ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hi Beo Fans,
Got a capacitor replacement kit for Beomaster 1200. Spent weekend installing all electrolytic caps and removing old ones.
When powering up resistor no. 421 started smoking (0.15ohm/1 watt). Transformer and rectifier very hot. Can anyone offer me help on why?
This is on the power supply board, you use voltage across resistor 421 to set trimmer pot, instead of 7.5mV I got reading of 390 and up before smoking resistor stopped play.
Much appreciated.
Was the Beomaster working before the re-cap??
If so, I'm not familiar with the particular Beomaster and so, I can only suggest looking for solder bridges / short circuits, incorrect capacitor polarity or incorrect capacitor values fitted.
Ban boring signatures!
That's one of the output stage dropper resistors.The heat and smoke comes from a too high current draw.Make sure, you have correct trimmers in correct positions, particularly thetwo 470 ohms trimmers on the amplifier board and that they are both soldered in correctly.
If that's all fine, you probably have one or two shorted output stage transistors or someother short in that channel output stage.
Martin
Thanks Martin, just to be sure on the transistors please see my photo in attachedpdf file. Incidentally I double checked the orientation of the capacitors and don't see any problems there, if you spot one please drop me a line. Again thanks for the input.
Looks fine.The output stage transistors are fitted to the bottom plate of the Beomaster, using it as a cooling fin.
OK, that's a rellief so you are referring to these (as in the new pdf)
Check the output transistors if they are faulty.
Make sure that you use the right trimmer to adjust the idle current. (R421)
Normally for jobs like these a Variac is used. With a variac you can vary the main voltage between 0 and 250V.
You could try using a 220V light bulb in series with the amplifier. Then you can set the trimmer to the lowest possible measured value.
But better, measure the value of the old trimmer and set the new one to the same value.
Hi Martin,
Checked the output stage transmitters and got following readings:
Am I right in saying that transistor 4 is good and the others are all bad? I used multimeter on "bleep" diode test, note that I did not take the transistors out of circuit for the test.
Thanks man