Golborne Vintage Radio

One part of the installtion that needs attention is the audio/video router. The current arrangment is both temporary and inadequate.

I'm building a simple relay based router, the photo shows progress to date.

I had plenty of 4PCO relays with 700R 24V coils. Jeremy (Pamphonica) kindly provided some tapped strip to mount them on. The wiring looks a bit rats nest but shoud be fine. It's built for function, not beauty. There are 9 inputs and 4 outputs.

From left to right the outputs are:

To Ch1 Aurora
To Ch9 Aurora
To UHF modulator
Preview

From back to front the inputs are:

BBC1 off air tuner
ITVoff air tuner
BBC2 off air tuner
Testcard for BBC1 (no video normally connected, internal to Aurora in absence of signal)
Testcard for ITV (no video normally connected, internal to Aurora in absence of signal)
Testcard for BBC2
DVD player
COW
Aux

Note that the tuner and testcard inputs are only made available to their respective outputs. Testcard music can be put into the testcard audio inputs when we acquire 1 or more MP3 players.

The video inputs and outputs will be buffered with EL2020 opamps. The audio will not be buffered. The input buffers will be on a bit of matrix board to the right of the relays. The output buffers will be to the front of the relays.

Video crosstalk is minimised by using a pair of CO contacts for each crosspoint. The wipers are earthed via the NC contacts when a crosspoint is switched off. I'll measure the crosstalk when it's all finished. Even with all inputs hostile it should be better than -50dB to 5MHz. I'll be very pleased if I get -60dB . Gain should be accurate to 0.1dB. Frequency response flat to 5MHz within 0.2dB.

Audio uses the same pair of CO contacts technique though probably not essential. I'm not hazarding a guess at audio crosstalk but it should be fine.

I hope I don't get any significant video to audio crosstalk, usually sounds like a buzz on audio.

The ribbon cable carries all the controls to a separate control panel.

Need to mount it in a 2U rack tray and hook it up to an old Cox back panel which has enough holes for all the connectors. I'll need a 12+12 PSU, may well have one in stock, otherwise knock one up out of a toroid, bridge, 7812 and 7912. Actually the relays work perfectly well on 18V so I may use a 9+9 PSU to reduce disspation.

Good work, Jeffrey! That looks like it will be robust enough to last a few years in the "rough service" environment of the Museum.
Simplicity rather than beauty is the watchword here, I am sure. And ease of maintenance.
Jeremy

I've not really contributed to this thread but I read most posts on the forum...You're doing sterling work.

Lawrence.

When deciding how to wire up the input and output buses I looked at several options.

The output buses are 25swg tinned copper sleeved with some very fine silicone sleeving of which I seem to have loads. It's a bit fragile but proof against soldering irons. The output buses are each a single piece of wire, bent into a tight loop for each connection.

The input buses are 20swg tinned copper, unsleeved. They are spaced above the rest of the assembly by using a small resistor to feed each crosspoint. The value is 12R, simply because I had a load of them around. They may contribute slightly to reducing interaction when relays are switching. They will also reduce the capacitative loading on the EL2020 input buffers. Most opamps don't like capcacitative loads very much, current feedback types like the EL2020 are worse than average in this resepct. In a typical video output stage they would be driving a 75R resistor as a series termination and hence very low C.

CFB opamps are interesting. For a given process they give more bandwidth and output slew rate than voltage feedback parts. The BW is also only weakly dependent on closed loop gain and is largely set by the value of the feedback resistor. The EL2020 was the first video bandwidth opamp that was really easy to use. I think of it as the video 741. Absolute godsend to designers like me working with analogue video. I think it arrived c1989.

http://cds.linear.com/docs/en/design-note/dn46fa.pdf

Nice home-made crossbar relay, Jeffrey!
There's an on-line auction of broadcast TV equipment every month in which are frequently analogue matrices of various sizes. This month's has just started but there are no suitable lots this time. These normally sell for less than £200 for an 16x8 or 8x8 A-V (Leitch, Quartz etc). I don't want to spoil your fun but if I see one and get it for a reasonable price in a future auction, I could donate it to the museum. Shall I bother?

We've actually been given a 16x8 av router but sorting out suitable control panels would be a microprocessor job. I'm happier wiring up some relays etc

I understand but I,m talking about 1U routers with a front-of-unit x-y control panel. No micro stuff there!

The problem is the XY control panel. Far too complicated for the Museum environment. It's got to be rows of push buttons, one per xpoint, or possibly rotary switches. Anything more complex will just be confusing.

I think I've got an XY control panel for the router I have here but ISTR it really wouldn't be right for the Museum. The right answer is a a control panel with push buttons and a PIC or Arduino or something to convert the button presses to the relevant commands on the RS232 (RS422?) interface to the router. A friend (not a GVR member) said he'd do this as he's got the relevant experience but he's so busy that he'll never find the time in practice.

Hence I'm taking a KISS approach and making a very simple and direct system that can be used and understood by anybody.

One minor thing I need to do is sort out the defaults when the head end rack is switched to standby. At present it routes the off-air tuners to the Auroras and modulators. This means that all the TVs at the Museum will get pictures on all channels at all times. Peter Sanders and John Wakely have said it would be nice to have testcard instead. So I'm planning a 2 way switch so that in standby either TC or off-air can be selected. I'm sure it can be done with a few diodes and perhaps a relay or 2. Again simple and direct methods. Just need to work it out.

Out with the Boolean slide rule

I don't suppose this would be any use as a backup:
http://www.techbid.co.uk/lot/98507/lot-800?search
It's a 6x6 matrix - Kramer 2066. Very simple front-panel controls and display and still sitting at its £15 starting bid.
Finishes tomorrow just before 4pm
Jeremy