FT-757 GX II - No Power - No lights

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Re: FT-757 GX II - No Power - No lights

Postby kg5umh » Sat Feb 10, 2018 11:42 pm

Dave,

I checked RL01 on the PA Board. It is pulling in when powered on. In transmit, it does not change and is fully closed.

Set the radio to 7.200, connected a wire from the center conductor of a coax jumper and laid it across the RF board. Turned on the receive radio (IC-706), transmitted on the Yaesu using the MOX button. Received a clear tone on the IC-706 at 7.201. Tone disappeared when the Yaesu was taken out of the transmit mode. Tried this on several 40m frequencies with same result. I think we are generating the frequency on transmit, just not getting through PA and out.

Checked the voltages on Q01 through Q11 - if I can attach a spreadsheet I will send the readings along with factory readings (in red). They are a little out of whack. But, I am presuming that I am getting RF to the PA and I might not be.

That's the status at this point -

Thoughts Thanks
Michael
KG5UMH - DM93am
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Re: FT-757 GX II - No Power - No lights

Postby w6nfu » Sun Feb 11, 2018 5:18 am

Michael,

I would like to see your PA Voltage chart results, especially of Q-6014.

In CW Mode with Drive turned up about 1/3 or more, key the Mic or press MOX and at the same time, short the Collector and Emitter of Q-6008 with a small screwdriver. This bypasses the TX control circuit to possibly eliminate one source of problem. Make sure a 50-ohm antenna for the band used or dummy load is connected to the output coax. If no results, then......

Try your experiment of Tx on the 757 while listening to the IC706, bur first unplug the small coax that runs from the Local to the PA at P-6006. Do you get the same results?

Would suggest doing all these Tx tests in the CW-W MODE with Drive about 1/2-way or the red line straight up. Just momentarily and if you do get output, decrease the DRIVE to about 20-watts with ALC/PO button pressed IN.

What test equipment do you have? I will attempt to call you at your work on Monday.

Dave - w6nfu
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Re: FT-757 GX II - No Power - No lights

Postby kg5umh » Sun Feb 11, 2018 2:35 pm

Dave,

Thought that I included an attachment of the transistor voltages - will try again.

Shorted Q6008 and still no output

Will try and get the Q6014 readings today.

Multimeter and an RF probe are about all I have for test equipment - Doing this a little at a time on the weekends - makes it a little harder - going to be out of town next week, so a Monday call will not be possible.

If I can confirm RF to the PA that would make things easier. As you know, once you get the "top" off it is difficult to keep flipping it over to the RF side. :)
Michael
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Re: FT-757 GX II - No Power - No lights

Postby kg5umh » Sun Feb 11, 2018 8:52 pm

Dave,

Interestingly, the Schematic shows a Q14, but I only find a Q13, which is a MC14011B, which is what Q14 should be --- This is a GXII so I need to check my schematics - I was looking at a GX model - Still the GXII schematic shows the Q14, but the board is labeled Q13.

I'll attach the Xcel sheet again - it might be going to the files section of the forum - Might be easier to abandon this and just go direct email

Thanks
Michael
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Re: FT-757 GX II - No Power - No lights

Postby w6nfu » Sun Feb 11, 2018 10:06 pm

Michael,

At this point, I'd like direct email and when you get the radio sorted out, then post defects and solutions for the Forum benefit. Okay? I have a lot of notes and photos collected over the years that might help.
Am good on qrz.com.

Dave - w6nfu
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Re: FT-757 GX II - No Power - No lights

Postby w6nfu » Mon Apr 30, 2018 5:08 pm

As intimated in the last post, here is what we found in this FT-757GXII and the resolution:


Problem: Good Receive, No Transmit

Symptoms: Radio turns ON, when MOX pushed ON in CW MODE, ON-AIR lite comes on. No output

Radio was set up on 7 mhz, Mode CW, small gray cable from Tx Out at J-1029 unplugged, DRIVE set about 3/4ths, with MOX pressed ON.....
RF Output at the Drain of Q-1049 checked with Oscilloscope RF probe and found to be over 1.3-V. Placing the probe directly on the case (Collector) of the Driver Transistor Q-1050 showed no amplification until pressing slightly harder with the probe, then over 6.0 V RF was indicated on the Scope, which is the expected output of about 6X gain or more.

Cure: The 5-screws of the RF Board and necessary cables were unplugged, the RF pcb Board was flipped up at the back and laid down over the front panel. Sure enough, the Emitter pin on Q-1050 was still soldered, but the foil trace broken right at the solder joint. This was resoldered and a wire bridge run from that pin to another tie-point for strength. Problem fixed. Output is now over 100-watts,

Because Q-1050 was loose on the pcb which probably caused the foil break, the Heat-Sink was re-glued to the pcb with a bit of JB Weld and is now solid.

Dave - w6nfu
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Re: FT-757 GX II - No Power - No lights

Postby w6nfu » Fri May 04, 2018 9:08 pm

Correction:
On my post of Feb. 9, 2018, it was stated towards the end:

Too, the voltage on D-2023 and D-
1026 diodes would drop from about 5-volts on Rx to almost 0 on Tx.
Cause: Measuring the voltage on Q-1022 Voltage Regulator revealed a
bad IC.


It should refer to D-2023 and D-2026 (not D-1026). Also, It should refer to Q-2022 (not Q-1022) Voltage Regulator.
All components on the RF Board are 1xxxx and components on the Local Board are 2xxxx, PA is 6xxxx, etc.
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Re: FT-757 GX II - No Power - No lights

Postby vk5ht » Thu Jan 03, 2019 3:33 am

Hi

Mine had no TX due to an after market power plug installed. All I did was put the jumper wire inside the DC plug and away it went. You can see the jumper on the circuit. Make sure its on yours.
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Re: FT-757 GX II - No Power - No lights

Postby w6nfu » Sun May 19, 2019 1:46 am

VK5HT has a good point. Its easy to overlook the obvious when, as in the case without the jumper inside the plug, the Rx is fine, and in Tx, the ON-AIR light comes ON, but no output. I've actually had this happen before by ordering and using a plug made for the FT-747 which will power up the FT-757gx, but without the jumper there is no Tx.
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