Any way to improve the screen brightness of an MZ80B

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parc
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Joined: Wed Mar 20, 2019 8:29 am

Any way to improve the screen brightness of an MZ80B

Post by parc »

The screen of my "B" is pretty dark and the 80 character mode is not really useable. Is there any way to improve the screen brightness of an MZ80B ?
psmart
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Re: Any way to improve the screen brightness of an MZ80B

Post by psmart »

I recently decided to replace all the capacitors on my MZ-80A screen as the screen size was getting vertically smaller and couldnt be compensated with the vertical size/linearity (any attempt just started scolling). After completion, screen is back to full size, as per photo below and brightness is more luminance and crisp.

Currently repairing another MZ-2000 monitor where vertical drive has gone, replaced the parts but now have a very interesting scenario when I have mirror image and only half a screen - the signals dont look right, more digital than analogue, so either a duff cap or resistor as the transistors look to be working! wip!
F1 - 1 (1).png
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psmart
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Re: Any way to improve the screen brightness of an MZ80B

Post by psmart »

Bit more info, the 80B monitor is identical to the 80A and 2000.

Try adjusting the brightness (R2008 - variable resistor) but if it goes too bright and you see the flyback lines then back it off until the main screen is dark. Try also adjusting the contract, this makes the characters brighter in general, (R2002 - variable resistor). If neither cure your issue then suggest capacitor replacement.
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parc
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Re: Any way to improve the screen brightness of an MZ80B

Post by parc »

Ok, both resistor at their maximum - and I still do not see the flyback-lines.

Do you really mean, to replace ALL capacitors ? Best way to do it ? Replace one and check - then next ? Any special candidates to replace first ?
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psmart
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Re: Any way to improve the screen brightness of an MZ80B

Post by psmart »

That is strange to not see the flyback lines, I have contrast turned to max and brightness about half way, any more and the flyback lines appear.

I made the decision to replace all given that electrolytics only have a limited lifespan as they dry out and most sources limit their useable life around 15 years. There is also a tantalum 1uF 35V which cries out to be replaced given a lot are going short circuit. The monitor runs quite hot so this will exacerbate the issue. It is also a lot of work to get the monitor out and separate the PCB to work with it on the bench so it made sense to change them all at the same time. I had another MZ-80A and the sound didn't work, all the signs showed it was the LA4200 Amplifier which I replaced and no change. I then took out the 3 electrolytics and one was open circuit, replaced and all ran smoothly.

The only other thing I can think of, not being a CRT expert is the gun/heater element or the phospor has aged. Both are not really replaceable so hoping it is the driver circuits. If you look at this link https://original.sharpmz.org/mz-80b/dow ... 80b_sm.pdf page 22-25 and the circuit on page 44-45, probe the test points (the ringed numbers on the circuit diagram correlate to the test points on page 23-25). I would look at test points 2 (Contrast) and 3 (Brightness) first to see if they have the correct amplitude. Also check the 12V supply line from the PSU as this could affect it. If they look ok then check the other test points such as 10,11,12,21,22,23,24 which drive the CRT but be careful, 21-24 are high voltage.

The capacitor list is as follows:
Quantity Value Voltage Detail Source
2 1000uF 16V Box
1 100uF 16V RS
1 10uF 250V RS
1 10uF 25V non-polarised RS
2 10uF 16V RS
1 1uF 35V Tantalum RS
1 1uF 250V RS
1 2200uF 16V RS
1 220uF 10V Stock
1 220uF 16V RS
1 220uF 100V RS
3 22uF 16V Stock
1 3.3uF 25V Stock
1 33uF 25V Stock
2 4.7Uf 25V Stock
1 4.7uF 50V Stock
1 4.7uF 16V Stock
1 4700uF 16V RS
1 470uF 16V Stock
1 47uF 50V Stock
1 470uF 16V Stock
1 100uF 10V Stock
1 4700uF 25V RS
2 2200uF 10V Stock
1 33uF 35V RS
1 3300uF 35V RS

If you decide to change them and dont have any, especially the ones marked RS, let me know and I can send them to you. You have to buy in bulk packs, normally 10 - 100, I decided it was worth it as I will replace all the caps as standard now on any 80A/80B/2000 which comes my way.
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psmart
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Re: Any way to improve the screen brightness of an MZ80B

Post by psmart »

Almost as if I tempted fate, I was using my MZ-80A and the screen, horizontally, started compressing followed by a bang!

On investigation it is C2017, the 10uF/25V bi-polar electroytic on the horizontal deflection circuit that has popped. These were Panasonic parts from RS but much smaller in size than the original. Looking at the circuit diagram the deflection coil is driven by 83vPP but without further tests I assume the voltage drop at is sufficient to reduce the voltage to below 25V, if not then there is a misprint in all the circuit diagrams or the bipolar capacitors from 40 years ago are more tolerant of spikes. There is no spec on the capacitor such as ripple current or ESR so will have to go with a higher voltage - so dont replace this one yet!!!
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psmart
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Re: Any way to improve the screen brightness of an MZ80B

Post by psmart »

Just to close the above, after studying a lot of articles, it is a common issue with Bipolar parts made today compared with 40years ago. Panasonic, Nichicon etc are all the same, small, higher (than vintage parts) ESR and susceptible to ripple currents. Nichicon do make some lower ESR parts but the best solution is to keep the original (desolder and test, it should be around 9-11uF) and re-install, alternatively use a polyester 10uF capacitor.

Managed to repair the 2000 monitor, time base issue with open circuit dry joint. The monitor is very bright in 80column mode so hopefully you have managed to resolve your brightness issue as it is the same monitor.
Sharp MZ/X Series Projects and FPGA Emulator
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