FT707 ALC problem...help!
 

Hopefully someone has seen this before, or can offer a starting point
to trace it down. (or a plausible theory - I'm getting that ****ed
off)

Problem - on some bands but not all, on TX SSB the front panel ALC
meter doesnt work - the TX still has output, but very distorted and
splattery EXCEPT if you go to the very top of the band via the VFO
when it miraculously works. Looking at the ALC control line on the
cro, when working a variable (speech rate) almost DC voltage is
applied to the relevant ALC inputs to the RF and AF and IF boards. In
the fault condition there is a modulation patttern on the ALC signal,
in step with speech peaks.

What gives - my theory falls over on this one, I suspect the LPF on
the output of the PA amp has gone faulty (as evidenced by VFO
settings) but to pull it to pieces to check is going to be a real pain
- this thing was designed by evil minded dwarfs who were determined to
make life difficult in servicing - try and do ANYTHING with the front
panel parts, or the rear PA module, and wires come loose and generate
more faults - this it has done a few times now, so its getting very
frustrating and time consuming!

As I said, on some bands it works, but on 80 and 15 (from memory, its
5 past midnight here at the end of a long day...) and the "where it is
in the band" symptoms dont help either.

Any helpful suggestions, feel free to chuckle and be glad its not
sitting on YOUR bench....

73 de VK3BFA Andrew

I would guess the PLL is going out of lock. Do a careful alignment.

When the PLL is on the verge, the signal gets distorted and unstable.

If there is no ALC, it's probably because there isn't enough power output to
overcome its threshold.

Just a guess; I've never worked on one of those rigs.

Bob

I would guess the PLL is going out of lock. Do a careful alignment.

When the PLL is on the verge, the signal gets distorted and unstable.

If there is no ALC, it's probably because there isn't enough power output to
overcome its threshold.

Just a guess; I've never worked on one of those rigs.

Bob

"Bob" wrote in message ...
I would guess the PLL is going out of lock. Do a careful alignment.

When the PLL is on the verge, the signal gets distorted and unstable.

If there is no ALC, it's probably because there isn't enough power output to
overcome its threshold.

Just a guess; I've never worked on one of those rigs.

Bob

Hi Bob,
a good theory and got me going - but then I realised it is an analog
superhet, ie no horrible nasty almost digital things. But a valid
point - will look at the o/p of the mixer on the affected bands - the
vfo runs a constant 5-5.5Mhz, so possibly the heterodyne osc. is
playing up - although, a rough check has shown the drive levels to the
PA board are constant irrespective of fault condition.
de VK3BFA Andrew

"Bob" wrote in message ...
I would guess the PLL is going out of lock. Do a careful alignment.

When the PLL is on the verge, the signal gets distorted and unstable.

If there is no ALC, it's probably because there isn't enough power output to
overcome its threshold.

Just a guess; I've never worked on one of those rigs.

Bob

Hi Bob,
a good theory and got me going - but then I realised it is an analog
superhet, ie no horrible nasty almost digital things. But a valid
point - will look at the o/p of the mixer on the affected bands - the
vfo runs a constant 5-5.5Mhz, so possibly the heterodyne osc. is
playing up - although, a rough check has shown the drive levels to the
PA board are constant irrespective of fault condition.
de VK3BFA Andrew