Rough TX-300

[calman46]

The hum is now in both channels non volume controlled.I haven't found any out spec voltages .. The right side sound is muted but I can hear with volume full on , the left is better but not working correctly. I'm looking for cold solder joints and dirty switches I did solder the chassis connections.

 

[calman46]

I believe its the output transistors . The units voltage are good . The transistors couldn't take the volts ( I was ready for the whole thing to catch fire on first power up) . I'm checking them the same way I did before , I have 1 good out 4 checked

 

[calman46]

I'll be back , I may some 2N2147. I have to put this on the shelf for now. Thanks again Fred

 

[calman46]

Ok , I put the 2N2147 in for output trans. set voltage across the E-resistors for 25mV. The left channel is working the right is real weak this on AUX on tape monitor the left is better but the test tone is not steady. There is hum in left ,I'll look for the cause . Then I'm done ,I'm surprised the left is working so good .

 

[fred soop]

If you have any doubts about the output stage, you can disconnect terminals 2K (driver transistor base) and 2L (feedback to power amp input stage) on the preamp boards and swap between channels. If the problem changes channels with the swap, then it's in the output stage. I suspect that it's actually back in the preamp someplace.

 

[calman46]

it switches ,I now have the hum in the left ,the right plays fine now. So its in the preamp ,the output stage starts at Q9/Q10 ?

 

[fred soop]

Wait a minute. You say "it switches", then you say that the hum is in the left (same as before). So what do you mean by "it switches"?

 

[calman46]

I think the hum is both just louder in the right. When I switched the wires the problem channel played good. I put wires back to their original boards and the problem channel (right) passes the signal a little louder now ( played around with the pots and switches ) but its not a steady tone and the output transistors on that side heat up considerably more . I'll be checking voltages audio amp board

 

[fred soop]

If the output transistors are running hotter on one channel, and the audio signal is of similar volume on both channels, then check the voltage across each of the 4 emitter resistors. There may be a difference in bias.

 

[calman46]

I did there is a problem with bias P3 is to high P4 is unstable both work . When I switch boards its OK . I'm thinking the Hum is a something shorting on the board as the test tone is not getting through steady . I need to check all the voltages on that board.

 

[fred soop]

P3 and P4 are bias adjustment pots and the voltage across them is not the actual bias. The voltage that needs to be compared is the voltage across the emitter resistors. That would be R68 - R71.

 

[calman46]

I have my meter across the emitter resistors for P3 &P4 . With 2K &2L disconnected I can adjust P3 &P4 correctly . I checked all the transistors voltages on both boards ,both boards match. Remember My TX300 has a factory mod on the audio boards which may explain the different voltages I have for Q206 &Q207. Q206 C -9v, B -3v , E -3v, Q207 C -32v, B -9v, E-9v. The mod puts a cap (-) on a added terminal strip with two 82K resistors. The (+) of this cap to 2G , one 82k to 2J and one 82K to B Q206 ,R222 is removed . I noticed with the speakers shut off the hum is still heard in the speaker. I was checking the common connections. I think its a switch or a pot that is shorting or a bad connection .

 

[fred soop]

With the speaker switch off, there is no way that anything can get to the speaker unless there is a miswire, short, or maybe really bad buildup on the speaker switch itself that is conductive.