First, have look on this topic to be sure your flash will be compatible with your DSLR.
You are certainly wondering how and why a 25 years old flash will be simple to use with a new DSLR?
The reason is very simple. These kind of flash also called Auto Thyristor or computer flash were able to perform the metering and then provide the required amout of light to get a good exposition.
The flash has to know the ISO setting of the camera, and the aperture setting. Then, using it's own light sensor, the flash will compute the flash power reduction required and the flash duration.
The only thing you have to manage on the DSLR is the shutter speed. You shall not exeed the maximun sync speed of your DSLR.
For the Sony A700 it is 1/250 without the steady shot enabled and 1/200 with the steady shot enabled.
Check your camera user guide to know the maximum sync speed not to exceed.
if you vastly exceed the maximum sync speed (e.g. 1/2000 for max sync speed of 1/250) the sensor will not see the flash light. If you slightly exceed the max sync-speed, you will probably notice the shutter on our picture having a fraction of the picture underexposed and the other overexposed delimiter by an horizontal line corresponding to the shutter position at the moment of the sync. By this way you can determine by yourself the maximum sync speed not to exceed.
First requirement is to check that your flash will fire. For that, ensure on your camera menu that the setting for the flash in "forced flash" or "fill in". This is the way to force the flash to fire each time you press the shutter button independantly of the ambiant light.
Test the flash step 1: Manual mode
Set the flash on manual mode full power, shot : you shall ear a loud "plof" with a powerfull light and notice a long recycle time. Then set the maximum power reduction (e.g. 1/64) and shot. You shall ear a mini "clic" with a mini flash and notice a very quick recycle time or no recycle time at all. (in both case the picture are no well exposed)
If this test fail, this certainly means that the power reduction circuitry of your flash is out of use. This flash is only able to fire at full power. I have got a sunpak 36DX in this case. (firing but unable to reduce it's power)
Test the flash step 2: Computer mode (auto Thyristor)
Set the flash on Computer mode, shoot in direction (and close to) of a strong light source (be sure the light sensor of the flash is in direction of this srtong light source). You shall ear a tiny "clic" corresponding to a mini flash and notice a very quick recycle time or no recycle time at all. Then perform this test in the complete dark or put a finger to close the light sensor: you shall notice a full power flash and a long recycle time.
If this test is passed, then you will enjoy your flash.
- Set the flash on computer mode.
- Set the camera on M mode
- Set the ISO on your camera and report the value on the flash.
- look on the available aperture value available on the flash for this ISO setting.
- On the upper image for a 100 ISO setting, available aperture settings are 11 (red), 5.6 (yellow), 4 (green), 2.8 (blue)
- Select one of the available aperture value on the camera and report this value on the flash.
- Select on the camera a shutter speed just lower of the maximum sync-speed.
- Press the shutter button.
Normaly you shall have a well exposed picture. Ensure with the histogram.
Now you can try to bouce the flash on the ceiling (without changing any setting) and you shall also have a well exposed picture
Experiment more, just be carefull when bouncing light of the color of the surface you are bouncing on. If not white, it will change the white balance of your pictures. Be also carefull when in direct flash not to be to close to the subject (in this case the build-in popup flash will do the job perfectly, or bounce the flash)
