basic electronics question

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pishta

pishta

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rebuilding a Crate BFX100 bass amp. IRFP140 https://www.vishay.com/docs/91202/91202.pdf (at end of red trace) N channel Output MOSFET was blown (shorted) so I replaced it. Checking the other passives before I powered it up I noticed that a .1ohm 3W cermet resistor R53 was in series and the ohm value was way off, like shorted 0 ohm. This is on the -40V DC leg going straight to the source pin of the MOSFET. What should this -40V voltage be through the .1ohm 3W resistor at the S pin of MOSFET Q6? I think this resistor going to short caused the MOSFET to get full -40V causing it to blow? Zener diodes D16-17 changing anything? Voltage drop through a resistor? 15V AC is test point value of Drain pin per schematic.
@KitCarlson
trace.jpg
 
You cannot accurately measure .X anything resistance. I'm sure that it DOES measure "a short." The purpose of a resistor like that is often for metering (operating drop across the resistor goes to a meter, like an external shunt ammeter), or for protection/ stabilization similar to a ballast, or to act like an emergency fuse.

In fact, setting up a current drawing situation is exactly one way to measure very low resistances........accurately measure the voltage drop with a known accurate current flowing.

I don't know anything about these amps, be aware the diagram could be drawn incorrectly as well

Also one way to test it at low power output settings and somewhat protect the circuit from further damage, is to connect the power supply with a series light bulb. A "real" light bulb. You'l have to experiment with wattage, I'd say at least 100 and maybe 200. If you can drum up some cheap sockets, you can parallel bulbs to get the wattage.
 
OK, I'm not sure my meters can even register .1 ohm as that is pretty low resistance! I realize the resolution of my meters (on hand) were not that small. Parallel bulbs to control the power, like the old model trains where the kid had 5 bulbs in a row and the train would go faster with the bulbs unscrewed. That resistor is probably good.
 
Also those types of power circuits can generate all kinds of spikes and what is known as "ringing." This is the same type of thing set up EG in an ignition coil. The (even though small) inductance in the circuitry and the very high current and switching is part of the equation. I'm actually very little help, specifically on those things, although I do assemble them --for the short term now--every day. Our power chair drive modules use 24 mosfets in what is called an "H" bridge.
 
R53 is likely sensor part of a current limit for Q6. The clip of schematic is not enough for me to get full understanding. Typically in a single low side switch, a source resistor is viewed by a base to emmiter if a small bjt, and the collector pulls fet gate low, for off when Vbe is about 0.65V. Q2 and Q4 may either provide bias for Q6, or part of current limit. C33 is large and may slow down kick in of limit. By pushing the amp to limit, it should be possible to view R53 voltage, and Q6 gate to see action of current limit. Assuming it works....

D16,17 are back to back 15V zeners, they should limit gate voltage to + - 15.7V. Q6 gate limit is + - 20V. Q6 is hefty, but could fail if open in D16,17, or long term overload. I would certainly test Q2, Q4. They are small, and if Q6 shorted, they might fail too, or failed Q6 without current limit.
 
I'm good. I baby sit, and neck deep in remodeling of wife's sewing room. Haven't worked with electronics for a couple years. Grandson is very smart, he has an interest in electronics. It's all good.
 
I am building the last of the drive modules for the "company." I will likely be gone not long after that job is done. They have given up on the "in house" electronics package for a number of reasons, and selling only a couple of different "3rd party" systems. They fired my supervisor (this is right to work state) and hired a guy who used to work there as super, who it turns out, the CEO came across randomly. Doesn't THAT smell? There will be far far "less" stuff to build after the in house package is done, only used from now on for warranty/ replacement sales

I could use the money, but don't HAVE to have it
 
Wish you the best. Recent work experience is helpful, in finding next one, and often better. It doesn't hurt to find another, when the end is near.
 
Dave, Ill post the entire schematic if you got a magnifying glass!
0001.jpg
 
try this...
452XXh2.pdf

Ive not plugged in since replacing that MOSFET so I got a handful of fuses ready. Just an opinion why that MOSFET would blow (continuity on D and G in both directions. ) Ive trsted almost all the capacitors out of circuit and all are spec. Just that one .1 Ohm resistor (just fooled me as a short) threw me, it is a high heat ceramic but has hot glue blob next to it so it doesnt get hot enough to melt glue or discolor board. all 3 of those resistors by the MOSFET are square cermet "cement" resistors, as everyone calls them erroneously.
 
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I'm sure Dave will know this, I'm not up on some of this. But look at the note at far top right "Adjust AP1 for 5mv across R53"

This is all DC coupled which means a change/ failure in a "far removed" part of the circuit can affect things down the line. also there are some TP readings that might be of value?

I did a brief search on this and found some reference to a service manual, but most links went to "phake" pay to download places
 
I'm better at electrical subjects than electronic but it looks to me like Q5 and Q6 are the traditional MOSFET complementary pair. Looking at the note at the top, the only reason for R53 is to get a reading for the idle source current for Q6. Looking at the rating sheets for the IRFP140, its maximum Source-Drain voltage is 100 VDC so I'm guessing the 40 VDC isn't a problem. If it were me I'd fire it up and adjust AP1 as per the note and call it good. But again, I'm better at electrical than electronic topics.
 
6PakBee said:
... If it were me I'd fire it up and adjust AP1 as per the note and call it good. ...
Click to expand...
BINGO! Pulled the trigger with a amp meter in series with the fuse, nothing....changed meter to DC amps...nothing. Put 3A fuse back in clip, held breath and powered on with an 8 ohm speaker on the output. Nadda..WTF? Oops, forgot the 2 PCB to chassis ground screws...try again! 3-2-1 Click--- Red LED came on, no smoke...turned up master to full and heard just the slightest hiss, turned it down and it throttled down, turned all the pots~ no static. Cycled all the EQ sliders...F yeah....hot damn! Ill call this a winner!
 
6PakBee said:
..adjust AP1 as per the note and call it good. ..
Click to expand...
When it says no load, does that mean speaker is disconnected?

<<< EDIT>>> I believe Solid state guitar amps can be run with no loads as opposed to tube amps that REQUIRE a load. My old CATV maintenance trainer told us to never energize an amplifier/repeater with no load.
 
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So what did you "do" to fix it? I mean what then turned out to be problem?
 
pishta said:
When it says no load, does that mean speaker is disconnected?

<<< EDIT>>> I believe Solid state guitar amps can be run with no loads as opposed to tube amps that REQUIRE a load. My old CATV maintenance trainer told us to never energize an amplifier/repeater with no load.
Click to expand...

Again, I'm better with electrical than electronics but considering that the speaker, either internal or external is the load on the MOSFET's, I can't see how you would set the source current as there is no path through the pair with the speaker disconnected. As to no load, I am not familiar with the controls on that particular amp but my guess would be that there shouldn't be any bass connected to the amp when the source current is adjusted. What I would do is power it up with the speaker connected and no input and see what you have for voltage. If you are close to the 5 mv, you are golden, if not, well then you have to regroup. Just one step at a time.
 
The 1 blown N channel MOSFET was the issue the whole time. Guy offered it to me for $40 telling me it buzzed then kept blowing the 2.5A fuse. I did a little research on it over lunch before buying it and found the majority of solid state amp issues of this type are blown output transistors (MOSFETS) and I had replaced a bunch of them in a flat screen power supply and a 2500W inverter. I knew they were only a few bucks a piece so I took a chance. I was a little gun shy of powering the thing up without finding the root cause as these MOSFETs were special order and didnt want to pop it again without running the schematic past you guys who know better than me. I was waiting for a "check diode D23, they ALWAYS crap out in that circuit" or something like that. I usually suspect capacitors in anything electronic as those are the first to go, (checked all of them out of circuit, no faults) but this one displayed a short (blowing fuses) and bad caps usually don't do that, they just don't power up or power at a fraction of the voltage they are designed for. They are usually easy to spot as they are bulged on top, like they overpressured and blew up the perforated metal 'lid', or if the circuit does manage to power up, you can sometimes detect a bad one with a laser thermometer as a hotter one. Even if they appear to be good, its pretty easy to unsolder a leg and put an ESR tester or better, a cheap ebay component tester on it and it will display the actual capacitance and ESR.
 
Kind of like moving parts in mechanical stuff, The electrical parts the carry the most load (current or highest voltage) are usually the first to go. Often the high power stuff on the output side.
 
Great you got it going! Some of what I wrote without seeing complete schematic was wrong. There is no active fault protection that I can see.

The mosfet may blow if upper and lower mosfets turn full on at same time, basically a short on power supply. A transient or instability may cause that. It may be possible to add a 2n2222 transistor where the base would connect to mosfet source, the emitter to -40V (other end of 0.1 Ohm) and collector to mosfet gate. If the current reaches 6.5A the 2n2222 will shut off mosfet. It will actually limit current at 6.5A, but that is well below 20A hot mosfet rating. The 0.1 Ohm is rated at 3W, at 6.5A dissipation would be 4.2W.
 
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