| Author |
Message |
    
Mr259mobile
New member
Username: Mr259mobile
Post Number: 1
Registered: 6-2014
| | Posted on Sunday, April 12, 2015 - 1:07 pm: | 
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I have a superstar 3900 with the EPT360014B PCB in it. The AM modulation will not go above about 20%. I have already replaced TR49,TR50, and TR51 with no luck. I get a good audio signal out of IC4 pin 1. The little bit of modulation that is there is clean sounding just really quiet. On SSB the radio operates as it should with full output. Any ideas?
Any voltage measurements would be helpful too. I have a communications analyzer and can inject a steady audio tone into the mic jack to help trace the audio through the radio.
Thanks in advance! |
Press_man
Advanced Member
Username: Press_man
Post Number: 836
Registered: 5-2008
| | Posted on Monday, April 13, 2015 - 11:09 am: |
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Check (BTW welcome to Copper) the mike plug and board for weak solder joint plus check the AM regulator. Check the caps for leakage or open. If rig is older you may have the 10 volt blues and need to change all the electrolitics to 16v or higher. Every radio I have are in need of recap.
73 Pressman
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Mr259mobile
New member
Username: Mr259mobile
Post Number: 2
Registered: 6-2014
| | Posted on Monday, April 13, 2015 - 2:36 pm: |
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Thanks Pressman,
As mentioned I already replaced the AM regulator, also I am getting good audio to IC4 and am getting around 4Vp-p out of IC4.1 with my signal generator going to the mic input, so I am pretty sure the problem is somewhere after that section. It would be great if anyone had some measurements of the audio amplitude that is expected to be at the base of TR49 when modulation is at 100%. |
Mr259mobile
New member
Username: Mr259mobile
Post Number: 3
Registered: 6-2014
| | Posted on Wednesday, April 15, 2015 - 10:52 pm: |
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Solved it! C190 was bad. |
Dale
Senior Member
Username: Dale
Post Number: 2206
Registered: 12-2002
| | Posted on Sunday, April 19, 2015 - 11:28 am: |
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mine has same problem
dale/a.k.a.hotrod
cef426
cvc#64
454 [dx numbers]
38lsb
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