Interesting. 10mv definitely doesn't sound right and neither does the frequency. That is odd though because 500k is to much of an exact round number.. bit of a coincidence kind of thing... which I don't like.
3.9321MHz is a valid and recognised value though so that adds weight that it should be that:
3,9321-HC18: Standard quartz, fundamental, 3.93216 MHz at reichelt elektronik
Are you using a divider probe? A 10:1 probe does have much lower capacitance than a standard 1:1 although an oscillator like this should have no real trouble with a normal 1:1
Its worth looking into this further I think. 10mv is way to low and the frequency doesn't sound right. There are the three tiny caps all of which are slightly non standard values for common parts. 47pF and 120pF are more usual. It might be worth substituting one at a time, just quickly desolder one and tag something else in its place.
The crystal could even be swapped with one a of a close value as a test... why do I think of old TV's and the (NTSC) sub carrier of 3.579545 MHz. Any old analogue CTV would have one in the decoder.
The 800mv pk/pk given in the manual sounds a reasonable value for such an oscillator.
3.9321MHz is a valid and recognised value though so that adds weight that it should be that:
3,9321-HC18: Standard quartz, fundamental, 3.93216 MHz at reichelt elektronik
Are you using a divider probe? A 10:1 probe does have much lower capacitance than a standard 1:1 although an oscillator like this should have no real trouble with a normal 1:1
Its worth looking into this further I think. 10mv is way to low and the frequency doesn't sound right. There are the three tiny caps all of which are slightly non standard values for common parts. 47pF and 120pF are more usual. It might be worth substituting one at a time, just quickly desolder one and tag something else in its place.
The crystal could even be swapped with one a of a close value as a test... why do I think of old TV's and the (NTSC) sub carrier of 3.579545 MHz. Any old analogue CTV would have one in the decoder.
The 800mv pk/pk given in the manual sounds a reasonable value for such an oscillator.
I have 10:1 probes, but I was using the one as 1x for these measurements.
Yes, I was surprised by how much lower the amplitude was than what's spec'd in the service manual. I'll try swapping out those three caps one by one and see what happens.
I wish I weren't old enough to remember (and have owned) old analog TVs. 😉While I don't have one now, I'm sure I can find one in a thrift store if need be.
Yes, I was surprised by how much lower the amplitude was than what's spec'd in the service manual. I'll try swapping out those three caps one by one and see what happens.
I wish I weren't old enough to remember (and have owned) old analog TVs. 😉While I don't have one now, I'm sure I can find one in a thrift store if need be.
I replaced those caps with two 120pf and one 47pf, but no change in the amplitude or frequency of the waveform. I'll start hunting for a crystal.
OK... it is certainly a weird fault.
I do think we are on to something with the oscillator, as those things normally run all the time a chip is powered. I can't really see it could be gated on and off as that just doesn't really make sense.
Crystals can and do fail, I've had it happen and I always used to have some 4.43361875Mhz (lol, you never forget those numbers, the UK PAL colour sub carrier frequency) and 8.866Mhz ones in the toolbox. It's not a common fault though.
There is another engineers trick... the wet finger (honest) pressed and touched around the vital parts of the circuitry. Can you get the oscillator to spark into life doing that, by touching around the oscillator pins?
I do think we are on to something with the oscillator, as those things normally run all the time a chip is powered. I can't really see it could be gated on and off as that just doesn't really make sense.
Crystals can and do fail, I've had it happen and I always used to have some 4.43361875Mhz (lol, you never forget those numbers, the UK PAL colour sub carrier frequency) and 8.866Mhz ones in the toolbox. It's not a common fault though.
There is another engineers trick... the wet finger (honest) pressed and touched around the vital parts of the circuitry. Can you get the oscillator to spark into life doing that, by touching around the oscillator pins?
Normally when someone says "stick your wet finger into this electrical apparatus" I raise an eyebrow. 😉
So just move it around the solder side near the crystal?
Also, is there any way to test the crystal with my frequency counter and an external power supply?
So just move it around the solder side near the crystal?
Also, is there any way to test the crystal with my frequency counter and an external power supply?
The only realistic way to test the crystal would be to place it into a simple oscillator and check if if it runs.
Figure 2 and figure 3 here:
http://www.ti.com/lit/an/szza043/szza043.pdf
If you compare fig 3 with pins 16 and 17 of your chip you will see that the Sony chip oscillator is also just a standard invertor type.
The wet finger can give clues but yes, you only use this on low voltage stuff that is fed from a transformer. Damp rather than wet really but you want it to be a little conductive so that it alters biasing and feedback and so on.
Figure 2 and figure 3 here:
http://www.ti.com/lit/an/szza043/szza043.pdf
If you compare fig 3 with pins 16 and 17 of your chip you will see that the Sony chip oscillator is also just a standard invertor type.
The wet finger can give clues but yes, you only use this on low voltage stuff that is fed from a transformer. Damp rather than wet really but you want it to be a little conductive so that it alters biasing and feedback and so on.
Ok, here's what I see with a damp finger touching one of the pins of the crystal. Certainly an increase in amplitude and frequency.
Interesting. Did you try operating the start/stop button while doing this and seeing if anything moved.
Unfortunately, there's no easy way to simultaneously access the solder side of the board and test for platter rotation. I did just check pin 1 of IC3 with Start pressed, and it is now reading around 12.7V, where before it was zero. And again with Start pressed pin 5 of IC1 is now showing around 4.8V. When I press Stop, that 4.8V briefly switches to 2V before going back to zero.
OK. It had to be tried. So we are still left with the non running oscillator.
(You can always run a finger along the actual pins of the chip from the top side)
(You can always run a finger along the actual pins of the chip from the top side)
The chip is mounted flush, with no pin exposure on the component side.
Try swapping in a different crystal?
Try swapping in a different crystal?
Here are two that are the correct frequency. I'm just not clear on what other parameters need to be considered.
ECS-39-17-1X ECS | Mouser
LFXTAL011220Bulk IQD | Mouser
ECS-39-17-1X ECS | Mouser
LFXTAL011220Bulk IQD | Mouser
I don't think it would matter in an oscillator like this, either would work. Oscillators based around an invertor tend to be 'brute force' designs. Not much finesse in the design but they work and are cost effective.
You realise we are down to trying this as much to eliminate it from suspicion as anything. Yes, they can fail but its not common... and yet we have pretty much exhausted all other possibilities. High frequency crystal oscillators used as clock signals in my experience always run 24/7. I've never seen one 'gated' in a servo circuit, or anywhere else come to that.
You realise we are down to trying this as much to eliminate it from suspicion as anything. Yes, they can fail but its not common... and yet we have pretty much exhausted all other possibilities. High frequency crystal oscillators used as clock signals in my experience always run 24/7. I've never seen one 'gated' in a servo circuit, or anywhere else come to that.
In other words, no guarantee that swapping in a new crystal will fix the problem. Understood. As you say, at least it will be one more possibility to cross off the list.
That's about the long and short of it I'm afraid.
And just looking around there... pin 16 is the inverter input and pin 17 the output. If you connect pin 16 to ground via a say a 1k (it should be OK shorting it to ground tbh... however play safe) then I would expect pin 17 to go high (5V). Same thing applies for connecting it to the 5V rail, the output should now go low (0V).
Its very odd. No contamination around the chip in that area, anything leaked? Don't suppose so 😀 but this is the kind of thing we have to check.
And just looking around there... pin 16 is the inverter input and pin 17 the output. If you connect pin 16 to ground via a say a 1k (it should be OK shorting it to ground tbh... however play safe) then I would expect pin 17 to go high (5V). Same thing applies for connecting it to the 5V rail, the output should now go low (0V).
Its very odd. No contamination around the chip in that area, anything leaked? Don't suppose so 😀 but this is the kind of thing we have to check.
So yep, shorting pin 16 via a 1k to ground caused pin 17 to rise to 5.41V.
I checked the board around the crystal, and no leakage or other strangeness of any kind. I did just go ahead and resolder the crystal.
I checked the board around the crystal, and no leakage or other strangeness of any kind. I did just go ahead and resolder the crystal.
so it could be then.