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Old 10th November 2004, 09:34 PM   #1
larryg is offline larryg  United States
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im trying to test one channel but power-up blows the neg. rail fuse. im using the same side of the psu that i used to test the other channel [tested ok] this would seem to be easy to spot, that meaning,
a wire in the wrong spot or a short to gnd. i've compared this channel with the other good channel but find nothing wrong.
i checked the output trannys for shorts or grounds but checks ok.
can anyone who has encounterd this problem help me narrow down the most likely area to check?
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Old 10th November 2004, 10:20 PM   #2
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Is this board attached to its own heatsink and MJ15003/MJ15004?
If so, then follow Prof. Leach's guide:
Hints if Problems are Encountered

If problems are encountered, a list of things to check is as follows:

* Double check all wiring.
* Check to see that the bias diodes D1 - D4 are installed correctly, that none are cracked, and that the attaching leads make proper contact.
* Check the orientation of each diode and the polarity of each electrolytic capacitor.
* Check the orientations of Q1 through Q11.
* Are all transistor part numbers correct?
* Are any NPN and PNP transistors interchanged? I have embarrassed many students when I found this error in their amplifiers.
* Do the leads to the heat sinks connect to the right points on the circuit boards? I recently saw a student's amplifier where the base leads for the npn and pnp output transistors on one channel were reversed on the circuit board. The bias potentiometer would not adjust the bias current in that channel.
* Are all resistor values correct? (The resistor color code is: 0-black, 1-brown, 2-red, 3-orange, 4-yellow, 5-green, 6-blue, 7-violet, 8-grey, 9-white. The third color band is the multiplier, i.e. the number of zeros. As an example, brown-red-orange is 12000 ohm).
* Check for short circuits from ground to the leads of each power transistor. This indicates a short circuit in the heat sinks.
* Is the loudspeaker output shorted to one or both power supply leads? If so, one or more power transistors are blown.
* Does the ohmmeter give a short circuit between the collector and emitter of any transistor? If so, that transistor is probably blown. Bad transistors usually become a short circuit from collector to emitter.
* I have occasionally seen protection circuit transistors Q8 and/or Q9 shorted from collector to emitter. The amplifier will be completely dead if this happens.

Also check for grounding between any transistor and the heatsink/chassis. Use an ohmmeter to check for short circuits from each transistor lead to a bare metal spot (not the black anodize finish) on the heat sinks.
Is P1 set to minimum value or wide open. i.e. set P1 on each circuit board for maximum resistance.

Diodes D1 - D4:
Double check the diode assembly. If a diode is backward, the output transistors can blow. You can check the diode polarities one at a time with an ohmmeter. The ohmmeter will not have enough test voltage to forward bias all 4 diodes simultaneously. If there is a short circuit between a diode lead and the heat sink metal, the amplifier will not work and the output transistors can be blown.

You already checked for solder bridges?

One of these things has been the culprit on any of the 10-12 Leach amps I've built.
Luck to you.

Prosit
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Old 10th November 2004, 10:39 PM   #3
larryg is offline larryg  United States
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thanks acenovelty! i went through the list prof. leach shows; the board is on its heatsink. i checked all the trans. for shorts to the
heatsink and collector to emiter. i've checked all diodes for polarity. i've checked each wire from the board to the output trans. the speaker wire, positive, is not connected. i've checked
each trans. for type and placement. i sure i'm not seeing something, for obliviously there is an error. also note that nothing on the board went up in smoke. where to look?
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Old 10th November 2004, 11:00 PM   #4
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Are diodes D1-D4 in correct orientation and correctly wired to R25/R26?
Did you put 100 ohm resistors in place of the power supply fuses that will limit the current. If something is wrong they will let out the magic smoke. The dc voltage drop across the 100 ohm resistors should be less than about 2.5 V (25 mA or less) if nothing is wrong.
Is P1 on the circuit board set for maximum resistance?
Power and audio grounds connected to chassis?
Is any part of this problem board/heatsink touching the chassis? Looking for a grounding fault to a power rail.
Plus voltage is connected to plus on the board/negative to negative?
Running out of ideas.

Prosit
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Old 11th November 2004, 02:45 PM   #5
larryg is offline larryg  United States
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i've checked all the diodes found ok. checked for short on the power rails to ground found ok. the board and heatsink were of to
the side and not touching the chassis. however l do have a ground wire connected from the heatsink to the star ground on the chassis. i did the same on the other channel and it worked ok.
should this ground not be connected? also checked each rail lead
for shorts to ground and found nothing. anyone have an idea where to look?
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Old 11th November 2004, 04:56 PM   #6
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Are you using T03 version transistors on the heatsink? Are they mounted directly to the heatsink or in a socket? The mounting screw(s) thru the heat sink can be just close enough to make contact and short out under power.
Any chance you can post a pic of the board/wiring? Hard to mentalize your layout.

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Old 11th November 2004, 08:04 PM   #7
larryg is offline larryg  United States
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thanks acenovelty! im using the to-3 trans. the sockets are the slim profile ones. i guess they are your basic sockets. they do have a nice locator, deep, for the screw holes. i might have found my problem, but not 100% sure. on the heatsink are the bias diodes. i visualy checked them before but decided to check them a little closer. i removed the shrink tubing from the 3 connections
and found that i had not soldered two of them,only twisted. pretty dumb i guess! would this have blown the neg rail fuse?
before i proceed any farther, i'll wait for your opinion. thanks for your help!
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Old 11th November 2004, 10:10 PM   #8
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Very likely you found the cause. And yes this would blow the fuse. Verify that the diodes are in series proper polarity order and connected to R25/R26 correctly. If your meter can check diodes you should do this before applying power again. Check one diode at a time by putting the probes on each side of one diode, then reversing the probes on that diode to be sure it did not get killed. Probably not, but good to check.
This time, let's put a 100 ohm 1/4W resistor in place of the fuse and bring up the board with a variac slowly to 110V AC. No magic smoke out of the resistor and you're golden. No variac? Then the 100 Ohm resistor will smoke real quick if something else in the chain is damaged.
Crossing my fingers for you.

Prosit
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Old 11th November 2004, 10:18 PM   #9
larryg is offline larryg  United States
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thanks acenovelty! i'll do as you suggest, though i don't have a variac. i'll check the bias diodes! thank you for your help!
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