Burnt out resistor

Thread Starter

Squidge75

Joined Feb 12, 2024
25
Good day everyone. I am hoping that someone here can give me some help or at least some sound advice on a problem I have.

Before I continue I must say that my experience with circuits is rather limited. Replacing parts is about my limit. However and here is my situation.

I have a NADC316 BEE amplifier. It is a few years old now and the right channel is giving me a lot of distortion. I tried all the obvious things, testing speakers, switching cables etc etc. I have changed the relay and also the Tone Defeat switch. All to no avail.

To send away my amp to a professional and to get it fixed will probably cost me more than the amp itself. So I have two options. 1. Buy a new amp and throw this one away which I would hate to do as I am someone who hates to throw almost perfectly good things away, the unfortunate world we live in today. or 2. At least attempt to try to fix it myself. And if it all goes wrong then I will need to buy a new one anyways and so no love lost.

On very close expection I noticed that one of my Resistors has blown. (see image). Now to replace this is straight forward but I know that the problem probably lies deeper within the circuitry. A possible broken transistor(s) in the chain. And this is where my problem lies. I cant read circuit diagrams/schematics so I dont know what chain it lies within.

I have the schematics and found where the part it so would need a willing soul just to help me define which Transistors I should be testing and possibly replacing.

Is there anyone out there who could help me out, if so then I can upload whatever material is needed.

Thanks in advance
 

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dl324

Joined Mar 30, 2015
16,917
Welcome to AAC!
I have the schematics and found where the part it so would need a willing soul just to help me define which Transistors I should be testing and possibly replacing.

Is there anyone out there who could help me out, if so then I can upload whatever material is needed.
It would be helpful if you posted the schematic.
 

Thread Starter

Squidge75

Joined Feb 12, 2024
25
Hi there, thanks for the quick response. And of course. Attached is the Product electrics manual. I couldnt extract the separate pages unfortunately due to passwords. However the Schematics are on page 14 and the main board diagram on page 20.

The Resistor that has blown is R181. I have made a close up of this area and highlighted the Resistor and transistors that I believe to be in the same chain.

Once again appologies for the Full PDF file and thanks again for the quick response.

Matt
 

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dl324

Joined Mar 30, 2015
16,917
With the unit off, use the diode check function on a DMM on the junctions of Q114. Positive lead on the base, negative on the emitter. Then positive lead on the base and negative on the collector. Then check the resistance between collector and emitter.
 

Thread Starter

Squidge75

Joined Feb 12, 2024
25
Ok fantastic. Ill check that over tomorrow morning as its going to require a little bit of dismantling of some other parts. Can I check the transistor whilst its still in the PCB or is it best to remove it? Thanks again
 

Thread Starter

Squidge75

Joined Feb 12, 2024
25
Hi there again, so have taken some readings of the resistors (I hope I got the EBC the correct way round). I have attached a pdf of the results. I took readings from the Q114 resistor but also the ones next to them to get a general overview of the readings. Also took the readings of the resistors on the Heatsink. Q115 and Q114 are certainly showing some odd behaviour which makes sense as I presume they are in the same chain. Q114 showing obvious overload.

What are your thoughts on this? I see that there are inconsistencies but not sure how to read it

Thanks again
 

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Thread Starter

Squidge75

Joined Feb 12, 2024
25
Sorry I have just realised that Q217/215 and Q115/117 are PNP resistors I think, so I may have taken the wrong readings with the DMM
 

Thread Starter

Squidge75

Joined Feb 12, 2024
25
I have just realised a slight mistake that I dont think alters much, or maybe it does. The resistor that has blown is R281 and not R181 as I first stated. But even so, that would make Q214 or Q215 the culprits. Not 114 and 115 as first mentioned. Sorry fo rhis mistake.

But the readings I took then make my problem even more strange, at least to me. Someone suggested I take some Voltage readings at several points around the circuit aroudn the suspect component, would this help? Or would it be best to remove the transistors and take readings once again, Thanks
 

dl324

Joined Mar 30, 2015
16,917
I have just realised a slight mistake that I dont think alters much, or maybe it does. The resistor that has blown is R281 and not R181 as I first stated. But even so, that would make Q214 or Q215 the culprits. Not 114 and 115 as first mentioned. Sorry fo rhis mistake.
You have the pin order wrong on Q114/214. Double check the pin orders and do the measurements again.
1707941313159.png
I didn't check pin order on any other transistors.
 

Thread Starter

Squidge75

Joined Feb 12, 2024
25
Ah yes, I just checked the pin order. The base pin is on the right and collector in the middle. Ill take some new readings tomorrow morning. Sorry for hte confusion and pointing that out to me. Rookie mistake.
 

Thread Starter

Squidge75

Joined Feb 12, 2024
25
Yep seems I got the base and collector miixed up. Il lpost a new set of readings tomorrow.

Thanks for the patience
 

dl324

Joined Mar 30, 2015
16,917
Yep seems I got the base and collector miixed up. Il lpost a new set of readings tomorrow.
Note that some of the transistors have a different pinout:
1707946345433.png
You'll have to get the datasheet for each device to check the pin order.

When you test junctions, make sure the positive lead is on the anode. If you get continuity across any junction or from collector to emitter, that transistor is bad.
 

LadySpark

Joined Feb 7, 2024
124
I have just realised a slight mistake that I dont think alters much, or maybe it does. The resistor that has blown is R281 and not R181 as I first stated. But even so, that would make Q214 or Q215 the culprits. Not 114 and 115 as first mentioned. Sorry fo rhis mistake.

But the readings I took then make my problem even more strange, at least to me. Someone suggested I take some Voltage readings at several points around the circuit aroudn the suspect component, would this help? Or would it be best to remove the transistors and take readings once again, Thanks
you should find either Q214 or Q215 shorted. But also check Q222 and Q213 in case the bias circuit went bad and took out the other parts.
 

Thread Starter

Squidge75

Joined Feb 12, 2024
25
Hi there, so then I have taken some new readings. Which I believe are now the correct way round. One can certainly see some activity on Q215 as you suspected Ladyspark. I took a reading from Q222 as well. Not sure if the O, and the high readings mean anything.
 

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LadySpark

Joined Feb 7, 2024
124
The transistor that shows the OL readings are a BJT transistor, would this suggest a faulty Transistor?
In resistance reading, you will see where the circuit it has gone awry, even though you have a physical indication of this.

Do you have diode function on your DMM? (a little diode symbol on the mode select knob of your DMM)

You need to use that, and measure a junction in both directions (measure one way, then reverse the meter probes).

Because it looks like everything is ok to install a new resistor. Which if meter readings don't find a fault, you have possibly the bias adjustment in the circuit wrong and over time nuked the resistor, or the associated transistor it about to short out.
 

Thread Starter

Squidge75

Joined Feb 12, 2024
25
Hi there, yep I have a diode function. Ive been using it to take the readings of the transistors. Sorry for being a bit of a newbie here but how would I go about measuring the junction in both ways of a PNP resistor? Thanks
 

LadySpark

Joined Feb 7, 2024
124
Hi there, yep I have a diode function. Ive been using it to take the readings of the transistors. Sorry for being a bit of a newbie here but how would I go about measuring the junction in both ways of a PNP resistor? Thanks
It doesn't look like it. looks like resistance readings instead of junction voltages. measure one way then flip the meter leads and read it in opposite polarity.

 

Thread Starter

Squidge75

Joined Feb 12, 2024
25
Hmm, strange. I have the setting onto the Diode option. See screenshot. This is taken from the suspect transistor. As an example it gives me me a reading of 576mV voltage drop with the -ve on the base and the +ve on the Collector. Am I missing something.

These are BJT Transistors that I am testing. the video seems to be referring to MOSEFT. Maybe the test is the same.
 

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