Thurlby Thandar Instruments TSX3510P
Programmable PSU doesn’t start up
Problem
We bought a broken TSX3510P that powered on with the display lighting up all segments but would not start normal operation. It displayed only ‘8’ characters on all the numerical displays and after some time left on would begin to beep repeatedly.
The keys did not respond and the output was disabled. The main power supply capacitor for the CPU board was getting quite hot and measured low capacitance compared to the rating. The CPU power showed a lot of ripple.
We replaced the capacitor with a similar but higher temperature rated part.
The beeping would eventually stop and the display started to display correct figures after a while.
Diagnosis
Powering just the CPU board with a variable bench supply, we were able to get it to start reliably every time the power was switched on.
We noticed that a GPIB controller IC was getting hot. Testing the GPIB interface revealed no response to any commands. This IC was removed from the CPU PCB and then the unit would start normally without using the variable bench supply.
Testing the PSU under load revealed that it could only supply about 10% of the rated power without shutting down. The inrush current limiting circuit was faulty, it was staying in the inrush limiting state at all times, causing a large voltage drop across the inrush limiting resistor at higher loads.
The problem was traced to a bad rectifier diode, causing a low voltage at an auxiliary supply, once this was replaced the inrush protection functioned correctly.
Solution
Replacing the electrolytic capacitors which measured lower capacitance than expected and replacing the overheating GPIB interface IC restored full function to the CPU board.
Replacing a diode in the inrush current limiting circuit allowed the PSU to supply the full rated power without shutting down.
It now responds to commands via GPIB interface and operates correctly under the full rated load.
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