6
\$\begingroup\$

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

Enter image description here

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):

Enter image description here

\$\endgroup\$
  • 2
    \$\begingroup\$ The two transistors are there to control the motor speed. Most likely something wrong in that area if the motor doesn't turn. \$\endgroup\$ – JRE Sep 29 '18 at 10:33
  • 1
    \$\begingroup\$ 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 \$\endgroup\$ – Jasen Sep 29 '18 at 11:05
  • 2
    \$\begingroup\$ ha! check R571 if that's good measure voltage from ground/negative on pins of TS451 and TS452 when the motor should be running \$\endgroup\$ – Jasen Sep 29 '18 at 11:15
  • 2
    \$\begingroup\$ R571 is there so that the driver circuit can sense the current going through the motor. With 11 volts across 4.8 Ohms it should be getting very, very hot - if not, it's defective so replace it. \$\endgroup\$ – Finbarr Sep 29 '18 at 11:47
  • 4
    \$\begingroup\$ 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... \$\endgroup\$ – Nook Sep 29 '18 at 12:03
5
\$\begingroup\$

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.

\$\endgroup\$
  • 1
    \$\begingroup\$ 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. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 29 '18 at 12:29
  • 2
    \$\begingroup\$ 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 \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 29 '18 at 12:31
  • \$\begingroup\$ 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. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 29 '18 at 14:24
  • \$\begingroup\$ do you read me? Capiche? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 29 '18 at 19:29
  • \$\begingroup\$ I dodn't visit this site every day... TS451 it's not an emitter follower, the emitter is the non-inverting input the base the inverting input and the collector the output. TS452 is an emitter follower \$\endgroup\$ – Jasen Sep 30 '18 at 4:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.