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Old 15th November 2012, 09:53 AM   #1241
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When they were manufactured at the beginning, they did the same. It was good if I had read this, couple of weeks before. I would have dropped my RMI-FC100 project. Now the thermaltrack order has already gone.
MAY GOD BLESS ME and I do not get these f.ke tranys.
Gajanan Phadte
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Old 15th November 2012, 11:23 AM   #1242
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Quote:
Originally Posted by DRZ1 View Post
Well, it's worse than i thought. At lightly warm temp NPN diode = 0.67V and PNP = 0.52V.... Schotky diode in place of them ?....
Denis
Bypassing one of the five diodes with a Schottky will work. Schottky have higher tempco than regular silicon diodes. Togheter with the somewhat higher serial rezistance needed (with very low tempco compared to the diodes) the resulting compensation should work. Sanken also used Schottky diodes togheter with junction diodes in their SAP series.
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Old 15th November 2012, 11:48 AM   #1243
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I can second what Franz said. I have bypassed one internal TT-Diode by a Schottky BAT46 diode, when I built my FC-100 prototype,
and the TEMPCO consisting of 4 TT-diodes and the Schottky works absolutely stable.
The VF across 2 emitter 0R1 resistors returns to the adjusted value (quiescent current / BIAS adjustment) within the blink of an eye!

I have ordered about 180 NJL0302DG/NJL0281DG from MOUSER currently (still waiting for delivery), needed for a current group-buy,
and wonder, how the TT-diodes will measure (behave).
I will tell you.

Best regards - Rudi_Ratlos
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Old 15th November 2012, 01:03 PM   #1244
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On one channel I seem to have a problem with the TT diode bias current. Recognising the risks of fuse blowing and worse I have avoided breaking into the diode string to do a current measurement; instead estimating the figure from the voltage across R17.

I=E/R .33V/150R=2.2mA I presume Q19,Q20 Ib's are shunting some current away from the diode string.

To get a more accurate figure I dialled up 100R in the pot, and while measuring the voltage across this things started to go awry. I need to understand what it is that DEFINES the 2mA TT diode current. The current mirror Q6,Q7 only ensures equality of the currents between Q14, Q16 rather than the value. The value is 8.25 mA minus 13.5mA/2 plus ?

Looks like I may have to back track somewhat and power up the front end only per post 640 and lift one end of R7 and R37 (base stoppers). Any insights appreciated. Wouldn't be surprised if another suspect Toshiba device has gone leaky etc.

Keith
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Old 15th November 2012, 01:52 PM   #1245
DRZ1 is offline DRZ1  Canada
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Quote:
Originally Posted by Rudi_Ratlos View Post
I can second what Franz said. I have bypassed one internal TT-Diode by a Schottky BAT46 diode, when I built my FC-100 prototype,
and the TEMPCO consisting of 4 TT-diodes and the Schottky works absolutely stable.
The VF across 2 emitter 0R1 resistors returns to the adjusted value (quiescent current / BIAS adjustment) within the blink of an eye!

I have ordered about 180 NJL0302DG/NJL0281DG from MOUSER currently (still waiting for delivery), needed for a current group-buy,
and wonder, how the TT-diodes will measure (behave).
I will tell you.

Best regards - Rudi_Ratlos
My NJLs was from Digikey here in CANADA. My first FC-100 was NJLs sample directly ordered from ON semi.

Good Luck
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Old 17th November 2012, 12:35 AM   #1246
DRZ1 is offline DRZ1  Canada
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Bingo, i added a Shottky diode in place of NPN ThermalTrack diode, keep the 3 PNP diodes and 1 NPN diode.

1 NPN , 3 PNP, 1 Schottky on the heat sink near the hottest transistor, that's it. Lightly over-compensated (best for me because my heatsink are borderline). seems to work great.
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Old 19th November 2012, 04:13 PM   #1247
RKH is offline RKH  United States
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Would someone be willing to show a quick sketch/schematic of this 'fix'. I want to understand it better before I try to implement it.

Thanks,
Ryan

Quote:
Originally Posted by DRZ1 View Post
Bingo, i added a Shottky diode in place of NPN ThermalTrack diode, keep the 3 PNP diodes and 1 NPN diode.

1 NPN , 3 PNP, 1 Schottky on the heat sink near the hottest transistor, that's it. Lightly over-compensated (best for me because my heatsink are borderline). seems to work great.
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Old 19th November 2012, 04:44 PM   #1248
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Ryan: as shown on the picture below I have bypassed one of the 5 NJL - TT-diodes by a Schottky diode BAT46.
The Schottky diode has a Vf of about 250mV, whereas Vf of an internal TT-diode is about 500mV.

Best regards - Rudi
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Old 20th November 2012, 06:19 AM   #1249
RKH is offline RKH  United States
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Thank you Rudi! That clarifies it for me.

Ryan
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Old 20th November 2012, 09:47 AM   #1250
AndrewT is online now AndrewT  Scotland
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I don't recommend the shottky modification unless you are prepeared to check properly that the tempco is close enough to correct with just 4 diodes left in place as the bias voltage generator.

I would much prefer to believe the designer when he/she says use 5 diodes for tempco.
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