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PostPosted: Tue Dec 18, 2018 3:22 pm 
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Oh, sure, that's not the problem. The problem is that I don't have an external source that emits 8Vpp video. (That's not a standard at all. JAMMA uses 5Vpp, and that's the highest analog video range I know of)


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PostPosted: Tue Dec 18, 2018 8:44 pm 
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Oh, okay. None of the other mods should have worked then either though. I do feel that something for changing the magnitude of the voltage would be external though anyway; you said JAMMA is 5Vpp, but what's SCART RGB?

Edit: looks like SCART RGB is 1Vpp. I guess the mod showcased in the 8-Bit Guy video is only working for SCART and not JAMMA.


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PostPosted: Wed Dec 19, 2018 1:59 pm 
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Converting voltages down is easy: resistor dividers aren't a problem. It's just six resistors (three if you don't have to worry about matching cable impedance) to convert JAMMA 5Vpp analog down to 75Ω 0.7Vpp or 1Vpp.

As a complicating factor, the output impedance of the JAMMA board may be sizeable: this example had such a high output impedance that it was only capable of sourcing ≈10mA into the 75Ω input impedance of the monitor anyway.

Officially, JAMMA doesn't have to emit the correct analog video voltages into anything less than 1kΩ. I suppose if you do have to start adding current buffers for that situation, the other voltage and/or current scaling problems are comparable.


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PostPosted: Wed Dec 19, 2018 10:20 pm 
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Aren't operation amplifiers used for increasing voltage? Sounds like by several magnitudes though, which obviously isn't ideal. And I didn't think a signal could have an impedance value... Unless you mean to say that, for example, a 6V 3Ω signal is a 6V 2A one.

Also, what TV do you have? 8Vpp just seems bizzare...


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PostPosted: Thu Dec 20, 2018 1:12 am 
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Every amplifying device as a "gain-bandwidth"; with a desired component bandwidth of somewhere around 14MHz and a gain of 11 needed, that requires something with a gain-bandwidth of 150 (or so). Furthermore, these signals need to be able to directly drive the final BJTs that drive the tubes themselves, so need to be able to source a lot of current.

Op-amps with that much bandwidth are not cheap.

A design that used a bunch of BJTs would work, but then you have the complication of needing to design something with rail-to-rail support.


Outputs, inputs, and cables have an impedance. In the case of cable, it's a fundamental rule of physics. In the case of outputs and inputs it's how "non-ideal" it is—even if it is there to prevent reflections.

From what little I've been able to tell, signals in the range of 8 to 20 Vpp are fairly typical on this final stage before the power transistors that drive the electron guns.

The TV here doesn't have separate "jungle" and OSD chips; the only place RGB component exists is on the output of the all-in-one chip.


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PostPosted: Thu Dec 20, 2018 6:07 pm 
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Sorry if I'm being paranoid at this point, but about what year was your TV made? I'm wondering if the combination chip is typically found in newer CRTs.


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PostPosted: Thu Dec 20, 2018 6:09 pm 
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Yeah, it's one of the last CRT TVs to be manufactured. 2006.


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PostPosted: Thu Dec 20, 2018 6:22 pm 
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Yeah, shoot, both of mine are from about that period so they might not be eligible.


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PostPosted: Thu Dec 20, 2018 6:51 pm 
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Mucking up service manuals for TVs without paying a scalper isn't too hard. (The problem with paying someone for a DL isn't the paying—although I resent that also. It's that you don't know if it's even useful until you already have it)


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PostPosted: Thu Dec 20, 2018 7:01 pm 
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I figure I'll just open it up and look around; I presume it would be pretty obvious. I also imagine whatever chips being used will have part numbers written on them with a data sheet hopefully online somewhere. I wouldn't think a company would make new chips for every TV model?


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PostPosted: Thu Dec 20, 2018 7:12 pm 
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What exactly is the TV you got ? elektrotanya has never failed me when finding any CRT service manual, new or old. Knowing the chassis number is the best way to get things going, it is often marked on the sticker on the back of the thing.

The chip where video comes out is usually the biggest one and the neck board wires will go near it. In worst case you only have its datasheet but more often that not the reference circuits in those are nearly same as what the device using the chip uses.

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PostPosted: Thu Dec 20, 2018 7:24 pm 
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Drew Sebastino wrote:
I also imagine whatever chips being used will have part numbers written on them with a data sheet hopefully online somewhere. I wouldn't think a company would make new chips for every TV model?
New chips for every model, no. New chips that never saw any release outside of CRTs and exclusively have in-house part numbers without any public documentation? Yes.

For example, the all-in-one in this TV is a "M61271M8-058FP" ... or 61371 ... or 61272 ... per the service manual. There's no information about it: only what you can infer from the service manual for the TV itself.

TµEE: my TV is a "sylvania" 6420FF; I can find the service manual for the 6420FE (previous generation?) and the 6427FF (same generation, larger tube) but not one for this specifically. But thanks for the pointer to elektrotanya!


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PostPosted: Thu Dec 20, 2018 8:05 pm 
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Wow, that's a great resource! I found the TV I was interested in ("RCA" 204F24) but sure enough, it appears to have an all together chip. The only doubt I have is that there's a sync out; what is the neck board responsible for?


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PostPosted: Thu Dec 20, 2018 8:23 pm 
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Neck board is usually just for driving the actual electron guns itself. If a "sync" signal goes to it it's probably a blanking signal to hide retrace lines.


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PostPosted: Thu Dec 20, 2018 8:30 pm 
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Neck board is what amps that 4...8Vish RGB to 150ishV RGB to drive the cathodes.

Generally TV stuff all goes by "chassis" which is the term used for the electronics in the housing. One chassis is usually used in a selection of models, i.e Philips L04E chassis is used in some 10+ variations with varying sizes and the service manual for it will cover all those variations too. The chassis number is usually written on the sticker on back of the TV or sometimes on the board itself.

Sometimes those large chips belong to a single family of them, which has ton of part numbers due to different ROMs inside each chip. If you figured out the name of that family you could probably find some info out there. It only helps if these chips were not purely in-house stuff. Sony likes to have such chips that only their devices use, with no documentation about them available anywhere.

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