FT-757GX only loud static on Rx, no output in Tx

Discuss anything related to any of the Classic Yaesu Transceivers

Moderators: KE7GUY, KC9FOZ, EA5/G0KOM, KG6JEV

FT-757GX only loud static on Rx, no output in Tx

Postby w6nfu » Fri Aug 24, 2018 11:19 pm

Recently worked on a 757gx, noting the following:

Symptoms: Only loud static on Rx, no output on Tx. Noise on Rx same with and without Antenna connected. No signals heard.

Tests: Voltage tests on JP1011 (not shown on schematic, but on RF Board pictorial next to heavy aluminum bracket corner.)
Red Wire 5.45Vdc on Rx, 7.29 Vdc on Tx (should read near 0 on Rx and 7.29 on Tx)
Brown Wire 7.29Vdc on Rx, 3.12Vdc on Tx (should read 7.29 on Rx and near 0 on Tx)

Solution: Clip diodes D-1112 and D-1113 which immediately solved the voltage problems at JP1011[/b]. Then on 40-m, one signal
was faintly heard and Tx would put out 10-watts. Still mostly loud static on Rx.

Diodes D-1112 and D-1113, D-1001 thru D-1023 were replaced with the BAV21 diodes. (About a 2-hour job.)
Alignment accomplished as per page 14 of the Technical supplement, Step I (first), then Step H, noting the correction to
set CW Mode to 8216.600 (not 8215.900). Output frequency matched exactly to Display by tweaking TC2006 on Local
Board. Unless a radio has had screwdriver treatment, it is usually unneccessary to do the complete book alignment.

Results: Rx is excellent, very quiet. Tx output excessive and needed to be decreased down to 100-watts. Output frequency
matches Display frequency, confirmed by stations contacted on SSB.

Hope this helps someone who owns a FT-757GX with similar, common problems.

Dave - w6nfu
w6nfu
 
Posts: 98
Joined: Sat Feb 19, 2011 9:07 pm
Location: Saugus, California, USA

Re: FT-757GX only loud static on Rx, no output in Tx

Postby PA0SJM » Sat Sep 29, 2018 6:56 pm

dr Dave
this is a very good post that you wrote. This can be a great help to find the solution of the problems with incorrect voltages on the collectors of the switched transistors Q1056 and Q1057. About diodes BAV21 ... if the diode 1SS83 is used then there is no use to replace them by the BAV21. The Vr (reverse voltage) of the 1SS83 is 250 Volt and the BAV21 has Vr 200 Volts.
The avarage forward current of the 1SS83 is 200 mA while this for the BAV21 is 250 mA. I do not see a advantage by replacing the diodes 1SS83 by the BAV21. Peak forward current for both diodes is 625 mA. Very good. In older doc's there is diode MA190 used. There I see Vr = 35 Volts !!!
PA0SJM
 
Posts: 3
Joined: Fri Nov 08, 2013 5:06 pm

Re: FT-757GX only loud static on Rx, no output in Tx

Postby w6nfu » Sun Sep 30, 2018 4:45 am

Hi Stef,

I'm finding that the 1SS83 diodes are harder to find anymore, especially from our suppliers here in the States.
The BAV21 is easier to obtain and can be purchased in quantity for just pennies each, even Amazon has them for under .08-cents each and free shipping. They seem to work just fine and as evidenced in my recently repaired FT-757gx, the Rx is very quiet with the antenna connected and very-very quiet with the coax disconnected. It is sure worth it to replace all the front-end diodes, both for Rx and Tx.

Have also had good results with the common 1N4148 diode but think that the BAV21 is a better choice, and I agree with you, the 1SS83 the very best choice if one can get them.

Dave - w6nfu
w6nfu
 
Posts: 98
Joined: Sat Feb 19, 2011 9:07 pm
Location: Saugus, California, USA

Re: FT-757GX only loud static on Rx, no output in Tx

Postby WB9LIV » Sun Oct 07, 2018 4:12 pm

I have a 757GX that I haven’t used in many years. It was drifting so a couple of years ago I put in all new trimmers, but still couldn’t bring it to the right frequencies, obviously crystal ageing. Left it until I heard ICM was going out of business, so I ordered new ones just in the nick of time. Just put them in. Didn’t tune yet, but tried transmitting, lasted about two seconds, then smoke puffs and a tripped power supply, was C6024, 10@25V tantalum. Replaced the failed Q6008 with a KSA1010YTU, replaced all the tantalums with 35 V rated units.

Still haven’t tuned, mostly okay it seems. Questions:

1. CW frequency 8216.6 rather than 8215.9 – I had not heard of this, can you kindly supply the reference material?

2. No transmit on FM, no big deal to me, but any typical solutions?

3. The fan pulses for a second when turned on, but I don’t remember if it’s supposed to kick on when transmitting or only when it reaches a certain temperature, or is speed variable with temperature? I don’t want to try transmitting for a long time to test this out, only to find it should have been running and now I have a different set of problems.

Al
WB9LIV
 
Posts: 3
Joined: Sat Oct 06, 2018 8:56 pm

Re: FT-757GX only loud static on Rx, no output in Tx

Postby w6nfu » Sun Oct 07, 2018 10:41 pm

Al,

1. See this Forum post for explanation of doing step I before H and the alignment of CW CArrier point to 8216.6 and not 8215.9
viewtopic.php?f=1&t=1133&hilit=ft+757+lsb
Never had C-6024 go out, but have changed out many Q-6008 with TIP32a.

2. No TX on FM. In Rx, if you turn the MODE SW to FM, does the static noise increase fairly dramatically? If not, locate D-1112 and D-1113 and clip the exposed wire on each
diode. If the noise is now there on FM with clipped diodes, then the installed diodes are shorted. The easy fix is to solder 1N4148 diodes in series with clipped ends of D-1112 and D-1113. Not saying this is your problem on FM Tx, but it is a starting point.

3. Your fan is normal, it always cycles for a second when POWER is initially turned ON.

Note: Original early 757gx radios did not have D-1112 and D-1113 installed. To solve front-end diode blow-out problems, Yaesu added two diodes on the underside of the RF board.. Later radios had the mod built-in, but Yaesu Technical Supplement Schematics and Parts Layout were never updated. Only in the 757GXII Manuals can these two diodes be found, both in the Schematic and Parts Layout pages.

Also, would suggest checking the Collector voltages of Q-1056 and Q-1057 in both Tx and Rx. Too, mentioned in a previous post, changing out all the front-end diodes seems to solve a multitude of problems. Feel free to email me via qrz.com

Dave - w6nfu
w6nfu
 
Posts: 98
Joined: Sat Feb 19, 2011 9:07 pm
Location: Saugus, California, USA

Re: FT-757GX only loud static on Rx, no output in Tx

Postby w6nfu » Fri Oct 19, 2018 4:56 am

Al,
Re-reading your post regarding xtal replacement and difficulty aligning the radio, I came across the following from Kevin, VE3RRH in the link referred to previously. He states:

"If you can not get the oscillator frequencies to set within the limits described you can try a little trick I discovered, go to page 16 and read step R. L1011 adjusts the frequency of the oscillator for USB and CW, by adjusting the frequency slightly with L1011, it may be possible to pull the settings for SSB and CW into a usable range for step H on page 14.

The adjustment is then compensated for by the rest of the alignment.

I have used this modification to bring one of my rigs into alignment when I could not get the USB / CW frequencies into spec. It worked very well and I can detect no problems with the setup when comparing operation to my other two 757's.
"

Ditto here and it has worked, sometimes saving the changing out of xtals or trimmer caps. Were you able to get your FT-757 aligned?
Dave
w6nfu
 
Posts: 98
Joined: Sat Feb 19, 2011 9:07 pm
Location: Saugus, California, USA

Re: FT-757GX only loud static on Rx, no output in Tx

Postby WB9LIV » Mon Dec 03, 2018 7:16 am

Well, not yet. My 40 year old counter couldn’t read the 45 MHz, but read the others just fine. I got a new counter but it had trouble with the 45 MHz too (maybe I didn’t need a new one). I used a signal generator to feed the counter and an oscilloscope, then used the x10 probe on a second channel and matched the frequencies. When I connected the “straight” probe from the counter, the oscilloscope showed a decrease in level and distortion at the test point. I don’t know where I got the straight probe from, but it had a very thin cable, so must have had too much capacitance to it.

Following through, I had that same problem with step H, LSB/CW/USB frequencies (Step I, BFO done first, no problem there). Tried the L1011 tuning method, one full turn out brought the CW/USB to the proper frequencies, but then the LSB was out. Couldn’t find a compromise that worked for all.

So, I’m going to replace the steering diodes in that section at least. When I feel like working on it again, that is. Getting tired of removing/replacing all the connectors to get to the back side over and over.
WB9LIV
 
Posts: 3
Joined: Sat Oct 06, 2018 8:56 pm

Re: FT-757GX only loud static on Rx, no output in Tx

Postby w6nfu » Wed Dec 05, 2018 10:24 pm

Al,

Two more things you might try before changing out diodes.
(1) Check the swing left and right on both SHIFT and WIDTH knobs to ascertain that the pointers turn equally right and left. Then position them at 12-o'clock using L1011. I'm sure you have already done this.

(2) You've tried adjusting L1011, so one more thing - this also does work. Add to your adjustments VR2007 (top-back of Local Board, half-way between center and the left side) along with TC-2004/5, VR2005, and maybe L1011. With VR2007,, it may be possible to pull the settings for SSB and CW into a usable range for step H on page 14. It is a juggling act to get everything to line up, but you are close.

As for the 45-mhz reading while doing Step F, the 15-MHZ Reference Frequency, seldom can I read 45-mhz with my HP counter for some reason. Instead, by placing the counter probe directly on the 15-mhz Xtal X-2006 and adjusting TC2006 to 15.000000, unless the Display reads slightly off frequency when receiving stations on cardinal frequencies. Then, lift the cover and just touch up TC2006 to where the other station sounds right.

A $12 Frequency Counter kit purchased from Ebay works quite well for basic alignment of the FT-757GX, at least to get one in the ballpark and the price is right. Feel free to contact me directly if you need.
Dave
w6nfu
 
Posts: 98
Joined: Sat Feb 19, 2011 9:07 pm
Location: Saugus, California, USA

Re: FT-757GX only loud static on Rx, no output in Tx

Postby WB9LIV » Wed Dec 19, 2018 2:17 am

I replaced all the diodes that had white-white bands on the 2xxx board with 2N4448s. It may be my imagination, but it seems that it's in a bit closer, but still out. I didn't try the VR2007 yet - I took out the 1xxx board and will soon replace the white-white diodes there. This is getting to be not fun. Receive protection diodes measure okay so I will leave them. After reassembly I'll just put it away for a while, I have other things to do.
WB9LIV
 
Posts: 3
Joined: Sat Oct 06, 2018 8:56 pm


Return to HF Transceivers

Who is online

Users browsing this forum: No registered users and 3 guests

cron