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Working on a FT757GX Radio, problems problems..

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M Philbrook
Guest

Sat Feb 29, 2020 12:45 am   



I know someone out there must of worked on one of these rigs at one time
or another.. Currently I am working on one that I bought new years ago
and I brought it out of storage and decided to restore it's operation
....

First, the FM on this rig never worked the way it should have, always
low audio level on receive but I never cared for that anyways back then
and never gave it any thought.. But I decided now to look into it. It
appears on the RF board, the one on the bottom behaves like it has
leakages in the circuit.. Why I say this is because I am getting enough
voltage on the emitter of Q39 which also has a DIODE (D109) connecting
to the FM decoder chip to partially squelch out the audio...

According to the print I shouold be getting near 0 volts here in RX
mode but I get around 3 volts which is enough to forward bias D109 and
mute the FM audio.

I looked at every component that is connected to that rail, things like
the TX switch that supplies 8 volts to that rail down to all the tracks
to disables the RX RF buffers etc..

I disconnectec diode after diode looking for leakage and found none
but thbere absolutely voltage getting there with no sign of where its
coming from...

So I noticed that if I loaded the line down with a 1k resistor it all
works great. Btw, there does not appear to be any shunting R to common
as it is now? Bad design maybe but normally you have a pull down R, I
can't locate one anywhere in that circuit.

SO I notice when I disconnect diodes to the various paths on the board
this leakage voltage was reducing itself..

I've come to a conclusion that the fiber board has come partially
conductive over time and due to the multiple paths for possible leaks
this all adds up and since I see no pull down R in the circuit any
where, I put in a 1K and not it seems to work...

Does all this make any sense at all ?

Ralph Mowery
Guest

Sat Feb 29, 2020 1:45 am   



In article <MPG.38c36cd82804c3dd98a0df_at_news.eternal-september.org>,
jamie_ka1lpa_at_charter.net says...
Quote:

First, the FM on this rig never worked the way it should have, always
low audio level on receive but I never cared for that anyways back then
and never gave it any thought.. But I decided now to look into it. It
appears on the RF board, the one on the bottom behaves like it has
leakages in the circuit.. Why I say this is because I am getting enough
voltage on the emitter of Q39 which also has a DIODE (D109) connecting
to the FM decoder chip to partially squelch out the audio..


Check the transistors around that circuit. I have a 757 that developed
some receive problems about 10 years ago and traced it to leaky
transistors. Look for any that the voltages do not match the schematic.

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