Hi all!
Just a question about small power amp that keeps fuses blowing.
I suspect that chances are that an output transistor is shorted (or maybe the two TRs are) in the left channel, as pictures below show the two emitter resistors a bit "cooked".
I'm trying taking the components out for measurements.
But an important thing came to mind. It's about the toroid transformer.
I took Ohm (resistance) readings on both primary and the two secondaries and had this:
Primary is about 8.3 ohms
Secondaries are between 0.1 ohm and 0.2 ohm - (sometimes go from 0 ohm to 0.3 ohms variating bliefly):
Do you think the toroid trafo is OK?
Thanks you all! 🙂
Just a question about small power amp that keeps fuses blowing.
I suspect that chances are that an output transistor is shorted (or maybe the two TRs are) in the left channel, as pictures below show the two emitter resistors a bit "cooked".
An externally hosted image should be here but it was not working when we last tested it.
I'm trying taking the components out for measurements.
But an important thing came to mind. It's about the toroid transformer.
I took Ohm (resistance) readings on both primary and the two secondaries and had this:
Primary is about 8.3 ohms
An externally hosted image should be here but it was not working when we last tested it.
Secondaries are between 0.1 ohm and 0.2 ohm - (sometimes go from 0 ohm to 0.3 ohms variating bliefly):
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Do you think the toroid trafo is OK?
Thanks you all! 🙂
You can get a better idea of your transformer using a moving coil meter like Avo8 or such like. The resistance will start off high and then reduce quickly as the iron magnetizes. Your DVM cannot read with any accuracy, less than 1R0 due to the leads etc. The primary sounds OK. Put mains on it through a 55Watt lamp. The light should go out.
Have you got permission to use the Philips Logo?
Have you got permission to use the Philips Logo?
Footprint series 75 - small power amp
Hello Jon, thanks for the answer.
The Philips logo is just for the use as an avatar, but if it is a problem for the Forum I can take away.
About the transformer, ok I'll make another test.
I've read somewhere if the secondaries are "less than 1 ohm" they should be fine... That's why I came for a confirmation on that.
Thanks again!
Hello Jon, thanks for the answer.
The Philips logo is just for the use as an avatar, but if it is a problem for the Forum I can take away.
About the transformer, ok I'll make another test.
I've read somewhere if the secondaries are "less than 1 ohm" they should be fine... That's why I came for a confirmation on that.
Thanks again!
Footprint series 75 - small power amp
An update.
The left pair of outputs were shorted. I replaced them and the amp came again to life.
However, after 10 minutes of playing at half the volume, the red protect light went on, I heard a hum sound and it felt quite warm...
I let it cool off and then it worked again. I let it play with the volume set to 25% for about ten minutes. Then the sound stopped and the protect light went on again. The hum sound returned and it also felt warm again.
So it looks like it overheats too soon!
An update.
The left pair of outputs were shorted. I replaced them and the amp came again to life.
However, after 10 minutes of playing at half the volume, the red protect light went on, I heard a hum sound and it felt quite warm...
I let it cool off and then it worked again. I let it play with the volume set to 25% for about ten minutes. Then the sound stopped and the protect light went on again. The hum sound returned and it also felt warm again.
So it looks like it overheats too soon!
Well, *something* burnt the first failed pair and you have done nothing to solve that, just replaced the dead transistors.
Can you get the amp schematic?
Otherwise it will be very hard to suggest any possible troubleshooting.
Can you get the amp schematic?
Otherwise it will be very hard to suggest any possible troubleshooting.
Hi,
Unfortunately there is very few info about this model on the internet. It's a model from 1994, and schematics seem impossible to find. The drivers are good, not shorted.
What caused the output transistors to blow was the use of more speakers (probably in parallel) for long time, according to the owner.
I'll try to see if bias are set high, but, yeah, without the schematics, it's very hard indeed.
Problem solved.
Bias across the emitter (left channel) was much higher than in the rigth one. Four times higher.
56mV at left, 12mV at right.
Set it down to the same values and no protection mode any more, neither hum sound nor getting hot.
Bias across the emitter (left channel) was much higher than in the rigth one. Four times higher.
56mV at left, 12mV at right.
Set it down to the same values and no protection mode any more, neither hum sound nor getting hot.
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