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9th December 2003, 11:10 AM
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#1 |
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diyAudio Member
Join Date: Apr 2002
Location: Culemborg, The Netherlands
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Learned a lesson today
Learned an important lesson today. Did some rework on my PSU, and connected it all together again. The HT of the driver tube measured 200V right channel and 150V left channel (under load). Without load both channels measured the same. Very strange, it used to match very closely. After tripple checking everyting I found out that the first cap in the PSU was dead. It is a millspec 2uF/250V PIO, that now measures only 0.38nF on my DMM
 Replacing the cap solved the problem.The cap was rated 250V DC (75 deg) / 150V DC (85 deg), so it was simply underrated. It's a design flaw that never showed, till now. The reason I did the rework on my PSU is that it got too hot inside. Probably the cap died on a hot day when the PSU was getting very hot during testing. It gets 250V DC without load. I'm very happy the thing didn't explode or shorten things.... |
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12th December 2003, 10:13 PM
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#3 |
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diyAudio Member
Join Date: Sep 2003
Location: Alps:Tube amp designs over 150W, SMPS guru.
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That C3 is a vulnerable position for any cap as it has to accept a massive peak voltage and considerable voltage swings. The oil/ paper versions used in older equipment (although bulky) were the best, although polypropylene or sim H.V types will work. Don't do as someone else did, fit an electroytic in this position. That is extremely dangerous practice.
rich |
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13th December 2003, 02:45 AM
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#4 |
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diyAudio Member
Join Date: Jun 2003
Location: Woodlands Circle
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though if he connected 2 4uFs back to back it wouldn't kill the cap...get those long life 105deg ones...
__________________
Kids in the back seat cause accidents...Accidents in the back seat cause kids... |
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13th December 2003, 02:57 AM
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#5 | |
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diyAudio Senior Member
Join Date: Aug 2002
Location: Belgium
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Hi,
Quote:
Cheers, 
__________________
Frank |
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13th December 2003, 12:22 PM
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#7 |
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diyAudio Member
Join Date: Apr 2002
Location: Culemborg, The Netherlands
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Replaced the cap with this one. But I also use Philips Longlifes 105deg in the supply.
Inside the PSU |
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13th December 2003, 04:40 PM
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#8 |
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diyAudio Member
Join Date: Sep 2003
Location: Alps:Tube amp designs over 150W, SMPS guru.
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Inside that incendiary box........the central / upper part of the photo.....that bit above the 10H 20mA choke........I can't quite make out the value of that series dual resistor chain on top of those caps ? It looks like 2.7R.......Yes /No ? My comp' monitor isn't the best for colour. Any more on the circuit ?
rich |
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13th December 2003, 05:48 PM
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#9 |
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diyAudio Member
Join Date: Apr 2002
Location: Culemborg, The Netherlands
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first prize
 2R7 it is ...used in the 5687 12.6V heater supply.Latest PSU Latest Amp Building pictures pictures 1-45: prototype 55-100: first version of PSU 100-end : 2nd/current version of PSU. Ralph |
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18th December 2003, 06:35 PM
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#10 |
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diyAudio Member
Join Date: Sep 2003
Location: Alps:Tube amp designs over 150W, SMPS guru.
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Yes I was curious what it was doing there, fooling me in thinking 2R7 in HT supply wasn't going to do anything ! So there it is, in the heater supply. Will look at more later
regard rich |
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