MTX JH1200

RiLoWa · 18th April 2012, 03:03 AM

I found that two IRF640N outputs (FET404 & FET6) were shorted, one on each side. I removed all fourteen to find the two, checking each one. None of the remaining twelve appear to be leaking. The TIP29C & TIP30C were checked in circuit, and are not shorted. All of the 10 Ohm gate resistors are ok. I presume the .1 Ohm, 5W resistors are ok, none are even remotely discolored.

I re-installed all of the IRF640Ns that appear ok and left out the two shorted ones at those above FET locations. The amp draws excessive current. I removed the four rectifiers and the amp idles at ~2A draw. Is that high for an unloaded power supply?

Does this amp require all fourteen of the outputs to be good and installed or are there more areas I need to look at?

ivkata · 18th April 2012, 03:12 AM

replace all of them another one might be burned inside but not appearing shorted

Perry Babin · 18th April 2012, 06:11 AM

Did you check the components on the driver board? They often fail when the outputs fail.

I would expect the amp to work with one transistor missing from each side.

2A is high but doesn't necessarily mean that there is a problem.

RiLoWa · 18th April 2012, 02:22 PM

I checked the A92s and A42s in circuit and they are not shorted.

Should I check the smaller components?

Perry Babin · 18th April 2012, 02:58 PM

How much was the excessive current draw?

Did it draw excessive current as soon as remote voltage was applied?

RiLoWa · 18th April 2012, 03:13 PM

It does blow a 25A fuse shortly (~1.5 seconds) after remote voltage is applied. Once the draw begins, it climbs rapidly. I think it was close to ~30A when fuse blew.

With a limiting resistor in line with B+, the amp cycles on and off because the voltage drops too low for the amp to stay on but it does not blow a 15A fuse this way.

Perry Babin · 18th April 2012, 03:55 PM

When it's cycling on and off, do you see rail to rail oscillation on any legs of the output transistors?

RiLoWa · 18th April 2012, 05:16 PM

I have not checked for that yet.

Where should I place the scope ground in this amp when I do?

Perry Babin · 18th April 2012, 06:24 PM

You can probably use the primary ground.

RiLoWa · 19th April 2012, 02:07 AM

I re-installed the rectifiers.

With my 3 Ohm resistor and a 15A fuse in line with B+ the power light comes on with ~4 amp draw, however the voltage at the amp's terminals is ~8.5VDC and does not change much as I adjust my power supply from 12V-16V.

Testing any of the three legs of the output located at FET18 shows nothing on the scope.

18th April 2012, 03:03 AM	#1
RiLoWa diyAudio Member Join Date: Aug 2010 Location: Illinois	MTX JH1200 I found that two IRF640N outputs (FET404 & FET6) were shorted, one on each side. I removed all fourteen to find the two, checking each one. None of the remaining twelve appear to be leaking. The TIP29C & TIP30C were checked in circuit, and are not shorted. All of the 10 Ohm gate resistors are ok. I presume the .1 Ohm, 5W resistors are ok, none are even remotely discolored. I re-installed all of the IRF640Ns that appear ok and left out the two shorted ones at those above FET locations. The amp draws excessive current. I removed the four rectifiers and the amp idles at ~2A draw. Is that high for an unloaded power supply? Does this amp require all fourteen of the outputs to be good and installed or are there more areas I need to look at? __________________ Tools - 1. TPI Scope Plus 440 2. B&K 2125 20MHz Dual Trace 3. Sorensen DCR20-50B 4. B&K 4011 5MHz Function Generator 5. Weller WESD51 6. A big hammer

18th April 2012, 06:11 AM	#3
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Did you check the components on the driver board? They often fail when the outputs fail. I would expect the amp to work with one transistor missing from each side. 2A is high but doesn't necessarily mean that there is a problem. __________________ Links >> Basic Car Audio Amp Repair * Basic Car Audio Electronics * New Site * Basic Switching Power Supply Design * Basic Computer Skills << Links

18th April 2012, 02:22 PM	#4
RiLoWa diyAudio Member Join Date: Aug 2010 Location: Illinois	I checked the A92s and A42s in circuit and they are not shorted. Should I check the smaller components? __________________ Tools - 1. TPI Scope Plus 440 2. B&K 2125 20MHz Dual Trace 3. Sorensen DCR20-50B 4. B&K 4011 5MHz Function Generator 5. Weller WESD51 6. A big hammer

18th April 2012, 02:58 PM	#5
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	How much was the excessive current draw? Did it draw excessive current as soon as remote voltage was applied? __________________ Links >> Basic Car Audio Amp Repair * Basic Car Audio Electronics * New Site * Basic Switching Power Supply Design * Basic Computer Skills << Links

18th April 2012, 03:13 PM	#6
RiLoWa diyAudio Member Join Date: Aug 2010 Location: Illinois	It does blow a 25A fuse shortly (~1.5 seconds) after remote voltage is applied. Once the draw begins, it climbs rapidly. I think it was close to ~30A when fuse blew. With a limiting resistor in line with B+, the amp cycles on and off because the voltage drops too low for the amp to stay on but it does not blow a 15A fuse this way. __________________ Tools - 1. TPI Scope Plus 440 2. B&K 2125 20MHz Dual Trace 3. Sorensen DCR20-50B 4. B&K 4011 5MHz Function Generator 5. Weller WESD51 6. A big hammer


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18th April 2012, 03:12 AM	#2
ivkata diyAudio Member Join Date: Dec 2010	replace all of them another one might be burned inside but not appearing shorted

18th April 2012, 03:55 PM	#7
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	When it's cycling on and off, do you see rail to rail oscillation on any legs of the output transistors? __________________ Links >> Basic Car Audio Amp Repair * Basic Car Audio Electronics * New Site * Basic Switching Power Supply Design * Basic Computer Skills << Links

18th April 2012, 05:16 PM	#8
RiLoWa diyAudio Member Join Date: Aug 2010 Location: Illinois	I have not checked for that yet. Where should I place the scope ground in this amp when I do? __________________ Tools - 1. TPI Scope Plus 440 2. B&K 2125 20MHz Dual Trace 3. Sorensen DCR20-50B 4. B&K 4011 5MHz Function Generator 5. Weller WESD51 6. A big hammer

18th April 2012, 06:24 PM	#9
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	You can probably use the primary ground. __________________ Links >> Basic Car Audio Amp Repair * Basic Car Audio Electronics * New Site * Basic Switching Power Supply Design * Basic Computer Skills << Links

19th April 2012, 02:07 AM	#10
RiLoWa diyAudio Member Join Date: Aug 2010 Location: Illinois	I re-installed the rectifiers. With my 3 Ohm resistor and a 15A fuse in line with B+ the power light comes on with ~4 amp draw, however the voltage at the amp's terminals is ~8.5VDC and does not change much as I adjust my power supply from 12V-16V. Testing any of the three legs of the output located at FET18 shows nothing on the scope. __________________ Tools - 1. TPI Scope Plus 440 2. B&K 2125 20MHz Dual Trace 3. Sorensen DCR20-50B 4. B&K 4011 5MHz Function Generator 5. Weller WESD51 6. A big hammer

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