Help - Extended Foreplay III smoked!

ike · 13337

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Offline ike

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on: January 30, 2010, 03:08:57 PM
My worst fear came into fruition, I just finished putting in the extended FPIII.  Smoke blew upon initial power up...
FYI: I accidentally twisted off one of lead of the 270 ohm resistor.  I replaced it with a 1/2 watt version instead of a 1 watt, couldn't find one in Radio Shack.  Could that have course the smoke... it was pretty scary!  thanks in advance


Offline JC

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Reply #1 on: January 30, 2010, 04:06:09 PM
I do not know this circuit, but you very well could have smoked your undersized resistor.  And, if it was carbon composition, it would tend to be pretty smoky, particularly if it scorched a printed circuit board while it was destroying itself.

Your Ohmmeter will tell you for sure, if it is not readily apparent.  Some de-soldering braid will help you remove it, if it is indeed mounted on a PCB.  And, two 560 Ohm, 1/2 Watts in parallel would net you 280 Ohms with 1 Watt of dissipation.  But, again, I don't know the circuit to be able to tell you if the extra ten Ohms is acceptable.

Jim C.

Offline 2wo

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Reply #2 on: January 30, 2010, 05:50:52 PM
Well, step one. is the 270 ohm burnt?

John Scanlon

Offline Paul Joppa

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Reply #3 on: January 30, 2010, 07:24:39 PM
There are four 270 ohm resistors, two in each power supply. They do not normally dissipate a great deal of power. However, if any of the diodes or capacitors in the power supply is in backwards, that will result in excessive current. It will also damage the capacitors and may damage the diodes.

Paul Joppa

Offline ike

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Reply #4 on: January 31, 2010, 04:28:49 AM
yes, the two 1/4" watt 270 ohm resistors is toasted, the side where I placed the 1/2 watt resistor... beside replacing the resistors on that one side.  Do I need to replace anything else, you mentioned diodes?  thank you


Offline Grainger49

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Reply #5 on: January 31, 2010, 05:20:52 AM
I think PJ is suggesting that you check the diode orientation.  You should have had them working well before extending but IIRC the Extended goes to dual mono power supplies.  That opens the opportunity to get diodes and capacitors in the wrong orientation.

Check and post back.


Offline ike

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Reply #6 on: January 31, 2010, 08:41:03 AM
just checked the orientation and it is correct.  The caps and diodes seems fine.  Regardless, do i need to replace them?  thanks guys!


Offline Grainger49

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Reply #7 on: January 31, 2010, 09:10:23 AM
Yes, from what you say the resistors have changed value.  Any that have changed value should be replaced.  But there is something that caused it.  That should be found before you apply voltage to it again.


Offline ike

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Reply #8 on: January 31, 2010, 11:06:51 AM
OK, I'll change all the parts in that side of the power supply.  One other thing that puzzled me when I was putting this together: On page 28, where a 1.2 ohm 3w resistor is piggybacked to the existing .62 ohm 5w.  The instruction states to connected to terminal H4 and H5.  However, the photo looks like it is connected to H2 upper and H5 lower. and that's what I did.  Could this have cause the burn out? thanks


Offline Grainger49

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Reply #9 on: January 31, 2010, 11:32:32 AM
Ike, I don't have the instructions so I can't help here. 

PJ might post maybe Paul B.


Offline Paul Birkeland

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Reply #10 on: January 31, 2010, 08:27:51 PM
Quote from: ike on January 31, 2010, 11:06:51 AM
OK, I'll change all the parts in that side of the power supply.  One other thing that puzzled me when I was putting this together: On page 28, where a 1.2 ohm 3w resistor is piggybacked to the existing .62 ohm 5w.  The instruction states to connected to terminal H4 and H5.  However, the photo looks like it is connected to H2 upper and H5 lower. and that's what I did.  Could this have cause the burn out? thanks

The 1.2 ohm just straps right on top of the existing .62 ohm resistor.  If you built a stock FP-III and it was in working condition, strapping the extra resistor on top will not cause the kind of destruction you describe.  I don't have the manual next to me, but the resistor combination will go between the H terminal that receives the two banded ends of the 1N5820 diodes and the terminal with the non-striped side of the blue electrolytic capacitor attached. 

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline ike

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Reply #11 on: February 01, 2010, 04:52:58 PM
Okay, I figured out my bone-head mistake...
I made a call this evening to Bottle Head HQ, spoke to Sean the very patient and knowledgeable tech support.  I told him about the replacement of the 1/2 watt 270 ohm resistor, instead of a 1 watt piece.  Then he corrected me by asking "you mean a 270 K ohm, right, Ike".  I seem to agreed but not completely sure.  As soon as I finished speaking with Sean and ordering the replacement parts from the Queen.  I uncovered my replacement resistor and there it was, a pack of 270 ohm resistor not 270 K.  mystery solve, for now... thanks everyone and especially to Sean, my hero.


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