Work in progress… read on …

A priori: As always, click on images to enlarge in new tabs.

Introduction.
I worked at the Dutch Radio Communications Agency and periodically
administrative obsolete equipment was offered to staff members before it was destroyed.

Nowadays, due to governance issues this seems impossible … anyway …

The procedure was co-workers could subscribe to a list. After a while
you were informed whether you wanted to buy the item for a scrap price.

Around 1998 I got my Yaesu FT-780R for around 10 Hfl (ca. 5 Euro nowadays).

This FT-780R was used in our monitoring station (NERA) ‘to enforce amateur satellites’
(I was told). I was also told this FT-780R was ‘custom modified’ so that it was not able
to transmit, in order not to damage other sensitive monitoring/receiving equipment.

After all, NERA was a monitoring station, not a transmitter site ! ; -)

When I got this FT-780R the receiver worked okay, but when you pressed PTT
the processor crashed, resulting in ’8888888 88′ on the display.

For whatever reason I left this FT-780R in a box for more than 15 years . . . until recently.

Below the bottom cover of my FT-780R is depicted.
I think not many radio amateurs use equipment used and owned by their enforcement agencies ; -)


Rationale.

A few months ago some friends in my neighbourhood decided to build a linear transponder using
2320.7 MHz in and 432.7 MHz out, BW = 15 kHz. I own two IC-402′s but these lack crystals for 432.7 MHz.
Despite I have a FT-857 I thought it would be a nice idea to have a dedicated rig for this transponder
in conjunction with a 2320 <–> 432 MHz transverter.

So… I fetched the old FT-780R from my storage box and decided to ‘remodify’ it.

First I looked up the circuit diagrams on the internet in order to investigate these ‘secret
non transmit’ modifications. I found a ‘user manual’ PDF, but circuit diagrams were split.

I ‘reassembled’ the circuit diagrams into one piece with a pair of scissors and took
pictures of the results as depicted below.


‘Remodification = repairment’

My assumption this FT-780R was modified in order NOT to transmit seemed valid at first glance.
My connotation of the word ‘modification’ involves (some degree of) reversibility.

Along the PTT line an ‘extra wire’ was hooked up to the processor board.
Removing this wire eliminated crashing after PTT.

RF output was absent but . . .  I could hear myself on a nearby receiver.  Promising!

Optimistically I started to inspect the final stage, consisting of a Mitsubishi M57716 module.
The relevant part of the circuit diagram is depicted below.

In/nearby the final amplifier stage I noticed three ‘issues’ (refer to right picture above):

1. The antenna relay did not switch per PTT because the ‘RL’ wire was dismantled.
2. Power supply leads of the last amplifier stage were removed both inside and outside.
3. Q2 (2SD235Y) was missing (??) and bridged so PO CONT is forced to 13.8V (??)

Issues #1 and #2 were solved as depicted below.


Issues #1 (left) and                                  #2 (right) solved.

Issue #3 is not a real issue concerning output. It  eliminates the function of the LO/HI power
button on the front. Of course I hadn’t a 2SD325Y but inserted a BD139 in the small pertinax board
next to the 7808 (Q1).

Anyway,  issue #3 is a very queer ‘modification’ in order NOT to transmit . . . (?)

After solving these ‘issues’ I pressed PTT in FM mode . . . .  NO output.
Perhaps the RF module was damaged or received no drive?

Indeed, I measured no drive, so . . .  further investigation was necessary.

The driver for the M57716 resides inside the PLL unit.
Relevant part of the circuit diagram is depicted belowt.


.                                           FT-780R M57716 driver stage.

First inspection of the driver stage didn’t reveal something strange.
Relevant power supply voltages (13.8V and TX 8 Volt) were there, but no drive output.

After careful inspection I couldn’t believe my eyes . . . .  Q05 (2SC2026) was ‘missing’ !! ?

Remember my connotation for the word ‘modification’ ?
For me a ‘modification’ owns a certain degree of reversibility.
In my perspective one of my ex colleagues from the technical department stripped
Q05 from the PCB, and very likely landed in the waste bin !!

I know lots of ex colleagues read and enjoy this blog.
So. . . when you read this and it was you, or you know who it was, contact me?

Being ‘in full swing’ I dismounted the PLL unit for inspection and insert a new Q05.

Below pictures of the ‘missing’ Q05 and dismantling of the PLL unit are presented.

I could have soldered a new Q05 on the top side of the PCB but I wanted a ‘clean repair’.
Of course I hadn’t a 2SC2026 so I chose a good old BFR90 instead. And old it is, 41 years !
Below pictures of the bottom side of the PLL unit are presented. I reckon very few people have seen this side ; -)

After reassembling the PLL unit I measured 5.5Vpp RF @432 MHz over 46.4Ω with a decoupled OA91 germanium diode.
This means corrected around 5.8Vpp, resulting in (5.8/√2)² / 46.4 = 362 mW drive (which is too much btw).

I reconnected the drive cable to the M57716 unit and gave PTT in FM mode . . . . NO output : -(

Thus, very likely also the M57716 module is defect! See pictures below.

At this moment a decision had to be made to replace the M57716 module. In conjunction with
a 2320 MHz transverter replacement is not really necessary. I can route the drive signal from the
input (pin1, right) to the output (pin5, left) of the module with a small coax.

On the other hand, it is elegant to restore the FT-780R for standalone work. So, I ordered a M57716 from Ebay.

Awaiting its delivery . . . more to come, stay tuned!