The Sinclair QL Forum

Hi,

I put all units to test last night, they perform good R/W, reading and copying files, even the unit 4, with 96R Erase resistance can R/W good with varying compatibility though.

Format yet to be tested.

I found this chart for ZX MD base and head board, unlike QL, there are two PCBs, as they are detached. There is 92R stated for Erase head here.

Could it be, unless I made a bad measurement, that Sinclair at one stage fitted ZX R/W specific heads in QL for some reason?

With a smaller resistance, could this mean a higher erase current?
Also ULA of Unit 4 appears visually different so perhaps, erase current is compensated here somehow with different resistors used?

Tomas

Hi Tomas,

Further to the erase head resistance: All 8 that I've measured here have a resistance of around 95-100 ohms (including one that had been removed from the head board). Measuring some ZX microdrives also gives the same value. None that I've found measure as high as the 750 ohms. The erase circuit comprises a switched 9V supply (write protect) with a series diode and series resistor (either 180 or 470 ohms dependent on the board revision). This gives a current of around 30mA or 15mA for the 100 ohm erase head and 9mA or 7mA for the 750 ohm erase head (for comparison, the ZX sevice manual gives the ZX erase current as 25mA - calculation gives a figure closer to 23mA). Would seem odd that all these values would give adequate erase performance but maybe I'm missing something?

Charles

Hi Charles,

I am trying to understand my measurement, also it sounded here in the post, that ZX uses different erase current to the QL, so I was lead to believe this is due to a different erase coil used.

Tomas

tcat wrote:Hi Charles,

I am trying to understand my measurement, also it sounded here in the post, that ZX uses different erase current to the QL, so I was lead to believe this is due to a different erase coil used.

Tomas

This will finally solve our questions since 30 years ago

Hi everybody,

I'm trying to sort some slightly unreliable QL microdrives here - I need to know how fast the motor is rotating. Where can I find this program that measures the RPM please?
Thanks!

Hi Ian,

Not sure which motor speed method you're after but I 've used a simplified (if it can get any simpler) version of the strobe described here: http://mehax.blogspot.co.nz/2011/02/ard ... scope.html
There are a few other posts (http://qlforum.co.uk/viewtopic.php?f=2&t=1511&start=20) which cover this as well.

Hope this helps,

Charles

Hi Charles,
that's fantastic, I'll check it out.
Thanks!

Hi,

I have done new tests on all 4 MD units here. Thanks to `mPort' I can test two external drives at the same time.

All four units can read good, so R/ test passed and is alright to me. Unfortunately I was not able to format but only on one unit, the one with erase resistance 96R (Unit#4).
I was also surprised not to be able to write to any of the units#1-3, while I swear I was able to copy files to them a month ago.

I did not measure erase resistance on my QL internal drives yet.

I wish to understand better, and I look for any possible explanations.
Thank you all so far.

Tomas

Hi,

Now I see the reason why MD units#1-3 do not format. The resistance I measure is between points 5,6 of the coil. See picture.
The readings of 760R I got on 5,6 are in one direction/polarity only, and is that on the the diode D12, which is in parallel to the coil. It is hard to admit but erase coils in MD#1-3 are most likely dead

Unit#4 gives reading 96R in one and roughly 100R in the other polarity on the diode D12 - so is GOOD and can format tape which it does happily.

I believe I could breathe a new live into Units#1-3 If I had spare R/W heads, unless ULA has also some problems, which I do not know how measure yet, though R/ tests alright here.

These are common stereo audio heads, I can imagine they must have been produced in thousands, but where to get them now?

Many thanks in advance
Tomas

tcat wrote: These are common stereo audio heads, I can imagine they must have been produced in thousands, but where to get them now?

Tomas,

I am not aware that any of the usual suspects for electronics grocery would still sell audio tape heads new. But if you search eBay for them, there's hundreds of results:

http://www.ebay.de/bhp/tonkopf

Regards,
Tobias