Composite Black and White monitor dead

Right. SOmewhere I have the Radio Shack service manaul for that. Of ocurse it's the 115V model, but the modifications infolved adding the step-down transformer between the power switch and the input to the rest of the unit and not much else. Yes., The US version is hot chassis (and nasty to work on as a result), the European (and I guess Australian/New Zealand) model is not. video Ah. I'll explain those termins in aomnet, I think this tells me that you don't need the official service manual yet. I don't think you're stupid, but I do thing you need to learn some more electronics before you can make sense of such manuals. Service manuals are written for people who understand how whatever sort of device works and just need the information on how _this particular one_ works. Te excat details of it, the schematic, parts lists, etc. Service manuals anre not, normally, tutorials in electronics. An alanlogy that might help. If I was servicing a car, I would try to get the factory workshop manau land sue that as my reference. But if we consider chekcing the spark plugs, that sort of manual will tell you where they are, it'll explain any odd thignsi n removign and fitting them (like you have to remove <foo> first), it'll give the correct gap and the tighening torque. What it won't do is eplain how to measure nad set the gap, how to use a torque wrench, or anything like that. Now, as for 'IF strip' and 'Optoisolaotr'. Let's cover those in that order. A TV set is basically a special type of radio receiver. At a high level, the incoming signa lfro mteh aerial is ahdnled much as in a radio set, and turned into a composite video signal and an audio signal. The audio signal is amplified and det to the speaker, the composite video signal is processed by the second main part of the TV which is essentially a video monitor. Now, a radio set basically consists of a tunable filter to pick out the signal that you want from all the radio signals out there, amplifiers ot make it large enoguh to be useful and a demodulator to extrace the wanted ifnroamtion from that signal. The problem is the filtering, really, You have to be able to adjust it to pick up different datations, but you then need several filters that can be tuned together, one for each stage of the amplification. Making all those filters come ot the same frequency all alopng the tuning scale is a major problem. The cure is soemthing called a 'Supersonic Heterodyne', normally shortened ot 'Superhet' receiver. In this, the incloming signal is mixed with a signal from an oscillaotr (known as the 'local oscillator') and the beat frequecny which has a frequesnce of the difference between the incoming siangal and hte lcoal oscillator is filtered out, amplified, and used. Now the trick is tha the oscillator frequency is varied when you tune the set. so the differnece frequency is always the same. Thus the fitlers for tht are fixed, they are set at manufacture. Yo can have ans many staegs as you (sensibly) like. The diffenrce frequency is known as the 'Intermediate Frequency' or 'IF' ebcausew it is between the incoming signal and the freuqency of the output of the demodulator (the information you actually want to extract). The amplifiers for this are, not supriisngly called 'IF amplifiers', the whole circuit is commonly known as the 'IF strip'. In the particular TV that was modified to make this monitor, the 'tuner' -- the aerial input circuit, the lcoal oscilaltor and the mixer circuit -- was a separate module fitted ot the case behidn that large blanking plate. It fet the IF signal to the IF amplifiers, which were assembled on a plug-in PCB that fitted into the main chassis. THis PCB also contaiend the demodulator and much of te audio circuitry. THe rest of the chasis was essentialyl a monitor (this layout might be why this particular TV was chosen for the convfersion). The conversion to make this TV into a motnro therefore cosnsited of : Remvoign (or never fitting) the tuner module Removing (or never fitting) the plug-in PCB with IF amplifiers,m demodulator, etc on it. This is the 'IF strip' I refered to Doing the same with those few remaining audio components on the chassis. This inmcluded the volume cotnrol which would have come throug hthe hole labelled 'V' on the front of the cabinet. It was a bit of good fortuen that 'V' also could eb 'video' :-) Finding soem device to feed composite video from the TRS-80 into the remainign 'monitor seciton' of the TV. THe last leads me on to the tem 'opto isolator'. The TV chassis, at least in the US model was directly connected to the mains. Therefore nothign driectly conencted to it could be earthed (groudned). You certianly wouldn;t want a computer where the logic was effecively conencted ot the mains in this way, you couldn't safely connect up a normal priner, for example. So waht is needed is some kind of isolation so that the monitor section can remain connected to the power line but also the computer can be earthed. The way this was done was somethign called an opto-isolator. Conceptally it's very simple/ It's an LED and a phototranssitor so mtht the LED shines on the phototranssitor. It looks like a 6 pin IC. THe more current you pass throug the LED, the more light (actuallly likely to be IR) hits the phototransistor, so that conducts harder too. It's not very linear in tis simplest form, but in this case it doesn't ened to be. The computer can drive the LED side, the phototransistor can be connected to the (hot chassi) monitor section, and the computer's video signal will end up gettign to the monitor. But as the only thing linking them is that tiny beam of light in optosiolator, there is no driect electrical connection. You can eart hthe comptuer with no probkems. AS I said, that's for the 110V model whcih is hot chassis. The 220V modeul has the isolatiing transofmrer on the mains input. It's no longer hot chassis, there is no ened for isolation on the video input. So in this model there is no optoisolator. Just a simple transistor amplfiier circuit. This on From what I rememember from workign on my Model 1 monitor, the scheamtic of the main part of the chasiss is pretty muc hthe same. The differences are that video inpotu bvoard (which is very differne, on the other hand the 230V oen is some simple you cna trace out a schematic in about 5 minutes) and the isertion of that mains isolating transfoemr. I thought that the stuff fed fro mthe 110V inptu was the same in both versions. The rectifier, regualtor, etc. -tony