illini
The Past is Alive
There's a coil in the motor that generates a signal as the motor turns.
That looks like the PWM output, not the feedback coil voltage.
Is this speed variation normal?
- Thread starter justinis
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That looks like the PWM output, not the feedback coil voltage.
Do you have a schematic for this thing? Make sure there aren't any of the evil VD1212s or something similar in the drive controller.
Get a strobe app for your phone. It's much easier than trying to use a lamp.
If it were PWM then the period should be constant with only the percentage of on/off time varying.
Since you have already been down the replace the capacitor road this just might be a case of an IC becoming unstable. I briefly looked through the service manual you posted to Drop Box and noticed there was no troubleshooting guide. Unfortunately my experience has been when there is no troubleshooting guide and the adjustments don't do it the tech is expected to simply replace the board(s). I don't think I can help more that that from here.
illini
The Past is Alive
That waveform is the PWM output; the duty cycle is being adjusted to approximate the 20hz coil signal.
You may be right and I agree the signal is representing a rhythmic oscillation of the platter speed. It could be a from of Variable Frequency PWM, but if I remember VF-PWM has either a constant on time or constant off time component. I just don't see either in the graph shown, but there are a lot of ways to skin that cat and if I'm wrong I apologize. In any event he's done all of the easier fixes without avail. I'm still afraid he is down to either replacing boards or trying to find other replacement components.
illini
The Past is Alive
I hope not, for his sake. I'm still not convinced his table has a speed problem.
justinis
Active Member
The schematic is on page 50 of the service manual I linked to. I have the ap-207 (no quartz drive). Thanks for the tip on the VD1212s. I can't find any. All the diodes look like standard flavors I'm used to.
Sorry, let me be clear. The feedback coil gets fed through an opamp to amplify it, and then to a digital logic chip (quad XOR). The graph I posed is the output of the first gate of the logic chip. I just posted that because it's easier to understand than the weird sine wave coming from the amplifier. My scope isn't good enough to capture the tiny signal coming from the actual coil. If you're looking at the schematic, my screenshot is from probing pin 3 on IC2.
Thanks! I don't have a record to produce the frequencies to use an app with. I should get one.
That would be a bummer, but I don't think it's impossible. A lot the board's components are still in production. I could replace every single one if I had to. I'd love to not have to go that route though, especially if there's no guarantee I won't get results.
I hope you're right! Here's why I think I do though:
1. The service manual says the coil frequency should be 20hz for 33rpm and 27hz for 45 rpm. It is neither, and changes over time as seen in the graph I posted.
2. The long periods where the period widens cause a HUGE amount of current to be supplied to the motor, so much so that the power supply voltage dips by up to 4V (it's normally 9.6VDC). Surely this can't be by design.
3. There are voltages drawn over the schematic at different points, and almost all of them are wrong when I measure on my board. I don't know if these voltages are AC/DC/RMS, so it's possible I'm just misinterpreting the schematic, but some of them are so off (such as the drive to the hall effects sensors, which measures 2V on my board but is 0.1V on the schematic) they just can't be right.
Many, many thanks to everyone who has offered advice!!
justinis
Active Member
Hi! Sorry for not updating this thread back in 2015. Eventually I discovered that the weird oscillations and voltages I was seeing were due to the fact that I made all my measurements with the platter removed. This caused the stabilization feedback loop to go haywire. Once I put the platter back on, everything made sense.
However, I could never completely get rid of the speed fluctuations seen in the original video. I tried everything I could think of: cleaned all the pots, replaced all the caps, ran all the procedures in the service manual, replaced an IC I was suspicious of. After a while I concluded that the table just didn't have great speed control. The AP-207 does not have quartz lock like the AP-307. I briefly considered trying to upgrade my 207 with parts from a 307, but ultimately I just decided to give the table away and buy a quartz-locked model. I ended up buying a Technics SL-1301 which I have been happy with. Speed is rock solid. I'll never buy another DD table that isn't quartz locked.
The AP-207 speed variations may not bother you, but they were audible to me. If they do bother you, I'm sorry to say that to the best of my knowledge the only way to get rid of them is to get rid of the table.
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^^ this