Trouble with point to point F5

NYCOne

Member
2010-07-13 6:27 pm
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?
 

NYCOne

Member
2010-07-13 6:27 pm
Pics

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).
 

Attachments

  • Front PTP F5.jpg
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  • Back PTP F5.jpg
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tinitus

diyAudio Moderator R.I.P.
2005-11-24 1:47 am
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
 

NYCOne

Member
2010-07-13 6:27 pm
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.
 

juma

Member
2005-11-08 11:27 am
Zemun
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...
 

NYCOne

Member
2010-07-13 6:27 pm
I know how to check a mosfet, and a JFET too.

I was hoping to get a heads up before desoldering.

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...
 

NYCOne

Member
2010-07-13 6:27 pm
You have learnt your lesson.
It might actually be quicker to build another one with new parts on a fresh piece of proto board

@Melon Head - I'm not sure what lesson I "learnt".

I did, however, take apart the entire blown circuit and test every component to see what damage had occurred. It was not as bad as I expected. I lost the IRFP240, IRFP9240, and one 0.47 ohm resistor. Everything else was intact.

I'll put this back together with replacements for the damaged parts. I'll likely shift the component placement a bit to make some new ideas easier.