Ramsey problem AM25C

Hello Kris,

The TIP31 operates hot and it may have caused a problem with the solder connections on the printed circuit board. Carefully check the connections. From your description the problem is most likely in the LM358 and TIP31 part of the circuit and not in the frequency generating part of the circuit

If you can measure some voltages and post them here it could help. Measure the voltages to ground at the three terminals of R23.

Measure the voltages to ground at all pins of the LM358 and post them here. I might be able to tell from these measurements where to look further for the problem.

Neil

Hello Neil

I check all te connections these are ok

here the results of measuring

-powersup. = 13.5V DC

*pot R23: medio stand

* 1.6V
1.18V* * 2V

*LM358:
1=0.18V
2=5.75V
3=3.12V
4=0.1V
5=1.6V
6=1.7V
7=3.1V
8=13V

*TIP31:
B=1.2V
C=7.9V
E=0.12V

The TIP31 become faster than normal very hot
LM358 = new
The transmitter gives full power, but no audio, turning R23 has no effect to te power of the transmitter.

kind regards

Kris

Kris,

The measurements help. An op amp operating within its capabilities will have almost zero volts between the + and - inputs. This is not the case for pins 2 and 3 in your circuit.

The voltages which do not look normal are:

LM358 (U1A) pins 2 and 3. The voltage on pin 1 should be about 8V but is .18V possibly because the voltage on pin 2 is more positive than that on pin 3.

TIP31 Collector (should be the supply voltage around 13V)

TIP31 base to emitter voltage appears normal for a darlington but because the voltage on U1A pin 1 is .18V the base voltage should be 0V. This suggests that Q8 is defective or that you confused Q9 with Q3 for your measurements.

With the voltage at TIP31 (Q9) emitter at .12V you cannot be getting "full power" and should be getting no RF output. Did you measure the voltage on Q3 by mistake?

The audio is not passing through U1A because it is not operating in its linear region.

With the voltages you measured it should not be possible to have .12V on TIP31 emitter and .18V on U1A pin 1 and have 5.75 volts on U1A pin 2. Either U1 is defective or there is a solder bridge or other problem with the printed circuit board. Are you sure the LM358 is not installed backwards?

The voltage at the TIP31 (Q9) collector puzzles me since it is connected to the + supply as is U1A pin 8 and these voltages should be the same. Check your readings again on these two points. The TIP31 should not be hot with the voltages given unless the emitter circuit is shorted to ground somewhere. What voltage to you measure from the junction of R5 and R34 to ground?

It is possible that there is more than one problem but the problem appears to be with the U1A circuit. I would replace the LM358 and Q8. It is possible that the "new" LM358 you installed is defective or was damaged.

Hope this helps. If you can give more information I will try to help.

Neil

Hello Neil

The transmitter has been restored !
the problem was Q8, this has been replaced

the measurement can differ because these transmitter is customized
for 1 watt RF output (not US)
R33 and R34 are bridged
R12 is replaced by a diode for a better and harder sound

Thanks for help with this problem!

with friendly greetings
Kris

ps.

Now I can my previous Ramsey also restore because they had the same problem

Kris,

I am pleased to learn that the problem is solved.

Replacing R12 with a diode could produce too much base current in Q8 which can damage this transistor. Try putting R12 back in the circuit and putting the diode (1N4004 or equal) in place of R33 and R34 with the banded end toward C18/L4. This has almost the same effect as what you did and increases the maximum modulation from 90% to 100%. This introduces some audio distortion but you probably won't hear it.

Neil

Neil,

I will like you write, replace R12, otherwise it will break again Q8.
and at the place of R33 and R34 the diodes 1N4004

thanks for the advice