FT-757 GX II - No Power - No lights

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FT-757 GX II - No Power - No lights

Postby kg5umh » Thu Oct 05, 2017 9:12 pm

Acquired this unit and the matching power supply recently. Was told that the unit would power up if the power cord (red-positive on bottom horizontal, black - negative on top horizontal) was wiggled the unit would light up. I purchase the proper J304 plug and wired as described in the FT 757 Owner's Manual (Positive on bottom horizontal, Negative on the top horizontal, two vertical pins connected together). I verified that the power supply was putting out power, measured at 14.5 volts. The radio does not come on when the power button is depressed. The MOX switch is in the out position (I had hoped this was the problem) and that did not make any change. I find that RL01 does activate (audible click, and can be felt). Voltage across J09 and J10 is 13.5 volts. The fan does spin, albeit very slow and sluggish. I measured 1.6 volts across the fan connection. The 13.5v output on the back of the radio measures 2.16 volts. The 8V out on the back of the radio measures 0.55 volts.
I have all the schematics and was going to try and follow B+ as far as I could. Was hoping that someone might have a shortcut, or specific items to check. Overall the radio is in very good condition for its age. I just passed my General so this was to be the rig to get me on the air.

Thoughts or suggestions appreciated - and yes I read all 127 posts regarding the FT757 that I could find on this site.

Thanks

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

Postby N8YX » Wed Oct 11, 2017 2:23 pm

Put an ammeter in series with the plus lead and see what you have in the way of current draw when the power switch on the supply is toggled. Then try same with the radio's switch.

I'm wondering whether or not the final transistors are shorted.
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Re: FT-757 GX II - No Power - No lights

Postby kg5umh » Wed Oct 11, 2017 7:25 pm

Thanks for the response.

Turns out the relay is causing the problem. After three other sets of eyes looked at the problem and the schematic it was unanimous to jump past the relay from J09 to the jumper between C33 and C34. Radio came back to life. Took cover off relay and could see the relay activate, but the contacts were so dirty that it would not make a connection. Cleaned the contacts somewhat and so far so good. Trying to find a replacement relay now.

Now have a low volume issue and a squelch control that does not seem to do anything on FM.

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

Postby kg5umh » Mon Oct 23, 2017 9:48 pm

I cleaned up the contacts in the relay. Put the relay cover on and the cover plate on the lid and reassembled the radio.

I then connected the radio to a MF874 SWR/Watt Meter and then a MFJ260C Antenna (Dummy) load to the SWR/Watt Meter. Set the frequency to 7.1750. Selected FM. Activated PTT. Radio meter pegs to the right and the red transmit light comes on. SWR/Watt Meter needle did not even budge or twitch. Rechecked all connections. The SWR/Watt Meter has two "sides", a 1.8 to 225MHz and a 125 to 525MHz. Tested the SWR/Watt Meter with my handheld. Connection on the 1.8 to 225MHz side and using a transmit frequency of 144.xxxMHz, no SWR/Watt Meter activity. (Found to be an improper switch setting on the MFJ874, function verified). I then connected to the 125 to 525MHz side and there I showed 1.2:1 into the Antenna Load.

Next I connected to a MFJ941E Antenna Tuner. I connected the Antenna (Dummy) Load to the Dummy Load port on the tuner and the transmitter to the transmitter port. Set the tuner to "Tuner/Dummy Load. Same radio frequency,(7.1750 FM) activated PTT and radio meter pegs to the right and the red transmit light comes on. No movement on the Antenna Tuner SWR Meter.

Got the radio manual out and went through all the steps it lists for powering on and transmitting. All buttons, knobs in the correct position. Tests were done on both AM and FM.

Of course with no "wire in the air" it is hard to tell if it receives.
Now at a loss. The radio "thinks" it is transmitting and giving the indication that it is. SWR/Watt Meter/Tuner show otherwise.

Suggestions?
Michael
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Re: FT-757 GX II - No Power - No lights

Postby N8YX » Tue Oct 24, 2017 2:31 pm

Have a look at the ALC circuit and associated adjustment. Next, the SWR fold-back adjustments. The two circuits are inter-related.

Do this with a 50-ohm dummy load attached to the antenna connector at all times, until you are sure of proper circuit operation.
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Re: FT-757 GX II - No Power - No lights

Postby kg5umh » Thu Jan 18, 2018 7:07 pm

Old post ---But the good news is that the problem has been traced to a bad Display Unit. So, I am on the hunt for one. One of the questions is whether the Display Unit from a FT-757 GX will work of if it must come from a GXII. Also I have found some display units for a 767 GX, but not sure if those would work - Suggestions -
Michael
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Re: FT-757 GX II - No Power - No lights

Postby w6nfu » Sat Jan 20, 2018 11:58 pm

Michael,

I can say for certain that the Display for the FT-757gx is different than for the FT-757GXII, totally incompatible.

Not to discourage anyone, but I was never able to repair even one faulty display board. Before purchasing a supposed working display from Ebay, first make sure your rig is transmitting on a ham frequency by using an external frequency counter. With a non-working display, I was able to get output on weird non-ham frequencies,....sometimes.

Hey, anyone out there, I'm still looking for a non-working 757 but the display must light up. Am good on qrz.com.

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

Postby kg5umh » Sun Jan 21, 2018 12:52 am

OK - if -5VDC is provided to the rest of the radio - could a CAT control program like FLDIGI be used to "control" the radio. Might be a "Frankenstein" but at least it would be usable.
Michael
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Re: FT-757 GX II - No Power - No lights

Postby kg5umh » Wed Feb 07, 2018 10:29 pm

After replacing a bad display board, I had a great working radio for about three hours.
Well, the no power issue changed after three hours to receive, no transmit. In checking the no transmit issue - I checked all voltages on the connectors on the Local Unit and the Amplifier. All checked good. Moved to the RF unit and found voltages on the connectors looking like they should or close. I did find that on transmit the amp draw jumped to 8 amps for a few seconds and started to drop off. I then, while the microphone button was down, tapped on RL01 on the RF board and I had transmit - as noted on an external SWR and Watt meter. I tapped the relay again and the transmit quit. RL01 is controlled by Q1005. The relay also serves Q1003 and Q1004. This relay breaks the connection on the receive side from the BPF for the duration of transmit (at least that is how I read the schematic and block flow chart). So, the next step is to remove the RF Board and check for bad solder joints. A replacement relay can be found for about $30 on eBay, but I hate to replace that and then find it was probably bad solder joint. Though I also do not want to just check the solder joints reassemble only to continue to have the problem because the relay is bad. While I have the board out, I might replace some of the known problem diodes. Cost of these should not be much -

Any other suggestions out there - those that might have gone down this path before?
Michael
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Re: FT-757 GX II - No Power - No lights

Postby w6nfu » Thu Feb 08, 2018 4:08 pm

Michael,

I do not believe that RL-01 is your problem even tho tapping on it seems to restore Tx sometimes. This relay is strictly in the Rx circuit and only controls the RF Amp which is switched ON and OFF by the RF AMP button on the front panel. It is bypassed completely in Tx. Tapping or poking on the RF board to restore momentary Tx indicates a solder or connection problem that you will find. Don't neglect applying pressure to points on the LOCAL board also.

When it will not Tx and the MOX or Mic is keyed, do you get the ON-AIR light? This is the main clue IMHO. Q-6008 controls the Tx-13V and ON-AIR light. If it is bad, it can be replaced with a TIP32. I have subbed Q-6009 with the common 2N2222a. Q-6008 and Q-6014 are the main control transistors.

Off subject, I just went nuts on a 757 with no Tx and it was found that the INH circuitry was the cause, not allowing 13-V on Pin 10 of Q-6014. By jumpering Pin 14 to Pin 12 with a diode, I could get it to Tx, eventually finding that the INH (whatever the heck that is as all manuals I have seen show conflicting connections) blue wire had been pinched and cut while closing down the top by former owner. But, on another 757 I recently got going, it didn't matter if the INH blue wire was cut or not, in fact it was cut and taped off. Go figure. Incidently, that "blue wire" goes from Pin 5 of J-6003 to Pin 5 of J-2010 when it is there at all!

Wish I had answers, but these radios still present mysteries to me.

Dave - w6nfu (good on qrz)
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Re: FT-757 GX II - No Power - No lights

Postby kg5umh » Thu Feb 08, 2018 4:37 pm

Dave,

Thanks for the information -

Yes, with the MOX in or the microphone button pressed, I do get an "on Air" light, and so far, every plug that I have found that shows to have 13.5v on transmit - does have 13.5v on transmit. It still sounds like I need to pull that board and look for a bad solder joint. What about replacing the diodes - 104,105, 107>116, 119 and 120?? This is what Carol had recommended in several posts, if you have the RF Board out. If so, I can't seem to find the 1SS83 (12283) diodes. Found an NTE 177 that is supposed to be a replacement and a 1SS83TA that has the same specifications as the 1SS83 - I also noted that Carol suggested using the same diode in all of those listed locations -

Thoughts?
Michael
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Re: FT-757 GX II - No Power - No lights

Postby w6nfu » Fri Feb 09, 2018 2:17 am

Michael, I would first open the cover shield over the PA and remove the black plastic cover over the relay. It will probably break, but doesn't matter. On several radios, it would Rx and the Relay could be heard to click on Tx, but was making no contact. With one hand click the Mic for Tx and with a finger, move the contact to insure that it is doing its job. This has actually happened to me, the contact needed cleaning or bending.

If you pull the RF board up, you might as well change all the diodes mentioned. I have used the common 1N4148, 1SS83, and tend to favor the common BAV21, all available from Mouser for pennies. I would use a BAV21 as replacement for the 1SS83. It has nearly the same specs as the original and can handle twice the voltage then the 1N4148/914 series. It takes about an hour and a half to change out all those diodes. I doubt that these diodes are the problem with your set, but it sounds like you outta look at the underside of the board and while doing that, change out the diodes, one method used here is as follows:
Loosen the front two side screws on each side so the front Panel slides forward about an inch. Remove the RF Board screws (5 total) and unplug only what you need to allow the back of the board to flip straight up. Do one diode at a time by pulling on the component with needle-nose pliers while touching the opposite side with the soldering iron. Sometimes a small flashlight is handy to make sure yer on the same diode!! To clean out the solder holes, use about 2" of .030 Stainless Steel safety wire bent with a T-Handle on one end, the other end sand down to a point on a bench sander. After old diode is removed, press this SS sharpened wire into the hole and apply the iron to opposite side. The needle will slide thru cleaning the hole, yet the solder doesn't stick to the SS. Then install the new diode.

In my notes, I found these two more entries that might help, the first one I make mention in the first paragraph above:

My FT757GX2 has developed an intermittant fault. It goes deaf on RX and at the same time
Produces virtualy no measurable TX, although a signal can be heard on another RX.
Usualy faulty on switch-on which clears after a few minutes then goes faulty again after a
short period. Can oscillate between the two states.
Any clues?
Your Intermittent Tx / Rx problem probably srems from a very dirty set of relay contacts on
the T / R Relay. You can dismantle the covers, locating the T/R Relay, remove the plastic
cover and clean both set of contacts with a piece if 3 X 5 cardboard (similar to a QSL card)
and using Denatured Alcohol so it doesn't leave a film by attracting more dirt.
AND
FT-757gx No CW Transmit,Poor SSB audio, Receive in all modes:
Symptoms: Could get CW Tx if SSB filter was bypassed from the Kathode
of D-1052 to the Kathode of D-1045 with a .01 capacitor. Also, when
adjusting the Carrier Point, as per Page 14, Step H, the frequency
would jump up 1.3 kc on Transmit. 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.
Cure: Voltage Regulator was replaced with Radio Shack 78L05.
Radio was re-aligned as per Manual and works fine.

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

Postby kg5umh » Fri Feb 09, 2018 1:26 pm

Thanks Dave,

Well, your comment about RL1 - Relay under the cover on the PA section - this relay was part of the original "No Power" issue. I took that cover off and cleaned the contacts. After cleaning the contacts I got reliable on/off function. Before I pull the RF Board, I think that I will go back and give that another check.

After replacing the display unit, the transceiver received and transmitted only on SSB. No transmit on CW, AM or FM. I could get it to transmit on all modes, if I powered on the radio with the marker and linear switches "in" and then after power on - releasing those switches to the out position. Never thought anything about it until now. I will check RL1 before I pull the RF board.

On the diodes, I am unclear. The parts list calls for two different diodes in the 104,105, 107>116, 119 and 120 area. Some show a 1SS53 and some a 1SS83. From what you are saying and I have previously read, that replacing them all with the same diode is not a problem. Just a little clarification before I place the order. I appreciate all the help. This old thing really sounds great on receive, and now it is a challenge to get it operational again.

Thanks again, I am sure I will have many more questions.
Michael
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Re: FT-757 GX II - No Power - No lights

Postby w6nfu » Fri Feb 09, 2018 7:07 pm

Michael,

I would check that relay again first, before anything else, and be prepared to slightly bend the spring arm to make good point contact. Push it closed while in TX with a small screwdriver or your finger.

Is the Memory battery good as indicated with the Display at turn-ON, showing the last freq. before you shut off the radio? This may be your problem of having to hold IN the Marker and LIN switch at turn-ON. CR1/3N batteries are harder to find, I just use the CR2 batteries which are larger, but wire them in, there's room. For testing, wire two 1.5 V batteries in series and clip-lead them across the installed battery

Do you have a frequency counter? If so:
1. On the LOCAL board, place probe directly on the main Crystal X-2006, it should read 15.0000, adjustable by TC-2006
2. Attach the probe to Resistor R-2062 vertical wire which is same as J-2008 only easier to get to, and do Step H on Page 14 in each Mode, making sure first that the SHIFT/WIDTH controls are straight-up. Adjusting CW with TC-2004, the reading should be 8216.600, not 8215.900 as incorrectly shown in Manual.

Mouser has the BAV21 diodes for .20-cents each. A French seller has 50-each 1SS83TA diodes on Ebay, $7 for 50, shipping included. I have 100-new BAV21's, more than I will ever need, send me your address via your email and I will send 25 of them to you for nothing. The specs are good and the Brits prefer them to the 1SS83's.

One last entry from old notes:'

FT-757gx No CW Transmit,Poor SSB audio, Receive in all modes: Seemingly your problem with your GXII....

Symptoms: Could get CW Tx only if SSB filter was bypassed from the Kathode
of D-1052 to the Kathode of D-1045 with a .01 capacitor. Also, when
adjusting the Carrier Point, as per Page 14, Step H, the frequency
would jump up 1.3 kc on Transmit. 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.
Cure: Voltage Regulator was replaced with Radio Shack 78L05 or any 78L05.
Radio was re-aligned as per Manual and works fine.

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

Postby kg5umh » Fri Feb 09, 2018 8:49 pm

Thanks Dave -

I'll look at the relay this weekend -

Thanks for the offer of the diodes - found 20 BAV21 for $3.00 shipped on eBay - they will be here mid next week -

I appreciate all the help and will report back what I find - Trying to keep everything I do documented so I know what I have and have not tried -

Thanks again.
Michael
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