Hobart 6T28-600CL Operation And Maintenance Manual page 36

OM-2244 / Operation and Maintenance Manual
DCS-600/ Series 500082 / Solid State Transformer-Rectifiers
b) Troubleshooting Shorted SCRs and Diodes
WARNING
Electric shock and fire can kill!


(1) Turn off the unit and turn off the input power at the utility source.
(2) Disconnect the leads from the transformer to the heat sink assembly.
(3) Wrap each disconnected transformer lead with electrical tape. Mark each lead to identify it so it
can be reconnected to its proper location.
(4) Check the SCRs and fly-back diodes with the ohmmeter to located shorted devices.
(5) With the SCRs and fly-back diodes disconnected, apply power to the unit. Make sure that the
other components are not the cause of the tripped breaker.
(6) If no problems have been found, check for a device that only breaks down to a shorted condition
when voltage is applied by reconnecting one device at a time.
c) Troubleshooting Excessive Output Ripple
An open gate or an open SCR cannot be checked with a VOM. If an SCR is not firing, the AC ripple
current will increase across the filter capacitors, but that will not blow fuses.
Follow these steps to locate a bad SCR:
(1) Connect an oscilloscope across the unfiltered output:

Connect the probe of an oscilloscope to the heat
sink

Connect the isolated neutral of the oscilloscope to
the braid of the fly-back diode.
The output should appear as six evenly spaced pulses
of about the same height. If an SCR is not firing, one of
the pulses will be missing.
Note: if every third pulse is low or missing, check the
balance adjustments, R9 and R10, before attributing
the problem to faulty components.
(2) Disconnect the gate lead for one SCR from the
applicable suppressor board. If the lead disconnection
does not affect the output waveform, that is probably
the bad SCR.
(3) If that SCR was not the source of the problem,
reconnect the gate lead.
(4) Repeat for other SCRs, one at a time, until the
defective device is located.
December 15, 2014
Do not touch energized parts. Always turn off utility power to the unit before
touching anything inside.
Do not leave the GPU on long enough to overheat or fail in the faulty condition.
Figure 1 Gate Lead Connections
Chapter 2-1
Page 4