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Freebander
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Registered: 04-2009
Posts: 63
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Yaesu 757 Only 2w


Hi guys I have a Yaesu FT757GX 11 will only push 2w pep used to do 100w i have taken it to my rig doctor and he found a lose screw floating about in side he has checked it all out but is at loss to why it will only push 2 watts finals are ok all technical data as been dowloaded off the net he has read through all the data but there is nothing in it to help me with this problem that i am encountering any help will be fine


thanking you steve163


PS the rig doctor i use is good

  
16/Jun/2009, 6:10 pm Link to this post Send Email to steve163   Send PM to steve163
 
108FB027 Profile
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Freebander
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Registered: 03-2009
Location: just west of Edinburgh
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Re: Yaesu 757 Only 2w


Hi Steve
What is the output on am/fm ?is it just ssb that is affected ?

Are you getting the correct movement on the alc setting on the meter when you talk into the microphone ?

have you tried it into a dummy load with an external power meter in line ?

Alan



---
Icom 735,
HRD V5,
Various homebrew antenna
16/Jun/2009, 7:06 pm Link to this post Send Email to 108FB027   Send PM to 108FB027
 
steve163 Profile
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Freebander
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Re: Yaesu 757 Only 2w


Hi Alan i will ask Nick he is the one that dose all the work on my sets all i know is how to swich them on lol but i am takeing it all in from you guys on FB Forums

ps I will post the answer in the morning


again thanks Alan ps sorry but its still all new to me
16/Jun/2009, 8:39 pm Link to this post Send Email to steve163   Send PM to steve163
 
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Freebander
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Re: Yaesu 757 Only 2w


Hi Steve

No problem,,
hope it`s something simple

Alan

---
Icom 735,
HRD V5,
Various homebrew antenna
16/Jun/2009, 8:43 pm Link to this post Send Email to 108FB027   Send PM to 108FB027
 
Nick163 Profile
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Registered: 06-2009
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Re: Yaesu 757 Only 2w


Ok this is the story with whats happened.

The radio was brought too me by steve and it was slightly off freq.

When i switched on the set i noticed that the power output was very low, Even with the drive up all the way it would only do 2w max, This is on all modes and on all bands, (the linear switch on the back is in correct position)

After i explained to steve and his permission to break the warranty sticker (not original sticker) and removing bottom case a small screw fell out, Further inspection revealed the main board having one screw missing which is the same as the others, So its been removed and dropped and not put back, So im guessing this screw has caused some damage somewhere.

Now, Ive downloaded the technical supplement so i could check the voltages on the 2SA1012 2SD880 & 7808 regulator on the PA unit and all check out fine, The 2SC1971 also has correct voltages but heres the strange part.

When i touch the positive probe of the meter onto the base of the 2SC1971 i get correct voltage and it shoots up to 100w output and pulls 25amp off the power supply, but the SWR goes way over (on a dummy load) and the freq counter goes haywire (not reading correct freq) and drive dosnt adjust (dirty output?) Because its showing 100w and 25amp the PA's must be ok ?

Anyway this is where ive left it and havnt tried anything else apart from testing the relays which are all working.

So if anyone has any suggestions (apart from binning [sign in to see URL]) Then please feel free.

Regards
Nick


Last edited by Nick163, 17/Jun/2009, 11:52 am
17/Jun/2009, 11:45 am Link to this post Send Email to Nick163   Send PM to Nick163
 
Hot Fusion Profile
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Management
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Registered: 03-2008
Location: South West England
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Re: Yaesu 757 Only 2w


I don't know about this one, but it sounds to me that by touching the probe onto the base of the transistor you are simply causing it to oscilate due to capacitive feedback from (presumably) the collector or connected circuitry.

This random oscillation could be at any frequency - probably well away from where it's supposed to be and therefore the stage's output impedance will be incorrect causing the odd SWR readings.

It does seem as if the PA stage may be OK, so perhaps look at the driver stage - hook up a 'scope and see if there's any RF present either to it or from it. (Disconnect the driver stage from the PA for this test.)

Good luck! emoticon

---
Andy - 26 FB107
17/Jun/2009, 5:44 pm Link to this post Send Email to Hot Fusion   Send PM to Hot Fusion
 


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