Yaesu FT-757GX will not transmit on 17M or 12M bands RX is OK

[n5xmt]

I have a FT-757GX that I picked up cheap. Display wasn't working Found the 500khz ceramic resonator on the display board had been broken off Replaced it and the radio now works as it should, except I have no transmit on 17 and 12M. Receive is fine on all bands and modes. Any ideas where to start? I have the service manual.
Thanks

 

[davenn]

Replaced it and the radio now works as it should, except I have no transmit on 17 and 12M. Receive is fine on all bands and modes. Any ideas where to start? I have the service manual

Hi there
thread cleaned up, I apologise that you had to get those poor responses ... I have deleted all comments as a cleanup

I am aware of the FT-757GX but never used one. I have a good collection of other makes and models haha

OK to your radio problem
which 500kHz filter did you swap ? give the part number so I can find it on the schematic

I doubt that there is a filter that would fix the display. There well be a ceramic resonator that is an oscillator for the display,
like there are for the microprocessor chips .... just trying to find the display/driver circuit on the schematic

 

[davenn]

I have the service manual.

OK, likewise
here is most of the cct for the display board


There is a 500kHz ceramic resonator ( oscillator for the display driver chip - TMS2370) across pins 37 and 38 of the TMS2370. Is that the one you swapped?

The important thing is swapping that to get the display would not affect any failure of one or more bands.
it's a 99% probability that you disturbed something else that has killed the 17 and 12 metre bands ...
a cable connector bumped out on the RF board somewhere.

cheers
Dave
VK2TDN

 

[n5xmt]

It's the 500KHz Ceramic Resonator on the display board. Double checked my OP to make sure that was what I wrote LOL
I guess I should have stated more plainly that replacing the resonator didn't kill anything. After the display was working and I started testing the radio, 12 and 17m would not transmit. Opened it up again this morning to see of I bumped anything, pinched any wiring etc. Inspected everything, reseated all the cables/connectors on the local unit board, RF board and the Display board, same symptom. I'm ordering some 1N4148 diodes to replace all the band switching diodes. Been reading that they commonly go bad.

 

[davenn]

It's the 500KHz Ceramic Resonator on the display board. Double checked my OP to make sure that was what I wrote LOL

All good, but I am still not clear ....

Replaced it and the radio now works as it should, except I have no transmit on 17 and 12M.

so that original comment still has me thinking that opening it up and moving things around to get access to the display board
has disturbed cables or whatever that has stopped 12 and 17m to stop working.
You didnt say they were working or not ... I am assuming, again, from that comment, they were previously working

Opened it up again this morning to see of I bumped anything, pinched any wiring etc. Inspected everything, reseated all the cables/connectors on the local unit board, RF board and the Display board, same symptom.

OK

I'm ordering some 1N4148 diodes to replace all the band switching diodes. Been reading that they commonly go bad.

Honestly, I have never seen that problem with those diodes in any gear radio or otherwise
but who knows, maybe it is a thing in the 757 ?

We both need to study the SM and schematic to get ideas on why those 2 bands dont TX but the others do.
some signal path tracing will be needed. Checking bandswitch contacts, band change relays etc

Do you have something to trace RF signals ?

Dave

 

[n5xmt]

When I bought it a month or so ago, I was told everything worked but the display, so I'll take that with a grain if salt that 12 and 17 probably weren't working... I have an rf voltage probe and a digital 50mhz Oscilloscope.. along with the usual dvm etc

 

[davenn]

When I bought it a month or so ago, I was told everything worked but the display, so I'll take that with a grain if salt that 12 and 17 probably weren't working...

Ahhh, OK, so may have well been no go already

I have an rf voltage probe and a digital 50mhz Oscilloscope.. along with the usual dvm etc

Good Am at work at the moment ... if i get time tonite at home, I will look through the schematic
and see if i can come up things for your to check

 

[n5xmt]

Thanks, I appreciate it

 

[n5xmt]

Ok. Just spent some time scouring the schematics. RF board sends the band switch lines for each band, 160, 80,40,30,20,17,15,12,10 to the Local board Local board send them to the LPF, but it combines 12 and 10 in one, and 17 and 15 into another. 30 and 20 into a 3rd and the rest are by themselves. There are 6 LPF's The signals are combined by diodes. I've seen a lot of youtube videos where they replace them for a lot of reasons, some they replaced them "just because". The factory diodes have a 10V reverse breakdown voltage rating... in a radio powered by 13.8 volts. The 1N4148's they use have a 75V reverse breakdown voltage I'm thinking the ones that pass the 12 and 17M signals are open. since I'll have the board out, I may as well change them all to alleviate issues in the future. I've attached the sections of the schematics showing them...
Going to attack this in the morning...

 

[bertus]

Hello,

I have seen the 1N4148 was replaced by an 1N4448.
It is has a little higher current rating.

Bertus

 

[davenn]

Ok. Just spent some time scouring the schematics. RF board sends the band switch lines for each band, 160, 80,40,30,20,17,15,12,10 to the Local board Local board send them to the LPF, but it combines 12 and 10 in one, and 17 and 15 into another.

OK you need to do some voltage comparisons between the different bands switches
on a working band(s) measure the voltages with that band switched in and out
Then do the same for the 17 and 12m bands ... find out (trace) where those voltages
are or are not present.
It really is a case of methodically working through the circuit and comparing working
sections to non-working ones


Dave