toyboy2131

Hi all, I've got a question for all those gurus out there that know about the FM900 series, after tweeking the PA, the radio's power level on the one I have transmits upto 45watts on the 2 meter band on 146.000 mhz (it's an "A" band radio), I find this kind of strange, aren't these radios supposed to be only about 25 watts or so max? Or do you think it might have been modded to get the extra power out? or can they all do the 45 watts but with the risk of melt-down and short life spans or something?

P.S. before anyone YELLS at me, I've wound my power back to a more reasonable level than 45 watts ...

thankx guy's & gurls

mitaux8030

Most radios will do more than their advertised output level, and then some don't quite make it to the mark. While we like to think electronics is a precise science, component manufacture is far from precise. Hence designers often over-spec things, and then put controls in to adjust them back to the specified level of design performance.
Individual component tolerance, slight component quality variations, slight manufacture variations - all have a bearing in how 'good' or 'bad' a radio, or any electronic product, will perform.

But you're right, running a radio that is designed for 25 watts at 10% duty cycle, at 45 watts for long overs typical of ham radio operation (say 50% duty cycle measured over a short period) is a very good way to put the FM900 on a fast track to failure.

I would actually set your radio so that you are using LOW power, 5 watts, for your ham radio activities. Just feel the radio case after a typical back-and-forth 20 minute QSO at 5 watts, then do the same at 25 watts to see what I mean. Even at 5 watts the case will be warm; 25 watts will be alarmingly HOT.

Yobbo

i got 60w out of a 40w motorola once used it as a tx on a RF link, worked really well, until the fire.......................dont let the smoke out, it`s bloody hard to put back in.

lee

i am a bit surprised that you get 45w out on that "a band" fm900

i dont recall seeing one do that pwr unless we had the power supply cranked up to 15 or 16v.

maybe you have a special "taxi power meter"

( a meter to appease the ethnic taxi drivers who always wanted "much more powers" than their rival drivers)

at that power level, unless its an FM91 with the extended heatsink that has been modified with a higher gain o/p tranny, your fun may be short lived.

mind you i did get 60w+ out of a W1 band FM92 on UHF cb by replacing the MHW709 with a MHW710 in the pa stage.

cheers lee

toyboy2131

Hahaha, no taxi meter here, just a revex W560.
My voltage supply is bang on 14v (unloaded) using an electrophone psa1210 which is 11 amps peak 10 amp @ 50% duty cycle.
This power was measured into a dummy load and not into a miss-matched antenna, which as we all know can cause bogus readings.
I have reduced power to 10 watts cause I'm a good boy, don't worry about the 250watt PA after the radio that only needs a 10 watt drive, no no no just kidding...
Lee, just wondered if the VHF version uses the same MHW709, or is it a different tranny?
Thank

lee

if i recall the vhf version had a pre driver, driver and then a "SDxxxxx" pwr tranny.
i have some higher power motorola devices that would replace the standard pa tranny and give 40w o/p somewhere around here, and have used them years ago when i was at Philips working on the fm900's.

Same problem was the overheating of the PA section with prolonged talking etc. Thats why an FM91 has the extended heatsink.

the uhf versions just use the MHW power modules and a MRF464 o/p tranny. Same as was used in the 50w base stations.

re the 250w amp, maybe you have got an ex paging tx pwr module too

also i recall that for about 25w op the vhf 900's did about 3.75A-4A, so i guess at 45w op it should be drawing about 7-8A on tx.

porkop

the difference between 25watts and 45 watts is less than 3db and hence less than 1/2 an S point (6db = 1 S point), so why not just run it in spec (25watts) and be done with it.

jm2cw

porkop

(jm2cw = just my 2 c worth)

bulbous

Porkop is quite correct, 3dB does not make for a whole lot of extra 'quieting'.

The numbers are, however, extremely important to the 'CB ego' ........

vmark

Guys, I have a few very old Philips FM747 UHF single channel radios ( & some STC single/multi channel radios) that I'm thinking of modifying to either 70cm or 62cm. However they need overtone ctystals to make the grade. So, if there's any one that has the following lying around their shack let me know:- 45.4138rx(=438.525),18.2718tx(=433.525) 49.683rx(=476.950), 19.8729tx(=476.950), 45.1333rx(=436.000), 18.1666tx(=436.000), 45.4333rx(=438.700), 18.0708tx(=433.700).
Of course I could just order them from Bright Star (Victoria) or Jan in America.... but I'm a tight ass!!
Or if you want one "as is" (some do not have cases) let me know as well.
Mark.

__________________
With 7 sat dishes in the yard & 6 sat receivers, I'd say I'm serious about the hobby! Welcome to my own Pine Gap.

rob916

How much are crystals now vmark? Just as a matter of interest.

__________________
Vista SUCKS

porkop

i dont think it is economically worth worrying about it. I just picked up a FM92 U Band from ebay for $16.00. So I suggest, keep an eye out, cheaper in the long run.

lee

you would be up for at least $60-$80 just for xtals i would think.
even a programmed 80-100 channel rt85 or fm900 on uhf sells for less than $100 nowdays
as well as those puxing,weiweir handheld thingo's are less than that too

vmark

Copy that... thanks.
Wilco over & out.

__________________
With 7 sat dishes in the yard & 6 sat receivers, I'd say I'm serious about the hobby! Welcome to my own Pine Gap.

rurals

does anybody have the specs on the FM92? like how many channels in total they hold etc

mtv

99 channels

lee

I have seen them modified, with 2 and 3 banks of 99 channels (198 and 297 channels). A switch was used to manually switch the channels banks.

rurals

hmmm wonder how many lowband VHF channels i can program to my new fm92 heheh

good thing i got the scanner fanattics handbook with a list of EVERY registered radio freq in aust with the company that owns it for each state hehe

btw if anybody wants a particular freq for any company/police/fire/ambo in aust ranging from 27mhg all the way to 800mhz just let me know and i can hit u up woth the radio freq, but for special freqs, i dont have voting/ctcss numbers for them

rob916

you can go here
Australian Communications and Media Authority: Register of Radiocommunication Licences
It is current ( as any government department)
lmao

__________________
Vista SUCKS

rurals

only prob with that website is its not very user friendly as a easy to read tabled book lol

rob916

Yeah, it certainly is not, but you can search by postcode, frequency, location etc.
The book would be out of date in a week, but still good to read in bed.

__________________
Vista SUCKS