[ms660]

Brief description as mentioned earlier, all pertaining to the photo's on Flickr..

Ok, let's start with the antenna input/ext. modulator input....Photo 9....Tuner and Interface board...

Input is self explanatory in that schematic, the tuner is at the top right, the other inputs to the tuner are power supply (12v nominal) tuning voltage (variable) and the AGC voltage (Automatic Gain Control) the output is at Pin 7, it's the IF output (the Intermediate Frequency) to keep it simple there are two Intermediate Frequencies, one that carries the sound information and one that carries the vision information, their frequencies are 33.5MHz and 39.5 MHz respectively, you should note that these two frequencies are 6MHz apart, this will crop up later in the description, it might also be worth noting that the sound carrier is Frequency Modulated (FM) and the vision carrier is Amplitude Modulated (AM) when it leaves the transmitter and that info and modulation regime is maintained during the frequency conversion process that's performed in the tuner, ie: the sound IF (33.5MHz) will be Frequency Modulated (FM) and the vision IF (39.MHz) will be amplitude Modulated (AM) so now in effect we have a sound carrier of 33.5 MHZ and a vision carrier of 39.5MHz coming from the tuner.

That's basically it for the tuner so far as the signal goes, next is the IF amplifier, the first stages are still on that board and consist of the circuit pertaining to transistors TR32 and TR33, the output from the tuner is fed to the base of TR32 via a band pass filter, what's a band pass filter? simple, a filter that lets a bunch of desired frequencies through while at the same time telling those that aren't wanted to shove off, bit like a doorman bouncer...The signal output from TR32's collector is fed to the base of TR33, the output from TR33 is taken from its emitter…..that's about it for that IF amplification stage.....33.5Mhz and 39.5MHz IF's in...33.5Mhz and 39.5MHz IF's out (but amplified) that point can be seen on the schematic, tagged as I.F. Output.

So far so good, next part of the schematic to look at is that contained in photo 5....

IF output from the previous board/stage is connected to P501 on the main board in photo 5, you can see that it feeds into another IF amplifier stage, TR161, the output from that are still the IF frequencies which are then fed from its collector to another filter, SWF101, that filter is what is known as a Surface Acoustic Wave Filter or SAW filter to those in the trade, basically it's a filter come shaper for shaping the IF response prior to demodulation, the output from the filter is then fed to the input of the vision demodulator IC, IC101, this IC has several outputs, one for the AFC voltage that corrects the tuning voltage for any small scale tuning drift, one for the AGC voltage for the first IF and tuner stages and one for the demodulated output from the vision demodulator, this latter output is the one that's of interest so far as the signal path goes, it's a composite video signal and contains all the video/colour/sync info and also the sound information in the form of a modulated inter-carrier sound frequency, remember the 6MHz mentioned earlier on? well this is it, a product of the demodulation process in the IC is that the Sound (33.5MHz) and Vision (39.5MHz) carriers are mixed together producing a lower frequency of 6Mhz (the difference frequency) this frequency still contains the sound info in the form of FM which will be separately demodulated. So to re cap signal wise...Sound and vision IF in...Demodulated vision out (composite video) and 6MHz intercarrier sound out....that's basically it for that stage.

Next we follow the sound and video along their two different paths...

Both the composite video and the inter-carrier sound signals output at pin 12 of IC101, let's follow the sound signal route first as that's relatively short and simple...The inter-carrier sound signal (6MHz) leaves pin 12 and connects to a 6MHz filter via coil L102, the output from the filter is fed into the sound demodulator IC601, the output from the sound demodulator (the audio signal) is at pin 13 of IC601 from there it's fed to a two transistor audio output stage (TR603 and T604) and from there to the loudspeaker.....That's about it for the sound.

Next is the composite video signal, also taken from the same output point as the sound signal, you can see that it's connected to the input of a 6MHz trap, TRF2406, the trap's there to prevent the 6MHz inter-carrier sound signal from getting into the video amplifier stages that follow on from it, the first video amp stage after the 6MHz trap comprises the circuit pertaining to TR210 and TR202, the output is taken from the emitter of TR202, from there it splits three ways....

One to the sync separator contained in IC301 via R301 etc and from the same line to pin 8 of P650 which feeds the composite video signal to the chroma module (colour signal processing) shown in photo 1, the third is from the same point as the others but via R207 and an unidentifiable resistor then by way of the chroma trap and via C204 to the base of the third video amplifier TR203, the output of which is coupled to the base of the fourth video amplifier TR204, from TR204's emitter the signal is coupled to the base of the fifth video amplifier TR205 via the luminance delay line, the output from TR205's collector is connected via P520 (this must be plugged in for a picture) to the luminance input (connection 8) on the CRT Drive Board (photo 1) we'll come back to that signal at that point in a bit.

Next is the composite video feed to the chroma module it comes in on pin 8 of P650 (already mentioned before) and the demodulated colour signal outputs R-Y, G-Y and B-Y are fed to the CRT Drive Board (photo 1) via P550, those three colour signal outputs are connected to the respective bases of the RGB drive transistors while at the same time the luminance signal feeds into all three of the emitter circuits of the RGB drive transistors, the combinations of which are fed to the three cathodes in the CRT, there's a lot that actually goes on in the chroma processing but too much to go into given the context of this post...but basically line sync and line pulse in and composite video in....R-Y, G-Y and B-Y colour signals out.

Hope that helps a little bit.

Lawrence.