Hi all,
My Yamaha TC-520 Cassette Deck appears to have a power transformer problem with the voltages supplying power to the single DC motor.
Voltages measured directly off the soldered wires (black & red) from the back of the DC motor are reading around 11.3 to 11.5 volts and the motor is labeled as a 12 volt device. When measured directly off the transformer secondaries I'm also seeing about 11.5 VCA
I have a schematic, but I'm not sure if I'm reading it right as it looks to me as the voltages from the transformer yellow lead should be around 14 volts leading to the DC drive motor. The secondaries have 5 wires coming out which are 2 yellow, 1 orange, 1 brown, and 1 red.
I will post the schematic below which is taken directly from the last page of the original instruction manual. I have had this deck for more than 35 years and the only things I have ever needed to do was replace the belts 3 times and Deoxit the potentiometers and switches.
Any help would be greatly appreciated.
My Yamaha TC-520 Cassette Deck appears to have a power transformer problem with the voltages supplying power to the single DC motor.
Voltages measured directly off the soldered wires (black & red) from the back of the DC motor are reading around 11.3 to 11.5 volts and the motor is labeled as a 12 volt device. When measured directly off the transformer secondaries I'm also seeing about 11.5 VCA
I have a schematic, but I'm not sure if I'm reading it right as it looks to me as the voltages from the transformer yellow lead should be around 14 volts leading to the DC drive motor. The secondaries have 5 wires coming out which are 2 yellow, 1 orange, 1 brown, and 1 red.
I will post the schematic below which is taken directly from the last page of the original instruction manual. I have had this deck for more than 35 years and the only things I have ever needed to do was replace the belts 3 times and Deoxit the potentiometers and switches.
Any help would be greatly appreciated.
The AC voltage from the transformer will be lower than the DC voltage you get from the rectifier. To generate a nominal 12 volts DC requires about 8 to 9 volts AC. The diode bridge charges the reservoir cap (C312) to the peak of the AC voltage. The rms voltage that your meter reads is the peak value divided by root 2
What is the actual problem with the machine?
What is the actual problem with the machine?
Hi Mooly,
The tapes being played have a significant amount of pitch variation and the tape playback speed seems off.
I tried to adjust the single DC motor for more speed and when it's adjusted it then just starts the whole pitch variation all over again. Endlessly readjusting for either more speed and then having to readjust back again. Unit has brand new drive belts.
Nothing is binding and it was recently lubed at all the recommended points.
The DC voltage on the motor solder joints (red & black) used to read about 12.20 volts DC and now read about 11.3 VDC.
Also when this problem first appeared over the weekend I noticed a heavy burning smell from the unit like something was overheated. Took the cover off and could trace the source of the smell to the transformer.
It is possible the motor itself has an issue and is drawing to much current, or there could be mechanical issues causing it to do that.
If the voltage is lower than it used to be then all things being equal that suggests more current is being drawn. The voltage across C312 will be higher when no current is drawn and will fall as the supply is loaded.
One test you could do is to put your meter onto a DC current range (use something like a 1 amp setting) and connect the meter in series with either lead going to the motor and see what the current is. So you would have to disconnect one lead to do that.
The current should be pretty low, I'm guessing perhaps 100 milliamps sort of region. It should be very low if you slip the belt off the motor and run the motor with no load.
The diode bridge and cap C312 should be cool or cold when the motor is not running.
If the voltage is lower than it used to be then all things being equal that suggests more current is being drawn. The voltage across C312 will be higher when no current is drawn and will fall as the supply is loaded.
One test you could do is to put your meter onto a DC current range (use something like a 1 amp setting) and connect the meter in series with either lead going to the motor and see what the current is. So you would have to disconnect one lead to do that.
The current should be pretty low, I'm guessing perhaps 100 milliamps sort of region. It should be very low if you slip the belt off the motor and run the motor with no load.
The diode bridge and cap C312 should be cool or cold when the motor is not running.
Mooly,
I can certainly test the current draw by de soldering or maybe just cutting the lead from the motor as it's very easy to access when I get home from work later this evening.
I take it that this is the procedure you recommend as the starting point in trying to trouble shoot this and the transformer is not relative to the playback issue?
Is there a test for the transformer in order to rule that out also or is the diode bridge and cap C312 more or less being cool determining that.
I could also take some pictures of the motor, transformer; and the power supply area around C312 if that helps.
Well ideally you would use an oscilloscope to look at the voltage across C312 and check there was not excess ripple and so on but I made the assumption, maybe wrongly, that you perhaps have not got access to one 🙂
That would also show if a diode were open circuit... but those are least likely problems really. The cap is a possible problem area, easily proved just by bridging it with another.
The only easy test for the transformer is to ensure it runs at a normal temperature and that the AC voltage it puts out is correct. You can also check the DC voltage is correctly related to the AC transformer voltage by multiplying the AC voltage you read (say 9 volts) by 1.4 to derive the DC value you see across C312. The diode losses should be accounted for as well (about 1.2 volt in total).
So if you measure 9 volts AC and you should read very approximately around 11 volts DC. When the motor is not running the AC and DC values will be higher.
I'll look in tomorrow, good luck.
That would also show if a diode were open circuit... but those are least likely problems really. The cap is a possible problem area, easily proved just by bridging it with another.
The only easy test for the transformer is to ensure it runs at a normal temperature and that the AC voltage it puts out is correct. You can also check the DC voltage is correctly related to the AC transformer voltage by multiplying the AC voltage you read (say 9 volts) by 1.4 to derive the DC value you see across C312. The diode losses should be accounted for as well (about 1.2 volt in total).
So if you measure 9 volts AC and you should read very approximately around 11 volts DC. When the motor is not running the AC and DC values will be higher.
I'll look in tomorrow, good luck.
Oil the motor as well..dry motors pull too much current.
Sewing machine oil or similar works.
And check the mechanical parts for sticking, if it had been out of use.
Do a clean and lube, there are spindles, rollers, slides, so many things.
All gone with memory stick players...
Sewing machine oil or similar works.
And check the mechanical parts for sticking, if it had been out of use.
Do a clean and lube, there are spindles, rollers, slides, so many things.
All gone with memory stick players...
Hi Korpberget,
Pinch roller is in excellent shape as it was replaced around 4 or 5 years ago along with three new belts.
Hi Mooly,
Voltage at C312 is 14.23 volts with just the power on and it's 11. 52 when a tape is in and playing.
Voltage at the DC motor is still about 11.3 to 11.5.
My digital temperature reads C312 around 83-F and TR313 around 87-F
So if I'm understanding you correctly with your first reply that the diode bridge is being feed from the yellow lead off the transformer; and the transformer is not supplying the 14 volts directly to the motor, but charges C312 to the voltage value 14.23 at idle and that feeds the 12 volt Dc motor.
The motor when play is engaged then draws down the voltage from 14v to somewhere around the 12 volts.
So this confirms that transformer is fine then.
Should I still put my meter in series to the motor or is this enough info to determine the motor is failing or weak?
And thanks for your help guys.
Lube it first. Could be a sticky motor.
Check all motions by hand.
Lube the pin that drives the pinch roller.
And let the oil soak.
Had that issue on my dot matrix printer, took several doses of oil, and I did not use it much. Only choice was a scrap motor, as maker had stopped that model. So I kept oiling it, and letting it soak the brass bushes.
It was a stepper motor for the platen, I was not confident about opening it.
After a week of idling it decided to work properly.
Motors are fairly cheap and easy to get, Mabuchi had a huge variety, no worries.
If you can, open it, lube it, check the commutator and brushes (stiff wire sometimes as brushes), and put a tiny amount of grease in the brass or sintered bushes.
Put light oil on the shaft, and fit it back.
Most are not intended to be serviced, take care.
Check all motions by hand.
Lube the pin that drives the pinch roller.
And let the oil soak.
Had that issue on my dot matrix printer, took several doses of oil, and I did not use it much. Only choice was a scrap motor, as maker had stopped that model. So I kept oiling it, and letting it soak the brass bushes.
It was a stepper motor for the platen, I was not confident about opening it.
After a week of idling it decided to work properly.
Motors are fairly cheap and easy to get, Mabuchi had a huge variety, no worries.
If you can, open it, lube it, check the commutator and brushes (stiff wire sometimes as brushes), and put a tiny amount of grease in the brass or sintered bushes.
Put light oil on the shaft, and fit it back.
Most are not intended to be serviced, take care.
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Hi Naresh,
I'm not seeing any real way to service this DC motor with light oil.
The rest of the deck was gone through mechanically and serviced regularly over the years.
Yes, digital storage devices are the rage, but I still like my analog devices as long as they are working right...
That's right. You need to understand the relationship between what your meter measures on an AC voltage range and what the actual voltage looks like in real time.
If the AC voltage at the left (from the transformer) reads 9 volts AC on the meter then that is an rms value. The peak voltage is from the centre 'zero' line to either the upper or lower peak and the peak to peak voltage is from top to bottom.
So 9 volts AC on the meter is actually 9*1.414 volts peak which is 12.7 volts. The 1.414 is the square root of 2.
So the diode bridge charges the cap to this peak voltage and that is the DC voltage you read across the cap. The negative half cycle also doe the same because of the four diode bridge. Two diode 'volt drops' have to be overcome before the bridge conducts on each half cycle and these are around 0.65 volts per diode. So the 9 volts AC doesn't quite give the calculated DC but its a little lower.
The transformer is small and has modest regulation. That means when you draw current (motor runs) the AC voltage falls and with it the DC.
It would be useful to see the motor current. With the belt removed the current draw should be very low and constant.
Just an educated guess, but if you've used the deck for many years it's possible (likely even) that the brushes have worn out.
Unfortunately it's unlikely you'll get one from the manufacturers, but there are a lot of similar ones available on 'thebay'.
Hopefully you will be able to find one that's a close enough match.
Unfortunately it's unlikely you'll get one from the manufacturers, but there are a lot of similar ones available on 'thebay'.
Hopefully you will be able to find one that's a close enough match.
Hi Mooly, and the rest of the helpers here.
Okay, I was able to work on the deck this afternoon and un-soldered the red power lead to the motor.
I then hooked-up my meter in series like this---> red power wire feeding the motor to an alligator clip on the one end of the lead that's connected to my meter "com" port and the other lead is connected to my mA port on my meter. The lead connected to my mA port is then placed to the spot on the motor where I just un-soldered the red power wire from.
Set my meter to 200m and then pressed the play button on the deck and the meter read "OL"
Tried the 20m on the meter and the meter also read "OL"
Tried 2m and the meter also read "OL"
I'm not getting any reading in milliamps.
Am I doing something wrong with my connections or is my meter not capable of reading the milliamps of the DC voltage.
Picture below is from my meter.
Attachments
OL is overload. You have possibly blown the fuse in the meter (I can see it says 500ma fuse on the front). If it has blown then 500 ma current is way over the top and either the motor or a mechanical overload is present. Never go lower on the meter range if you see an OL reading, you need to go higher and that would be the 10A range.
So first check the meter. Just pop the cover off and check the fuse. The meter may or may not work correctly on other ranges with a blown fuse.
Now do as Naresh suggests and we will use the 10A range. Be sure to set the meter to 20m and we now use the COM and 10A sockets and check the current. If it is high then try again after slipping the belt off. The motor alone with no mechanical load should draw very little current, perhaps as low as 25 milliamps.
So first check the meter. Just pop the cover off and check the fuse. The meter may or may not work correctly on other ranges with a blown fuse.
Now do as Naresh suggests and we will use the 10A range. Be sure to set the meter to 20m and we now use the COM and 10A sockets and check the current. If it is high then try again after slipping the belt off. The motor alone with no mechanical load should draw very little current, perhaps as low as 25 milliamps.
Okay, I had to have the slide switch in the AC position.
With the belt on and play button engaged without a tape in the deck and the meter on the 20m position I got a reading of 51.0 ma.
With the belt off the reading was bouncing from 36.0 to 68.0 ma.
With the belt on and play button engaged without a tape in the deck and the meter on the 20m position I got a reading of 51.0 ma.
With the belt off the reading was bouncing from 36.0 to 68.0 ma.
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Disregard the readings above as they are not correct.
With the belt on and play button engaged without a tape in the deck and the meter on the 20m position I got a reading of 19.2 to 20.8 ma.
With a tape in it was bouncing from 19.7 to 22.6
With the belt off the reading was bouncing from 0.35 to 0.78 ma.
With the belt on and play button engaged without a tape in the deck and the meter on the 20m position I got a reading of 19.2 to 20.8 ma.
With a tape in it was bouncing from 19.7 to 22.6
With the belt off the reading was bouncing from 0.35 to 0.78 ma.