Above is a " reverse engineered " rough layout of a circuit board by which we had to "trace out" and derive a working
electronic circuit layout to effectively service and repair a 30 output video amplifier / distribution box for our client
to use and work from.
Someone had attempted (though very poorly) some 10 + years ago, to build a video duplication box with 30 outputs with
using a now obsolete hex transistor array, which normally under digital or logic switching would function as per the
data sheet, however it was very much a different output when deployed as a video amplifier, with odd stray parasitics
due to the use of the same chip "die" within the LM3086N. Bad choice of video amplification components by the previous
designer of the 30 way video distribution amplifier.
Despite these problems, the owner "struggled-on" for years with a poor resolution output for his Tape Duplication business.
As there was no circuit available, we contacted the owner and explained that in order to repair the unit, we would need to
devise some form of workable schematic diagram, this was before we discovered the stray parasitics and other inherent
"noises" within each LM3086N chip. The LM3086N is basically a six NPN transistor array in a 14 pin DIL package.
This all takes time to carefully check for 100% accuracy and correct component orientation as well as the correct data
sheets and identification of each part. The circuit above took 2 hours to trace out and a further 2.5 hours to carefully
draw the relevant 100% accurate working electronic circuit. Thus adding further costs to the client/customer's account.
While this method works fine for a single sided printed circuit board with parts on one side and copper foil on the other,
it becomes rather complex when there are two sides to the circuit board and parts cover the track work. Impossible ?
Not always, however, with multilayer circuit boards, there can be as few as three layers , top, bottom and middle.
Now comes a new entry into the mix. A multi-layer PCB with four, five and up to 32 layers in a single manufactured PCB.
That is impossible to trace out using conventional methods. A full schematic will help, however, unless you posses the
schematics in full, complete with all revision notes, you are wasting your time, and effectively our time also. It's not on.
Note: The circuit (above) was "reasonable", however the "LM3086N chip transistor array" exhibited thermally induced
parasitics and a basic lack of performance, inherent picture instability, features not conducive to a client's projected long
term usage in a business where quality matters, in fact it is an absolute requirement.
This more than likely resulted from aging stock and manufactured in circa 1994 and no longer made by NS, So, after an
in depth consultation with our client, it was resolved that the unstable circuit boards were to be removed from the Video
box and returned to our client, we set to and designed a fresh "new circuit" approach was to adopted thus providing our
client with 50 good solid video outputs from one video input source. Our client was highly delighted with our efforts and
paid us accordingly.
The scrapped circuitry was fully removed and using the fresh new idea and approach, a viable circuit design was skillfully
produced, prototyped and tested by us at Unitech and field tested at the clients premises with all benefits measured in
terms of reproduced picture quality on a series of video tapes and the final test of a copy from a copy, this is the
"acid test " on how well a copy is being made, the results were delightful, however, more importantly, our client was
very happy with our efforts and especially with the results. As a bonus from prototype testing, we were able to include
such benefits as 10 stage video signal attenuation to cater for various video input strengths. Not all video tape signals
are weak and not all are strong either.
Research and development on a project such as this takes time and money, more time and more money usually, however,
the end result is what counts, a very satisfied client, delighted with the extra efforts and attention to detail we took to make
sure each video output was fully stabilised and that all 50 video outputs functioned correctly, thus providing an excellent
amplified video signal Video to all 50 outputs simultaneously to each and every VCR (VTR), with no distortion or color
fade or drop out, even with the annoying Macrovision.