Blown resistor, cant figure out what it was.

[mikay786]

So I wired amp back up wrong after taking apart to clean pot, one pin up on connector and have blown a resistor.

have spent two hours staring at it and at screen of what is supposed to be the service manual but cant make heads or tails of it.

Any help would be greatly appreciated.

[budm]

Re: Blown resistor, cant figure out what it was.

Is that the complete circuit of the board? I do not see any 680 Ohms (two of them on the board) resistors in the diagrams.

[lookimback]

Re: Blown resistor, cant figure out what it was.

It looks to me like it should be one of the 10Ω resistors which connect between the switch, and the left and right inputs.

Edit: If this is even the correct schematic.

[mikay786]

Re: Blown resistor, cant figure out what it was.

Perhaps, but the other around it aren't 10ohm. You would think there would be 2 10ohm close by each other. Ill have to have a go at reading the ones in there again tomorrow. Heres the front of the unit

The blown resistor is in between bridged mono stereo part

[mikay786]

Re: Blown resistor, cant figure out what it was.

Right schematic: I don't think it is, it says it is but the values on the caps are all wrong.

[mikay786]

Re: Blown resistor, cant figure out what it was.

Is that the complete circuit of the board? I do not see any 680 Ohms (two of them on the board) resistors in the diagrams.

That's just the front panel board with the volume controls, I think schematic might be for a different model or revision. Its similar and looks familiar but a few things look off to me.

[stj]

Re: Blown resistor, cant figure out what it was.

schem's wrong in the pdf - wrong part number on it, wrong wiring too - totally different board.

[budm]

Re: Blown resistor, cant figure out what it was.

Can you please clean up the section I marked so I can see the traces? The cap and the glue are in the way.

[vinceroger69]

Re: Blown resistor, cant figure out what it was.

just saw this thread the 22 pin ic looks like it may off overheated as some of the printing on it has come off so maybe worth trying to test this ic too (although the ic could just have smoke residue etc from the burnt resistor over the printing i guess).

[mikay786]

Re: Blown resistor, cant figure out what it was.

Can you please clean up the section I marked so I can see the traces? The cap and the glue are in the way.

It looks like it bridges over to a pin on the switch

[mikay786]

Re: Blown resistor, cant figure out what it was.

I'm thinking using the other half of the board I might be able to figure out what the resister was as looks like two half of the board. Looks like the resister is linked to pin 21 on the IC. So drawing a path on the board on each side should reveal its identity

This is how far ive got but sides are slightly different so struggling a bit

Edit think I'm nearly there found a similar patern

[mikay786]

Re: Blown resistor, cant figure out what it was.






Am I on the right path here? Gets a little confusing on left hand side from the 25v cap and is a little different to the right hand side.

[budm]

Re: Blown resistor, cant figure out what it was.

'Am I on the right path here? Gets a little confusing on left hand side from the 25v cap and is a little different to the right hand side.' As you can see, the burnt resistor one end is connected to the Negative leg of the cap and another end is connected to the second pin from the top of the connector. That was my thought too is that the resistor for the left circuit (looking at the bottom side of the board, will be the same as the one connected to the top pin of the connector but the top pin connector is connected to the Diode not resistor. I can trace the connection now since you have clean up the board.
The cap on the right has Negative to GND (PIN 3 of the connector) and the positive is connected to the pin 21 of the IC, PIN 21 of the left IC is also connected to the positive of the cap on the left.
PIN 4 of the connector looks to be for VCC.
The burnt resistor must be quite low value. Do you know what the VCC feeding that pin 4 is? Did you check the Yellow LED to see if it is OK?

[mikay786]

Re: Blown resistor, cant figure out what it was.

The yellow LED was ok when working, don't know now as is all apart.

Don't know what the VCC was either.

Does the resister need to be bang on or can I get away with a small resister?

[budm]

Re: Blown resistor, cant figure out what it was.

You can check the LED with the meter.
Can you please tell how you put in the connector wrong so I understand how the over current occur and I can see if it may have damaged more than just the resistor.
The connectors are not keyed and two of them are 4 pins so I can see why the mistake happened.

[SteveNielsen]

Re: Blown resistor, cant figure out what it was.

What is the function of the slide switch?

[budm]

Re: Blown resistor, cant figure out what it was.

OK, after looking at your new pictures and my virtual PCBA, yes both L and R are set up the same, the resistor is 1K Ohms, one end of that burnt resistor is connected to GND (pin 3 of the 4-pin Black connector), the other end is connected to pin 2 of the connector, if you look at L CH, you will see that it also has 1K resistor between pin 1 and GND (3).
The mistaken connection must have had high Voltage enough to burn 1 K resistor.
Good pictures always make tracing the circuity much easier.

[mikay786]

Re: Blown resistor, cant figure out what it was.

The slide switch is the set mono/stereo

I put the connectors one place up. So bottom pin was not connected. (4pin mid board vertical connector with L/R marked on them. The bottom pin was unconnected. Right would have been connected to L. And what ever the bottom half of was connected to would have gone into R

- not connected to connector
L - R
R - 1
1 - 2
2 - unconnected

That should better visualise

Thinking the IC may have been damaged but it's only £3 on ebay

[budm]

Re: Blown resistor, cant figure out what it was.

Check the Diode next to the burnt resistor, the ICs are probably OK.
pIn 21 is the signal input pin, the diode is for rectifying the audio signal.

[mikay786]

Re: Blown resistor, cant figure out what it was.

Test diode like you do a transistor so only goes one way? Not tested a diode before but have done a transistor

Can I do in circuit?

The IC did have some blackness on the pins but that probably came off the resistor.

ill do that now