The TM5000/500 mainframes have a bulk supply from which is derived 5.0 volts.? The value of that supply is not important unless it's 8.0 volts or less.? The regulators of that time used NPN transistors, which generally want to see 3.0 volts across them (7805, as an example of technology).
One thing that can pull down regulator outputs is the famous shorted tantalum capacitor.? They don't have to be shorted, just leaky as #$@!!!!.
Look at the regulator (which is on the module).? There's generally a current limiting resistor (Tek used a bit of an odd design).? Look for a series resistor of less than an ohm in the 5 volt line.? Measure the voltage drop across that and see what it works out to current.? Older designs put a base/emitter junction across that, and use that transistor to steal base current from the pass transistor.? This limits the output voltage (reduces it in this case) when more current is drawn.? Tek doesn't do this, but the idea is similar.? Too much voltage drop on this resistor shuts down the power supply.
What's the voltage drop?
Harvey
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On 2/14/2024 8:54 PM, Frank Mashockie wrote:
Thank you all for your replies! So it looks like I've got a problem with the SG503 power supply then. Because ~4.74V is probably not enough to power the counters. And the LED drivers require 5.25V per the datasheet I found (could not find a recommended minimum). Guess I need to keep looking. If anyone would be willing to take some measurements (running the SG503 with a TM5000) I would greatly appreciate it!
-Frank
