FT-757GX No Audio - Fixed -- 757GX vs 757GXII Manuals

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FT-757GX No Audio - Fixed -- 757GX vs 757GXII Manuals

Postby w6nfu » Sat Feb 23, 2019 7:04 pm

PROBLEM; Last evening turned my FT-757GX on and heard nothing, no sound from the speaker. Setting the Marker to ON and tuning across the 40-m band, the S-Meter would spike every 25-khz. Transmit was good up to full-power as indicated on the radio and external Watt-meter.

With bottom cover removed and FT-757GX Technical Supplement with schematics in hand, it was soon noticed that parts placement in the audio section did not match my radio. Bringing out a FT-757GXII Supplement, it was evident that my 757GX audio stages had been upgraded to the 757GXII layout by the factory at some point.

FIX: Using an external speaker with clip-leads and pins, the audio came alive when Capacitor C-211 was bypassed. Close inspection of this 470uf/16V electrolytic showed swelling and goo at the bottom, the first bad electrolytic I have ever found in a 757gx. Pulling up the RF board, this cap was replaced with a 25-V speciman, restoring the Receive function. C-211 was the same and in the same location, but other components surrounding Q-1037 were different.

It is difficult to work on the FT-757GX using the Technical Supplement (manual) alone. When early 757GX's were modified by Yaesu with addition of D-112 and D-113 diodes, it was not reflected in the manual, but is shown in the FT-757GXII manual. There are many – many discrepancies in the Voltage Charts in the GX manual, often corrected in the GXII manual.

SUGGESTION; Bottom line, working on your own 757GX, it is advantageous have the GXII Supplement in hand. There are major circuit differences in the band switching sections, but the remainder is close and voltage charts are more correct. One deficiency of the GXII Supplement is that there is no Interconnect
schematic to help when working on the GXII models.
Dave - w6nfu
w6nfu
 
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Re: FT-757GX No Audio - Fixed -- 757GX vs 757GXII Manuals

Postby PA0SJM » Sun Feb 24, 2019 10:51 am

hi Dave, nice to see you again after long time. And as you know , no messages show that all goes well. My 757gx works always without problems , hope this remains so. Congrats that you could find the defective part .. also I can agree with you that capacitors do very good still after so many years.
The use of the GX II Supplement has only the problem that the identification of the parts is different of the numbering in the Supplement of the 757GX . You are right that the schematics and pcb layout are different , also my 757GX has circuits following the GX II Sup. All well here in Edam ... we had very soft winter with only little frost .. totally different as I saw how this was in US. Best 73 !
PA0SJM
 
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