Good morning Dave:
 
We received your inquiry below regarding the old Tektronix modular test products, but I am afraid we will be of little help to you.  The service manual you have is more than what is available from us.
 
TEGAM, Inc. did acquire this line in 1995 from Tektronix, but both of the units you list below (DM5120 and TM503A) were discontinued long before 1995, so we never produced or serviced them at our facility.  We discontinued the entire line in 2003 as well, and moved calibration and repair service to Dynamic Technology a few years later.
 
We do still carry parts inventory, though for products we never produced, we may not have any of those parts unless the same part was used on a module we did manufacture.
 
Sorry we could not be of more assistance, but these units are well over 20 years old and were never produced or service by TEGAM, Inc.
 
Kind regards,
 
Scott Lambert
TEGAM, Inc.
 
Problem: If possible I need information on a Tektronix DM 5120, I have one that when powered on displays "ERROR 453" I have a Tek service manual on this but there is no mention of this "ERROR" or any description or remedy for it.
I also have a Tektronix TM 503"A" I need the right SCHEMATIC FOR.
NOTE: I spoke with technical support at Tektronix, and allowed that Tegam had purchased these products from them. advised that I contact Tegam technical support for these items.
Thanks
Dave