Mixing
What does this part of a circuit with two NPN transistors do?
Clash Royale CLAN TAG#URR8PPP
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My grandmother asked me if I could fix her old portable record player, so I took a look at it and the motor wouldn't spin, so I opened it up and found a schematic inside. Nothing seems bad or broken in the machine, but when I measure the voltage between the motor + and - it's only getting 0.3 volts DC while it should be between 9 and 12 volts.
Upon further testing I noticed that when wiring the motor - directly to ground it works fine, yet in the schematic it goes through a little part of the circuit with two NPN transistors. Why is that part there and what does it do? Why not just wire it directly to ground?
BTW: I already took out and tested both transistors on a multimeter, nothing odd to them
As suggested by Jasen I checked the voltage on R571, and it's 11.09 volts on one side of it and, near to nothing on the other side. It's also a really weird resistor (picture added):
transistors dc-motor
edited 4 mins ago
brhans
8,37921826
asked 4 hours ago
Nook
306
 |Â
show 5 more comments
up vote
2
down vote
favorite
My grandmother asked me if I could fix her old portable record player, so I took a look at it and the motor wouldn't spin, so I opened it up and found a schematic inside. Nothing seems bad or broken in the machine, but when I measure the voltage between the motor + and - it's only getting 0.3 volts DC while it should be between 9 and 12 volts.
Upon further testing I noticed that when wiring the motor - directly to ground it works fine, yet in the schematic it goes through a little part of the circuit with two NPN transistors. Why is that part there and what does it do? Why not just wire it directly to ground?
BTW: I already took out and tested both transistors on a multimeter, nothing odd to them
As suggested by Jasen I checked the voltage on R571, and it's 11.09 volts on one side of it and, near to nothing on the other side. It's also a really weird resistor (picture added):
transistors dc-motor
edited 4 mins ago
brhans
8,37921826
asked 4 hours ago
Nook
306
2
The two transistors are there to control the motor speed. Most likely something wrong in that area if the motor doesn't turn.
– JRE
4 hours ago
1
check that the contacts 491/492 are clean and pass electricity. you may need to move the tone needle end of the tone arm towards the centre of the platter to close the contacts. transformer will be +/- 5% on voltage same as mains
– Jasen
3 hours ago
1
ha! check R571 if that's good measure voltage from ground/negative on pins of TS451 and TS452 when the motor should be running
– Jasen
3 hours ago
1
If you bypass the speed regulator, records will sound awful. Everything will play too fast, especially when using AC power, and the speed will vary with the battery voltage. That circuit is in there for a reason, and you need to figure out what's wrong with it. You've got 9V across the circuit as a whole, but only a small voltage across the motor itself. The rest of the voltage has to appear somewhere (across either TS452 or R571) -- so find out which component it is, and then you can figure out why.
– Dave Tweed♦
3 hours ago
2
I desoldered r571 to check it on the multimeter and got no reading, but noticed that one of the little coil wires wasnt connected so i carefully sanded it clean, soldered to where it looked to have been once, checked it and it reads 6.7... close enough, placed it back on the circuit board and now it works again without jumpering anything so...
– Nook
2 hours ago
 |Â
show 5 more comments
up vote
2
down vote
favorite
up vote
2
down vote
favorite
My grandmother asked me if I could fix her old portable record player, so I took a look at it and the motor wouldn't spin, so I opened it up and found a schematic inside. Nothing seems bad or broken in the machine, but when I measure the voltage between the motor + and - it's only getting 0.3 volts DC while it should be between 9 and 12 volts.
Upon further testing I noticed that when wiring the motor - directly to ground it works fine, yet in the schematic it goes through a little part of the circuit with two NPN transistors. Why is that part there and what does it do? Why not just wire it directly to ground?
BTW: I already took out and tested both transistors on a multimeter, nothing odd to them
As suggested by Jasen I checked the voltage on R571, and it's 11.09 volts on one side of it and, near to nothing on the other side. It's also a really weird resistor (picture added):
transistors dc-motor
edited 4 mins ago
brhans
8,37921826
asked 4 hours ago
Nook
306
My grandmother asked me if I could fix her old portable record player, so I took a look at it and the motor wouldn't spin, so I opened it up and found a schematic inside. Nothing seems bad or broken in the machine, but when I measure the voltage between the motor + and - it's only getting 0.3 volts DC while it should be between 9 and 12 volts.
Upon further testing I noticed that when wiring the motor - directly to ground it works fine, yet in the schematic it goes through a little part of the circuit with two NPN transistors. Why is that part there and what does it do? Why not just wire it directly to ground?
BTW: I already took out and tested both transistors on a multimeter, nothing odd to them
As suggested by Jasen I checked the voltage on R571, and it's 11.09 volts on one side of it and, near to nothing on the other side. It's also a really weird resistor (picture added):
transistors dc-motor
transistors dc-motor
edited 4 mins ago
brhans
8,37921826
asked 4 hours ago
Nook
306
edited 4 mins ago
brhans
8,37921826
asked 4 hours ago
Nook
306
edited 4 mins ago
brhans
8,37921826
edited 4 mins ago
brhans
8,37921826
edited 4 mins ago
brhans
8,37921826
8,37921826
asked 4 hours ago
Nook
306
asked 4 hours ago
Nook
306
asked 4 hours ago
Nook
306
306
2
The two transistors are there to control the motor speed. Most likely something wrong in that area if the motor doesn't turn.
– JRE
4 hours ago
1
check that the contacts 491/492 are clean and pass electricity. you may need to move the tone needle end of the tone arm towards the centre of the platter to close the contacts. transformer will be +/- 5% on voltage same as mains
– Jasen
3 hours ago
1
ha! check R571 if that's good measure voltage from ground/negative on pins of TS451 and TS452 when the motor should be running
– Jasen
3 hours ago
1
If you bypass the speed regulator, records will sound awful. Everything will play too fast, especially when using AC power, and the speed will vary with the battery voltage. That circuit is in there for a reason, and you need to figure out what's wrong with it. You've got 9V across the circuit as a whole, but only a small voltage across the motor itself. The rest of the voltage has to appear somewhere (across either TS452 or R571) -- so find out which component it is, and then you can figure out why.
– Dave Tweed♦
3 hours ago
2
I desoldered r571 to check it on the multimeter and got no reading, but noticed that one of the little coil wires wasnt connected so i carefully sanded it clean, soldered to where it looked to have been once, checked it and it reads 6.7... close enough, placed it back on the circuit board and now it works again without jumpering anything so...
– Nook
2 hours ago
 |Â
show 5 more comments
2
The two transistors are there to control the motor speed. Most likely something wrong in that area if the motor doesn't turn.
– JRE
4 hours ago
1
check that the contacts 491/492 are clean and pass electricity. you may need to move the tone needle end of the tone arm towards the centre of the platter to close the contacts. transformer will be +/- 5% on voltage same as mains
– Jasen
3 hours ago
1
ha! check R571 if that's good measure voltage from ground/negative on pins of TS451 and TS452 when the motor should be running
– Jasen
3 hours ago
1
If you bypass the speed regulator, records will sound awful. Everything will play too fast, especially when using AC power, and the speed will vary with the battery voltage. That circuit is in there for a reason, and you need to figure out what's wrong with it. You've got 9V across the circuit as a whole, but only a small voltage across the motor itself. The rest of the voltage has to appear somewhere (across either TS452 or R571) -- so find out which component it is, and then you can figure out why.
– Dave Tweed♦
3 hours ago
2
I desoldered r571 to check it on the multimeter and got no reading, but noticed that one of the little coil wires wasnt connected so i carefully sanded it clean, soldered to where it looked to have been once, checked it and it reads 6.7... close enough, placed it back on the circuit board and now it works again without jumpering anything so...
– Nook
2 hours ago
2
2
The two transistors are there to control the motor speed. Most likely something wrong in that area if the motor doesn't turn.
– JRE
4 hours ago
The two transistors are there to control the motor speed. Most likely something wrong in that area if the motor doesn't turn.
– JRE
4 hours ago
1
1
check that the contacts 491/492 are clean and pass electricity. you may need to move the tone needle end of the tone arm towards the centre of the platter to close the contacts. transformer will be +/- 5% on voltage same as mains
– Jasen
3 hours ago
check that the contacts 491/492 are clean and pass electricity. you may need to move the tone needle end of the tone arm towards the centre of the platter to close the contacts. transformer will be +/- 5% on voltage same as mains
– Jasen
3 hours ago
1
1
ha! check R571 if that's good measure voltage from ground/negative on pins of TS451 and TS452 when the motor should be running
– Jasen
3 hours ago
ha! check R571 if that's good measure voltage from ground/negative on pins of TS451 and TS452 when the motor should be running
– Jasen
3 hours ago
1
1
If you bypass the speed regulator, records will sound awful. Everything will play too fast, especially when using AC power, and the speed will vary with the battery voltage. That circuit is in there for a reason, and you need to figure out what's wrong with it. You've got 9V across the circuit as a whole, but only a small voltage across the motor itself. The rest of the voltage has to appear somewhere (across either TS452 or R571) -- so find out which component it is, and then you can figure out why.
– Dave Tweed♦
3 hours ago
If you bypass the speed regulator, records will sound awful. Everything will play too fast, especially when using AC power, and the speed will vary with the battery voltage. That circuit is in there for a reason, and you need to figure out what's wrong with it. You've got 9V across the circuit as a whole, but only a small voltage across the motor itself. The rest of the voltage has to appear somewhere (across either TS452 or R571) -- so find out which component it is, and then you can figure out why.
– Dave Tweed♦
3 hours ago
2
2
I desoldered r571 to check it on the multimeter and got no reading, but noticed that one of the little coil wires wasnt connected so i carefully sanded it clean, soldered to where it looked to have been once, checked it and it reads 6.7... close enough, placed it back on the circuit board and now it works again without jumpering anything so...
– Nook
2 hours ago
I desoldered r571 to check it on the multimeter and got no reading, but noticed that one of the little coil wires wasnt connected so i carefully sanded it clean, soldered to where it looked to have been once, checked it and it reads 6.7... close enough, placed it back on the circuit board and now it works again without jumpering anything so...
– Nook
2 hours ago
 |Â
show 5 more comments
1 Answer
1
active
oldest
votes
up vote
4
down vote
accepted
To get constant speed from a PMDC brushed motor, you regulate the back EMF.
To measure this you need to subtract the voltage drop in the motor's internal resistance from the voltage seen on the motor terminals. You can get that voltage by Ohm's law if you know the current through the motor.
R571 measures the motor current. The chain R566-R567-R568 measures the voltage across the motor-R571 combination, and TS451 compares some fraction of the total voltage against R571 + V_be. When the motor voltage is too high TS451 conducts more, and this reduces the current available for TS452 base, which reduces the current it passes.
edited 49 mins ago
Peter Mortensen
1,56131422
answered 2 hours ago
Jasen
8,3791326
An Emitter follower is not a current sense regulator, but the collector drop is a current limiter for TS452 for over current protection (OCP) in case of a motor short. ALL Emitter FOllowers must have an emitter resistance smaller than the load ( Rcoil < <R Motor) The coil R571 pulls down the motor for >nominal speed, lower=faster. The Pot pulls the emitter up to reduce the Motor voltage for a calibratedEMF is a rating of kV/RPM if there was no friction and with slight friction this motor voltage is slightly more to produce the exact voltage to drive at the right RPM e.g. 33 1/3 RPM.
– Tony EE rocketscientist
2 hours ago
So a solder joint was the fault, but the explanation of Theory of Operation was incorrect. No matter. it was fixed and led to a solution
– Tony EE rocketscientist
2 hours ago
like total wrong theory. R571 could be a smaller or bigger value slightly and that current does not get sensed to measure speed, it is the EMF or voltage controlled speed that regulates RPM. Its the collector R that senses over current in TS421 to shutdown or limit rather TS452. So voters who did +1 beware. but nice effort.
– Tony EE rocketscientist
13 mins ago
add a comment |Â
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
4
down vote
accepted
To get constant speed from a PMDC brushed motor, you regulate the back EMF.
To measure this you need to subtract the voltage drop in the motor's internal resistance from the voltage seen on the motor terminals. You can get that voltage by Ohm's law if you know the current through the motor.
R571 measures the motor current. The chain R566-R567-R568 measures the voltage across the motor-R571 combination, and TS451 compares some fraction of the total voltage against R571 + V_be. When the motor voltage is too high TS451 conducts more, and this reduces the current available for TS452 base, which reduces the current it passes.
edited 49 mins ago
Peter Mortensen
1,56131422
answered 2 hours ago
Jasen
8,3791326
An Emitter follower is not a current sense regulator, but the collector drop is a current limiter for TS452 for over current protection (OCP) in case of a motor short. ALL Emitter FOllowers must have an emitter resistance smaller than the load ( Rcoil < <R Motor) The coil R571 pulls down the motor for >nominal speed, lower=faster. The Pot pulls the emitter up to reduce the Motor voltage for a calibratedEMF is a rating of kV/RPM if there was no friction and with slight friction this motor voltage is slightly more to produce the exact voltage to drive at the right RPM e.g. 33 1/3 RPM.
– Tony EE rocketscientist
2 hours ago
So a solder joint was the fault, but the explanation of Theory of Operation was incorrect. No matter. it was fixed and led to a solution
– Tony EE rocketscientist
2 hours ago
like total wrong theory. R571 could be a smaller or bigger value slightly and that current does not get sensed to measure speed, it is the EMF or voltage controlled speed that regulates RPM. Its the collector R that senses over current in TS421 to shutdown or limit rather TS452. So voters who did +1 beware. but nice effort.
– Tony EE rocketscientist
13 mins ago
add a comment |Â
up vote
4
down vote
accepted
To get constant speed from a PMDC brushed motor, you regulate the back EMF.
To measure this you need to subtract the voltage drop in the motor's internal resistance from the voltage seen on the motor terminals. You can get that voltage by Ohm's law if you know the current through the motor.
R571 measures the motor current. The chain R566-R567-R568 measures the voltage across the motor-R571 combination, and TS451 compares some fraction of the total voltage against R571 + V_be. When the motor voltage is too high TS451 conducts more, and this reduces the current available for TS452 base, which reduces the current it passes.
edited 49 mins ago
Peter Mortensen
1,56131422
answered 2 hours ago
Jasen
8,3791326
An Emitter follower is not a current sense regulator, but the collector drop is a current limiter for TS452 for over current protection (OCP) in case of a motor short. ALL Emitter FOllowers must have an emitter resistance smaller than the load ( Rcoil < <R Motor) The coil R571 pulls down the motor for >nominal speed, lower=faster. The Pot pulls the emitter up to reduce the Motor voltage for a calibratedEMF is a rating of kV/RPM if there was no friction and with slight friction this motor voltage is slightly more to produce the exact voltage to drive at the right RPM e.g. 33 1/3 RPM.
– Tony EE rocketscientist
2 hours ago
So a solder joint was the fault, but the explanation of Theory of Operation was incorrect. No matter. it was fixed and led to a solution
– Tony EE rocketscientist
2 hours ago
like total wrong theory. R571 could be a smaller or bigger value slightly and that current does not get sensed to measure speed, it is the EMF or voltage controlled speed that regulates RPM. Its the collector R that senses over current in TS421 to shutdown or limit rather TS452. So voters who did +1 beware. but nice effort.
– Tony EE rocketscientist
13 mins ago
add a comment |Â
up vote
4
down vote
accepted
up vote
4
down vote
accepted
To get constant speed from a PMDC brushed motor, you regulate the back EMF.
To measure this you need to subtract the voltage drop in the motor's internal resistance from the voltage seen on the motor terminals. You can get that voltage by Ohm's law if you know the current through the motor.
R571 measures the motor current. The chain R566-R567-R568 measures the voltage across the motor-R571 combination, and TS451 compares some fraction of the total voltage against R571 + V_be. When the motor voltage is too high TS451 conducts more, and this reduces the current available for TS452 base, which reduces the current it passes.
edited 49 mins ago
Peter Mortensen
1,56131422
answered 2 hours ago
Jasen
8,3791326
To get constant speed from a PMDC brushed motor, you regulate the back EMF.
To measure this you need to subtract the voltage drop in the motor's internal resistance from the voltage seen on the motor terminals. You can get that voltage by Ohm's law if you know the current through the motor.
R571 measures the motor current. The chain R566-R567-R568 measures the voltage across the motor-R571 combination, and TS451 compares some fraction of the total voltage against R571 + V_be. When the motor voltage is too high TS451 conducts more, and this reduces the current available for TS452 base, which reduces the current it passes.
edited 49 mins ago
Peter Mortensen
1,56131422
answered 2 hours ago
Jasen
8,3791326
edited 49 mins ago
Peter Mortensen
1,56131422
edited 49 mins ago
Peter Mortensen
1,56131422
edited 49 mins ago
Peter Mortensen
1,56131422
1,56131422
answered 2 hours ago
Jasen
8,3791326
answered 2 hours ago
Jasen
8,3791326
answered 2 hours ago
Jasen
8,3791326
8,3791326
An Emitter follower is not a current sense regulator, but the collector drop is a current limiter for TS452 for over current protection (OCP) in case of a motor short. ALL Emitter FOllowers must have an emitter resistance smaller than the load ( Rcoil < <R Motor) The coil R571 pulls down the motor for >nominal speed, lower=faster. The Pot pulls the emitter up to reduce the Motor voltage for a calibratedEMF is a rating of kV/RPM if there was no friction and with slight friction this motor voltage is slightly more to produce the exact voltage to drive at the right RPM e.g. 33 1/3 RPM.
– Tony EE rocketscientist
2 hours ago
So a solder joint was the fault, but the explanation of Theory of Operation was incorrect. No matter. it was fixed and led to a solution
– Tony EE rocketscientist
2 hours ago
like total wrong theory. R571 could be a smaller or bigger value slightly and that current does not get sensed to measure speed, it is the EMF or voltage controlled speed that regulates RPM. Its the collector R that senses over current in TS421 to shutdown or limit rather TS452. So voters who did +1 beware. but nice effort.
– Tony EE rocketscientist
13 mins ago
add a comment |Â
An Emitter follower is not a current sense regulator, but the collector drop is a current limiter for TS452 for over current protection (OCP) in case of a motor short. ALL Emitter FOllowers must have an emitter resistance smaller than the load ( Rcoil < <R Motor) The coil R571 pulls down the motor for >nominal speed, lower=faster. The Pot pulls the emitter up to reduce the Motor voltage for a calibratedEMF is a rating of kV/RPM if there was no friction and with slight friction this motor voltage is slightly more to produce the exact voltage to drive at the right RPM e.g. 33 1/3 RPM.
– Tony EE rocketscientist
2 hours ago
So a solder joint was the fault, but the explanation of Theory of Operation was incorrect. No matter. it was fixed and led to a solution
– Tony EE rocketscientist
2 hours ago
like total wrong theory. R571 could be a smaller or bigger value slightly and that current does not get sensed to measure speed, it is the EMF or voltage controlled speed that regulates RPM. Its the collector R that senses over current in TS421 to shutdown or limit rather TS452. So voters who did +1 beware. but nice effort.
– Tony EE rocketscientist
13 mins ago
An Emitter follower is not a current sense regulator, but the collector drop is a current limiter for TS452 for over current protection (OCP) in case of a motor short. ALL Emitter FOllowers must have an emitter resistance smaller than the load ( Rcoil < <R Motor) The coil R571 pulls down the motor for >nominal speed, lower=faster. The Pot pulls the emitter up to reduce the Motor voltage for a calibratedEMF is a rating of kV/RPM if there was no friction and with slight friction this motor voltage is slightly more to produce the exact voltage to drive at the right RPM e.g. 33 1/3 RPM.
– Tony EE rocketscientist
2 hours ago
An Emitter follower is not a current sense regulator, but the collector drop is a current limiter for TS452 for over current protection (OCP) in case of a motor short. ALL Emitter FOllowers must have an emitter resistance smaller than the load ( Rcoil < <R Motor) The coil R571 pulls down the motor for >nominal speed, lower=faster. The Pot pulls the emitter up to reduce the Motor voltage for a calibratedEMF is a rating of kV/RPM if there was no friction and with slight friction this motor voltage is slightly more to produce the exact voltage to drive at the right RPM e.g. 33 1/3 RPM.
– Tony EE rocketscientist
2 hours ago
So a solder joint was the fault, but the explanation of Theory of Operation was incorrect. No matter. it was fixed and led to a solution
– Tony EE rocketscientist
2 hours ago
So a solder joint was the fault, but the explanation of Theory of Operation was incorrect. No matter. it was fixed and led to a solution
– Tony EE rocketscientist
2 hours ago
like total wrong theory. R571 could be a smaller or bigger value slightly and that current does not get sensed to measure speed, it is the EMF or voltage controlled speed that regulates RPM. Its the collector R that senses over current in TS421 to shutdown or limit rather TS452. So voters who did +1 beware. but nice effort.
– Tony EE rocketscientist
13 mins ago
like total wrong theory. R571 could be a smaller or bigger value slightly and that current does not get sensed to measure speed, it is the EMF or voltage controlled speed that regulates RPM. Its the collector R that senses over current in TS421 to shutdown or limit rather TS452. So voters who did +1 beware. but nice effort.
– Tony EE rocketscientist
13 mins ago
add a comment |Â
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2
The two transistors are there to control the motor speed. Most likely something wrong in that area if the motor doesn't turn.
– JRE
4 hours ago
1
check that the contacts 491/492 are clean and pass electricity. you may need to move the tone needle end of the tone arm towards the centre of the platter to close the contacts. transformer will be +/- 5% on voltage same as mains
– Jasen
3 hours ago
1
ha! check R571 if that's good measure voltage from ground/negative on pins of TS451 and TS452 when the motor should be running
– Jasen
3 hours ago
1
If you bypass the speed regulator, records will sound awful. Everything will play too fast, especially when using AC power, and the speed will vary with the battery voltage. That circuit is in there for a reason, and you need to figure out what's wrong with it. You've got 9V across the circuit as a whole, but only a small voltage across the motor itself. The rest of the voltage has to appear somewhere (across either TS452 or R571) -- so find out which component it is, and then you can figure out why.
– Dave Tweed♦
3 hours ago
2
I desoldered r571 to check it on the multimeter and got no reading, but noticed that one of the little coil wires wasnt connected so i carefully sanded it clean, soldered to where it looked to have been once, checked it and it reads 6.7... close enough, placed it back on the circuit board and now it works again without jumpering anything so...
– Nook
2 hours ago