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Old 7th June 2009, 07:13 PM   #1
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Default Documenting SimpleSE FRED Problem

I haven't noted quite this issue mentioned previously.

First power up, all transformers connected - no switches, including stand-by. No tubes. Edcor 120V:740V transformer. Blew 2A fuse.

Replaced 2A fuse, tried again thinking bad luck. Powered up, blew 2A fuse and put a hole in a C-90 thermistor.

Disconnected power transformer, powered transformer, no problem.

Put El Cheapo meter across secondary and powered up, blew fuse. Looked like shorted transformer secondary but it measured a pretty reasonable 49 ohms.

Checked HV connector on board and found high resistance continuity.

Removed DESI-12 FREDS (TubeLab specified parts) and all is well.

Initial power up shorted FREDS even though they weren't in-circuit (no switch). Subsequent testing of transformer secondary with El Cheapo meter shorted meter (meter rated at 750V, guess there's no margin in that rating).
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Old 8th June 2009, 12:02 AM   #2
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Look at the schematic. If a FRED shorts, it WILL short the high voltage secondary regardless of whether SW1 is closed or not. If you must use the FREDs, make sure you've got the Fairchild parts, not the IXYS.

See the memo here.
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Old 10th June 2009, 01:59 PM   #3
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Thanks, and thanks for the link.

I realize the FREDs will short the HV. I was trying to emphasize that it wasn't related to switching transient (stand-by switch) as I had read on the TubeLab site.

But thinking about it, I guess a switch is a switch regardless how far upstream it is.
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Old 10th June 2009, 10:08 PM   #4
chrish is offline chrish  Australia
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With that transformer a GZ34 is going to give you around 450V (+/-), just what you want, and slow warmup is good for your output valves. I'd just keep the valve rectifier and cut out the SS completely.
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