STE power supply problem / switches on and off constantly

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Chandler
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STE power supply problem / switches on and off constantly

Postby Chandler » Tue Feb 20, 2018 9:34 pm

I have a DSP 508A type PSU in an STE which looks like it hasn't been used that much

This problem randomly started the other day, after some use it switched off and then on again, so i changed all the caps and now it switches on and off very fast so obviously the computer doesn't start at all

If you can imagine the green power LED goes on and off constantly, say on/off in a second constantly in a loop

I have since changed the caps again and the same problem

I have also changed the BU508A transistor on the heatsink and the opto coupler, same problem

Does anyone have any idea what else i could look at? :?:

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Re: STE power supply problem / switches on and off constantly

Postby Dal » Tue Feb 20, 2018 9:44 pm

Disconnect the PSU from the board and check the 5V line (preferably on an oscilloscope if you have one) to see if it’s constant. If OK, then it might be worth replacing the 4700uf cap on the board.
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Re: STE power supply problem / switches on and off constantly

Postby czietz » Tue Feb 20, 2018 9:57 pm

Check if there is any protection (overvoltage or overcurrent) circuit that might be erroneously triggered.

siriushardware
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Re: STE power supply problem / switches on and off constantly

Postby siriushardware » Sat Mar 10, 2018 9:24 pm

Disconnecting the power supply output from the ST motherboard and seeing if the output voltages are steady and normal is not an absolute test for any SMPSU because it is possible for one to output good voltages under no-load conditions but then collapse or oscillate under what should be permissible load conditions.

What are the specified maximum output currents on 12V / 5V from a typical ST PSU?

The ideal (simple) way to load test a power supply is to use suitably high wattage resistors of appropriately low resistance calculated to draw almost the maximum allowed current for each rail, but as most people won't easily be able to lay hands on such resistors, a good cheap and widely available alternative would be a selection of automotive filament light bulbs wired either singly or in series or parallel to draw near maximum load from each rail.

Plus, the use of bulbs as loads gives an immediate visual indication whether the output is steady or oscillating on and off.

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Re: STE power supply problem / switches on and off constantly

Postby 1024MAK » Sun Mar 11, 2018 12:07 pm

The regulated rail is the 5V output. So it's this that should be loaded. I suggest 6V motorcycle lamps for use as test loads. You don't have to load the PSU anywhere near maximum for a basic test. A 5 to 10W lamp (or combination) should do. Until the 5V line is working, don't worry about the other outputs.

One possible cause of a switch mode PSU cycling, is the loss of the 'normal running' power feed to the control circuitry. Then the control circuitry will use the 'start-up' resistor feed, it will run very briefly, then 'stall' (as the 'start-up supply cannot supply enough current), then the cycle repeats. So look for dry solder joints/poor connections, broken tracks and check any individual diodes.

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Re: STE power supply problem / switches on and off constantly

Postby siriushardware » Mon Mar 12, 2018 6:20 pm

For a basic test, I agree that any middling load will do, but as a (general) electronics repair technician for many years I have seen quite a few cases where an SMPSU would hold up its output until the load exceeded a certain percentage of its specified capability, and then collapse or oscillate off and on. Sometimes 30%, sometimes 80%.

Usually power supplies which do this are suffering from tired capacitors, and so the extent to which they can or can not supply current depends on just how tired the capacitors are (but note that capacitors have already been ruled out as the cause in the OP's case). For that reason, I always do test SMPSUs at close to their maximum rated load.


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