kjello

AK Subscriber
Subscriber
Hi all gurus or other helpful souls of this beloved forum

I have a question regarding pcb mounted transistors without heatsink.
As you can see in this picture there are some heat damage on the board, (amplifier probably took some abuse) my questions are; would it be better to leave the "legs" a bit longer to get more air between the transistor and the board? or is it better to just mount a heatsink down at the board?
Will the transistor legs "bend" by the heat if leaved too long, and what would be too long?
17949672_10154447740897551_613076961_o_zpsbghs9xfi.jpg


And

This resistor is a 4,7 ohm 1/4 watt, 5% I have 8 of them damaged (pic)
Now I found some new ones on the www which claims to be the same specs, but they do not have the BLACK stripe, other stripes are the same, Im a bit confused by this.
Can they still be 4,7 ohms?
18901096_10154558949947551_1736209265_o_zpsvpyuty1m.jpg


Best regards
KO
 
Last edited:
Transistors mounted without heatsinks must dissipate heat through their leads, so shorter is better in that respect, but they're more likely to get damaged by soldering heat. Pick your poison.

The black band doesn't signify resistance value. It's meaning might not be standardized at all, but there's bound to be someone here who knows more on that topic.
 
i guess we are looking at resistors here not transistors ?
resistors can be mounted above the board no problem .
insulator rings on the legs are often used .
 
I would mount a small heatsink on the transistor and be done with it.

This is the first time I see a black band on resistor like this. You might want to go with 1/2W if you have multiple failure with the 1/4W.
 
This resistor is a 4,7 ohm 1/4 watt, 5% I have 8 of them shorted (pic)
Shorted ? Are you measuring in circuit or out of circuit ? Those resistors usually "open" or go real high ohms. 4.7Ω may just look shorted to your meter because they are low resistance.
 
Agree. They usually open, not shorted. All the ones I burned, they look burned also.....with smell and all. They usually don't die quietly.
 
Sorry guys, I mean open resistors of course...
I should not be writing in my sleep :) I have corrected my questions.
these bad resistors are in circuit with power transistors that are also broken
Now the good ones read 4,7 ohms on my meter (the ones from the working amplifier channel)
 
Last edited:
i guess we are looking at resistors here not transistors ?
resistors can be mounted above the board no problem .
insulator rings on the legs are often used .

Im sorry for my confusing writing
First question was regarding transistors, second question resistors.
 
I would mount a small heatsink on the transistor and be done with it.

This is the first time I see a black band on resistor like this. You might want to go with 1/2W if you have multiple failure with the 1/4W.

OK
So a 1/2 watt will behave like a 1/4 watt in circuit ?
My guess in this case, is that the resistors was overloaded when the power transistors died.
 
Last edited:
Shorted ? Are you measuring in circuit or out of circuit ? Those resistors usually "open" or go real high ohms. 4.7Ω may just look shorted to your meter because they are low resistance.

They are reading Mega ohms in- and out of circuit.
 
Well,the schematic indicates a 4.7 ohm resistor. I'm thinking the ''F'' indicates either fusible or flameproof.

EDIT:
Also,the exclamation point indicates a component with a ''safety'' requirement. If you are measuring very high resistance (megohms) then these have failed under overload.
 
Last edited:
OK
So a 1/2 watt will behave like a 1/2 watt in circuit ?
My guess in this case, is that the resistors was overloaded when the power transistors died.
If it is a fusable resistor, you need to find out why it burn.

You have the schematic, post the whole thing!!! With the schematic, we can tell you a lot better what you should and should not do.

Indicate what transistor on the schematic that is getting too hot.
 
If it is a fusable resistor, you need to find out why it burn.

You have the schematic, post the whole thing!!! With the schematic, we can tell you a lot better what you should and should not do.

Indicate what transistor on the schematic that is getting too hot.

Link to schematic: https://www.hifiengine.com/manual_library/nad/218-thx.shtml
So far I found:
Q326(C3788), Q318(A1478) shorted
Q338(D1975), Q342(D1975), Q344(B1317) shorted
Q332((A1837) shorted
R358, R360, R366, R368, R374, R376, R382, R384 all 4R7F reading mega ohms out of circuit.
R370 3W 0,22 ohm cement resistor reading mega ohms out of circuit

All this in the right channel power amplifier board
According to previous owner, damage probably caused by shorted speaker output.
 
Back
Top Bottom