong, but it appears that when the thermistor gets hot, it increases resistance which LOWERS the voltage to the regulator control pin, which in turn LOWERS the voltage output which INCREASES the voltage potential to the fan. This is the opposite of what I was expecting when I first looked at this. With the comments I read about this amp running hot and the channels cooling unevenly, I decided to replace the thermistors with resistors and replace the fans with new quiet computer fans with high MTBF ratings. After biasing, it now draws about 350 watts at idle and runs very cool (but it does warm up the room with the exhaust air). I really like this amp, it is like Mike Tyson with soft velvet gloves...lots of detail and air, but great bass and seemingly endless power.