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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Fri Sep 24, 2021 12:03 pm 


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Climhazard wrote:
Josh128 wrote:
strayan wrote:
You’ve also got a pretty good opportunity to do some blind tests to see if you can tell the difference between s-video and rgb :mrgreen:


Oh crap, I hadnt considered that-- this could very well be an S-video feed. Do you know something we dont? :lol:


Hey Josh, I'm the guy you sold those 2 sets to.
That is indeed just straight s-video from the SNES. Both sets look great!
I've had a chance to do some more testing this morning and I have noticed one issue with the s-video input. Whenever there is a bright flash on screen or a game goes suddenly from a dark screen to a very bright screen the whole screen will flicker, almost like the TV is losing sync for just a fraction of a second.
Here's a short video example.
https://imgur.com/a/prNH9nw
I've tested SNES, PS1, and Sega Saturn so far and it's happened on all of them so it's not a console issue. It also only happens on the s-video input, composite works perfectly fine. I don't currently have anything on hand to test out the VGA input.
I'll try to open her up once I have more time this weekend and look for anything loose but if anyone has a more specific idea on what to look for I would appreciate it. I'm far from an expert on CRTs, especially consumer sets, my experience is limited to doing a few recaps and manual deflection adjustments on some older arcade monitors.


Clim, I was just able to try GC via svideo last night, and used the 240p suite to switch full color screens. Even on the newest model "Ultravision Digital" unit, Im seeing the same behaviour. Its almost like the set loses sync for an instant when switching to full white screens. Maybe the standard Svideo signal is overdriving / clipping the input on the video demux / jungle circuit. My guess would be to add some resistance to the luma / Y input circuit on the TV. Dont know when I'll be able to play with that though.

yatzr - Do you have an svideo capable SNES, GC or DC you could try this same test on your set(s)? Curious to see if this is a universal issue or just affects some units.


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Sat Sep 25, 2021 1:07 am 



Joined: 28 Aug 2021
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Josh128 wrote:
Clim, I was just able to try GC via svideo last night, and used the 240p suite to switch full color screens. Even on the newest model "Ultravision Digital" unit, Im seeing the same behaviour. Its almost like the set loses sync for an instant when switching to full white screens. Maybe the standard Svideo signal is overdriving / clipping the input on the video demux / jungle circuit. My guess would be to add some resistance to the luma / Y input circuit on the TV. Dont know when I'll be able to play with that though.


Oh hey! My first post was finally approved by the mods after 2 weeks! lol
I've pretty much completely switched over to using the VGA input now. I was able to pick up a few Extron IN1508 scaling switchers with VGA out and those work very well. For inputs it's got 2 composite, 2 s-video, 1 component, 2 VGA, and 1 DVI so I can run just about anything I want.
Right now I have my MiSTer running through VGA, PS2 running through component, and Saturn running through s-video with the scaler outputting 480p and an inline VGA scanline generator going to the VGA input on the TV. It takes a little bit of messing with the scaler settings on each input to get things just right but so far everything looks great with pretty much no noticeable input lag as far as I can tell.
It would be interesting to see if that s-video issue is widespread and if it can be fixed. s-video picture quality was very good but as I mentioned in my last email to you the TV seems to use a different deflection circuit for the VGA input since on composite and s-video I have some noticeable warping on the edges of the picture, even after adjusting with the service menu, while on VGA the geometry is almost perfect.
Overall those issues are minor to me since I'll probably only be using it through VGA from now on. These really are fantastic sets.


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Sun Oct 03, 2021 1:23 am 



Joined: 23 May 2013
Posts: 32
I think I'm also just gonna stick to the vga input on mine as well for now. The gbs control does great for my 15kHz stuff, and having the geometry controls via the remote is super handy. Not having to hear the slightest 15kHz whine is a little bonus as well :lol:
I might get some more 15kHz rgb mod ambition later on, but I'll miss those easy access geometry controls.


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Sun Oct 03, 2021 10:04 pm 


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^^
This is my feeling as well. With a fast scaler like the RT5X Pro, even GunCon 2 will work via the VGA port. To my knowledge, the only possible downside of running line doubled +scanline 15KHz stuff via the VGA port is that you have to crank up the brightness a bit vs if you run native 15KHz-- but the set has plenty brightness capability so its not a problem-- and of course you need a scaler like a GBS-C (standalone) or the OSSC or RT2X Pro/5X Pro + a DAC to go to VGA.

On a similar subject, using CRTEmudriver + GroovyMAME, Ive recently tested running 24KHz games with centered timings in a 640x480/31KHz mode and using the controls to stretch the raster to fill the screen, and had perfect results. The end result is you could stick this monitor+chassis in a MAME cab and get pixel perfect 15/24/31KHz games with it. Pretty cool.


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Mon Oct 04, 2021 8:15 am 


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How well do features like the OSSC's pre-ADC gain setting compensate for the loss of brightness with 240p games? I enjoy doing RGB mods, but given the apparent quality of life improvements with these monitors' VGA mode, I think I might end up just running everything through a line doubler. If it's set up properly, it sounds like everything would be pretty seamless.


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Mon Oct 04, 2021 8:32 am 



Joined: 19 Mar 2017
Posts: 617
If you have an Extron RGB interface with boost control would that help compensate?

Edit: Why yes, yes it will https://www.facebook.com/groups/4445602 ... 607332718/


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Mon Oct 04, 2021 12:33 pm 


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How exactly does that extron device work? Does it overdrive/clip the RGB signals to get them brighter or something? If the set is still drawing ~480 lines, half of them black, then how technically would the non-black lines be any brighter than they are when the set is either turned to full brightness OR you set your scaler down from 100% scanlines? Wouldnt this device basically just be doing the same thing as setting a lower scanline darkness %?

It is, after all, still feeding an analog RGB signal to the set. Other than overdriving the signals, which could introduce clipping, Im having trouble understanding what it actually does.


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Mon Oct 04, 2021 6:44 pm 


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Josh128 wrote:
How exactly does that extron device work? Does it overdrive/clip the RGB signals to get them brighter or something? If the set is still drawing ~480 lines, half of them black, then how technically would the non-black lines be any brighter than they are when the set is either turned to full brightness OR you set your scaler down from 100% scanlines? Wouldnt this device basically just be doing the same thing as setting a lower scanline darkness %?

It is, after all, still feeding an analog RGB signal to the set. Other than overdriving the signals, which could introduce clipping, Im having trouble understanding what it actually does.


Pretty sure it's basically just acting as a line driver/gain boost... really moreso meant for long cable runs, but if used with care and subtlety it should help without introducing clipping/blooming et al.

If you can find one for an affordable price, track down an X-Vue Gamma Boost/HDFury GammaX or a X-Vue Box1020. Same Gamma adjustment circuit used in the HDFury Nano GX.
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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Wed Oct 06, 2021 8:10 am 


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Josh128 wrote:
How exactly does that extron device work? Does it overdrive/clip the RGB signals to get them brighter or something? If the set is still drawing ~480 lines, half of them black, then how technically would the non-black lines be any brighter than they are when the set is either turned to full brightness OR you set your scaler down from 100% scanlines? Wouldnt this device basically just be doing the same thing as setting a lower scanline darkness %?

It is, after all, still feeding an analog RGB signal to the set. Other than overdriving the signals, which could introduce clipping, Im having trouble understanding what it actually does.


According to the manual, that function boosts the signal from 0.7v p-p to 0.8v p-p. I've never used that function myself, but I suppose it depends on the monitor whether or not clipping occurs. I'm pretty sure most displays are flexible enough to handle it.


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Sat Oct 09, 2021 6:38 pm 


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edit: this has already been discussed
Spoiler: show
So the way the VGA and 15khz signals are switched is kind of funny.

TV and VGA modes have two different OSD chips, and different RGB paths. On the neckboard I552 is responsible for switching between the VGA and 15k jungle RGB outputs at the final stage. Prior to that, they have their own separate OSD overlays, control chips (for color balance, blanking, etc).

Unfortunately the 15khz jungle does not offer a second unused RGB input for teletype, etc. It has the traditional RGB overlay, so the old-school hack is applicable here.

It might be nice to run the RGB from the VGA port over to the OSD, and use one of the composite video inputs as sync (with a buffer so as to not terminate the TTL sync improperly). That'd let you skip mounting a separate port for RGB. However, despite all the switching circuitry in the monitor, it doesn't present a convenient injection of RGB that will retain the OSD and work as expected.
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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Sat Oct 09, 2021 7:26 pm 


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Im still wondering if double terminating the signals from the VGA port can be done without losing brightness from either without using a switch. If you can manage something that works, with or without a switch, that'd be fabulous. Since trying the initial OSD mux via a separate SCART port and getting a clear but dim signal, I havent tried again. Curious to see what you come up with.

BTW as an update Im almost out of sets. Im keeping two or three for myself and have four or five left after that. Some have some screen dings like yours, I just got a little cerium oxide buffing kit and plan to use with a sub 1000 rpm rotary tool to see if I can help out these small spots. Never tried it before. Please share your results and technique, if you have one. I'd appreciate that.

Also, so far out of all the sets I had 0 DOAs that I couldnt fix, but one has a partial horizontal "parabolic" collapse that gets worse as you brighten the image, and has a static popping sound intermittently. I couldnt pinpoint the issue but suspect the flyback winding is arcing internally or the flyback internal cap is arcing. I swapped mainboards from my unit with the missing phosphor in the center and fixed the issue, but I'd like to replace the flyback and try to fix this mainboard. I cant find the same flyback online, thinking about trying something that has similar pin voltages to see if I can repair it. You have any experience with cross-referencing flybacks?


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Fri Oct 15, 2021 2:00 am 


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Had a unique problem on a set I was reflowing/testing today. Got a strange almost-full horizontal collapse in 15KHz modes, but 31KHz mode worked fine. It would also make a high pitched whining sound when switching to 15K modes.
Image

Looked at the service manual for the 15/31K switching circuits and first tried two of the switches S701 and S702 thinking there might have been some partial/bad contacts, but no luck there. Then tried the transistor Q756 at the end that conducts in 31k but opens at 15k.
Image
Image


Voila-- got the 15KHz modes working again!
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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Mon Oct 18, 2021 1:37 am 


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Man, I am surprised nobody has got a fully-working 15kHz RGB mod goin’ yet (regardless of being on a shared VGA input w/ 31kHz or not)
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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Mon Oct 18, 2021 2:01 am 


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In hindsight, I think my original 15kHz RGB mod would have been brighter had I used the sets own internal blanking signal instead of relying on what comes out of the Genesis. It actually looked damned good, just not quite bright enough. This is the image with brightness and saturation cranked up a bit.

Spoiler: show
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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Thu Oct 21, 2021 4:57 pm 


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I had an issue with the newest set of the bunch, the "Ultravision Digital" one from 1999. Despite a great picture, the set suffered from a very aggressive "breathing" on bright images that caused the image to "squeeze in" from the sides about 1 cm when going from say a full red screen to a full white screen. Also very noticeable when opening mostly white windows in Microsoft Windows.

It really bugged the hell out of me, so the first thing I thought was failing caps or resistors in the power supply and horizontal output areas. Checked a bunch of values vs a set that didnt have the the issue and came up with nothing. Then I changed main power supply regulator and the large cap that is right near the HOT. Again, no change.

The next thing I looked at was the way ABL is handled. ABL is a feedback loop used in modern CRTs to regulate flyback B+ voltage to prevent drastic size changes in the picture when going from dark to bright images. Without it, bright images become larger and dark images become smaller, which is very noticeable. I first changed out the jungle /video board, because the jungle does have an ABL pin on it. The service manual is not very helpful in describing how or if its used though. Changing the board made no difference.

So it turns out that the PA1 SIZE PWB handles a variety of functions, such as horizontal size, position, pin, and ABL. So naturally I swapped it with another sets to see if the problem followed. No change. So I continued looking and saw how they are taking the ABL signal from the flyback and using it to drive a couple of transistors in series with a +12V rail and the primary winding of T771, which causes inductance changes in the secondary winding through which the B+ current flows, thus regulating the B+ coming out of the flyback.

I started looking at what modifications I could do that could potentially affect the ABL agressiveness and the easiest thing to try was to increase the resistance of the circuit that drives the transistors and transformer primary above. R781 is the initial resistor in the ABL signal path on the card, and its a 150R resistor. Normally, increased driving voltage/current happens when images go bright, which makes the secondary of the transformer conduct less and reduce B+, so an increase in resistance should theoretically tone down the ABL changes. I went with a 220R resistor put in series with R781 to start, and to my surprise, the issue was GREATLY improved. It improved so much that the ABL handling looked very much like all the other sets. It may or may not be able to be further improved, I may try a bit more resistance to see if theres any change, but Im pretty satisfied. I wish I could have located the actual culprit causing the issue instead of doing this hack, but it worked so well I decided to post it here, in case anyone else runs across a similar issue on their sets.

Full Size: https://i.imgur.com/ZgyGRtj.jpg
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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Mon Oct 25, 2021 8:45 am 



Joined: 17 Mar 2018
Posts: 231
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Very interesting, thanks for sharing! I have a similar problem on a Bang & Olufsen TV (Beovision 1). Sadly, I don't have a service manual for it and was going to do a "spray and pray" recap one of these days.


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Fri Oct 29, 2021 10:04 pm 


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I quit hoping for the RGB mod when yatzr_ showed me his running 240p through a gbs control and leaving the monitor in PC mode. It looks amazing, has no perceivable lag, awesome picture, damn near perfect geometry. He’s putting a couple together for me and I suggest anyone that scored one of these do the same.


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Fri Oct 29, 2021 10:19 pm 



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I'm going a different road and putting an arcade chasis on it when I test the yoke. Wanted this for my supeegun shmups setup. Beats my current 19 incher.
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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Sat Oct 30, 2021 12:22 am 


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Last of the Mohicans! Just dropped off the final monitors Im selling. These are going to Texas, Oregon, and Washington. Crap dudes this was SO MUCH work I am so glad I am done. My back still hurts from packing the ones you see below. Fingers crossed they all arrive safely to their destinations. I still have 4x Zenith presentation monitors that only do 30fps in VGA mode, so useless for gaming. I may look to RGB mod those eventually and sell them. The Hitachis were the real deal though.
Image

Mike, I dont know if you missed it but I posted the yoke coil inductances earlier in this thread. Variation should be negligible from set to set. Hey if you are 100% set on using an arcade chassis on it, what are you planning to do with the chassis & neckboard? Which chassis are you going to use? If it doesnt work out and you would be interested in another bezel, the set I have with the missing phosphor doesnt need it and I could possibly ship it to you if you would be interested.

Eddie, you are right about the GBS control being perfect for these sets. Im not too worried about the 15KHz mod either because of that. A Retrotink 2X or 5X coupled with an HDMI>VGA converter is even better as the colors are more accurate, the scanlines are more uniform (remember certain colors on the GBS, like blue, dont show scanlines very well for some reason) lag is lower, and even GunCon 2 works. That said, the picture difference is not worth the cost difference, but for someone looking to use the GunCon 2 perhaps it would be worth it.


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Sat Oct 30, 2021 8:04 am 



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Hi Josh,

I am not dead set on using the arcade chassis yet, but it was a workable option since I planned to decase it due to the bezel damage.

I would definitely like a working bezel for it and then would keep it VGA only and use a line doubler. I don't even care what the shipping runs at this point. I also have something you might be interested in as a gift to return the favor.

Actually going to other Mike's tomorrow since I haven't even had the chance to collect it with work being the way it's been.

I know I'm not alone in saying this, but thanks again for doing this. Must have been such a pain in the ass.

I'll be in touch.
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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Sat Oct 30, 2021 9:23 pm 


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Thanks for helping out the crt gaming community Josh! I missed out on buying one of these but happy to see them go to good homes.


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Thu Nov 04, 2021 5:27 am 



Joined: 30 Jun 2020
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Aw man, I'm sad I missed out on these. Though my financial situation has not been the best so probably wouldn't have been a good idea for me to buy. either way, thank you for putting in all this time and effort to get these amazing displays to the community. Hopefully we can find more of these in the wild! I saw a listing for one not too far from me a while back from the city government but I was mere hours too late to get it. :(


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Thu Nov 04, 2021 6:19 pm 


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Location: TX
Image

Image

Image

Image

Image

Photographing CRTs is hard.

This is a fantastic set. It has very sharp S-Video input, and the CVBS input is the cleanest that I have seen.

I started testing this with my GBS-C, but I was unsatisfied with the image being produced. GBS-C sucks at scanlines. The photographs embedded in this post are of the output from my OSSC in 2x mode with 100% black scanlines. Super sharp and just a fantastic look. I can use this to make Garou:MOTW on the Dreamcast look the way that it should. Native 480p games are unbelievable on this bad boy, but they are more difficult for my phone camera to focus on so I do not have good 480p images.

This is easily the most desirable videogaming monitor that I have ever come across, and I am now selling my old primary monitors.

Thank you, Josh, again, from Texas


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Fri Nov 05, 2021 12:28 am 


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^

Cool man, glad you like it! Your pics almost look like they would be of a high TVL SVGA monitor , while in person the set looks more like what you'd see in a Model 2 or Model 3 cab. Its refreshing seeing a slot mask set after playing mostly with aperture grill and shadow mask CRT for the last decade or so.


I find S-video is really sharp on it as well. Personally, I found the GBS-C to look really good aside from the standard scanline variations you get with different colors (like blue, which for some reason has no scanlines). But yeah, IQ wise the digital scalers will give more accurate scanlines due to the GBS-Cs strange handling on certain colors.


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Sat Nov 13, 2021 11:12 am 


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Thanks again to Josh for taking the time to save these monitors! He was kind enough to send 2 of them to me over a distance of 4000 miles (mostly over sea) and they made it in great shape. These things are awesome - basically a 31khz arcade monitor in a consumer grade shell. This is the perfect monitor to play Xbox 360 shmups - the Cave ports all render at 640x480 and are absolutely perfect on this TV. Dreamcast games are beautiful and look just like an old Naomi cabinet. I've been so distracted by playing 480p games that I haven't gotten around to trying out the 15khz S-video mode yet.

I did have some issues with one of the TVs, however. After a few days of normal operation, it wouldn't power on and you could hear an audible ticking sound from the power supply. It turned out the HOT was shorted. So, during the time it took to order and receive a replacement, I opened up both TVs and thoroughly inspected the chassis and neck boards for bad solder joints. I did find a few that needed to be reflowed, although I couldn't tell if it made a difference or not. Fortunately, replacing the HOT solved the problem and it's been fine since then. Apparently, a shorted HOT puts the power supply in overcurrent protection mode and it doesn't even provide standby power.

The part number for the HOT listed in the service manual, however, is wrong. It should be 2SC5124.

I also found that on both neck boards, the cathode voltage filter cap was bad (C859). One of them had very high ESR, while the other had leaked and tested at 0 capacitance. Replacing it appeared to improve the picture slightly. I expect there will be more faulty caps to be found on the main chassis.

There's a limited number of these TVs out there, and given their age it's inevitable that they'll start to fail and will need repair. I think any of us who encounter maintenance issues should document them in this thread. Also, if anybody has a unit that can't be fixed, PLEASE save the chassis before throwing it out! Some of the parts aren't available any more and we need to keep them available for the community if possible.


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Sat Dec 04, 2021 9:46 am 



Joined: 08 Dec 2011
Posts: 33
I am probably picking this tv up. I have gscartsw and an ossc and a gbs-control aio
https://www.retroupgrades.co.uk/product ... 01%20frame.

I am all rgb and scart with my consoles. Is there anything else i need to use this tv to get 15hz 240p?

Ill also be using a dreamcast and I have the scart cable from retro access for it but is there anything else ill need?


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Sat Dec 04, 2021 2:18 pm 


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A GBS-C that has SCART input or an OSSC+ HDMI to VGA dongle is all you need. Where are you getting the TV from?


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Sat Dec 04, 2021 10:31 pm 



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through a friend i believe its coming out of state. I am excited i wanna try some of my arcade boards on it.


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Sat Dec 04, 2021 10:46 pm 


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8bitForLife wrote:
Ill also be using a dreamcast and I have the scart cable from retro access for it but is there anything else ill need?


I'd recommend against using the Retro Access SCART cable. This TV doesn't like composite sync - you'd be better off using a VGA box or cable. A cheap Dreamcast VGA cable would probably work fine.


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 Post subject: Re: Hitachi 27" 27MM20B Dual-Scan CRT TV??
PostPosted: Sun Dec 05, 2021 2:21 am 


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Agree with Matt on the DC VGA, thats the way to go on this TV. Curiously, Ive had no problems using composite sync with CRTEmudriver on it. One thing about this TV is that bright flashes of white on S-Video will ALWAYS cost a brief sync loss. Happens on every set Ive tried it on. Think some resistance on the luma input might help, but I havent tested it. That aside, S-Video looks amazing on these sets.


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