The two large resistors in the middle of the board..
Basically, I was asked to fix this amplifier, opened it up, and the original parts had gone brown with heat... One measured open circuit, the other measured about 610ohm... The colours made me assume it was a 620ohm, so I source two replacements, 5watt carbon film (didn't want wire wound in case it created issues) to replace the original 2-3 watt parts..
Well, the amplifier now works, makes music, but after 10 minutes on the bench, the new resistors were too hot to touch (measured over 90 degrees)
So, what do these parts actually do? Is it possible I've refitted the wrong value part, and that's why they're hot, or could something else be wrong that caused these parts to fail in the first place...
I know its a rather vague question, but I don't want to have to remove the board again to trace the circuit out!
Thanks!
Basically, I was asked to fix this amplifier, opened it up, and the original parts had gone brown with heat... One measured open circuit, the other measured about 610ohm... The colours made me assume it was a 620ohm, so I source two replacements, 5watt carbon film (didn't want wire wound in case it created issues) to replace the original 2-3 watt parts..
Well, the amplifier now works, makes music, but after 10 minutes on the bench, the new resistors were too hot to touch (measured over 90 degrees)
So, what do these parts actually do? Is it possible I've refitted the wrong value part, and that's why they're hot, or could something else be wrong that caused these parts to fail in the first place...
I know its a rather vague question, but I don't want to have to remove the board again to trace the circuit out!
Thanks!
I notice you submitted just now! (Good service here)
As you say only guess-work possible unless someone recognises the amplifier. I did not notice the make and type anywhere. Perhaps if you state that one might find a schematic somewhere.
Too hot to touch might be in order. Vaguely high power carbon resistors can withstand up to 80 - 100 degrees, and one's finger starts burning at about 50 degrees. Also, if still possible (you may have to remove the board again!), I would mount them a little higher up from the p.c. board, and further away from those electrolytic capacitors next to them and perhaps from each other - heat is a particular enemy of electrolytics. The longer leads will also keep some heat away from the solder joints. Apply plenty of solder.
Waiting for others to reply ....
As you say only guess-work possible unless someone recognises the amplifier. I did not notice the make and type anywhere. Perhaps if you state that one might find a schematic somewhere.
Too hot to touch might be in order. Vaguely high power carbon resistors can withstand up to 80 - 100 degrees, and one's finger starts burning at about 50 degrees. Also, if still possible (you may have to remove the board again!), I would mount them a little higher up from the p.c. board, and further away from those electrolytic capacitors next to them and perhaps from each other - heat is a particular enemy of electrolytics. The longer leads will also keep some heat away from the solder joints. Apply plenty of solder.
Waiting for others to reply ....
Its a soundstream LW1.350.
As I said, I measured the resistors at over 90 degrees (although this might have been slightly inaccurate, we used to have an oven at work that was at 70 degrees, and you could touch it for a few seconds before being burnt.. lol)
Anyway! I didn't mount these resistors higher, as they're 2 or 3 times the wattage of the originals, so thought their thermal capacity would suffice!
As I said, I measured the resistors at over 90 degrees (although this might have been slightly inaccurate, we used to have an oven at work that was at 70 degrees, and you could touch it for a few seconds before being burnt.. lol)
Anyway! I didn't mount these resistors higher, as they're 2 or 3 times the wattage of the originals, so thought their thermal capacity would suffice!
Looks like the dropping resistors for the ±15V supply for the opamps in a car amplifier, you should be fine with what you have done.
Best I can tell at a glance, they are power resistors. I prefer metal oxide to carbon film, but to each his own. Both types are rated to handle 200 degrees C (392 degrees F), because they DO get hot. Again, I'm assuming they may be current limiting resistors? Someone else can give a better explanation, I'm sure. But they WILL get hot, that's why they are rated for the higher wattage and temperature. Looks like the manufacturer of the amp cut the specs too close to the bone, and one of them burned out. When you're building thousands of units, a few pennies a part makes a big difference in operating costs. 🙂
...powered the amp up again, 14volt supply, and the resistors have just hit the 115 degree Celsius mark....
This doesn't seem right...
This doesn't seem right...
125 degrees... ambient is 20 degrees... not sure what'll happen in a car on a 40 degree day, when its 50 degrees in the car... :/
1.35W with 29V across them, maybe 43mA.
That may just be the way things are, hot.
If you changed the opamps to something that drew less current you could up the value of the dropping resistor and it would dissipate less power.
That may just be the way things are, hot.
If you changed the opamps to something that drew less current you could up the value of the dropping resistor and it would dissipate less power.
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Can you measure the voltage across the resistor ?(quickly!) Photos of both sides of the PCB might assist observers work out what the rest of the circuit is.
It is not impossible that the existing 2W rated resistors are there to protect something else (was it Quad who did that? No - it was Leak ) and the death of the one indicates a fault somewhere else.
The capacitors to the left of the resistors are on the +-15volt rails. SO I changed those for some 330uF (didn't have any 220uF at hand)
Made no difference...
One resistor gets slightly warmer than the other, but not excessively... So I'm beginning to think the temperature is normal.
Made no difference...
One resistor gets slightly warmer than the other, but not excessively... So I'm beginning to think the temperature is normal.
Hi, yeah that's HOT ! But it must have worked for some time like that 😀
I would be tempted to fit 2 x resistors of half the value in series @ 2W to 3W each. Soldered vertically into the PCB, & then across the tops of them. Do this for both positions. This WILL reduce the temps quite a lot & give better reliability too 🙂
I would be tempted to fit 2 x resistors of half the value in series @ 2W to 3W each. Soldered vertically into the PCB, & then across the tops of them. Do this for both positions. This WILL reduce the temps quite a lot & give better reliability too 🙂
I was trying to avoid using multiple resistors as its only a single sided board. Which is why I went from 2 watt parts to 5watt... Should I try and measure the current draw on one of the 15volt rails, after the resistor? There are also zener diodes next to the resistor but I assume they must be working to have the proper +-15 volt rails.
You have nothing to do if there is 15V on the zeners. Measuring the current draw on the 15V supply is not easy to do, so I don't recommend it.
Good quality power resistors can go up to 175C with high dissipation, so 100C is not a real problem, I guess.
Sajti
125 degrees with the case open at 20 degrees ambient probably means 160+ degrees when in a car... Who designs this crap!
I met burned resistors just like this in Sansui Electronics, an amp and a K7 recorder, and just this week, in a NAD amp. The body of the resistors becomes nearly black but sometimes their value is not modified.
I suggest you mount your two resistors elevated 1-2 cm from the board to avoid to heat it too much and better cooling.
I suggest you mount your two resistors elevated 1-2 cm from the board to avoid to heat it too much and better cooling.
Not a crap. Use proper part there. The Welwyn vitreous enamelled resistors specified up to 300C. So 160C can't be a problem...
The 5W version has 68K/W thermal resistance. This means 92C heating.
Sajti
"Measuring the current draw on the 15V supply is not easy to do,"
I already calculated it in post #9, and gave the permanent fix too.
Keep in mind solder melts at 179°C.
I already calculated it in post #9, and gave the permanent fix too.
Keep in mind solder melts at 179°C.
😕 160 degrees .....Unbelievable, what an over-costcutted crappy design.
Maybe it is possible to feed the opamps +/- 18 or even +/- 20 V. Any Volt less to drop would result in somewhat less heat. If the opamps are suitable for higher voltages you just need to replace the zeners (I would do so anyway as they too got quite some heat). First check all the opamps for what type they are. A schematic would be essential. Replacing the opamps for less power consuming opamps would be OK too. If the original ones are bipolar types the OPA49860 are suitable as they can have +/- 22V max. so +/- 20 to 21 V would work.
Preregs to +/- 30 V would make things better too as heat would be distributed. Too bad the LM317/337 and 78/79xx can't have 43 V at their inputs. Maybe the owner never uses the tone control and can opamps be removed 😛
It is good that you replaced the electrolytic caps near those resistors. They must be dried out with these temps. Maybe the larger ones to the right are also "less than optimal"...
Maybe it is possible to feed the opamps +/- 18 or even +/- 20 V. Any Volt less to drop would result in somewhat less heat. If the opamps are suitable for higher voltages you just need to replace the zeners (I would do so anyway as they too got quite some heat). First check all the opamps for what type they are. A schematic would be essential. Replacing the opamps for less power consuming opamps would be OK too. If the original ones are bipolar types the OPA49860 are suitable as they can have +/- 22V max. so +/- 20 to 21 V would work.
Preregs to +/- 30 V would make things better too as heat would be distributed. Too bad the LM317/337 and 78/79xx can't have 43 V at their inputs. Maybe the owner never uses the tone control and can opamps be removed 😛
It is good that you replaced the electrolytic caps near those resistors. They must be dried out with these temps. Maybe the larger ones to the right are also "less than optimal"...
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