RL02 Tracking

[Aaron Jackson]

On 22 December 2020 at 22:17 GMT, shadoooo via cctalk wrote:
>> The ready lamp flashes not when the servo burst is >visible, but when the
> heads are just before it.
>> Why? Well, I probably set the gain too high on the read/write module.
>
> Hello,
> I'm sure you read the manual, however I add some explanation to be sure.
> The best head position is not where a servo track is at maximum amplitude
> (head is exactly centered to a servo track), but where you read two servo
> tracks with the same amplitude (so head is exactly between two servo
> tracks, in middle position).
> Comparative measurement of two servo tracks allow the servo control to
> understand the position of the head in respect to data track.
> If the best position for data track is not where servo have same amplitude
> probably the head is misaligned or the spring support of the head bent /
> deformed.
> To analyze head circuits you need a good oscilloscope, you should be able
> to see burst of servo tracks and data tracks too, with two channels you can
> understand if analog to digital threshold / pulses signal conversions do
> work as expected.
> Time ago I fixed an RL02 having a malfunctioning head amplifier circuit.
> The gain was too low. When I increased it rotating variable resistor, the
> circuit begun to ring (barely auto-oscillate), so was nearly unstable. Data
> and servo signals were corrupted, but this was visible only zooming on
> oscilloscope after careful trigger alignment.
> I don't remember exactly what I did, but some capacitors needed
> replacement, then I tuned head gain while loading a platter to the best
> position for operation, maybe slightly lower than manual recommendations.
> Then it worked perfectly.
>
> Andrea

Hi Andrea,

Thanks for this. My understanding is that the S1 and S2 servo bursts are
passed through a comparator and then integrated. When the head is
directly over the target track, S1 (becomes E1 after integration) is a
perfect saw tooth, while E2 'saw tooths' away from 0V.

After performing the amplitude adjustment, the potentiometer was very
close to max, and I was getting around 650mV from the cartridge, but the
noise floor was quite high. The manual also says that the outer guard
track should not have an amplitude greater than 2.5V. Presumably the
amplitude on the platter is higher the further out the track is, perhaps
due to the amount of space available given 40 sectors per track.

With this amplitude set as mentioned, loading the cartridge results in
this head unpredictable oscillating behaviour, but as I mentioned, the
ready light only comes on briefly, and only when the heads are before
the outer guard. Basically, as far as they could be off the platter
before being lifted by the ramp.

Reducing the amplitude slightly prevents this oscillating behaviour, but
the ready lamp does not come on at all. Increasing it to the very max
causes very extreme oscillation, sometimes aggressively unloading the
heads.

I happen to have three of the read/write modules, and they all exhibit
this behaviour. Not to say that caps don't need to be changed, but it
seems unlikely that all of them would need this.

I have a fairly decent scope and have been able to do some very nice
captures of the servo bursts. They are not perfectly sinusoidal but I
think close enough given the age of the heads.

I'll go through the caps at some point anyway to double check. Looking
at the schematic I'm guessing they are either ceramic or tants.

thanks again,
Aaron


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