Hi All,Well i changed out the dual Transistors and I got the left channel working with no problems, so i decided to replace the balance of transistors the 2sc1384 and the 2sa 684 now i have a bigger problem the right channel V702 glows and i had smoke, i triple checked to make sure i did not mix up the transistors when i replaced them and made sure the correct transistors were in the right places, i need to know what i can do now i am thinking of putting back the old transistors and seeing what happened any advice please
Pat
Pat
Hi All, i rechecked the transistors that I replaced and they are OK, they are in the proper locations and appropriate orientation this is really baffling when i switched out the798 everything was fine both channels worked perfect no problems does anyone have a picture of the board from the solder side i did take pictures before i begin any work and i would like to compare it to what i have. It looked like their was a solder joint that was bridged from the Q702 to the jumper wire i just want to verify.
Pat
Pat
Hi Mooly, I changed out the 798 dual transistors with SA992 after i did that everything was perfect so i decided to change out the rest of the transistors the 1384 and the 684 that is when things went wrong the right channel started to act up there was a little smoke and now the V702 bulb dims I tripled check the position and orientation and they are in correct what did i do wrong???
Pat
Pat
So the left channel is still OK.
If you are seeing 17 volts when setting the bias then suggests that one or both of the 0.22 ohm resistors are open circuit (I assume you are measuring the bias voltage across these).
If so then that indicates that the output transistors may well have failed.
Its no good just replacing parts in hope, you have to be methodical and check everything along the way. You must also use a bulb tester when powering it up to prevent major damage occurring.
So the first thing to check is whether the 0.22 ohms have failed and whether the output transistors read short (or nearly so) from collector to emitter.
If you are seeing 17 volts when setting the bias then suggests that one or both of the 0.22 ohm resistors are open circuit (I assume you are measuring the bias voltage across these).
If so then that indicates that the output transistors may well have failed.
Its no good just replacing parts in hope, you have to be methodical and check everything along the way. You must also use a bulb tester when powering it up to prevent major damage occurring.
So the first thing to check is whether the 0.22 ohms have failed and whether the output transistors read short (or nearly so) from collector to emitter.
OK thanks Mooly, I am a newbie and really appreciate your help and advice, i did use the dbt after i replaced the 798's and everything was ok so i just assumed that by switching out the transistors everything would be ok WRONG learned a lesson their that will never happen again and yes the left channel is still ok. I will go back to my shop and start trouble shooting this. BTW I did replaced the output transistors so they could have blown i will recheck them
Thank you so much for your time
Pat
will get back to you in a couple
Thank you so much for your time
Pat
will get back to you in a couple
One for tomorrow.........
If the transistors are short circuit and the resistors OK then the bulb tester glowing is normal. It is doing that because there is almost a short circuit across the supply.
You need to take this slowly and replace the faulty output transistors.
When the output transistors are removed, check again that the 0.22 ohms are still OK because its possible with shorted transistors that you are just reading rail to rail via the short. Better still, lift one end of each of the resistors and check. Something doesn't sound right there... you couldn't have 17 volts across 0.22 ohm and not see smoke and flames. Check those out of circuit and be sure.
Also check the driver transistors are OK, but don't start swapping parts with the good channel
that is a recipe for disaster.
If the transistors are short circuit and the resistors OK then the bulb tester glowing is normal. It is doing that because there is almost a short circuit across the supply.
You need to take this slowly and replace the faulty output transistors.
When the output transistors are removed, check again that the 0.22 ohms are still OK because its possible with shorted transistors that you are just reading rail to rail via the short. Better still, lift one end of each of the resistors and check. Something doesn't sound right there... you couldn't have 17 volts across 0.22 ohm and not see smoke and flames. Check those out of circuit and be sure.
Also check the driver transistors are OK, but don't start swapping parts with the good channel
that is a recipe for disaster.
Hello!
As I can see in the quoted post, you used transistors from China, which means they are fake. Fake transistors do not function (they are essentially jumpers), and this is why your amplifier burned up.
Get new parts from Mouser or Digikey, but you will have to use modern equivalents with different names. Whatever transistor you find in an old amp, will no longer be in production. All old, out of production parts that you can still buy, are fake. Actual NOS parts are so rare they practically don't exist.
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Hi and thanks for the info so as i read this i just placed an order for replacement transistor with Digikey i ordered to replace the 1384 the KSC2383 and for the 684 i ordered KSA1013
all my problems started with i replace these transistors
Thanks
.Pat
all my problems started with i replace these transistors
Thanks
.Pat
When you measured the bias you said you had around 17 volts showing, compared to the correct 9 millivolts on the other channel.
The bias is measured across those resistors and so if you calculate the current and power then 17 volts across 0.22 ohm is 77 amps and 1300 watts.
So the only way you could have actually had 17 volts would be if the resistor was open circuit
Do you see ?
Another possibility could be broken print i.e. the resistor isn't going where it should because of print damage.
The only way to approach this is to build the amplifier back up with new transistors for the suspect ones and to then power it using the bulb tester and fault find based on the voltages you then see within the circuit.
The bias is measured across those resistors and so if you calculate the current and power then 17 volts across 0.22 ohm is 77 amps and 1300 watts.
So the only way you could have actually had 17 volts would be if the resistor was open circuit
Do you see ?
Another possibility could be broken print i.e. the resistor isn't going where it should because of print damage.
The only way to approach this is to build the amplifier back up with new transistors for the suspect ones and to then power it using the bulb tester and fault find based on the voltages you then see within the circuit.
I will take a closer look and take my time going trough this i will take some pic's of the board the thing that worries me now is that the traces are getting worn out from me taking out the old and new transistors i am going to have to bridge some of these, but i will take pic's and post it
Thanks Again
Pat
Thanks Again
Pat
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