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Superstar 3900 problem
- Thread starter Robb
- Start date
That reading was taken in AM mode.
Remember; there is no modulation in AM mode either; just a blank ~15w carrier.
SSB works outstanding . . .
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I edited my earlier post. There is full 15W output because TR51 is biased completelyon in AM when it shouldn't be.
This is weird. I can put tr52 in backwards or forwards and there is no change in the radio.
Even tried that with a new 2sc945.
TR52 isn't the problem - go figure . . .
Even tried that with a new 2sc945.
TR52 isn't the problem - go figure . . .
Without trying to state the obvious are you sure the transistors your using are genuine, there's fakes aplenty out there.
can't help wondering if C212 has shorted/been bridged in some half assed swing/power mod, not knowing whats been added/subtracted from original design makes diagnoses more difficult.
can't help wondering if C212 has shorted/been bridged in some half assed swing/power mod, not knowing whats been added/subtracted from original design makes diagnoses more difficult.
does it make full power with the 754 removed? if it does not then id say the 754 or something driving/switching the 754 is caput,
comparing voltage readings to the chart i posted link to should tell you what is/is not doing what it should be.
comparing voltage readings to the chart i posted link to should tell you what is/is not doing what it should be.
Naw; the radio worked just fine before.
Gonna check C212 and replace it and see what pops up.
No fake transistors; pulls from parts radios that check out fine.
C212 is fine . . .
C212 is fine . . .
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Could something in the AM modulation circuit have gone open?
So as to cause the carrier to go wide open/full?
New to radio repair and troubleshooting; but am willing to do whatever it takes.
Again; SSB works perfect now that I restored TR32 and replaced R228.
I will remove the 754 and check in here with that result as requested, Bob.
So as to cause the carrier to go wide open/full?
New to radio repair and troubleshooting; but am willing to do whatever it takes.
Again; SSB works perfect now that I restored TR32 and replaced R228.
I will remove the 754 and check in here with that result as requested, Bob.
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so you have established the voltage is coming from the 754 and the 754 is ok using diode test on multimeter,
check the voltage of the ssb switch against the chart,
check the voltage on the other transistors in the am reg / mod circuit against the chart,
make your own chart in tx/rx for comparison, it will give you a better picture of what's not as it should be,
check the am/fm carrier vr is working as it should using ohm meter,
you need to find out what is turning the 754 hard on in all modes.
check the voltage of the ssb switch against the chart,
check the voltage on the other transistors in the am reg / mod circuit against the chart,
make your own chart in tx/rx for comparison, it will give you a better picture of what's not as it should be,
check the am/fm carrier vr is working as it should using ohm meter,
you need to find out what is turning the 754 hard on in all modes.
UPDATE:
Without doing anything to the radio (not that I can recall anyway), it has now begun to modulate on AM when keyed. The carrier is still very high; but it modulates! Haven't tested any transistors yet.
But TR41 is getting fairly warm to the touch w/o talking. The audio IC is also warm. Never got warm like that before. Warm; not hot. D78, R200, 2SA473/TR41 and the audio/IC8 peak @ 125f/temp. D78 seems to get the warmest. Using the temp probe on my DMM. 2sb754 is mild/warm.
TR52 goes to 260 degrees/Fahrenheit; is that what you meant by 'hot' Bob?
Gonna test TR49, 50, 52, 53, & 54 after dinner as requested . . .
Without doing anything to the radio (not that I can recall anyway), it has now begun to modulate on AM when keyed. The carrier is still very high; but it modulates! Haven't tested any transistors yet.
But TR41 is getting fairly warm to the touch w/o talking. The audio IC is also warm. Never got warm like that before. Warm; not hot. D78, R200, 2SA473/TR41 and the audio/IC8 peak @ 125f/temp. D78 seems to get the warmest. Using the temp probe on my DMM. 2sb754 is mild/warm.
TR52 goes to 260 degrees/Fahrenheit; is that what you meant by 'hot' Bob?
Gonna test TR49, 50, 52, 53, & 54 after dinner as requested . . .
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Found the fault . . .
The 2sa934 had a very small split on the side. Noticed this after I pulled it out. It is at TR50. Replaced it with an NTE-294. Cost was $2.
Radio temps and AM operation are all back to normal.
Thanks for all of the insights from you all that taught me so much.
Still need to learn more!
The 2sa934 had a very small split on the side. Noticed this after I pulled it out. It is at TR50. Replaced it with an NTE-294. Cost was $2.
Radio temps and AM operation are all back to normal.
Thanks for all of the insights from you all that taught me so much.
Still need to learn more!
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