| Author |
Message |
    
PC751™
| | Posted on Wednesday, May 15, 2002 - 10:54 pm: | 
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Don't know if anyone else has had this problem, but I need some help on where to look for what is cauing my 2950 to have such high deadkey (35 w), but no modulation. FM and SSB mod is ok, although I notice that the FM key is also 35 watts. Has no variable RF control as far as FM And AM, but the variable for SSB power is OK. Have checked Q53 and Q51. Are these supposed to check open, or shorted? Read somewhere that a reverse polarity would short AM power regulator, anyone know which part this would be? Any help would be appreciated. |
Tech671
| | Posted on Thursday, May 16, 2002 - 10:34 pm: |
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Usually is the 2sb754 regulator. |
bullet/151 southern Indiana
| | Posted on Friday, May 17, 2002 - 11:00 am: |
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that would be my guess as well
good place to start |
ss8541
| | Posted on Friday, May 17, 2002 - 6:01 pm: |
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if the emitter-collector junction of the 754 is shorted you will have the full voltage of the vehicle batt/pow supply going to the finals and driver. this will give you that 35-40w carrier you have. it could also be that the 473 is bad, cranking the the 754 wide open. i doubt this, but it could happen.
i would replace the 754 with an 817 and the 473 with a 1012. for one, this eliminates both, and secondly since you are changing parts anyway, might as well uprade both.
richard |
PC751
| | Posted on Saturday, May 18, 2002 - 10:44 am: |
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Thanks guys. I have finally fixed this headache.
I started checking for shorted diodes and transistors in the AM Mod amp section, but could find none. I usually thought if the 754 shorted, it would give these symptoms, I had this problem with a different radio at one time. So, I knew where abouts the problem was. After getting frustrated from checking with the meter for shorts, I decided to start pulling transistors, looking for one that might be open. You wont find a transistor that checks open while still in circuit, so I started pulling and testing. Low and behold, the first transistor I pulled, Q54, a C945, was checking open from Collector to Emitter. Replaced transistor with new one, checked all connections, and bam, there was our AM Modulation. So, I hope this may help someone solve a headache. I dont know what caused the transistor to open, I guess a faulty one. But, it works now. |
ss8541
| | Posted on Saturday, May 18, 2002 - 11:36 pm: |
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hmmm, this is very strange. q54 is the am audio kill switch while in fm/cw/ssb. it stops am modulation in these modes. it is isolated dc wise from the rest of the am mod amps/power regs by a cap. so it shouldn't play any role in the 35w carrier you had.
and with the emitter-collector junction being open, you 'should' have had a different effect mod wise. am mod 'should' have worked fine, while fm and ssb were not only fm or ssb modulated, but both modes would also be am modulated at the same time since the am audio kill switch is basically disabled.
but that is why i like electronics, especially radio. what 'should' happen isn't always what does happen. |
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