resistor burning up

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Re: resistor burning up

Postby plush » 30 Mar 2018, 12:28

ballfire wrote:
any advice?


Check other parts then.
90% of every repair is diagnostics.
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Re: resistor burning up

Postby deltafred » 30 Mar 2018, 19:01

ballfire wrote:any advice? please anybody around here..until this time i cannot fix my amp..the fatiness of my amp lost..


It is not quite as easy as you telling us which resistor is getting hot and that the sound has changed for us to be able to tell you how to fix your amp.

I have already given you my diagnosis from the very limited information that you have given us but I may be completely wrong because I don't have your amp on my bench.

To confirm whether it is oscillating above the audio frequency range you would need to look at the output waveform with an oscilloscope.

If it is oscillating then the next challenge is to find out what is causing it. This could be a bad solder joint or a component out of spec, including the output IC and those in the power supply.
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Re: resistor burning up

Postby ballfire » 02 Apr 2018, 05:28

deltafred wrote:
ballfire wrote:any advice? please anybody around here..until this time i cannot fix my amp..the fatiness of my amp lost..


It is not quite as easy as you telling us which resistor is getting hot and that the sound has changed for us to be able to tell you how to fix your amp.

I have already given you my diagnosis from the very limited information that you have given us but I may be completely wrong because I don't have your amp on my bench.

To confirm whether it is oscillating above the audio frequency range you would need to look at the output waveform with an oscilloscope.

If it is oscillating then the next challenge is to find out what is causing it. This could be a bad solder joint or a component out of spec, including the output IC and those in the power supply.



good day..


thank you sir deltafred for the reply..



last week i checked all the components in the board, i removed all the cpacitors on the board check it one by one and theyre all ok..

i checked all the resistors with my meter and they are all okey..

i changed the burned resistor into 1 watt and with new zener diodes with same specs now they are all okey they dont burned except for the large capacitors
its getting a little hot..

i checked all the IC chips on it and they are ok also..i change the transistor with new ones exact specs..

i check the amplifier chip and swap it with new one but the same amplifier chip spec..but still the "tingy" tone still persist..

but i dont understand .i checked all but the twangy tingy tone still there..

you mention ocillope instrument damn i dont have one even in the near music store here they are lack of this repairing inside the amp parts they only sell amps but they dont care amp "tones" you know i mean..

by the way my amp brand name called "x-trek" gx-30 model..i search the internet for the schematic but i dont have found one..but only physical picture of this amp


good for you sir theres lot of people there in your country concerning guitar amps i mean get serious on the tones of it..but here in our place..i guess im the only know here what is freestompbox site is..
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Re: resistor burning up

Postby deltafred » 02 Apr 2018, 09:14

Can you measure the voltage that is across the smoothing caps, the voltage across the zener diodes and the voltage across the 220 ohm resistors, and post your findings here.
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Re: resistor burning up

Postby ballfire » 03 Apr 2018, 05:56

deltafred wrote:Can you measure the voltage that is across the smoothing caps, the voltage across the zener diodes and the voltage across the 220 ohm resistors, and post your findings here.



good day

ok im gonna check it ..tomorrow i have the result..

thanks
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Re: resistor burning up

Postby ballfire » 04 Apr 2018, 03:18

good day..

these are my readings


220 ohms resistor at +V rail : 14.6 dcv
220 ohms resistor at -V rail : 14.2 dcv

2200 uf (35v) capacitor on the +V rail: 27.6 dcv ( this capacitor dont get hot)
2200 uf (35v) capacitor on the -V rail: 27.6 dcv (this capacitor reall get hot when im using the amp)

12 volts zener diode at +V rail: 12.84 dcv
12 volts zener diode at -V rail: 13.5 dcv

1000 uf cap (25v) capacitor at +V: 12.84 dcv
1000 uf cap (25v) capacitor at -V: 13.5 dcv

* the amp power transformer specs indicate on the label :

ac or the primary : 230 volts
dc or the secondary : 21 volts

* amp fuse : 250 volts 500mal
* amp diode bride: 4 ampere 1000 volts

During my test i detach the speaker of my amp desolder the wire positive and negative terminals
when i turn on the amp ,i notice that the one large 2200 uf at -V rail didnt get much hot just a little warm..the other 2200 uf didnt even get hot..
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Re: resistor burning up

Postby phatt » 04 Apr 2018, 09:37

At a guess the Bridge rectifier may have a blown diode,, That would heat up the Cap.
Also Check that the negative Cap is connected the right way round,, i.e. Positive to Common Rail.
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Re: resistor burning up

Postby deltafred » 04 Apr 2018, 09:52

ballfire wrote:During my test i detach the speaker of my amp desolder the wire positive and negative terminals
when i turn on the amp ,i notice that the one large 2200 uf at -V rail didnt get much hot just a little warm..the other 2200 uf didnt even get hot..

I still think it is oscillating above the audio frequency range, disconnecting the speaker will have an effect on that.

Check the 1 ohm and 0.22uF (R5 and C5) connected to the output pin 4 in the schematic below. If either of these are faulty the circuit will oscillate. (This is a typical schematic taken from the datasheet so some values may be different).

LM1875_split_supply.jpeg
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Re: resistor burning up

Postby ballfire » 05 Apr 2018, 03:06

phatt wrote:At a guess the Bridge rectifier may have a blown diode,, That would heat up the Cap.
Also Check that the negative Cap is connected the right way round,, i.e. Positive to Common Rail.
Phil.



the orientation of that heated capacitor is just like the marshall power supply i posted above..i think its right..

what do you mean" Check that the negative Cap is connected the right way round"

>>>do you mean i will reverse the 2200 uf the negative side to the ground now not on the -V rail side??

if the diode bridge is the problem can i use a 2 ampere diode bridge??instead of 4 ampere??

thanks
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Re: resistor burning up

Postby ballfire » 05 Apr 2018, 05:41

ballfire wrote:
phatt wrote:At a guess the Bridge rectifier may have a blown diode,, That would heat up the Cap.
Also Check that the negative Cap is connected the right way round,, i.e. Positive to Common Rail.
Phil.






the orientation of that heated capacitor is just like the marshall power supply i posted above..i think its right..

what do you mean" Check that the negative Cap is connected the right way round"

>>>do you mean i will reverse the 2200 uf the negative side to the ground now not on the -V rail side??





if the diode bridge is the problem can i use a 2 ampere diode bridge??instead of 4 ampere??

thanks
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Re: resistor burning up

Postby ballfire » 05 Apr 2018, 05:44

deltafred wrote:
ballfire wrote:During my test i detach the speaker of my amp desolder the wire positive and negative terminals
when i turn on the amp ,i notice that the one large 2200 uf at -V rail didnt get much hot just a little warm..the other 2200 uf didnt even get hot..

I still think it is oscillating above the audio frequency range, disconnecting the speaker will have an effect on that.

Check the 1 ohm and 0.22uF (R5 and C5) connected to the output pin 4 in the schematic below. If either of these are faulty the circuit will oscillate. (This is a typical schematic taken from the datasheet so some values may be different).

LM1875_split_supply.jpeg



ok sir i will check that section..if theres something wrong there..
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Re: resistor burning up

Postby plush » 05 Apr 2018, 10:47

FTFY
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Re: resistor burning up

Postby phatt » 05 Apr 2018, 12:32

Oh sorry if that was confusing. :oops:
Yes keep it like the original drawing.
Common is also often referred to as Ground.

Regards the diodes, generally wise to always replace with same ratings BUT In this case those 1N4007 diodes are overkill as 1N4004 would work fine.
So what is written on the Diodes in your Amp?

BTW some pictures of the pcb might help,, hint :wink:

But take DeltaFred's advice first and check that Zobel network on the output of the power chip. Good catch Fred I missed that one :thumbsup
If that checks out then get a plastic stick and gently poke around the PCB you may just have a broken solder pad somewhere on the pcb and gentle pressure can often find the offending component.
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Re: resistor burning up

Postby deltafred » 05 Apr 2018, 15:16

As Phatt says, check for bad solder joints and cracked copper tracks especially where a track joins a solder pad.

Phatt. I was thinking about one of the diodes in the the bridge rectifier being faulty but that would probably affect the cap voltage.
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Re: resistor burning up

Postby ballfire » 06 Apr 2018, 05:28

deltafred wrote:As Phatt says, check for bad solder joints and cracked copper tracks especially where a track joins a solder pad.

Phatt. I was thinking about one of the diodes in the the bridge rectifier being faulty but that would probably affect the cap voltage.



good day..

i check that section on the pin 4 of the power amp for the resistor and the capacitor but theres no problem there..

im getting tired of the repairing of this amp i think im gonna set aside this for a meantime and look for other solutions maybe im gonna trace the signal path of this circuit and fnd where the problem is..i think it will take so long..

anyway thanks for deltafred, phatt, plush, okgb, and alexradium for replying this thread..thank you very much..

more power to freestompbox people here..
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Re: resistor burning up

Postby ballfire » 17 Apr 2018, 03:13

good day everybody..

i fixed the problem of my amp i follow the advice of phat on diode bridge may is the problem ..after i change the diode bridge section..the amp back its own mojo tone and im very happy but after i played for an hours and next day testing it .. then the amp busted again and the output becomes bassy and fatty tone the whole output is like rumbling bass on maximum but i didnt turn up the bass knob on maximum !!! damn it :x its busted again..!! arrghh
im really going to hopless about this amp..its fustrating..


anybody where to find a 4 ampere diode bridge where i can buy it?? maybe its the diode again? i have only 2 ampere diode bridge here it works if i install it???

or anybody advice??
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Re: resistor burning up

Postby ballfire » 18 Apr 2018, 05:32

anybody here with great knowledge of amplifiers im poor on this thing please help..


by the way this cap always heating up can i reverse this cap ??? maybe i should reverse it maybe its the problem....see picture below
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Re: resistor burning up

Postby plush » 18 Apr 2018, 10:18

ballfire wrote:by the way this cap always heating up can i reverse this cap ??? maybe i should reverse it maybe its the problem....see picture below


No! You should not reverse it.
It's the typical implementation in bipolar power supplies. You should desolder and check if it is faulty. If not, then check other components. Remove any load from power section, then check.

The main crap-feature of your power supply is that the output voltage is "stabilized" using zener diodes, they are prone to adding noice to your power section and, due to their pretty high tolerancy, they don't provide symmetrical bipolar voltage.
If I were you, I'd redesign the whole power section using low voltage drop regulators, similar to lm7812 and lm7912.

p.s. also do check rectifier diodes for shortages.
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Re: resistor burning up

Postby ballfire » 19 Apr 2018, 05:27

plush wrote:The main crap-feature of your power supply is that the output voltage is "stabilized" using zener diodes, they are prone to adding noice to your power section and, due to their pretty high tolerancy, they don't provide symmetrical bipolar voltage.
If I were you, I'd redesign the whole power section using low voltage drop regulators, similar to lm7812 and lm7912.



do you mean like this see picture below..

i will remove the two 220 ohms and the zeners ???? where i will put the two large capacitors??what are the capacitors now in the railings??


by the way sir plush what ppossible other component makes the other 2200 uf getting hot??? is possible another capacitor is busted in the power section? that makes the 2200 uf getting hot??
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Re: resistor burning up

Postby phatt » 19 Apr 2018, 09:31

Follow what Plush said; Remove the two 220 Ohm resistors (just un solder one end of each resistor).
This will remove the preamp from circuit so the only parts left that can cause a fault are the *Bridge rectifier, the *2 large filter caps* and the *power amp chip*.

IF that negative capacitor still runs hot then there are only 3 things that could cause the fault.
The Neg cap is faulty,, the LM1875 is shot or the diodes in the bridge are stuffed.
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