Remember when computer monitors were modified TVs?

Let's take a look at this Zenith ZVM-121 from 1982. This is back from the days when many computer monitors were just modified television sets.
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[@adriansdigitalbasement2]

For the vertical size, I changed out R614, replacing the 1.2meg resistor with a 300k. This gave the size potentiometer much more range. I noticed on the Zenith ZVM-122/123 that Zenith used an identical circuit design to the ZVM-121, but used a 1.0meg resistor for R614, so that confirmed a smaller value here would give more range. I first tried a 1.0meg but it wasn't enough range, only a little more... And the next smaller size I had in stock was 300k so I threw that in, and it worked. At extreme deflection there is some amount of rollover, as the amplifier for vertical deflection is likely being pushed to the limit (think distortion on an audio amp, clipping) so I can now adjust it to the maximum deflection before it starts to "clip" or rollover, and it nearly fills the entire CRT now. Much better than it was before. Edit: lots of comments on B+, and I didn't mention I also checked B+ and it is spot on, 12.7v. I did try increasing it slightly and observed no increase in deflection. I think the monitor is working exactly as designed.

[@ya837]

I think the monitor does not have enough supply voltage so it cannot generate enough high voltage and good deflection and focus. Can you please test if the power supply voltage is correct?

[@briangoldberg4439]

@@ya837 that's what i was thinking. good first step is to check all the important voltages

[@adriansdigitalbasement2]

​@@ya837yes it's spot on, 12.7v. I did test it but forgot to mention it.

[@KlodFather]

@@adriansdigitalbasement2 - The Radio Shack board in the TRS80 monitor uses an optoisolator chip. Nothing more complicated than that. I reverse engineered the board many many years ago and looked it up in an IC master which said it was an optoisolator. Those typically had 5kv isolation on those chips. And yes it was an RCA TV because I found one at a garage sale and used it for parts to repair the RadioShack monitor :)

[@eDoc2020]

​@@adriansdigitalbasement2 The main B+ is OK but the HV is specced at 12kV. I suspect all the flyback-derived voltages are low, including the 110V which biases the vertical output. I'd suggest checking the caps near the flyback, especially C517.

[@WilliamCohn]

I was part of the Monochrome TV group as an electrical engineer at Zenith. One of my projects was to modify the current 12 inch B&W to be a computer monitor (ZVM121). The TV chassis model number was 12MB1X. We later made monitors for the computer industry for use in computer terminals.

[@vwestlife]

The TRS-80's original monitor (a rebranded RCA XL-100) was hot chassis, and instead of using a transformer, they added an opto-isolator on the video input.

[@shmehfleh3115]

Yarg! Good to know.

[@greendryerlint]

It also seemed to be about the lowest resolution of all of the early monitors. We used to joke it was a 10x10 pixel display.

[@naikrovek]

black and white tubes have such great clarity with things like 80-column text because there's no shadow mask in front of the phosphors. straight shot from the electron gun to the phosphors.

[@datadude67]

Plus we have to break from our modern 'fixed grid of pixels' thinking to embrace the analogue geometry of triangles and cones. As the monitor gets smaller, because the phosphors are much tinier than the resolution of the source data, the apparent (and actual) resolution gets better.

[@jimbotron70]

Also the ones with ember and green phosphors.

[@rickedwards6150]

When I was an enlisted programmer in the USAF in the late 80s, Zenith had the contract to supply the PCs we used. They came with a monochrome monitor even though every one I ever saw was ordered with a CGA monitor as an add-on. There must’ve been a hundred of those monitors sitting in a storage room having never been taken out of the box.

[@annieworroll4373]

Wonder what the odds are they're still there, forgotten about? I know the government does sell off surplus property like that eventually, but there's no way an organization that big doesn't have some warehouses and closets full of old junk policy says they should get rid of but they simply forgot about.

[@simonscott1121]

Being a digital guy, I really appreciate these analog focused videos. My dad used to do TV repair etc and was a radio/analog guy, and digital always confused him. For me, it's the exact opposite. I can't even visualise what inductance is :D

[@rarbiart]

Adrian, in Europe hot chassis were the standard up to the late 1970ies/early80ies. it started to change not only for composite input, but for headphone jacks. it was probably cheaper to make a proper switching power supply with full separation than to run the audio over transformers. For the style of "repurposed OEM chassis": I remember your TRS80/Tandy monitor in Germany as well, running on (at the time) 220V. And for the OEM chassis construction: the ORION branded TV going as the Amstrad Color screen in 1984: it even had a low resolution tube with vertical "dots", like on those camping TVs.

[@CasualSpud]

I remember saving my paper route money to get a 13 inch colour tv for my Ti99/4a... holy crap I'm old.

[@JamesPotts]

I remember I could hear those Zeniths from across the room in my high school's Apple II lab, if they were left on without the computer running.

[@davidprocopio9021]

1982: Used a tv with a RF modulator. 1985-2015: Dedicated computer monitors. 2016 - present: Back to using a TV (55" 4k). Things come full circle!

[@UserUser-zc6fx]

Same. I'm using a 43" Vizio as my monitor.

[@argvminusone]

Can it display 80-column text now? 😂

[@michelleshaw337]

Sort of - TVs became monitors. Interesting note: Sony's Trinitron tubes were some of the nicest high resolution displays for colour monitors. Expensive - but damn they were nice to work with.

[@JenniferinIllinois]

Me too (well only 1080p TV) and boy does Flight Simulator II look incredible. 🤣🤣🤣,

[@UserUser-zc6fx]

@@michelleshaw337 They look awesome. I've got one of the 17" late 90s Vaio monitors and use it on a 486 and it looks amazing.

[@starquake48]

Very interesting monitor. Never Zenith before...

[@jaimeramoscarranco]

Jajaja 😂 that's a good one

[@jonorgames6596]

..opens ze door...

[@PeteWord]

I love the spooky basement CRT tour with phosphor color surprises! Another great video. Thanks Adrian!

[@wun1gee]

I love all of your content but anything you do with a TRS-80 I share with my old boss. I worked for a Radio Shack right out of high school (2001-ish) and the owner was a TRS-80 guru. He holds the pattent for the first TRS-80 dial-up modem. He gets such a huge kick out of seeing anything TRS-80. He says it's a huge, wonderful trek down memory lane. Keep it up!

[@andyhu9542]

Note that green CRTs run on lower anode voltage than white ones, so the 8kV reading may be correct.

[@daw7563]

We definately had live chassies in Europe with 230V back in the day, don’t ask me how I know 😊

[@wimwiddershins]

When I finally got a dedicated monitor for my C64 it was almost as great as getting the computer itself. Finally, I could use the computer whenever I wanted to. The "monitor" was a crappy old TV, but I didn't care.

[@GuukanKitsune]

And now it has come full circle because TVs are now modified computer monitors.

[@MindCaged]

Just the comment I was about to make, modern tvs are just computer monitors with specialized-limited computers, especially smart tvs. Unfortunately that means they don't turn on/off nearly instantly anymore, they have a boot-up/shutdown.

[@argvminusone]

​@@MindCaged There was a time when you could also use a computer monitor as a TV, by installing a TV tuner in your computer. Dunno if that's still a thing, though.

[@abysspegasusgaming]

@@argvminusone PCI-E cards exist of TV tuners, so it can still very much be done. Most of them are BNC and advertised as capture cards, though they are also quite pricey.

[@AOClaus]

​@argvminusone yeah, I had one in the very late 90s in a Pentium.

[@TransCanadaPhil]

@@argvminusone Around 2000/2001 or so I had one of those ATI “All in Wonder” cards on my computer. It was an ATI branded video card that also let you connect standard analog cable television or an antenna directly into a coax connector on the back of the card. it came with windows television viewing software so you could tune in your local cable channels, record, etc either in a window or full screen alongside your other desktop applications. Worked great, and it was amazing especially back then when video on desktop PCs wasn’t common; there was no such thing as video streaming yet. I’d sit on my PC and use IRC in a chat window while my local TV channels played in another window. Very high tech for the time and my friends were amazed I could watch standard analog Cable TV on my computer (not streaming). I remember I had the 2000 Sydney Olympics playing on the TV window from the local CBC tv station while chatting away with friends on IRC on the same monitor and this was all the way back in 2000; full frame rate analog video playing right overtop my Windows 98 desktop along with other apps without a hitch and since it was just analog video it took almost no system resources to do.

[@rivimey]

The massive advantage of mono CRT screens is there is almost no "resolution" inherent in the screen design. Although the circuits will have a bandwidth and there will be other constraints the CRT has no "dot". Conversely the older colour tubes have a very specific dot, so they had an ideal resolution...

[@tramadol42]

My uncle's monitor (bought from one of the few Tandy dealers in West Berlin in the early 80s) looked even more like a TV. I remember that you could see through the grilles where the (non-existent) loudspeaker was placed into the housing and see something glowing inside it. I can also remember that the housing of this thing was always statically charged. As children, we thought it was funny because it tingled and our hair stood up when we ran our fingers over the casing. Looking back: a miracle that nothing happened to anyone

[@unnamedchannel1237]

Something wasn’t earthed correctly 😅

[@mikaelanuzzo4051]

Wow, that is the exact display I had hooked up to the Apple ][+ I used as a kid in the mid-1980s. Thanks for delivering a wonderful hit of nostalgia. 😊

[@evensgrey]

In the late 70's, my family not only had a TRS-80 Model I, we also happened to have the model of TV that was adapted into the monitor for the TRS-80. You have the controls exactly right.

[@TransCanadaPhil]

Yes I used a little 14 inch zenith tv with my Commodore Vic 20. It only had the two screw type terminals so you had to use one of those 300 to 75 ohm matching transformer adapters along with an RF switch box. In fact, later on I also used a TV for my Commodore Amiga 500 as late as 1991/92. Nice thing about using a TV as a monitor in those days is it was easy to switch back and forth between using it with the computer or just to watch TV. Just flip the RF switch box back to regular Cable tv to watch tv on the same set in my bedroom as a kid.

[@CATech1138]

"unbal" not matching transformer...the difference?...coax has a grounded shield and the signal is only on the center wire making for an unbalanced connection....twin lead has a balanced signal on both wires....the term for the device that converts the signal from unbalanced to balanced is unbal...yay, right?

[@TransCanadaPhil]

@@CATech1138 ahh ok, memory is a little rusty. All I remember is that our TVs back in the early to mid 80s used to only have those 2 screw type connectors, not even an RCA composite, and certainly not S-Video or component (that was much later) with a rotary dial channel selector. You had to use that coax adapter to the two screw connector thing-a-ma-Bob to connect the cable tv and any game consoles/computers to the TV. What a revelation around 1987/88 when we got a TV with actual RCA composite inputs for the first time; used that to connect the NES. Then I got a TV (Sony Trinitron) in 1995 that had S-Video inputs and that felt so advanced. Got a TV with component input around 2002 for the first time. Then my first TV with HDMI in 2007.

[@PeterBellefleur]

As a kid in the 80s, my parents had the exact model of this that was the consumer b/w TV model in their bedroom. I remember playing our 2600 on it a ton because I wasn't allowed to use the "good" TV in the living room too much as there was this prevailing urban myth that too much video game playing would somehow hurt the TV. I always wondered if that was just made up to keep us from playing video games all day. But you are 100% right. There was a rabbit ear mount right above where the antenna input is. Ours was also molded in black plastic instead of the beige, but the exact same molds were used.

[@tcscomment]

as long as the signals are correct and in spec (especially VSync/HSync), there's no way a console/computer could ever damage a CRT screen.

[@unnamedchannel1237]

@@tcscommentwell it could . Some games have menus that don’t move a logo of some sort : these could stay on the screen long enough to cause “burn in” which is essentially when the screen is always showing a shadow of what ever was left on the screen. Or Maybe the kid puts game on pause . Goes outside gets distracted leaves TV on pause for hours on end. They is why Computers have screen savers they had a practical reason not just for pretty sake like these days .

[@tcscomment]

@@unnamedchannel1237 ah, I forgot about that.

[@PeterBellefleur]

@@unnamedchannel1237 Well in those days, games didn't have pause. :)

[@nickwallette6201]

^ This is exactly right. Back in the NES era, you would get a warning in the booklet that use on projection TVs was not recommended due to potential screen burn-in from static displays. Back then, projection displays were just three high-output CRTs shot through a color lens, bounced against a mirror, and onto the back of a diffraction lens. (There were also direct-projection TVs that used three CRTs shining directly onto a screen.) This wasn't SUCH an issue with normal direct-view CRTs, unless you left your game console on 24/7 for weeks at a time. :-) I think it was just ignorance that it made the rounds as a problem with video game consoles on any given TV.

[@toddr104]

The Zenith N121A looks like the TV version of that monitor.

[@Alcarods]

The Zenith N12xx series are all nearly identical!

[@JamieStuff]

Regarding the TRS-80 monitor: It's believed to be a minimally modified RCA AU 121W TV. In fact, it's said that Tandy chose the silver and black color scheme to match that of the TV. And yes, if you remove the cover with the TRS-80 logo, the tuner holes are there. (At least they were with my monitor.)

[@VintageTechFan]

German here, our TVs were also almost all hot chassis, only small ones which were portable and could run on 12V too had transformers, also the Grundig "Büchsenchassis" (tin can chassis). The last one was called because all the circuits were on plugin modules (in metal cans), and there was a service adapter which said "replace this can". This was meant that simple sales contractors could "repair" most faults in the customers home. The cans were then meant to be sent back to the factory to be refurbished. Since they didn't want this people to work on a hot chassis, they built an insulation transformer into the TV. It never turned profitable, by the way. Insulated power supplies only appeared widely with the introduction of AV inputs from the factory, and then were almost universally switch mode.

[@Inadvisablescience]

Like the Zenith monitor, the TRS-80 monitor also is a TV set. The panel on the right hides the holes for the VHF and UHF dials. The posts for the dials are even still there.

[@quadmods]

15:36 this exact scenario happened to me running a coax from one room to an other. Ended up on two circuits with the old TV plugged into a miss wired outlet thus blowing up the VCR on the correct circuit in the other room.

[@jeromethiel4323]

The TRS-80 model 1 and 2 monitor WAS a TV. If you pried off the TRS-80 logo (on the right hand side) there were tuner knob holes and volume knob holes. It was very much a repurposed black and white TV.

[@edhughes8619]

Nice to finally see a Zenith Data Systems product. Loved building my Heathkit H89 system, along with my Heathkit H25 printer.

[@tommyvanpelt2408]

Finally, someone else with Heathkit equipment. I still have my H89 that my father and I built. Still working I might add.

[@edhughes8619]

@@tommyvanpelt2408 I built several Heathkit kits, Weather station, Reverb, multimeter, tachometer, TV, 8 channel radio control, just to mention a few. I really enjoyed the kits

[@ChristopherHailey]

My first monitor was a portable black and white TV that had a composite video input that I connected to an Atari 800. It was a lot sharper than an RF adapter.

[@misterkite]

My TRS80 was hooked up with one of those little RF switches (marked TV on one side, and Computer on the other).

[@herrbonk3635]

That Zenith monitor indeed looks like the typical 12" portable black and white TV from the 1970s Japan/Taiwan. But afaik, these were usually designed for 12V battery operation as well, using a small internal transformer for 220V to 12V. Where I live, "hot chassi" was a thing of the 1950s, perhaps 60s. Large color TVs of the 1970s didn't have that design either.

[@argvminusone]

I noticed that it has a pretty low power draw for a CRT.

[@xsc1000]

Where do you live? In Europe hot chassis TVs were used till the end of 70s when switching power supplies appeared. Only the small portable TVs designed to run on 12V had transformer. Only in USSR all TVs had large power transformer, so especially color TVs were very heavy, more than 10kg about european ones.

[@Chris.Davies]

Yep - my Dad and I turned an old 12" B&W TV into a monitor capable of a massive 40 x 40 characters, IIRC. It was truly incredible to have a computer at home, an OSI Superboard II, with 8K of RAM, and running at 1MHz. But our converted crappy old TV was a good deal lower quality than the 72 columns of the DEC VDUs attached to the PDP-11 at school. :)

[@wun1gee]

That Zenith monitor - the TV version was white on the front and black on the back. I had the TV version when I was a young kid. Very early 90s. I had my NES hooked up to it. I got very good at playing Dr. Mario on a black and white TV. Haha. I was the only kid on the street that had my own TV in my room. I remember watching Star Trek TNG on that TV in black and white when I was like 8 with the volume down low because I was supposed to be sleeping...

[@chadhartsees]

Love the TV/Monitor Archeology! "Yup, cutouts for the tuner and the speaker!"

[@millermonsterair]

37:20 idk man, i am almost 40, been around loud music my whole life and i can STILL hear that damn high pitched tv noise.... dog whistles used to REALLY hurt my ears and would cause me to hit the floor and curl up from how intense it would sound to me. everyone else ive known couldnt hear a dog whistle and didnt believe it until we did some "blind testing"... same with older tvs. i used to complain to my mom and she would just call me crazy and delusional. the fact that you admitted to being able to hear it at one point in your life, it confirms that i, in fact, am not crazy and heard what i heard. i still hear it tho, but not as badly. dog whistles dont bring me to my knees any more, but they still hurt like hell

[@SeanBZA]

27:00 the transformer is original equipment, as the entire set operated off 11.6VDC, as they often included as standard a 12V input for car use. the missing components are the varicap tuner, which only needed a 36V supply from one of the LOPT windings, as otherwise all voltages were LOPT derived, like the hater voltage, and the 200V for the cathode drive, and the 800V focus supply. The 11.6V supply runs the IF chip direct, along with the tuner, and this then gave a 36MHz IF signal to an IF block, which separated the sound off it, and demodulated it, along with converting the video down to baseband for processing in the horizontal and vertical drive, and the video, after a sound trap filter, was fed to the CRT base socket to be amplified to a 200V swing and applied to the cathode. Later versions all the IF and video processing was done by a single chip, which also handled the AGC and AFC for the tuner, using varicap diodes so you had a tuning drawer and 8 linear pots to tune in stations.. Only the last few iterations of colour sets did the entire tuner and video and audio get put into a single jungle chip, with a separate microcontroller doing all the tuning, reducing chip count down to 2. They also had higher power draw, so had SMPS power supplies, locked to the line rate, so as to reduce dot crawl from beat between the transformer and the line output stage, and those got a power supply of 130VDC.

[@BollingHolt]

So in addition... the N122P I rescued this morning... the back of the set looks nearly IDENTICAL to yours except that mine is black. You're right, that oval is where the antenna is mounted. Mine entered the world in January of 1981, just like me! ;)

[@cabbitkisser2620]

back the 1980s. i was 13 at the time. i owned a trs-80 coco 2. i never had a trs-80 monitor. i hooked my computer up to a goldstar 13" tv. that is all i had at the time. so i was happy for what i had

[@nickblackburn1903]

@ 9:08 Adrian its not an abomination, its beautiful! Great video thank you, I still use 12" B&W tvs for my ZX80 & 81 and they work brilliantly :) I believe you may find a lot of people interested in learning how to repair CRTs. Good job!

[@TheLemonhawk]

I used a Sony 9" TV as the monitor on my IMSAI computer. I solder in an RCA plug so I had direct video. The Sony has a transformer power supply so it was isolated from 110v. I think the video board was a Hercules.

[@antiriad86]

Long time fan here! I am from germany and I inherited my granddads TRS-80 Model I - It uses the same model of monitor fitted with an europlug. Works well, but the flyback has definitely seen better days.

[@altebander2767]

At least in Germany there were some Bildschirmtext (BTX) terminals based on TV sets. The sets were designed to be able to switch into a special 625/60Hz mode to have less flicker, and they re-use the connection to the teletext decoder to connect a BTX terminal module inside. And yes, they obviously have the normal 6 MHz RGB bandwidth to be able to display crisp 40 column text (at 10x12 pixels per character) or 80 column text (at 10x6 pixels per character).

[@BollingHolt]

I just rescued a Zenith N122P this morning! I don't know much about it, and there isn't much about it on the Internet at first glance...

[@ricke573]

I bought a Zenith ZVM-121 in 1981 along with my Apple ][+, attaching it to the output of my Videx 80-column board. I still have it and the Apple up in the attic. I had a Sakata color monitor for 40 column mode.

[@WelcomeToMarkintosh]

LOVE that Zenith monitor! It’s the design esthetics. CLASSIC

[@michaelcalvin42]

The reason the amber CRTs show up as purple under a UV flashlight is because the amber phosphor does not fluoresce under UV light. You're just seeing the visible near-UV (i.e. violet) light from the flashlight reflected directly off the phosphor surface.

[@Duewester]

I had a JCPENNEY branded BW TV that was a match for my trs-80 monitor.

[@foxhack5011]

"It doesn't weigh that much" And I just saw the plastic buckle when you lifted it less than an inch...

[@SeanBZA]

Still got one of those, with a burn on it from the camera it was used for.

[@fattomandeibu]

Huh. That part about vertical hold affecting refresh makes massive sense to me. I had a really old TV, which was PAL/50 only, but I was able to "tune in" my Dreamcast which was PAL/60 by adjusting the vertical hold. Had to change it back when wanting to watch TV or play another system, but that now makes sense.

[@Carstuff111]

Good to see (and read) that another CRT is good enough to preserve and use. My great grandparents LOVED Zenith TVs well into the 1990s. Their last TV was a Zenith console TV with an IR remote and channel preset ability, stereo sound, and a beautiful wood finish. They bought it to replace their Zenith TV that had the Space Command remote with only 4 buttons. My grand father being the tinkering type, had spare parts for their last TV, they used that same TV from the late 1980s or VERY early 1990s, till basically the day they died in 2008, and the TV as far as I know was used for years more by my aunt and cousins.

[@ZagnutBar]

We hooked up our first Apple ][+ in 1979 to my parent's television set with the same kind of RF adapter you used for an Atari.

[@staggerwings]

@ 4:28 in the Video- "Now I've shown this trick before..." Little flashlight magically teleports from workbench to Adrian's hand... :)

[@Graham-ce2yk]

Thanks for covering this, I did a quick search and I think there may be only one other video covering the Zenith ZVM-121 on KZbin.

[@AndrewBryantFuentes]

Great vid! Back in those days, we were accustomed to the loarger border space under the text. It was just normal that way.

[@tommyvanpelt2408]

The zenith monitor was intended to be used with the z-100 line of computers which was an interesting machine in that it had two processors; a z80 and a 8086. It could run HDos, cp/m-86, and dos. It was available either assembled or as a kit although i can't remember if the monitor was available as a kit or not though i suspect it was because i built the H89 with internal monitor and several tv's.

[@Saturn2888]

I had a color TV as a kid that looked like that Zenith. I played my NES on it in 1992. My dad bought it when he came to the US in the 70s.

[@bluephreakr]

The blank panel on the right side of the television where the controls were could have been handy for notes and bind legends for applications.

[@mikebarushok5361]

I worked on a pile of IBM CGA monitors and one of the designs was very obviously a color TV without the RF and demodulator circuits. Wish I could remember which TV manufacturer. It meant that I could use the Sam's photofact for the schematic which showed voltages and scope traces. The deal was that I could scavange parts but couldn't buy any parts and I was paid a flat rate for each one repaired. A few were serial numbers covered by a recall so, those were replaced for free by IBM and they didn't want the old ones back subject only to a signed statement for each attesting that I destroyed the ones being replaced .replaced.

[@thirstyCactus]

I wonder if there is a general power supply issue, because the vertical size, focus, and hi-voltage are all a bit out-of-range. Or maybe I missed something; was also browsing ebay :D

[@jussikuusela7345]

1:50 this is paradoxical because in the age of virtually everyone carrying a far better than SDTV portable camera, we still get stationary "security cameras" where you get to try and recognize the perpetrator by a few pixels.

[@carolimhoff2231]

Look at a newspaper's stippled image, the bigger you blow it up, the lighter the contrast, and the smaller you condense it, the darker it appears. This happens due to the space between the dots in the stippled image. The same could be true in the electronic image on the CRT monitor. The larger or smaller the image could be affecting the brightness and clarity of the image. I know I am comparing an 'apple' to an 'orange', but just a thought.

[@myleft9397]

Do you remember when you used to buy cheap TVs and shove the guts into video game cabinets? I do because it was my father's business :D If a video game monitor burnt *in, or got necked, *or was just plain too difficult to fix, the cheapest Radio Shack or Sears TV that was the same size did the trick! [*edit for typos]

[@HarryBroadwell]

In the early 80s I worked for a computer store that sold Commodore, Apple, Franklin Ace, and PC clones and Zenith monitors (along with others). We never had any problems with the Zenith monitors. Wish I still had the one I bought for my C64.

[@jussikuusela7345]

Purple glow from amber screen when excited by UV-A is probably because UV-A excites the blue sensor cells in the eye and the amber phosphorescence excites more of the red than the green ones.

[@bborkzilla]

I have one of those Zenith monitors - I bought it with my Commodore 64 back in the 1980's

[@petergunn551]

tech tip: put a sticky note on the screen with the paper angled out about 1/8 inch. if the paper moves when you turn on the power. you have high voltage.

[@jaycee1980]

Those bipolar caps were usually so large because they are basically two regular electrolytic capacitors connected "back to back"... ie in series but with +ve connected to +ve for example. a 10uF bipolar would be made of two 20uF capacitors connected as such. Nowadays you can get some nice film capacitors e.g. WIMA MKS2 series which are large enough capacitance, for example MKS2B051001N00MS is 10uF 50V and yet still has a lead pitch of 5mm. This is usually what I use when replacing these bipolar caps in CRT's

[@jonathanreedpike]

Security monitors often showed multiple cameras at once, the higher rez probably was required so camera data numerals would work even if shrunken from full screen.

[@uploadJ]

This required the use of a box to combine the multiple images, all swept (V and H freqs in phase) at the same time ( and later they could digitize and recombine the images) to pull off this trick. Camera chains back in the 70's required all cams to be slaved from a common time base to synchronize the sweep rates ...

[@lorensims4846]

I used a black and white TV with an RF modiulator with my Atari 800 because I was more interested in programming than with playing games. Atari never sold a "monitor" though my 800 did have a monitor port. I found that the Commodore 1702 monitor was the very best Computer monitor for my Atari 800. Later I used it as my television by feeding the signal from my VCR through it. It gave a very clear picture.

[@raelik777]

The entire ZVM-12X series and ZVM-13X series of monitors are really interesting... and pretty weird. You had the ZVM-121 which is clearly just a rebadged and modified black-and-white TV, but then you had the ZVM-130 and ZVM-131 which were the same idea but they actually put the color TV tubes in nice computer monitor style cases (the same case design they use for all but one of the 13X series). They were still just medium resolution color displays though, only really good for 40 column mode. The 130 had composite, chroma and luma inputs, and the 131 traded the chroma/luma for a TTL RGB input with a DB-25 port that could support multiple different computer types if you had the correct cables. They used this with every other 13X monitor... except the 134. That one is so weird. First, it uses what appears to be a rebadged TV case like the 121... but it's a high resolution CRT that can do 80 column. Also, it used a DB-15 instead of a DB-25 for it's RGB input, and had its own set of cables instead of using the ones compatible with the other RGB capable 13X monitors. The 133, 135, and 136 were all very similar, with the 135 coming out first, the 133 being a slightly cheaper version of it with a REALLY interesting 133T model that was TEMPEST shielded, and the 136 which was the 135 with a long-persistence phosphor tube (not sure why you'd intentionally want that). In the 12X series, they also added the 122 and 123, which are amber and green monochrome displays perfect for 80-column text. I personally own a 122 (with a REALLY high-pitched whine from a sketchy flyback) and a 135. which is considered by most to be the best monitor out of the whole bunch.

[@mfree80286]

"REALLY interesting 133T model that was TEMPEST shielded" There was a stretch of time where Zenith was winning a lot of DOD contracts. When I was in high school the ROTC got a load of retired Zenith 286 machines from the Air Force.

[@mckinnon42]

35:26 When I was a kid, my dad was always adamant that we had to unplug electronics because they would 'waste power' even when off. Monitors like this might be why he thought that.

[@prestongivens3594]

Ah! Murphy lurks around your shop floor just like mine! He takes hostages all the time! 🤣

[@greendryerlint]

You can use a green or violet laser pointer to trace a line on a monitor or TV also and the power of the green and violet lasers is enough to excite the phosphors and leave a glowing trace too, which will tell you the color. (don't look at the direct reflection of the laser)

[@ravenbarsrepairs5594]

You mean today? At work they've put a couple Toshiba TV's in the hallway, used to display employees relevant computer data. One is used to show a "improvements in progress" spreadsheet, and it's gotten long enough without finishing anything, that the text is nearly unreadable....

[@Hans-KRC]

Need a bigger TV! Or is that on the list of improvements in progress...

[@HammysHangout]

the UV flashlight is also good to check tubes for SCREEN burn.. ( common for arcade monitor repair / swapping )

[@shmehfleh3115]

The first Atari arcade machines. Computer Space and Pong, used hacked-up, tuner-less Zenith B&W televisions, too.

[@coyote_den]

Fun thing about phosphors fluorescing under UV: That's how plasma display panels work. They're basically a grid CCFL cells that emit UV, which is converted to RGB by the same phosphors a CRT has on the face. Not just UV either, there is no such thing as a white LED. White LEDs tend to be a yellow phosphor in front of a blue LED, or a white phosphor in front of a UV LED.

[@michelleshaw337]

My HS had a bunch of those monitors, and I do recall the display area having significant "margins" around it

[@KennethScharf]

The Rat Shack monitor probably used an opto-isolator chip. Some of them were wide band enough to pass video speed signals. I think the power transformer in the Zenith is just a 1:1 isolation transformer, it seems to have only 4 wires. If that ISN'T a cut down TV circuit board, then the transformer could be a LV secondary with a regulator. And actually, it does look like the PC board is NOT from a TV with parts missing, but a new design based on the video and HV TV circuits, but with ONLY parts for a monitor.

[@urgtuiop5455]

The bipolar cap is large to handle high current spikes from the yoke inductance. The bandwidth of the monitor can sometimes be improved by dropping a Schottky diode across the high voltage transistor on the yoke PCB driving the gun. Base to emitter. Stops the transistor from going into saturation.

[@urgtuiop5455]

Edit. Oops base to collector.

[@herrbonk3635]

The black and white TVs from the late 1970s that I used with computers had a large coil in the video signal path. Guess it was intended to reduce noise of interference or something. However, I replaced it with a short and the picture became visibly sharper. That was my level of understanding :)

[@herrbonk3635]

(I discovered this when I removed the HF demodulator btw, in order to inject the video signal directly into the tv.)

[@urgtuiop5455]

They might have been trying to restrict the bandwidth of the monitor section down to match the broadcast signal bandwidth. Some of the monitors I played with had overshoot/undershoot artifacts at edges of characters so a bit of filtering was necessary.

[@xsc1000]

@@herrbonk3635 Maybe it was color trap reducing NTSC/PAL interference.

[@Alcarods]

I was one month old when that monitor was manufactured. I feel old!

[@glenn9854]

I was 34 years old at that time. You feel old, I am old!

[@nickolasgaspar9660]

Amstrad CPC computers came with modified TVs but with an RGB connection.

[@atomicskull6405]

The situation was a lot better in europe, SCART was available since 1977 and in France SCART was mandatory on all TVs sold from 1980 onward.

[@xsc1000]

If you had ZX Spectrum, you had to modify computer to add composite out. Composite in was in any new TV :-)

[@BG101UK]

After the early 1980s in the UK monitor-style portable TVs became quite popular, like the Fidelity CTV140, the set I used with my Commodore 64. The back had knock-out positions for AV/SCART inputs (and unpopulated parts on the board) but it seems these were probably only fitted on the remote control model; just a UHF input on mine. I later fitted a headphone/external speaker jack to improve on the horrid side-facing speaker.

[@kaitlyn__L]

Haha, when you turned the oscillator down and it briefly dipped below your notch filter’s cutoff (which I thank you for, by the way!)

[@Kudeghraw]

Had a TI-99/4A and the TV was the monitor. NES systems had an adapter that fit old TVs as well and that was huge coming off the Atari age.

[@darkwinter6028]

It’s worse than that with a hot chassis and miswiring - if you have the TV’s composite ground connected to the live 120v wire, and you connect that to a device with it’s composite ground connected to the 120v neutral or ground you will now have a direct short on the 120v live, running through the video cable: instant fireworks and blown mains breakers/fuses.

[@ManuelSLaraBisch]

What do you mean, remember? I was using two HDTV sets as the monitors on my main PC as recently as two months ago!

[@AnonymousDisparity]

Modified TV? Well aren't you fancy! My Atari STE had a Sanyo 10" Color TV. And worked on Channel 3 or 4 :P

[@securitycamera8776]

Computers intended to be viewed on TV sets have large borders on the display because TV sets are overscanned. The only way a computer display could be sure to be visible was to stay away from the edges. There is nothing wrong with the monitor. Remember round picture tubes?

[@JASPACB750RR]

When I was a kid I was hooking up a sega to the rca/coax jack on the back of the tv. And as you do, you feel around with your finger to blindly line up the cable. In doing that, the moment I touched that jack it shocked the ever loving piss outta my little hand and made my mouse taste like a penny! For the longest time I was terrified of hooking up anything through coax, but that never happened again. It would make sense it being a one off thing. That outlet or the tv plug could’ve been wired or plugged in wrong and making that jack hot.

[@richchinnici6182]

The first IBM PC compatible computer I owned was a Zenith Data Systems Z148 with an amber monochrome Zenith monitor.

[@TheGmr140]

Cool video thanks 😊

[@ruawhitepaw]

I saw text scrolling fast across the screen, and thought the video was over. Then I realised it's scrolling on the monitor within the video, not the video itself. XD

[@SeanBZA]

Capacitor most likely to fail are the 160V ones, and the non polarised one in the horizontal stage, there on the input board by your set.

[@ReneKnuvers74rk]

47:50 or, because the beam traverses more width of the screen the phosphor is not radiated as much and thus doesn’t light up as bright. Maybe the effect is most profound on ‘slow’ green phosphors.

[@fluffysheap]

1980: Computer monitors are just TVs with specialized input connectors 2000: Computer monitors are advanced displays capable of resolutions and image quality far in excess of ordinary televisions 2020: Computer monitors are just TVs with specialized input connectors

[@semifavorableuncircle6952]

These low value bipolar electrolytics were something rather special, used thick un-etched foils to support a surprisingly high AC current (in that size, a few A). Such capacitors are no longer made, as outside of CRT devices there is not much use for these and more importantly, foil capacitors have gotten smaller and cheaper. If you need to replace one, only reasonable choice is a MKT or MKP film capacitor (any 10uF film C will support the same or higher AC current). Any regular small bipolar electrolytic is only intended for low level signals and will blow within minutes.