uPC1238 substitute (or what)

[Harvey]

Hi.

I am working on a audio stage monitor. It blew its fuses. The amp, a
uPC1238, was shorted.

Had trouble finding one on the internet, so after some research I
decided to try substituting it with an LM1875 or TDA2003 which I
ordered from Futurlec.com. Tried using the LM1875 but got a loud hum
from the speaker, and the part heated very quickly. Tried power on only
for a few seconds at a time. With a couple dozen trys, ther were a few
times when it did power up okay, quietly. Had light bulb in series with
power cord (40 or 100W) to limit current.

Tried more things with the LM1875T in the Monitor, but no help. Finally
hooked up the scope and found that the amp was oscillating at over
100kHz, as I suspected. The specs say that the LM1875 is unstable for a
gain of under 10. The gain appears to be about 9 in this design.

Tried a TDA2003 Amp in the Monitor, and got a similar hum in the
speaker. But this time it was not oscillating, but the output was stuck
on the negative rail. Anyone have any idea whats going on there?

Decided I needed to get the correct part. I did find one finally -
waiting for order now.
But the negative rail had me puzzled.

Any thoughts would be appreciated.

Harvey

 

[Cyrus®]

The best subs for your uPC1238 ( TO-220, 5 PIN IC ) is ECG1378 Or NTE 1378.

With best wishes
Cyrus

 

[Franc Zabkar]

Hi.

I am working on a audio stage monitor. It blew its fuses. The amp, a
uPC1238, was shorted.

Had trouble finding one on the internet, so after some research I
decided to try substituting it with an LM1875 or TDA2003 which I
ordered from Futurlec.com. Tried using the LM1875 but got a loud hum
from the speaker, and the part heated very quickly. Tried power on only
for a few seconds at a time. With a couple dozen trys, ther were a few
times when it did power up okay, quietly. Had light bulb in series with
power cord (40 or 100W) to limit current.

Tried more things with the LM1875T in the Monitor, but no help. Finally
hooked up the scope and found that the amp was oscillating at over
100kHz, as I suspected. The specs say that the LM1875 is unstable for a
gain of under 10. The gain appears to be about 9 in this design.

Tried a TDA2003 Amp in the Monitor, and got a similar hum in the
speaker. But this time it was not oscillating, but the output was stuck
on the negative rail. Anyone have any idea whats going on there?

Decided I needed to get the correct part. I did find one finally -
waiting for order now.
But the negative rail had me puzzled.

Any thoughts would be appreciated.

Harvey

The application circuit for the TDA2003 adds an optional Rx and Cx in
the feedback loop between pins 4 and 2 for high frequency rolloff. I'd
also check the cap connected to pin 2 of the uPC1238, assuming it
follows the application circuit in the datasheet. This would determine
the DC cutoff point. Ripple on the supply rails may also contribute to
hum at the output.

- Franc Zabkar