Repairing a crystal (1.008MHz)

[davegsm82]

Since I posted a wanted in the section on here I had no luck finding a replacement crystal for the Terminal board which I want to get going.

In light of this, I decided to see what I could do with what I already had.

The crystal can be seen, in bits, here --> https://www.vintage-radio.net/forum/...81&postcount=7

In the pictures below you can see how I have managed to put it back together and test it, without the use of a frequency counter as I don't have one;

1) holding the pins (the right one has come loose) by soldering to an inductor. I was going to glue it but decided against it.

2) Came up with the idea of using a piece of PCB material with apropriate pin spacing to hold the pins in the right place.

3) Replace the crystal disk between the 2 springy terminals (now that they are sturdy enough) and paint on some silver conductive paint (although its not really needed!).

4) Build the shown circuit that I came up with, this uses a 4093 Schmitt trigger Quad Nand gate IC. Left most gate is an AF oscillator, about 5-600Hz. This gates the next oscillator which runs at the crystal frequency, 50% on, 50% off. The right most gate is simply a buffer onto which you can add a foot or so of hook-up wire to act as an antenna.

5) Use a sensitive radio scanner (Icom IC-R20) set to the crystal frequency. Then simply connect the IC to a 9v battery and listen for the tone.

This worked a treat! the crystal frequency was absolutely spot on, you could only just hear the tone 9khz up or down from 1.008MHz. I didn't bother checking for upper and lower harmonics, the frequency is so spot on that I'm not worrying.

Now all I have to do is rebuild the rest of the terminal board and hopefully we'll see first light from it again.

Dave.

[Pamphonica]

Very fine job, Dave. A true work-around!
Circuit details should be with you in a couple of days.
regards
Jeremy

[cmjones01]

Very neat! Much nicer, in restoration terms, than using either an alternative crystal or a replacement of the right frequency.

[davegsm82]

Thanks chaps, i've since packed the bottom of the pins inside the case with epoxy resin, after doing some experimentation I found a particular resin which sets absolutely rock solid.

Some cheap epoxies seem to stay very soft, the one I used was ever-build, comes in a double barrel tube which dispenses epoxy and hardener in even amounts.

Dave.

[G8HQP Dave]

You may have shifted the frequency a little, but in this application that won't matter.

[davegsm82]

UPDATE:

For no apparent reason I decided to have another play with the Character generator board tonight, so I set about repairing the dodgy solder connections on the larger components (mains TX, filter caps and regulator).

After I'd done this, I dug out an old Black and White CRT tv, one of those little ones that you get at the boot sales for 20p.

firstly the TV was underscanning and unstable, so I opened it up and resoldered a few dry's around the Vertical deflection circuit. After this was done I was greeted with a nice full raster of snow.

I hooked the generator board up to my dual bench PSU along with the TV as I couldn't be bothered to find a 12v 1A PSU. Everything at the right voltage, switched on and a quick fiddle of the tuning control yelded a snowy yet visible picture.

In a fit of curiosity I switched the VL/VH/UHF switch to VH, to find amazingly that the picture becomes almost crystal clear. No interference other than the character generator vertical lag which is unaviodable.

At present its just a screen full of garbage, but it proves that the rebuilt crystal is working, and that the board is capable of outputting something intelligent. Next step for tomorrow is to try and feed it some RS232 and see what I can get on the screen.

I've attached a picture of the running TV.

On the very bottom line, 2nd character from the left you can see an underscore, this is the blinking cursor for where the next text will be printed. Each time you power up the board, the garbled text is different.

Dave.