Mods & Repairs

Yaesu FT-920 No FM Receive & No AM Transmit

Yaesu FT-920This Yaesu FT-920 was advertised fairly locally with the faults of having no received audio on FM, although transmit working fine, and also no transmit on AM but receiving OK.

After short consideration, I came to the conclusion that the asking price was a bit too high to take a risk on a partially working radio. Within a matter of days, I had been sent details of the radio from two different people, the second time the asking price had been reduced from the original price.

It was at this point I gave the radio a bit more thought. I came to the conclusion that the asking price was still a bit more than I would want to pay for a radio with these faults and made a reasonable (to me) offer.

My offer was based on the fact that as it stood, the radio was working as an SSB/CW radio. The problems could be a very simple fix, or at worst it could be a CPU fault which would then become an uneconomic repair.

Once back home and on the bench and connected to my signal generator, the radio did indeed receive an FM signal but produced no audio from the speaker.

With this, the first suspect, the optional FM board, was checked for security on it’s mounting pins – it simply plugs in and relies on the connector to hold it in place. A quick tap and a wiggle produced an intermittent sound from the speaker, could this be the first problem sorted?

A very close inspection of the mounting pins and sockets on the main board and the FM board revealed nothing amiss.

Old & New 455KHz filtersNext step, checking the 455KHz filter. Tapping the filter brought the sound back temporarily between taps. These filters have often been a problem area in a range of Yaesu radios produced around the same time period.

A new 455KHz filter was then fitted, and the FM board installed back in the radio, unfortunately, this didn’t cure the problem and sound was still intermittent.

yaesu ft920 fm board filter removedAt this point is was obvious the fault was somewhere on the FM board. The board was then again removed and this time a close inspection of all the soldering on the back of the PCB was made.

Under the microscope, an almost invisible crack in a solder joint was spotted, the joint was reflowed and once again the board returned to it’s place on the main PCB of the Yaesu FT-920.

Optional FM Board - Bottom SideSuccess – the FM receive problem was now fixed. Now on to tackle the no transmit on AM problem.

This problem with the Yaesu FT-920 was a bit more elusive. I spent quite a lot of time studying the schematic diagram, checking voltages and signal paths, pretty much to no avail. I was almost at the point of giving up with the no transmit on AM problem.

It wasn’t until I was searching online for any solutions or possible ideas that I came across a snippet of information, quite by accident, that I hadn’t even considered.

I wasn’t aware that when these radios left the Yaesu factory, there was a “dummy” AM filter fitted where the optional narrow AM filter would be installed. It also transpired that the default menu setting for the AM filter was “Off” – quite obvious really, why would you have it turned on if no filter was fitted.

With no dummy filter in place and the menu option being off, any attempt to transmit on AM would cause the display to show “ERROR”. Turning the filter on within the menu system, the display no longer showed error, and while the Yaesu FT-920 showed transmit, no RF was actually produced…

After much searching, further information on the dummy filter was unforthcoming, and unlike a lot of Kenwood radios, simply connecting the input and output pins of the filter holder made no difference, so a simple by-pass was out of the question.

The answer – I needed to source either a narrow AM filter, or a dummy filter as originally shipped. Due to both these optional items for the Yaesu FT-920 being out of production for several years, this was going to be an almost impossible task.

Optional FM Board, AM & CW Filters FittedAs pure luck would have it, a week or so later while doing yet another search on eBay for either of the aforementioned filter one appeared in the results. Not only was this person selling the AM filter, but also the narrow CW filter.

Without any hesitation whatsoever I hit the buy it now button for both filters. Subsequently, the seller offered me a deal and knocked the postage (a few pounds) off the total price as I was buying the pair.

Within a couple of days or so, the filters arrived and were installed into the Yaesu FT-920. The relevant menu settings were made to enable both filters, and finally the good old Yaesu FT-920 was once again transmitting on AM and the whole radio working within specification as it should.