| Re: Kustom 100 blowing fuse [message #27328 is a reply to message #18016] |
Sun, 19 May 2019 15:13   |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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If you touch the two meter leads directly together what does your meter read? I will guess that you are reading the resistance of the two meter leads, maybe 0.3 ohms plus the resistance of the ballast resistor 0.51 ohms.
No the off value of the resistor will not cause a fuse to blow. The main reason that fuses blow is shorted transistors, diodes or other parts.
In very broad terms, the power amp is designed to work as a balanced system. The upper part amplifies the positive half of the audio signal and the bottom half amplifies the negative side of the signal.
When something happens that upsets the balance between the two halves there is a larger amount current drawn and that's when the fuse blows. If you really want to fix your amp, we will help you get there, but you need to start taking a more systematic approach to the repair, otherwise you will continue to throw away time and money on parts.
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| Re: Kustom 100 blowing fuse [message #27346 is a reply to message #18016] |
Tue, 04 June 2019 23:28 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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The generic number for that diode is 1N3754.
It has not been made for about 20 years now, so they are difficult to find. Is your's dead?
You can replace it with a typical signal diode like a 1N4148, but you will need to try and figure out a way to mount it close to the power transistor heatsink. It is supposed to sense the temperature of the output transistors and adjust the bias as the transistors heat up.
As for your capacitor question, I assume that you are asking about the tantalum caps. The dot usually identifies the positive lead, but there is a marking style that uses a color dot to signify part of the caps value (color code). Are the caps marked with the usual number markings?
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| Re: Kustom 100 blowing fuse [message #27348 is a reply to message #18016] |
Wed, 05 June 2019 12:26 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Last night I pulled out my Hustler chassis and looked at the tantalum caps. There were a few different styles, but all were value marked with numbers and some had red dots and some had white dots which noted the positive lead.
So to answer your question, yes the dots are the positive leads.
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| Re: Kustom 100 blowing fuse [message #27354 is a reply to message #18016] |
Wed, 05 June 2019 23:45 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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The transistors leads will line up with the pc board correctly as long as the replacement transistors have the same case and pin out as the original ones. Of course, the transistors could be inserted into the wrong places on the pc board.
I couldn't get your image to open so I don't know what the diagrams show.
If you want to take photos of the board and email them to me, I will review them for you.
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| Re: Kustom 100 blowing fuse [message #27356 is a reply to message #18016] |
Thu, 06 June 2019 01:36 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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I checked my board and there are 4 tantalum caps on there. Looking from the filter cap side of the board, there are 2 27uF caps in the center of the board. They both mount with the positive leads facing towards the filter caps.
At the lower right side of the board there is a 1uF cap with the positive side towards the filter caps.
Nearby is a 33uF cap that has the positive side facing away from the filter caps.
I've sometimes been able to grind away some of the epoxy filler from the bottom of the 1N3754 diode around the broken off lead. Just enough to solder a wire to the stub end. I would suggest that for the time being just replace it with a 1N4148 or similar diode until you get the problems straightened out. In fact you could just use a jumper wire to replace it for the time being.
Still looking at the board from the filter cap side, the two driver transistors are located at the upper left side. The outermost one should be a 128 and the inner one should be the 129.
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| Re: Kustom 100 blowing fuse [message #27358 is a reply to message #27356] |
Thu, 06 June 2019 12:08 |
timetraveler
Messages: 25
Registered: April 2019
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Junior Member |
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the c2 (.1uf) its a larger black toward top center) capacitor has a white line on one side im guessing that is the negative side and looking at the schematic the negative side goes towards the filter caps is that correct?
Im checking the output transistors to make sure they arent grounding to the chassis. I put one test lead on the back of the actual transistor and the other test lead to the chassis and i have it on ohms setting. it is reading ohms when i do this. am i doing that correctly? I used the plastic spacers and the heat goop stuff (messy as heck)
[Updated on: Thu, 06 June 2019 12:10] Report message to a moderator |
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| Re: Kustom 100 blowing fuse [message #27361 is a reply to message #18016] |
Thu, 06 June 2019 15:34 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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C2 is not polarized, so it shouldn't matter how it is inserted into the board.
Does your meter have a diode test setting? If it does use it to check for continuity from the output transistor cases to the chassis. If the transistors are not connected to the rest of the circuit, there will be no reading between the case and the chassis. If the red wires are connected to the transistors, there will be a slowly rising reading as the filter caps will be charged by the meter battery.
No matter what, there should not be a steady zero ohm reading from the case to the chassis.
Steve, if you are going to walk him through this repair, I will defer to you. He is getting too many instructions to follow and he is bound to get mixed messages from too many of us.
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| Re: Kustom 100 blowing fuse [message #27368 is a reply to message #18016] |
Fri, 07 June 2019 15:28 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Before you order new parts, remove the red wire from the transistor and pull off the connector plug. If the transistor case still reads any resistance other than open in reference to the chassis, then you know that the insulator or the washers have failed.
The insulators are made from mica and are available from places like Mouser. They may also be sold on Amazon or eBay. Look for TO-3 insulator or TO-3 mounting hardware.
Please check the nylon step washers on the two mounting screws as well, because they can crack and cause the screws to touch the chassis as well.
If you need them and can't find them, I can look them up for you.
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| Re: Kustom 100 blowing fuse [message #27379 is a reply to message #18016] |
Sat, 08 June 2019 12:01 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Then there is no problem with the insulators and mounting hardware.
Test each transistor with the diode test function of your meter. You can leave the red wires connected or remove them, it doesn't matter. Pull the black connector plug and note which pin the blue wire was connected to. That pin is the base. The yellow wire connects to the emitter.
Test by putting the red lead to the base pin and then touch the black lead to the emitter pin and then the collector mounting screw. You should get a reading of approx. 0.6 volts on the meter with both connections. Next reverse the two meter leads putting the black lead on the base and touch the emitter and collector with the red lead. You should get an open reading on the meter (the same as when the two leads are not touching anything).
Finally test the collector to emitter junction by touching the red lead to the collector mounting screw and the black lead to the emitter pin (yellow wire). Also test by reversing the two meter leads. In either test, there should be an open reading on the meter.
Something to watch out for is the fact that on these chassis', the two output transistors are mounted in 180 degrees from each other, that is the emitter and base pins will not appear on the same side.
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| Re: Kustom 100 blowing fuse [message #27384 is a reply to message #18016] |
Mon, 10 June 2019 19:47 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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The next tests should be of the transistors on the pc board. You need to test all 7 of them. Start with the driver transistors Q3, Q4, and Q5. Then test the protection circuit transistors Q6 and Q7, and finally the two input transistors Q1 and Q2.
There are 4 PNP transistors and 3 NPN transistors. The difference will mean that you will get low readings from base to emitter and base to collector with the red meter lead connected to the base or with the black lead connected to the base of the transistors. Typically the NPN's Q3, Q4 and Q6 will read low with the red lead on the base and the PNP's will read low with the black lead on the base.
There will be a little problem with testing the two protection transistors as there are low value resistors that connect the base and the emitter. As long as there are no zero readings, you can probably assume that the transistors are okay. If there is any real doubt, then remove them from the board to test them.
Finally test all 5 diodes in the circuit. With your meter, they should read just like a transistor junction, low reading in one direction and open in the other.
Tell me what transistors and diodes you have replaced, and what you used as replacements for them.
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| Re: Kustom 100 blowing fuse [message #27387 is a reply to message #18016] |
Wed, 12 June 2019 12:09 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Good, you're making some progress.
Right now, are all of the sections of the amp connected to the power supply. Preamp, FX and power amp? If they aren't, reconnect them and see if the bulb stays dim.
If all goes well, keep the light bulb limiter in circuit and add the speaker to the mix and see if the light bulb stays dim. If it does, plug a guitar in and see if the amp is passing signal.
What did you do about the bad thermal diode?
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| Re: Kustom 100 blowing fuse [message #27388 is a reply to message #27387] |
Wed, 12 June 2019 21:24 |
timetraveler
Messages: 25
Registered: April 2019
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Junior Member |
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replaced the thermal diode. didnt think to do the light bulb test with a guitar plugged in but i played through it without the limiter and it sounds nice. i guess i can still test it with the limiter hooked up. thanks for all the help. do you guys work on other amps? might need your help again
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