Microdrive boards

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Dave
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Microdrive boards

Postby Dave » Mon Feb 05, 2018 8:40 pm

I'm taking expressions of interest in replacement microdrive boards.

In 2013 I designed a replacement microdrive PCB that is much less prone to interference from the video hardware on the QL. I'm now doing a revised version as I move the design files to a new tool. It uses all SMD components except the ULA, and all components are moved to the back of the PCB behind a large ground plane.

It requires people sent me a microdrive unit with working head and ULA. I remove these parts and place them on the new PCB, machine the plastic frame slightly to accommodate the new component positions and mail them back. I then tune the motor speed to ensure maximum formatted capacity while retaining pre-written microdrive compatibility. The boards are a very slightly modified board based off the Samsung SG18 microdrive unit used in the very latest production runs. These are the gold standard of microdrives.

The price would be £30 for one or £50 for two microdrive units, including PCBs, rework, head alignment, set-up and return shipping. The ribbon cable is fitted with turned pins, which makes fitting to the QL much easier too. You get your original PCB back too.

If I get expressions of interest equaling 20 microdrives, I'll order the PCBs. If you express an interest, you're not committed to buy and your boards would be freed up for someone else. If I get interest for 10-19 units, I may have to raise the price by around £5 per unit.

If you're interested, say how many units you'd be interested in...

CURRENT INTEREST: 14 units
Last edited by Dave on Fri Mar 16, 2018 3:57 pm, edited 5 times in total.


martyn_hill
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Re: Microdrive boards

Postby martyn_hill » Mon Feb 05, 2018 8:49 pm

Hi Dave

Given my perverse fascination with our beloved MDVs, I can't help myself but to put my name down for 4 units, please :-)

M.


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Re: Microdrive boards

Postby RWAP » Mon Feb 05, 2018 8:58 pm

Because of shipping to / from the USA and potential charges etc, is there no source for replacement microdrive heads?

I have plenty of ULAs if you need those rather than de-soldering them from an existing microdrive unit.

I might have an interest in some of these , but when a microdrive is not working, I am never sure if it is the ULA or the read/write head - I don't do electronics :o

I have 3 spare microdrive units here, but they are completely untested so not worth sending them to be improved in case they are non-working in any event.


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Re: Microdrive boards

Postby martyn_hill » Mon Feb 05, 2018 9:26 pm

I was thinking something similar - but having researched replacement heads, came up blank.

Seems a lot of extra hassle with the return shipping thing, but very happy for Dave to have my business!


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Dave
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Re: Microdrive boards

Postby Dave » Mon Feb 05, 2018 10:53 pm

I'd take in untested or not working units if they also come with a new or known working ULA and the head has no or very light signs of wear. I have six spare heads. It would balance out across the run. If there were a HUGE demand, I can get the combo ERP heads from a specialty company, but they're expensive.


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Re: Microdrive boards

Postby Dave » Mon Feb 05, 2018 11:08 pm

martyn_hill wrote:I was thinking something similar - but having researched replacement heads, came up blank.


Most of the original heads are ok. If they show a wear band, they almost always can be re-lapped once. Wear only becomes a problem when it starts to bend the tape and force an air gap.

Re-lapping is a polishing process where the head is held against a VERY fine belt sander with a computer controlled tension and speed to create the exact curve. Then the re-shaped head is polished to a very high sheen against a rigid surface (normally glass).

I might not be able to order the heads, but I can definitely make the information of which exact supplier and model number they were once I identify them.


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Re: Microdrive boards

Postby martyn_hill » Tue Feb 06, 2018 11:23 am

Hi Dave - it's reassuring to hear that the heads are less-likely to be the cause of faults (beyond contact-wear.)

Measurements of faulty MDV units during my efforts to reproduce a legible set of signal traces (ala Appendix D5 of the Service Manual - see another thread) indicated that the first stage of pre-amplification from the ULA (pins 4 & 5, 14 & 15) showed problems, which left either or both the ULA or tape-head as suspect.

As we have an apparently reasonably supply of replacement MDV ULAs (I was pleasantly surprised by Rich's inventory!), plus your potential revved board gives hope for our beloved (or much malligned, depending on point of view!) MDVs.


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Dave
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Re: Microdrive boards

Postby Dave » Tue Feb 06, 2018 5:23 pm

The ULA often fails. All heads are different, and have different impedances. Impedances on AC signals are complex. If Sinclair had thought of it more as an antenna and done a little matching, quality and reliability would have been a lot higher. Often with these heads they used, the impedance ended up being on the lower end of the range, allowing higher current flows that heated the IC.

Of the 10 microdrive units I examined here, three had one or the other side of the RP head be much lower impedance (though I only have equipment to test DC impedance not AC impedance) and those always had the hottest 2G007s, of which two had failed.

My board will have a zero ohm link that can be substituted to a higher value to prevent that.

The other obscure but common problem I saw was that two of the microdrive units had erase tab detect switches that contained enough tarnish to cause poor contact. Both poor contacts were between pins 1 and 3, which is the position that enables the erase circuit. This caused improper erase function.


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Re: Microdrive boards

Postby Starfrog » Tue Feb 06, 2018 7:04 pm

Interested in 2


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