FM900

Note: Checkout the FM900 noisy VCO fault repair at the bottom of this page...

Guy VK2KU The project day to program and realign the FM900 Radios was a big success, as can be seen in the photos on this page and by the large number of modified radios appearing on air. Everybody pitched in and a large amount of work was carried out on the day.

FM900 Tuneup A big thank you to all the members in attendance and to those who supplied equipment on the day (Laptops, Test equipment, etc.) and a special thank you to Neil & Robin Fallshaw for turning their house over to the club on the day.

Lunch Should we call Dave a pi-coupler?

VK2KU Guy right in the thick of it... and is that Gordon making some fine adjustments with the yellow tuning tool?

Click to download and then print an MS Word file or an MS Excel file of the BMARC FM900 frequency allocation chart.

There are two FM900 EPROM enhanced firmware designs on the amateur market.

One system by BMARC club member David VK2AFI has several advanced on the fly user definable scanning facilities as well as numerous other refinements including button controlled anti-repeater facility to release many of the channels, on the fly priority channel programming, S-meter display, front panel low power selection etc etc. This system requires the I/O,timer, RAM expansion IC NSC810 to be installed. The rest of the expansion kit is already installed in the BMARC FM900's. David has negotiated to purchase a limited supply of these chips.
Click here for more information.

The other system by Blue Mountains WICEN member Dave VK2UDM has been released through the Blue Mountains WICEN group. Among other refinements, it includes direct 2 digit frequency display. Click here for more information..

Below is an extract from APCNews 16 August 2000...

Dear Dr Fixit,

I have just bought an FM900 at the local Hamfest, and when transmitting, other amateurs report low audio and a rumbling or rushing noise on my transmission, also a rumbling can be heard in Receive mode, can you please help?
Well, Rumbling old chap, it seems that you have the common "Noisy VCO" found in many of the FM900 series radios.
The best way to test this is to listen to a fully quieting signal in receive mode, with no modulation, then turn up the volume, and tap the main body of the radio with your knuckle, screwdriver handle or similar and listen to the received signal.
You should not hear any significance sound from the speaker. If you do , it's likely that you have a noisy or microphonic VCO.
This is caused by the ceramic substrate in the VCO de-laminating from the metal housing, any small vibrations cause capacitance changes within the VCO module, and so you "hear" these vibrations in the receiver as audio.
On the TX side its basically the same problem, symptoms and cure. Using the same tapping technique as before, listen to your Transmitted signal without modulation, on another receiver and tap the radio as before.
If you hear a loud noise on your TX signal, the TX side of the vco is also microphonic.
The other sure way to check, is to measure the voltage on the VCO output control voltage pin.
Whilst tapping, check to see that the voltage is stable. If it varies with the tapping, then the VCO is microphonic.
The only cure is to re fix the substrate to the metal housing, either by resoldering it yourself, (good luck), or you can try this with a hot air gun if your game.
By far the better option though Mr.Rumbling, is to take it to a competent technician who is experienced at these type of repairs.
The result will be a wonderfully performing radio once again, which even today, is still pretty pager free, and basically bullet proof; and remember, these repairs work on all the 900 series radios, regardless of the band.

(Lee VK3GK & Dr.Fixit)