TCM 240020 repair

[Roman78]

I own a very cheap TCM 240020 mini stereo system. This one is for my children in there room. Yesterday the magic smoke came out...

I dissembled the broken device and found a burned resistor next to a ac/dc regulator (i don't know the correct English word for it).

The resistor is infinite.

See the pictures. Now the question is, what value could it have had?

I could not find any circuit diagram from this device. The board has 248-T80U02-147 printed on it.

And yes... it is Resistor 404.

[Devil_rider]

Re: TCM 240020 repair

Without schematic not easy but we can help with a good guess if you start drawing the schematic on that part of the circuit. Where in circuit is the resistor, if it is feeding a regulator , what regulator is it and maybe plug it in and measure voltage on the empty pads. Also from the rating plate of the device you can see max current which can help calculate the value of resistor if that is inline with main power source. Help us to help you.....

[R_J]

Re: TCM 240020 repair

You need to post a clear, in focus picture of the trace side of the board so we can see where the resistor is connected. post the highest resolution you can.

It may just be a low value fuse resistor feeding a regulator ic, it looks like it was running hot for a while.

DB1101 with the [-~~+] is the bridge rectifier

[Roman78]

Re: TCM 240020 repair

Here are two pictures.

The burned resistor is just behind the positive side of the bridge rectifier. In Germany it is called Gleichrichter.

There goes 14V in, at least that is printed on the circuidboard near the 2 big red wires.

I'll could try to draw a diagram using kicad. Never used it, though.

[jiroy]

Re: TCM 240020 repair

Did you test the bridge rectifier ? , can you identify its name ?
The burnt resistor is in the rail of Ic 401 . Can you identify Ic 401 name ?

[Roman78]

Re: TCM 240020 repair

On the bridge rectifier ist written " GBU 8D AC"

The burned resitor goes to the IC401 indeed. To the right leg. The middle leg is goin to the negative of the bridge rectifier.

[jiroy]

Re: TCM 240020 repair

Okay . If you can test the bridge , treat it like a 4 diodes , put your caliber on diode mode test : ~ to - , ~ to + , the other ~ to - , the other ~ to + ..
and next What's Ic 401 name ?

[Roman78]

Re: TCM 240020 repair

Bridge looks normal. One direction closed other one open.

IC 401 is a... gee where are my glasses....KA7805

[jiroy]

Re: TCM 240020 repair

Bridge looks normal. One direction closed other one open.

IC 401 is a... gee where are my glasses....KA7805

That's a 5v+ dc regulator . If you can test it , without plugging the unit , by injecting +9v or +12v dc on pin 1 , and the negative on pin 2 , you can find the +5 on pin3 , otherwise , it would be burnt .
Meanwhile , i'll check for the resistor value .

[eccerr0r]

Re: TCM 240020 repair

BOO... poor design. The resistor is indeed a low value resistor that was meant to "protect" IC401, the 7805, due to voltage on the output of the bridge rectifier.

What is the voltage across the + and - terminals of the bridge rectifier, is it that 14VDC?

TBH if you can put a larger heatsink on IC401, any low value resistor would be fine there. Based on the size of the heatsink and assumption that 14V is correct, I'd guess a 1W 4.7R or perhaps 0.5W 2.2R resistor was its original size/value and it died. I'd start with that, measure the current with a 4.7R there and then perhaps choose another resistor for longer life. Do you have any random spare resistors around this value to make measurements?

[jiroy]

Re: TCM 240020 repair

Any value between 1 and 10 Ohms is fine , but should be marked with a fifth white band as it's a protection mainly .. I already calculated its value , and I suggest 5.6 ohms .
But he didn't reply yet , and since it could be that neither bridge or regulator are the original problem , and I need an affirmation , otherwise the resistor will burn again .

[Roman78]

Re: TCM 240020 repair

Sorry for the delay. There is a large heatsink on the KA7805. It is temporally removed.

The KA7805 seems to be working, i put 12V in and get 5.004V out of it.

I have a lot of 0.25W Resistors, but not many with a higher capacity. I found a 6.4 Ohm, hmm must be 1W, but not sure about that. Oh and i have 1.4Ohm 5W

[Roman78]

Re: TCM 240020 repair

Oh and i just found a 2.2 Ohm. That one is huge maybe 5 or more Watt. Won't fit on the circuit board.

[jiroy]

Re: TCM 240020 repair

Okay . It's time now to test Q403 since the big smoothing capacitor is supplying both IC401 and Q403 .. And to save your time , if Q403 is found good , then you should replace the big capacitor along with ordering a 5.6 ohms resistor .. This way , we would deal the 2 possibilities of 1) a sudden electrical surge or 2) the deterioration of the capacitor .
The third possibility is if there was a short beyond IC401 or Q403 , but we won't know that till you change the 2 mentioned parts .

[eccerr0r]

Re: TCM 240020 repair

The heat the resistor generates is heat the 7805 doesn't heat up. So that's the tradeoff here. BTW this resistor does not need to be a fusable type as this is a low power circuit, but it needs to be sized appropriately to last!

If you have 3 pcs of 15Ω ½W resistors, connect them in parallel, that would be acceptable. What is in your arsenal?

I don't know how big of a heatsink is on your 7805 but the bigger it is, the smaller (in ohms) resistor is needed for the ballast. The smaller the resistor (in ohms) the power rating can be lower as the 7805 would take the heat instead...

[jiroy]

Re: TCM 240020 repair

Oh and i just found a 2.2 Ohm. That one is huge maybe 5 or more Watt. Won't fit on the circuit board.

No , We need the contrary , it should be a protection resistor , 1 watt and marked white as its fifth color band , like for an example : Green , Blue , Gold , Gold , White .

[Roman78]

Re: TCM 240020 repair

The Q403 is a B1185 http://www.datasheetcafe.com/b1185-d...et-transistor/

Do i test that the same way?

The big capacitor still has his value of 3300uF I measured 3356uF

I got a big box of 0.25W Resistors here. There are 2 heatsinks on the 7805. Size about 20mmx15mm an 10 high.

[jiroy]

Re: TCM 240020 repair

The Q403 is a B1185 http://www.datasheetcafe.com/b1185-d...et-transistor/

Do i test that the same way?

The big capacitor still has his value of 3300uF I measured 3356uF

I got a big box of 0.25W Resistors here. There are 2 heatsinks on the 7805. Size about 20mmx15mm an 10 high.

It's better to change the capacitor anyway , especially if the unit is excessively used for more than 5 years . And by the big capacitor , I mean the big one near IC 401 and on its same rail .
The resistor is protective , and that mean is its flaming breaking point should be fast . you can put anything else as a normal resistor , but you would risk burning more expensive parts in the future .

Test the b1185 , like 2 diodes , on mode test , and it's a PNP Power Transistor.

[Roman78]

Re: TCM 240020 repair

I just removed the B1185 and put it into my testing device.

[jiroy]

Re: TCM 240020 repair

I just removed the B1185 and put it into my testing device.

Okay . Just notice that I meant by the big capacitor , the big one near IC401 ..