resistor burning up

Tube or solid-state, this section goes to eleven!

Re: resistor burning up

Postby ballfire » 20 Apr 2018, 05:28

phatt wrote: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.
Phil.



good day..

thank you sir phatt for replying this again..yeah exactly!! last night i look closely on that section what a coincidence opinion..i check carefully of that large capacitor and the other involve parts there ..im wondering there something there makes this capacitor very hot ..you right what you said i will really dissect it tonight and swipe it meticulously edge by edge on the pcb tracks..


but last time i already change the power amp ic with new one.. but ""a new problem came up""..when i turned on the amp it makes a sudden stops every two minutes a then the signal will appear again..then stop again so i put back the old same power amp ic..and the amp its ok again...

by the way can i change the two 220 ohms resistor into 150 ohms?? like those on marshall mg 30 watts amplifier because im thinking maybe its ok because my amp is 30 watts amp also?? can i swap my Lm1875 with tda 2050 power amp ic just like on that marshall mg 30??
ballfire
Resistor Ronker
 
Posts: 262
Joined: 12 Apr 2011, 06:37
Has thanked: 18 times
Have thanks: 0 time

Re: resistor burning up

Postby phatt » 21 Apr 2018, 04:34

Long before you try to reinvent the wheel let's just find out why that Cap is heating up. Hint :wink:
You have 3 things that could be faulty check those first,,, then you can explore other options.
Phil.
phatt
Transistor Tuner
 
Posts: 1018
Joined: 04 Aug 2010, 06:17
Location: Morayfield SE Qld AU
Has thanked: 0 time
Have thanks: 251 times

Re: resistor burning up

Postby ballfire » 23 Apr 2018, 05:18

phatt wrote:Long before you try to reinvent the wheel let's just find out why that Cap is heating up. Hint :wink:
You have 3 things that could be faulty check those first,,, then you can explore other options.
Phil.



good day ..

i check now that power section i found out that the power amp ic is the problem ..i changed the filtering cap and the diode bridge but still the filtering cap is getting hot so i suspected the problem is in the power amp ic..
ballfire
Resistor Ronker
 
Posts: 262
Joined: 12 Apr 2011, 06:37
Has thanked: 18 times
Have thanks: 0 time

Re: resistor burning up

Postby phatt » 23 Apr 2018, 06:17

OK then replace the LM1875.
Be aware that the metal tab MUST BE isolated from the heat sink with a wafer pad.
Before you power up the new chip check that the metal tab is actually isolated from the heat sink. Use your meter, if you get a beep it's bad,, should be open circuit between LM1875 Tab and Heatsink.

I'm fairly sure the tab is internally connected to pin 3 (neg supply rail) so if the tab is inadvertently grounded it will shut down.
You mentioned this start stop issue when you tried to change the chip so go check.
Use a new wafer pad and heat paste as it will help to dissipate heat.
Phil.

For this message the author phatt has received thanks:
ballfire (24 Apr 2018, 05:35)
phatt
Transistor Tuner
 
Posts: 1018
Joined: 04 Aug 2010, 06:17
Location: Morayfield SE Qld AU
Has thanked: 0 time
Have thanks: 251 times

Re: resistor burning up

Postby ballfire » 24 Apr 2018, 05:31

phatt wrote:OK then replace the LM1875.
Be aware that the metal tab MUST BE isolated from the heat sink with a wafer pad.
Before you power up the new chip check that the metal tab is actually isolated from the heat sink. Use your meter, if you get a beep it's bad,, should be open circuit between LM1875 Tab and Heatsink.

I'm fairly sure the tab is internally connected to pin 3 (neg supply rail) so if the tab is inadvertently grounded it will shut down.
You mentioned this start stop issue when you tried to change the chip so go check.
Use a new wafer pad and heat paste as it will help to dissipate heat.
Phil.



good day..

you're very right sir ., i think thats the other reason why the filtering capacitor if heating up ..because of the tab of power amp ic is connected to the ground it should not be connected!! i thought it sometime but i never separate the heatsink and the tab of the power amp..because based on my reading on the internet the tab they say its a ground point also of the power amp ic but now i know now theyre wrong..

thank you very much sir phatt for information..i added some knowledge about power amp ic those tab it should not be grounded!! and my amp power amp ic tab is connected to the ground!! what a bad design!

i really salute people like you sir its really good that youre guiding people without knowledge on this thing..

maybe im gonna make a separate board of my amp power amp ic and the tab should not connected to the ground..i guess its a long days again in working it..

again sir thank you..
ballfire
Resistor Ronker
 
Posts: 262
Joined: 12 Apr 2011, 06:37
Has thanked: 18 times
Have thanks: 0 time

Re: resistor burning up

Postby phatt » 24 Apr 2018, 12:22

I am pleased that we have been of some help. :thumbsup :thumbsup :thumbsup
Re the LM1875 grounding confusion;
This chip can be run from a single supply, in that situation pin3 (and the Negative Tab) becomes circuit Common so only in that situation can the tab be grounded.

BUT when using a split supply pin3 is then at negative potential So the TAB Must then be isolated.
Keep learning it takes time to absorb these things, 8)
Phil.
phatt
Transistor Tuner
 
Posts: 1018
Joined: 04 Aug 2010, 06:17
Location: Morayfield SE Qld AU
Has thanked: 0 time
Have thanks: 251 times

Re: resistor burning up

Postby phatt » 24 Apr 2018, 12:27

I should add,, the old chip is likely shot so use the new one with isolation wafer and it should be ok.
Phil.
phatt
Transistor Tuner
 
Posts: 1018
Joined: 04 Aug 2010, 06:17
Location: Morayfield SE Qld AU
Has thanked: 0 time
Have thanks: 251 times

Re: resistor burning up

Postby ballfire » 25 Apr 2018, 07:44

phatt wrote:I should add,, the old chip is likely shot so use the new one with isolation wafer and it should be ok.
Phil.



good day..

ok sir i will follow your instructions..

again thank you very much..will see what will happend..
ballfire
Resistor Ronker
 
Posts: 262
Joined: 12 Apr 2011, 06:37
Has thanked: 18 times
Have thanks: 0 time

Previous

Return to Amplifiers

Who is online

Users browsing this forum: No registered users and 3 guests