What is the model number of your analog meter?I measured resistance measurement with the one I took out and compared it to a new 2SC2240 and they were identical as well except for one measurement. Measured across the C and E using an analog meter at x10k (the highest my meter goes to) red probe on C and black probe on E - the old one appears to be more ‘conductive’ than the new one.
Thanks @JoeJester, I use a Fluke 179 DMM. The before mentioned voltages were measured using it.Thanks for telling me about your meter. The only concern I would have is if you used it to measure voltages in a higher impedance area as the could be loaded down by the meter causing an incorrect reading.
My other concern with the meter, which was proven unnecessary, was the current you were applying when testing the transistors. For instance the classical SIMPSON 260 analog, 20k/volt meter could supply something like over a couple hundred milliamps on one of the resistance settings. I wouldn't want to guess how many transistors that were made defective till the found out that the meter was the culprit.
Its just a reminder to me to be mindful that not everyone will be using a 10 M ohm input impedance digital multimeter.
Basically I was monitoring across the emitter resistors. I just wanted to see whether it goes out of whack, and since it remained stable I took output transistors from an unused channel and replaced the diodes.What did you monitor when the diodes were in the circuit?
I haven't got access to an oscilloscope at the momentIf you monitored with an oscilloscope ... did you see any thing more than the dc bias that got the diodes to conduct? If you didn't, do you have access to an oscilloscope?
The output transistors are still holding on. It seems to be working at my place it was the case the first time as well. The output transistors shorted out at the owner's place last time - I did check at his place when I took the amp it did work, it was only after when I got back he phoned me to say that the amp had failed. The problem was on the same left channel. I did check whether the speakers had shorted, but it had a crossover circuit and probably a series capacitor. So I couldn't measure whether there was a driver shorted. When we were playing music through I didn't remark that there was any difference in the Left and Right speakers.Those transistors failed for a reason. Counterfeits could be one reason, unless they were the originals.
Hi Yami,There are no adjust points for the DC offset. My question are:
1.Does DC offset got anything to do with the biasing or biasing circuitry?
2.Are the above mentioned values acceptable for the DC offset?
3.Could the DC offset be taking out my output transistors? Maybe something in the crossover circuitry of the speaker shunting to ground putting a short across the terminals. (just thinking out loud)
Do you know the circumstances surrounding the failure? What device was the source audio, how loud was the volume setting? Etc.
Well, you can test every input and ensure the.output reaches the power specified.
I certainly would do that so you don't have a second return for repair.
Abuse the hell out of it .....
Thanks @JoeJester & @Martin_R, yes I checked the zobel network and it seems to be in order.Hi Yami,
To answer the above points
1. The DC offset is due to Vbe mismatch between the differential input pair Q5000 and Q5010. If you put your finger on either transistor the heat from your finger will cause the offset to change.
2 Accepted offset is less than 50mV generally, so you're in the right ballpark.
3 No, the DC offset won't blow the transistors, massive DC offset in the region of volts may destroy your speaker though. To this end manufactures add relays in the output stage to disconnect the speaker under fault conditions. You have checked the zobel compensation components haven't you?(post 10)
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by Jake Hertz
by Jake Hertz