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Old 7th November 2010, 06:49 AM   #1
NYCOne is offline NYCOne  United States
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Default Trouble with point to point F5

I'm trying to build a number of F5s for experimentation with a balanced IRFP version.

I did a point to point on a blank PCB and when I test it, the 3w 100 ohm resistors get extremely hot very quickly. I'm sure I did something wrong, but does the 3w 100 ohm resistor tell what?
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Old 7th November 2010, 08:18 AM   #2
juma is offline juma  Germany
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Quote:
Originally Posted by NYCOne View Post
...does the 3w 100 ohm resistor tell what?
Yes it does, without a signal present only a negligible ammount of current should run through it.
Your 100R resistor is either connected to wrong point in circuit or your amp's output sits on rail voltage (there ia a large voltage across that resistor).
Clear, high resolution picture would help.
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Old 7th November 2010, 02:01 PM   #3
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It could be just a wrong trimmer position at initial startup. Check both to make sure they are at zero ohms.
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Old 7th November 2010, 04:31 PM   #4
NYCOne is offline NYCOne  United States
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Thanks. I'll check both tonight.
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Old 7th November 2010, 06:03 PM   #5
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When I did my first point to point F5, in my excitement I forgot to connect the trimmers to the power rails.
Fortunately when I powered it up I used a current limiting power supply which saved my ****.
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Old 7th November 2010, 08:46 PM   #6
NYCOne is offline NYCOne  United States
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OK, don't make fun - I have a terrible soldering iron, and my first circuit I ever tried was a couple of months ago. I'm sure I could have done better on a number of fronts.

Please note, I twisted the 1K and the 100K (R9 and R10) on the input together (not yet soldered), and the same with the two 10 ohm resistors (R1 and R2 - I grounded them for the initial test, but I want to leave them unattached for the balanced version). When I tested the board, I grounded the 100K (R10) and R1 and R2.

Here are the front and back of the PTP F5 (one side).
Attached Images
File Type: jpg Front PTP F5.jpg (577.7 KB, 366 views)
File Type: jpg Back PTP F5.jpg (684.0 KB, 345 views)
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Old 7th November 2010, 09:22 PM   #7
tinitus is offline tinitus  Europe
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I suppose you have a drawing of your layout

might be better to look at

but it looks like a few of your solder points might be close to shorts
more specificly, Im looking at one of your trimpots
check all that are very close

and there appears to be some cold soldering, on one side especially
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Old 7th November 2010, 10:52 PM   #8
NYCOne is offline NYCOne  United States
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I think I have good joints, it looks cold at some PCB spots, but the legs of the resistors are twisted and soldered too. I think I may have a bad trimmer. I'll know better after I try it later tonight.
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Old 26th November 2010, 05:30 AM   #9
NYCOne is offline NYCOne  United States
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In finally had a chance to track down the problem, and I'll say I'm very stupid -

I connected the output devices P-side to N and vice versa.

I corrected the problem, and the 100 ohm resistors are cool now. The issue is what damage I may have done to the components. The 2SJ74 side of the circuit seems to bia correctly, but the 2SK170 side of the circuit reads 0v when I check the bias around R3, and 0v when I check around R11. What might be fried on the 2SJ74 side?

Is it possible I blew out my MOSFETs too?

My only test equipment is a $7 DM.
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Old 26th November 2010, 11:11 AM   #10
juma is offline juma  Germany
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Since there was a considerable current running through 100R resistors, there is a chance that your MOSFET is dead - check it and you'll know - please don't ask "how do I check if the MOSFET is OK?" 'cause you'll be the 10,000th member with the same question and they'll be obligated to reward you with a 7-day Bahami cruise tour...
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