Troubleshooting Amp

[Mark245]

Hey everyone.

I am trying to fix one of the channels that has quit working on my Onkyo 5 channel amp. I found that on one of the transistors, the Vbe read by my diode tester is three time that of all of the other identical transistors on the other four channels. It is 1.8v and only .65v for all the other channels. What does this mean? I think it means there is a problem here because when I power up the amp, I get a Vbe of 25v. Does it mean that this transistor is burned out? (It is one of the ones attached to another larger transistor for thermal tracking I guess).

Any help is greatly appreciated.

Thanks,

Mark

[nigelwright7557]

Originally posted by Mark245
Hey everyone.

I am trying to fix one of the channels that has quit working on my Onkyo 5 channel amp. I found that on one of the transistors, the Vbe read by my diode tester is three time that of all of the other identical transistors on the other four channels. It is 1.8v and only .65v for all the other channels. What does this mean? I think it means there is a problem here because when I power up the amp, I get a Vbe of 25v. Does it mean that this transistor is burned out? (It is one of the ones attached to another larger transistor for thermal tracking I guess).

Any help is greatly appreciated.

Thanks,

Mark

If the Vbe is not around 0V7 then the transistor is probably blown.

I would certainly have a lookaround the volts around that area in case something else has gone too.

[Mark245]

Yes I will look into it. Actually what happened in the first place I think is the output of this channel got shorted and blew the output transistors. I think a lot of stuff got taken out. I replaced the output transistors and a couple of resistors and diodes. Even though I am having trouble with that transistor giving vbe of 25 v the channel surprisingly works. I noticed that there was probably a problem when I was trying to set the idle current. I am supposed to get 6mV at the test point but I get 1.4mV no matter what I set the 1k pot to. It works though somehow. Is this Ok?

Thanks

Mark

[nigelwright7557]

Originally posted by Mark245
Yes I will look into it. Actually what happened in the first place I think is the output of this channel got shorted and blew the output transistors. I think a lot of stuff got taken out. I replaced the output transistors and a couple of resistors and diodes. Even though I am having trouble with that transistor giving vbe of 25 v the channel surprisingly works. I noticed that there was probably a problem when I was trying to set the idle current. I am supposed to get 6mV at the test point but I get 1.4mV no matter what I set the 1k pot to. It works though somehow. Is this Ok?

Thanks

Mark

It could be the transistor in the bias network has gone when the output transistors blew. Its easy enough to check.

[unclejed613]

i recently repaired a Denon 5 channel amp that had a blown channel. i see a lot of them, but this one was unusual since the channel was fried all the way back to the diff amp, and most of the Denon amps i've seen only have the outputs blown, and sometimes the bias transistor. this particular Denon didn't have the usual zeners between the output device bases, and so i think that the difference is that the more expensive Denons have the zeners there to limit the damage from a shorted output, but the cheaper ones don't.

it seems you have learned one of the "tricks of the trade", using the other known good channels as a reference.

[anatech]

Hi unclejed613,

but this one was unusual since the channel was fried all the way back to the diff amp, and most of the Denon amps i've seen only have the outputs blown, and sometimes the bias transistor.

Yes. This damage is more typical of a Yamaha product.

Hi Mark,
If you have any outputs blown, always replace all transistors at least one level past the first good part you find. For outputs blown, you should be replacing all the outputs in that channel, plus the driver and bias transistors. That's a minimum. Always check the emitter resistors and all other low value resistors, especially any between the output and / or driver bases. If these open, your bias current will be too high (you may see this after you fix the current faults).

If you don't have the service manual, you should try to get one. Small details can make a large difference!

-Chris

[Mark245]

Thanks everyone I did not know all that information. I am just getting into working on audio equipment. I am just playing around with this amp trying to get it to work and learn stuff. Once I get it to work I will build some of Randy Slones amps.

Mark