Recently, Ive dug out my old N1500 machine that I last looked at about 2 years ago. At that time, it was plagued by the Lockfit Transistors popping off, first in the regulated PSU, 12V rail, then in the Head-Servo, after that, the Capstan-Servo, Then FM Video gain stages and finally playback sound.
All in all, I changed 11 of 'em in the space of about 4 or 5 hours actual playing time, over the course of a week messing round.
It then developed a very poor E-E picture and poor recording, so I guessed another somewhere had failed and put the machine aside.
I decided to replace All--Or as many as is practical--of the horrible Lockfits in the machine. Thankfully, there's not a huge variety of types used, mainly BC147, BC148, BC149, BF194, and BC195/6
I bought 40 of each of the closest newer or equivalents (BC147, 148, 149, BF194 = BC107, 108, 109 and BF494/6/8 used) and set to work.....
The job was mostly completed in 3/4 evenings of messing round and in the end just a couple were left. I counted the ones removed, and Ive got 112 Lockfits in a small ceramic dish.....
This restored playback and E-E, but I also wanted to be able to run straight Video into and out of the machine. Play out is already catered for on a DIN socket which gives out what is in effect, 'S-Video' signal, a separate Chroma and Luma signals. I found that these can be combined to make for a fully usable compo video signal, or left separate and used via an S-Video input on a monitor/TV set.
For Inputting compo-video a little more work was required.
Looking at the schematics of the Tuner/IF block suggested that the Luma and Chroma are detected separately, and fed their separate routes finally being combined at the recording amplifier before the video-heads.
As an experiment, I cut the track at a point just after U508 pin 1, U508 being the Luminance detector, and prior to the luma delay-line driver, TS4050 and via a 10uF cap, I input CVBS/full composite video signal from a set-top-box.
A test-recording was made, which surprisingly worked pretty well, all be it Monochrome, and a little low-contrast level. A small adjustment to the rec-luma level sorted that, making it approximately the same level as a previous recording.
A similar approach was tried for the Chroma. The Chroma Detector U516 outputs a Chroma signal ( the test-waveform shows a full composite signal at this point on the schematic) on pin 1 The track from this pin was cut and via a 0.22uF cap was also connected to the composite video source from the set-top-box.
Another test-recording gave very acceptable results! While this modification was only an experiment, it proves concept so at some stage, I'll make it switchable between Added Input, and the built-in Analoge tuner. As the luma/chroma is being input After any AGC at least that infernal Macrovision shouldnt upset it (I hope!)
The Video-heads in this machine, while working are pretty worn, its Very sensitive to the tape used, any slight damage/crease etc will cause picture disturbance or with one tape-no usable function at all, while a good tape gives nice results.
Various attempts have been made to help the problem, such as tape path cleaning and alignment, back-tension clutch cleaning etc,--all sorts of messing round, but it came to the point that I have to accept the fact the heads just are Not up to it anymore--No surprise really, They are of the very earliest type, not having the connecting wires coming up through the top in those little black round things, all wiring is done underneath the disc.
A few months ago, I won a new/NOS boxed head-drum assy for an N1700 from the usual spares-auction-site, Fleagay for a pretty cheap price. It was decided to carry out the period modification to this machine and fit the new 1700 heads.
This modification involves machining down the Capstan Motor pulley to half its original size and also to machine the capstan shaft itself. The original article was in the Television Magazine around 1982 ish also called for the addition of another pick-up head for the capstan servo.
--Experience over the years has shown this addition isn't completely necessary, capstan servo still locks solidly with just the original one.
Many years ago, I did this to a machine and still had the various parts I had used. At that time, try as I did, I couldn't machine the actual Capstan shaft, it was just Too hard. I made a complete new shaft, but again had problems due to the fact, my machining wasn't good enough to make an accurate cut at the point the tape is gripped by the pinch-roller. --All I ended up with was screwed up tapes, so I gave up on that, and settled for just halving the tape-speed.
I fitted the machined Pulley and brake-disc to the capstan motor, and carefully fitted the new head-drum, after adding just a little extra epoxy to the head-chips, (the original epoxy seems to have a reputation for failing leading to a head chip being flung off into orbit) and made a test recording.
The results were very impressive, producing a very clean although a little soft-focus picture due to the 3MHz limited video bandwidth. Colour was excellent and there was very little if any edge-noise often seen with old video recordings. All in all, pretty happy the machine is usable although isn't any official video format as its tape-speed is still too high at 7 odd cm/S instead of the N1700 6 odd cm/S, but at least a whole movie can now be recorded onto one single cassette.
A little bit more work to be done, like sorting an audio input, curing the hum on the sound channel and making things a little more permanent adding a Tuner/AUX input switch etc....
I would like to than Baz, (Red to Black) for his help in supplying some service-info, as all the info I had up to recently is in Dutch. His manual was excellent in locating various little issues I had with the tape lacing and helping find a dry-joint causing no colour on record.