resistor burning up

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resistor burning up

Postby ballfire » 20 Mar 2018, 03:12

Good day..

i have this problem with my guitar amp, actually is not a tube amp. its a 30 watt guitar amp..
the other day when im playing it with my guitar suddenly the tone change the output it
change a little bit kind of output,,
the tone gets thin a little bit..so i opened the amp and to see wahts happening inside i notice there was a resistor near the power amp chip its
a 1/2 watt resistor its value was 220 ohms and it burned up!! so i decide to change it with new one the same value same size..so i soldered it to the board then put back the amp..
then i plug my guitar and start playing again within 30 minutes to 45 minutes and my same tone back again with the output and its good to me..,it working..perfect to me.

then after a day i opended it again to see if the resistor fine..im shocked and it burned again but there was no change in tone at all of my amp..what is this resistor up to??

anybody encounter this problem??is there other parts makes this resistor burned as in super hot??

thank you for anybody for advice..
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Re: resistor burning up

Postby okgb » 20 Mar 2018, 03:19

I'd try a higher wattage resister and stand the leads off the board so it has air space underneath it

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Re: resistor burning up

Postby ballfire » 20 Mar 2018, 04:41

okgb wrote:I'd try a higher wattage resister and stand the leads off the board so it has air space underneath it




thanks for the reply ..do you mean theres no problem with my amp?? ill just increase the wattage of the resistor??

is this normall?? im confused??
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Re: resistor burning up

Postby alexradium » 20 Mar 2018, 08:34

You need to know what that resistor is supposed to do, then you can understand if it is under rated or there is a fault in the Amp.
Schematic and Amp model, pictures.

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Re: resistor burning up

Postby plush » 20 Mar 2018, 10:31

The thing that you described indicates that this resistor is not the reason, but some other component is definitely faulty and causes too much current to run through this resistor and burn it.
Replacing this resistor with more powerful one won't fix the problem and can cause further more damage.
I'd start with checking the caps (one of them might have short), trannies and chips.

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Re: resistor burning up

Postby ballfire » 21 Mar 2018, 03:10

plush wrote:The thing that you described indicates that this resistor is not the reason, but some other component is definitely faulty and causes too much current to run through this resistor and burn it.
Replacing this resistor with more powerful one won't fix the problem and can cause further more damage.
I'd start with checking the caps (one of them might have short), trannies and chips.




last night i look on the power section where the resistor connected i notice it was on the (-12v) of the amp..
and the resistor is beside the zener diode(12volts) and connected on to it ..and check also the filtering large capacitors there..
im really hungry to debugg or repair this thing im into it..i will check everything until i find the main problem


thanks for the reply..
Last edited by ballfire on 21 Mar 2018, 03:14, edited 1 time in total.
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Re: resistor burning up

Postby ballfire » 21 Mar 2018, 03:12

alexradium wrote:You need to know what that resistor is supposed to do, then you can understand if it is under rated or there is a fault in the Amp.
Schematic and Amp model, pictures.



thanks for the reply i will check the other components..
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Re: resistor burning up

Postby phatt » 21 Mar 2018, 07:51

If this burning resistor came with an Amp Brand name, a model name and even better a schematic or link to one, then there is a good chance it can be fixed. hint. :wink:
Meantime check if there is a DC voltage on the speaker output terminals.

DC at speaker should be under the 100mV range if it's 10~20 Volts then disconnect the speaker until you have fixed the issue.
Sounds like the Zener might be the regulation for the preamp but without more info it's a guess.
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Re: resistor burning up

Postby ballfire » 21 Mar 2018, 08:41

phatt wrote:If this burning resistor came with an Amp Brand name, a model name and even better a schematic or link to one, then there is a good chance it can be fixed. hint. :wink:
Meantime check if there is a DC voltage on the speaker output terminals.

DC at speaker should be under the 100mV range if it's 10~20 Volts then disconnect the speaker until you have fixed the issue.
Sounds like the Zener might be the regulation for the preamp but without more info it's a guess.
Phil.

by the way its not a branded amp..but it has a brand name on it its a local manufacturer i think..

what do you mean if you have a burn resistor it might be the dc output is higher??

i just bought a new zener diode 12volts , im thinking it might be the problem.. :cry:

maybe i just check other parts..


by the way i notice some "white residue" around the power amp ic..what is this ?? the power amp ic is the problem?? is the power amp ic near to deteriorate??
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Re: resistor burning up

Postby plush » 21 Mar 2018, 11:47

ballfire wrote:by the way i notice some "white residue" around the power amp ic..what is this ?? the power amp ic is the problem?? is the power amp ic near to deteriorate??



send us nu... gutshots
:oops:

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Re: resistor burning up

Postby deltafred » 22 Mar 2018, 00:37

Please read this thread - viewtopic.php?f=73&t=22602

We don't have a lot to go on yet, here is what I get from reading your posts.

It is a 30W amp
with a 220 ohm 1/2 watt resistor that keeps overheating
and a 12v zenner
it has a -12v power rail
and has white stuff round the output IC

We really would like to help you fix your amp (that is why we keep coming back to this thread) but you are not really doing much to help us so post some photos and we should be able to help you fix it. (Otherwise we will get bored and go read other more interesting threads.)

The white residue around the power output IC is probably heat sink compound (but that is just my best guess and it could be corrosion, mould, cocaine, deodorant stains, hot melt glue, flour, icing sugar, etc, etc. Without seeing a photo I am just guessing.
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Re: resistor burning up

Postby ballfire » 22 Mar 2018, 01:51

deltafred wrote:Please read this thread - viewtopic.php?f=73&t=22602

We don't have a lot to go on yet, here is what I get from reading your posts.

It is a 30W amp
with a 220 ohm 1/2 watt resistor that keeps overheating
and a 12v zenner
it has a -12v power rail
and has white stuff round the output IC

We really would like to help you fix your amp (that is why we keep coming back to this thread) but you are not really doing much to help us so post some photos and we should be able to help you fix it. (Otherwise we will get bored and go read other more interesting threads.)

The white residue around the power output IC is probably heat sink compound (but that is just my best guess and it could be corrosion, mould, cocaine, deodorant stains, hot melt glue, flour, icing sugar, etc, etc. Without seeing a photo I am just guessing.



thank you for the reply..


good day

haha cocaine? no such thing here sir..


okey i will take some pictures of my cheap 30 watt amp.. i just embarass to take some pictures because i think you guys have great branded amps..anyway i will take some pics im just busy right now..

thank you for your replies..
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Re: resistor burning up

Postby ballfire » 22 Mar 2018, 01:54

ballfire wrote:
deltafred wrote:Please read this thread - viewtopic.php?f=73&t=22602

We don't have a lot to go on yet, here is what I get from reading your posts.

It is a 30W amp
with a 220 ohm 1/2 watt resistor that keeps overheating
and a 12v zenner
it has a -12v power rail
and has white stuff round the output IC

We really would like to help you fix your amp (that is why we keep coming back to this thread) but you are not really doing much to help us so post some photos and we should be able to help you fix it. (Otherwise we will get bored and go read other more interesting threads.)

The white residue around the power output IC is probably heat sink compound (but that is just my best guess and it could be corrosion, mould, cocaine, deodorant stains, hot melt glue, flour, icing sugar, etc, etc. Without seeing a photo I am just guessing.



thank you for the reply..


good day

i think i just fix it i change the zener diode 12volts on power rail maybe it was already busted ..and now the resistor doesnt become hot when i turned on the amp..but i will observe it for a week..

by the way is this normal to those 2 large filtering cap a 2200 uf 35volts become hot when the amp is on and you plug your guitar on itand play for an hour???
i touch it after i use the amp its hot..is this normal??
thank you for your replies..
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Re: resistor burning up

Postby deltafred » 22 Mar 2018, 09:38

Hey don't be embarrassed by your amp, I still have my first amp and home made speaker cab that was made from bits of old radios because I couldn't afford anything else.

The smoothing capacitors should not get hot.

Your amp could be oscillating at a frequency above the audio range (that your speaker cannot reproduce and you wouldn't hear if it did). If this is the case then that could cause all the symptoms you describe - different sound, resistor and smoothing capacitors getting hot. This is of course pure speculation because I don't have your amp on my bench.

What is the number of the power output IC, some of the older ones were prone to instability.
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Re: resistor burning up

Postby ballfire » 23 Mar 2018, 05:27

deltafred wrote:Hey don't be embarrassed by your amp, I still have my first amp and home made speaker cab that was made from bits of old radios because I couldn't afford anything else.

The smoothing capacitors should not get hot.

Your amp could be oscillating at a frequency above the audio range (that your speaker cannot reproduce and you wouldn't hear if it did). If this is the case then that could cause all the symptoms you describe - different sound, resistor and smoothing capacitors getting hot. This is of course pure speculation because I don't have your amp on my bench.

What is the number of the power output IC, some of the older ones were prone to instability.



good day sir..

im really appreciate you reply and advice sir..i think you are experience in repairing amps..i salute you sir..

anyway i think you're right ..i didn't hear something on the speaker but some thing is wrong on it that's makes very sense to me..

by the way the power amp ic is LM1875T is this the culprit of having the resistor getting burned or the large capacitors getting hot when im using the amp?? this power amp ic has a white residue on it sides i scraped it but i think is not a glue is like a powder stuck on the sides or maybe its from the ic itself..

i took a picture shot on the board section ..sorry for my camera is a little blured.. scroll down please..
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Re: resistor burning up

Postby ballfire » 23 Mar 2018, 05:32

If you look at the pictures there's a marked 100 ohms on the board underneath the 220 ohms. im also confused here! but the actual resistor is 220 ohms but the board indicates there a 100 ohms..i just followed the previous value installed on it..but it seems ok when i turned on the amp the 220 ohms dont burned..

what is the really right value here?? 220 ohms or 100 ohms??


by the way based on my trace on my amp the power section is like on those marshall mg10 practice amp
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Re: resistor burning up

Postby ballfire » 23 Mar 2018, 05:52

the power section of my amp looks like this..i just now change the value based on my amps value of resistor and capacitor..
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Re: resistor burning up

Postby DrNomis » 25 Mar 2018, 05:05

ballfire wrote:the power section of my amp looks like this..i just now change the value based on my amps value of resistor and capacitor..



It's possible that the -12V supply Zener Diode (ZD 2) could have developed a short to ground, so maybe replacing both of them (ZD 1 and ZD 2) with new ones may fix the fault, I would also replace both of the 220 Ohm resistors with higher rated ones, maybe anything from 1 Watt to 5 watt (maybe go with a couple of 5 Watt 220 Ohm resistors for reliability).

C33 and C34 might need replacing too, as well as the two 1000uF caps in parallel with ZD 1 and ZD 2.


That white stuff around the power amp IC is Silicone Heatsink grease which is used to provide an efficient transfer of heat from the power amp IC to the Heatsink in order to keep the power amp IC from overheating.
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Re: resistor burning up

Postby ballfire » 26 Mar 2018, 07:17

DrNomis wrote:
ballfire wrote:the power section of my amp looks like this..i just now change the value based on my amps value of resistor and capacitor..



It's possible that the -12V supply Zener Diode (ZD 2) could have developed a short to ground, so maybe replacing both of them (ZD 1 and ZD 2) with new ones may fix the fault, I would also replace both of the 220 Ohm resistors with higher rated ones, maybe anything from 1 Watt to 5 watt (maybe go with a couple of 5 Watt 220 Ohm resistors for reliability).

C33 and C34 might need replacing too, as well as the two 1000uF caps in parallel with ZD 1 and ZD 2.


That white stuff around the power amp IC is Silicone Heatsink grease which is used to provide an efficient transfer of heat from the power amp IC to the Heatsink in order to keep the power amp IC from overheating.


thanks sir for the reply..i already change the zener diodes , resistor and capacitor on power section area..but theres something else happen to the output i mean the over all tone of the amplifier in the output it became twangy or lost its fatiness even i change the equalizer knobs still the twanginess appear during or everytime i play my guitar..what is this? is this the result of burned resistor??..i check the opamps theyre all ok..

any advice sir? these last days i focused so much on repairing my amp but the last only problem is the overall output became less fatiness..
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Re: resistor burning up

Postby ballfire » 28 Mar 2018, 02:30

good day..

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