I doubt that you will ever find a EP53A. Look for a TU-7 which is the EP53A and a pulser combined to cal the 535/545 scopes. Actually the TU-7 works for most 500 series scopes. How many plugins do you have for your 535? The TU-7 is used to standardize the gain of the 535 mainframe as well as adjust the risetime response of the 500 series scopes. So if you only have 1 plugin you can just set the gain of the plugin as outlined in the manual for the plugin and everything will be just fine. If you have more than 1 plugin then just pick one as the "standard" and then adjust the others gain to match. Read the description of the TU-7 on . This will give you a better idea of what this plugin does.
The TU-7 is really only necessary if you have multiple plugins being used with many 500 series scopes.
Bill
