RL02 Tracking

Aaron Jackson Aaron.Jackson at nottingham.ac.uk
Tue Dec 22 07:38:25 CST 2020

On 22 December 2020 at 10:33 GMT, Aaron Jackson via cctalk wrote:
> On 22 December 2020 at 08:01 GMT, Christian Corti via cctalk wrote:
>> On Mon, 21 Dec 2020, Aaron Jackson wrote:
>>> The status currently is that the heads will load, and the ready lamp
>>> flashes as the heads wobble back and forth very slightly, trying to lock
>>> onto the outer servo guard band. Probing TP2 of the read/write module, I
>>> can see the S1 servo burst flash (roughly in time with the ready
>>> lamp). If I disconnect power to the servo motor, I can manually move the
>>> head onto the outer guard band (less than a mm away) and monitoring the
>>> position signal (TP15 on the drive logic module) shows this to be close
>>> to 0. So, I'm very confused.
>> I've had almost the identical problem with a RL02 drive from a friend. It
>> would start and load the heads, but the ready light would flicker or not
>> even come on. After a measuring session I discovered that if I manually
>> select the other head, everything would be fine. And if I disabled the
>> servo motor I could manually lock onto the track on side 0, but then side
>> 1 was out of alignment. So the alignment between the two heads was wrong
>> and I had then aligned one of the heads so that the two would lock onto
>> their corresponding track. The drive works fine since then.
> That's very interesting to know. Thanks! I have already performed the
> head alignment and they looked well aligned when switching between the
> two heads, but I didn't have any luck getting it to lock onto the first
> track with the head select jumper in. I'll mess about with this some
> more, it is at least reassuring that someone elses drive has displayed
> similar behaviour.

Alright been playing around with that for an hour or so. I think I'm
beginning to understand what has happened.

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.

... Why did I set the gain too high?  Probably both my cartridges are
weak and to compensate for this, I set the read/write module's amplitude
too high during the "Read Signal Amplitude Adjustment" in Chapter 3 of
the tech manual.

I think having this amplitude too high amplifies the noise too much,
which makes the drive believe it's found the first track, but is
struggling to read the servo bursts because they're not actually there
there, hence the slight head oscillation.

Does this sound like a reasonable theory?

If so, I suppose now might be a reasonable time to ask the list if
anyone in the UK has a known *good* cartridge which they'd mind selling
me for a sensible price? I know there aren't too many of them floating
about the UK these days.

Thanks everyone who has given me some pointers so far


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