Microdrive Unit Design

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tcat
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Re: Microdrive Unit Design

Postby tcat » Mon Dec 04, 2017 8:36 pm

c) The presence or absence of the foil screen appears to affect reliability in either slot #1 or #2 (i.e. reliability improved with a foil-screen in both positions.) NB - in my QLs, I've switched-out the linear 5V regulators for various designs of switched-regulator - these may well generate more noise near slot #2.

Applying documented mod, may also rectify noise in slot#2, as the power is then led on the copper side, not disturbing MDV2 ULA, right beneath. While I think ULA noise protection in slot#1, acts as shield against noise from TV modulator?

Tomas


Dave
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Re: Microdrive Unit Design

Postby Dave » Mon Dec 04, 2017 11:33 pm

Buck type regulators are electrically much less noisy than the 7805 in the QL, which puts out a huge spike of noise through its ground plate (heatsink!) at around 75-200 KHz.... Generally, the buck regulators switch at 500-600KHz but radiate far less (9-12dB less) than a 7805.

A lot of noise is induced into the cable connecting it to the QL, but it's small compared to the signal. However, a lot is also induced into the head itself, and that is the main area of concern. None of the heads I have seen have been shielded internally.


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Pr0f
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Re: Microdrive Unit Design

Postby Pr0f » Tue Dec 05, 2017 7:19 am

I suspect some of that noise on the old 7805 regulator could be reduced by soldering a couple of smallish value smd's across the vin-gnd and gnd-vout pins on the regulator, right by the regulator body, as it's got a about 1-1.5 inches of wire to radiate from as well, and any bypass caps are quite some way from the regulator chip itself. I used that technique in an audio preamp that had similar noise issues, and it worked far better than I had hoped.


martyn_hill
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Re: Microdrive Unit Design

Postby martyn_hill » Tue Dec 05, 2017 11:16 am

Dave wrote:A lot of noise is induced into the cable connecting it to the QL, but it's small compared to the signal.


Hi Dave - were you referring in this line to the 2x 7-way cables connecting the MDV to the mainboard, or to the voltage reg 3-way connector?

However, a lot is also induced into the head itself, and that is the main area of concern. None of the heads I have seen have been shielded internally.


Should any shielding be grounded, or would another foil-shield like that used around the ULA (and electrically isolated) by helpful fitted immediately behind the tape-head? How do you go about actually measuring the impact of shielding like this?

M.


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Re: Microdrive Unit Design

Postby Dave » Tue Dec 05, 2017 4:02 pm

martyn_hill wrote:
Dave wrote:A lot of noise is induced into the cable connecting it to the QL, but it's small compared to the signal.


Hi Dave - were you referring in this line to the 2x 7-way cables connecting the MDV to the mainboard, or to the voltage reg 3-way connector?


The lesser amount of noise that is problematic to microdrives comes from the 7805 - the larger amount comes from the MC1377 thru the RGB port and also back to the J1 connector. 7805 noise can be reduced by two electrolytics just soldered to the 7805's legs.

For a comprehensive noise reduction program you would:

Sources:
Remove the MC1377 and use other video output ;)
Replace the 7805 with a buck regulator that has large electrolytic caps on the buck reg board
Add a large electrolytic cap across +5v and GND on the 8301, two places in DRAM, 8049, and near J1

Receivers:
Shield the ribbon cables that go from the QL to the microdrive boards, best way is replace the individual supplied gaskets with a single gasket that covers the back of both units, well grounded at both ends that covers the cables too.

Should any shielding be grounded, or would another foil-shield like that used around the ULA (and electrically isolated) by helpful fitted immediately behind the tape-head? How do you go about actually measuring the impact of shielding like this?


I just measure with an oscilloscope, which is a flawed method as attaching the probe can change the antenna properties of the part being monitored. However, it is useful for like for like comparisons and I can get a relative dB reduction level accurately.


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Re: Microdrive Unit Design

Postby martyn_hill » Tue Dec 05, 2017 4:49 pm

Thanks Big-D!

Dave wrote:Sources:
Remove the MC1377 and use other video output ;)


I typically use the RBGVH lines (via SCART), so I guess I can safely remove the noisy MC1377, right?

Add a large electrolytic cap across +5v and GND on the 8301, two places in DRAM, 8049, and near J1


By 'large', is that in the 22uF range or greater? Would a 16v electrolytic be safe enough for those 5V lines, without getting too big?

Receivers:
Shield the ribbon cables that go from the QL to the microdrive boards, best way is replace the individual supplied gaskets with a single gasket that covers the back of both units, well grounded at both ends that covers the cables too.


Hummm - I'll take a look to see how that might physically fit. Thanks!


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Re: Microdrive Unit Design

Postby Dave » Tue Dec 05, 2017 5:43 pm

martyn_hill wrote:I typically use the RBGVH lines (via SCART), so I guess I can safely remove the noisy MC1377, right?


I'd go over 22v for the capacitor on the input->ground side and >10v for the +5V->GND side. Capacitor sizing in this area is an art, as all QLs have aged slightly differently, so whatever you have that's a decent capacity will help. As long as you're using voltage suitable parts, having the 'wrong' farad value wouldn't hurt, just help differently.


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Pr0f
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Re: Microdrive Unit Design

Postby Pr0f » Tue Dec 05, 2017 8:05 pm

larger as in >10uF caps will get rid of ripple and lower frequency noise, but unless you go with low ESR types, higher frequency noise needs smaller caps with a low ESR to filter that out. Consider also, that larger caps have more foil area in the electrodes, and this increases inductance within the capacitor itself and acts against the capacitance, making them less effective filters. Caps designed for Switch mode supplies are ideal, but consider bypassing with a 10-100nF ceramic for higher frequency and RF suppression.


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Re: Microdrive Unit Design

Postby Dave » Tue Dec 05, 2017 8:31 pm

Or just get a buck regulator from Tetroid... Mine are better, but not worth the shipping costs to the EU.



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