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Repair flag GS on PFD and msg on GNS430 G/S has failed

A flag GS appeared on the PFD display, the Garmin GNS430 msg on page about a malfunction of the glide path receiver board – G/S has failed.
The glideslope receiver board was replaced, the GS flag disappeared on the display.
It is interesting to find the cause of the faulty board and how does the flag appear on the display from the glide path glideslope receiver board? Via arinc 429 bus or directly by voltage signal?

OSF, Russian Federation

Earlier there was a similar case with a post:
there with a field-effect transistor problem.

OSF, Russian Federation

Wanted is the electrical circuit of similar glideslope receivers.
The installed board has the number 105-00212-11 VER.1
Pls help.

OSF, Russian Federation

Garmin have never published any circuit diagrams for their GNS products. Even the MM is useless.

You may be able to track down the area of the PCB (by tracing wires from the RF connector) and then work out the MOSFET type from its markings. Garmin designers from that era were mostly ex King guys so probably same circuitry as a KX165 etc.

Shoreham EGKA, United Kingdom

It is clear that Garmin does not publish schematics in the public domain.
Glideslope receivers + schemes are the same, modulation nodes are 90 and 150 MHz.
Perhaps someone has such schematics?

OSF, Russian Federation

Try this one.

Shoreham EGKA, United Kingdom

Peter – big thanks for docs!
What kind of flag can the GS on PFD?
Superflag or something else?
The flag on the PFD appears after entering the active VLOC frequency.
Could field-effect transistors BF995 or synthesizer be to blame?

Last Edited by engineer at 27 Apr 20:34
OSF, Russian Federation

Low-level of super-flags will only be used if the EFIS display is supplied with analogue glide slope deviation. If the glide slope connection is via ARINC 429 then the flag will only be a word on the 429 data-bus.

Avionics geek.
Somewhere remote in Devon, UK.

I decided to describe the malfunction and the steps taken.
GNS430 – the malfunction is as follows:
The GS failure flag “hangs” on the PFD display.
There are two GNS430s available:
1.011-00280-10 mode level 6,8,9 (main sw ver 4.04 gps sw ver 3.03)
2.011-00280-10 mode level 6,7,8,9 (main sw ver 5.03 gps sw ver 3.03)
The first GNS430 has a faulty glideslope board – p / n 105-00212-11 ver.1
The second GNS430 has a working glideslope board – p / n 105-00212-10 ver.2
A working glide path receiver board is installed in the first GNS430 – the flag with PFD is gone, that is, it was confirmed that the matter is only in the GS receiver board.
I began to study the issue of the cause of the breakdown in the glideslope board p / n 105-00212-11 ver.1
Powered GNS 430 from a power supply unit with a voltage of 25 V and a current of 1 A.
The unit started up, measured the voltages on the J25 connector of the glideslope board, and also compared several voltage points with the working board.

On this cool forum I read that perhaps the problem is related to the double-gate mosfet (tetrode BF995), which is in the input stage.

Also on the forum, they advised the documentation on the glideslope receiver BENDIX / KING KX165A – I looked through its diagrams, the principles of operation of each stage and, by analogy, called the control points on the glideslope receiver GNS430 board.
With the board connected, on the message page MSG GNS430 writes – G/S has failed.
When the board is disconnected from the NAV BOARD, the message changes to – G/S is not responding.
These actions suggested that the board may be “conditionally live”, and the point is in some cascades / elements, but not in the INTEL central processor – if, of course, it’s in it, then I think it’s unrealistic to fix it …
Then I connected via arinc429 to the P4006 connector (legs 23 and 24).
Received data on labels 173 and 174.
A working glideslope board successfully accepts the input signal and the DDM changes from zero within acceptable limits (no LOC/GS antenna connected)
The faulty one issues the SSM status bit – as FAIL, this signal is likely and forms the output of the flag on the PFD – GS.
In general, I do not know which glideslope section / module of the board is defective.
I did not check with the signal from the IFR4000 generator applied to the board input, except with the GNS430 installed on board and the standard antenna.
Advise based on your experience – it may be the cause of the BF995 mosfet or it is in other parts of the glideslope of the board (amplifiers, attenuators, mixers, etc.

Perhaps someone will help me find the cause of the breakdown, or my information will help someone later.
Last Edited by engineer at 29 Apr 16:27
OSF, Russian Federation

For almost a week I have been studying the glide path receiver board, calling the elements and measuring megahertz from the generator, but have not yet found the cause of the breakdown.
I was lucky to find the wiring diagram for the communication module, but so far it hasn’t helped much either.
I am trying to find a malfunction by comparing the nodes of the board.
In the photo, the upper board is faulty, the lower one is serviceable, but differs in other radio elements or their location, which makes it difficult to find a breakdown.
Maybe someone came across a diagram for the glide path module from the harmine?
Please share.


OSF, Russian Federation
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