MCI JH24 Bias Oscillator Crystal

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tardishead

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Aug 11, 2004
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I tried to test record only to discover that the bias oscillator was missing a load of parts including the 6.72MHZ crystal.
Do I need exactly this frequency or will anything close do? I can find 6.5536MHz at RS
 
I presume you get the closest xtal and use the trimmer cap to dial in the frequency but at what cost. Do you get less amplitude in the 210khz sine? Does that cause problems?
 
Without having looked at the schematic, it seems the bias frequency of 210 KHz is a result of dividing the crystal frequency by 32. That would mean you will get a frequency of 204.8 KHz when using a 6.5536 MHz crystal. Not a very big difference. (2.5%)
Most tape machines use a free running oscillator and one can question how stable the frequency of that type of oscillator is.
I don't foresee any problems.
 
It has a 10pf trimmer cap as well so I presume you can get closer to 210khz
But does the amplitude of the sine drop? Is it best to get the frequency  with the biggest amplitude?
 
tardishead said:
It has a 10pf trimmer cap as well so I presume you can get closer to 210khz
Frequency accuracy is not essential; however, remember there are many tuned LC's in the record/repro electronics, that will need to be readjusted when aligning the machine. Bias traps have generally enough variation range to allow tuning.

But does the amplitude of the sine drop?
It should drop slightly because the HF coil will be slightly out of tune.

  Is it best to get the frequency  with the biggest amplitude?
The issue is finding the point where the distortion of the bias signal is lowest. The measurement is not a direct one; it is done by adjusting for lowest recording noise.
 
tardishead said:
It has a 10pf trimmer cap as well so I presume you can get closer to 210khz
Don't expect that you can detune a crystal oscillator much more than 1 KHz!
(Any attemps to detune it more will usually cause the oscillator to stop.
 

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Right I am getting a better picture.
Thanks for posting the schematic.
So looking at the circuit - the trimmer allows you to decrease the frequency a little (more capacitance) but not by much you were saying - until it stops.
 
tardishead said:
Right I am getting a better picture.
Thanks for posting the schematic.
So looking at the circuit - the trimmer allows you to decrease the frequency a little (more capacitance) but not by much you were saying - until it stops.
I have a feeling this capacitor is not really used to tune frequency, because, as Ruud mentions, its range would be minimal; I'd rather think it allows waveform tuning.
 
tardishead said:
You mean adjust for best sine shape?
The signal from the crystal oscillator may not be a pure sine, but I suspect it may influence the way it triggers the divider.
EDIT: having read the datasheet for the timebase generator, it appears that the oscillator is a textbook inverter-based oscillator. The trimmer is used to adjust the feedback ratio, making sure the oscillation starts unconditionally whilst ensuring best stability (too much OLG in an oscillator results in frequency drift).
 
The datasheet says the Xtal must be cut for a 10pFd load instead of the common 30pFd load.
 

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tardishead said:
This is all a bit over my head.
Any suggestions as to how to proceed?
As I wrote, tune it for best S/N ratio; since bias amplitude is stabilized by the CMOS divider, there is no gain variation in the record process, so you just have to record a low-level low-frequency tone and listen to the noise.
 
The RS Components crystal https://uk.rs-online.com/web/p/crystal-units/8141730/
says for a load of 12pF, therefore it should be fine.
 
While waiting for a crystal I discovered I had a 6mhz crystal
I tried it in the circuit.
Getting oscillation now but channels do not erase when I press record.
6mhz divided by 32 is 187.5khz. Is this too far from 210khz to see any effect at all? I was hoping to see at least partial erasing.
How tight are the tuning circuits for erase and record generally?
 
tardishead said:
While waiting for a crystal I discovered I had a 6mhz crystal
I tried it in the circuit.
Getting oscillation now but channels do not erase when I press record.
6mhz divided by 32 is 187.5khz. Is this too far from 210khz to see any effect at all? I was hoping to see at least partial erasing.
How tight are the tuning circuits for erase and record generally?
It's difficult to tell, but I would think they are pretty selective, since they are the last protection against waveform distortion. I would not expect the range of adjustment to be wide enough to allow retuning so far apart.
 

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