Batboard

I am going to be putting in a 900 mhz ham repeater, what should I use? Specificly, I would like to know about ease of conversion, and frequency stability, and well, generally what works best.

Try AR902Mhz@yahoogroups.com

Hi Mike,
I just finished putting my 900 MHz repeater together. It is in "test mode" and will be on the hill here in Silicon Valley CA sometime in the next few months. I went through the sames learning curve and I settled on the following...

Rx: Motorola Maxtrac 800 MHz mobile converted to conventional.
Preamp: some kind of surplus Rx preamp a friend gave me
Tx: Motorola Maxtrac 900 MHz mobile with output power pot mod and outpout set to 3 watts acting as an exciter.
PA: Motorola PURC 70 watt paging PA. 3 watts in = ~80+ watts out
Celwave 4 cavity duplexer
PS: Motorola PURC paging PA PS (the PA sucks about 40 amps!)

Still need to finalize on the antenna.

Hope this helps.
George
KJ6VU

<the PA sucks about 40 amps!)> 550+ watts in; 80 watts out! Something must be getting AWFUL hot!

Tom KB5DPE

My UHF MSF5000 with a 110w PA, single circulator, and factory filter / duplexer draws about 530 watts from the AC line and only gives me 60 watts to the antenna. Those ferro-resonant transformer power supplies are simple but quite inefficient. I would agree that something should be getting hot under those circumstances, but that's the price one pays for essentially a brute-force, non-electronically-regulated power supply that can live through just about anything.

Bob M.

For what it's worth, when the PA is putting out 80ish watts, the heatsink gets warm and convection cooling may be OK but I run a few fans blowing up the fins anyway. Not very efficient but all works pretty well!
George
KJ6VU

Did you have to alter the VCO to get it to lock? I've been fighting with a 800MHz maxtrac and got it programmed, but when I hit the monitor button, I get white noise but not on that frequency. WTF, over?

Do you have a talkaround [dual range] VCO?
Did you remove Q205?
Do you have the right firmware?
Did you change the front end filters?

See: http://batboard.batlabs.com/viewtopic.php?t=11273

[quote="xmo"]Do you have a talkaround [dual range] VCO? dunno
Did you remove Q205? I have no service manual so... ???
Do you have the right firmware? it thinks it's conventional
Did you change the front end filters? Not yet. Should still recieve something if I'm holding a 2.5W HT up next to the radio anyway, right?

To move the 800 radio into the 902 rx band all I had to do was clip out the transistor (Q205), put in a conventional firmware prom and initialize / program the logic board. It went really quite easily. The rx sensitivity is very good.
George

unless you removed or replaced the filters, the 800 filters should barely be letting anything thru at 902...

mine was deaf as all get out till i swapped them for the 925Mhz filters.

kcbooboo wrote:My UHF MSF5000 with a 110w PA, single circulator, and factory filter / duplexer draws about 530 watts from the AC line and only gives me 60 watts to the antenna. Those ferro-resonant transformer power supplies are simple but quite inefficient. I would agree that something should be getting hot under those circumstances, but that's the price one pays for essentially a brute-force, non-electronically-regulated power supply that can live through just about anything.

Bob M.

Interesting, Bob. This would have scared me to death when I begin setting up my station. Now I know better. Thanks

Tom KB5DPE

I even measured the DC current used by just the PA, and it was in the range of 18-20 amps at 14-15 volts when putting out rated power. The PA is around 30-40% efficient, which seems low for a class-C amplifier. The rest of the input power has to be going somewhere. Certainly the control and RF trays don't take the remaining 200 watts, so some/most must be the ferro-resonant transformer heating up. Anyway, the station's don't really get hot unless they've been transmitting for an hour or so. I did put a fan on mine just to circulate some air, and that's all it needs to stay essentially room temperature.

There are enough MSF5000s running right now that if operating heat was a problem, it would have been taken care of by now.

Bob M.

so were did you hook the alc line up to or is it running wild and hot ??? the little green wire in the plug of four not bashing just asking i have many of these anps

n8obu: what ALC wire are you referring to?

MSF5000s have a forward power sensor and a couple of temp lines that feed the controller. What it does with them is its business.

MaxTracs etc sense over-current in the PA which cuts the controlling voltage back.

Bob M.

The green wire in the MSF PA control plug carries the forward voltage which is cabled back through the interconnect board to the power control circuitry on the station uniboard.

In addition to being able to shut down the PA due to overtemp condition, the uniboard circuitry contains the "forward control loop". In other words, the power control is a closed loop system.

It is true that the PA will operated outside of an MSF with drive provided by some other device since the controlled stage is not on the PA itself but is in the IPA circuitry. That type of operation is an open loop mode with no fold back capability if the PA output drifts too high.

kj6vu wrote:Hi Mike,
Tx: Motorola Maxtrac 900 MHz mobile with output power pot mod and outpout set to 3 watts acting as an exciter.
Hope this helps.
George
KJ6VU

George, does the resistor mod only modify the temp sense circuit and not affect the receiver?

I did the mod which works perfectly, but my receiver is dead??? related???
I don't have any schematics to refer to...

Thanks

The resistor mod made to the power control loop only disables the internal software timer which would normally cut the RF power back if the radio was in transmit for a certain time.

This timed power cutback happens even if the radio is receiving proper cooling.

Even with the resistor mod there is a backup protection circuit that measures the PA's heatsink temperature which will cutback the output power under extreme heat conditions. This is why you still need a fan on the heatsink, this last backup temperature protection is the protection circuit that you don't want to disable.

Just to clarify things, the MaxTrac and Radius radios do NOT sense the PA temperature, but they DO sense the PA current and will attempt to keep things stable under a lot of strange conditions. The output power is cut back by the microprocessor only when it "thinks" the radio would be getting hot based on the amount of continuous transmit time. This varies but seems to be between 30 and 60 minutes. I believe that by using a manual power control potentiometer, this prevents the microprocessor from lowering the output power when it things things have heated up. The article can be found at http://www.repeater-builder.com/maxtrac ... ntrol.html

The GM300 DOES sense PA temperature, so when the heatsink gets too hot, the microprocessor will cut back on the output power. I haven't seen or heard of a GM300 in the 800 or 900 MHz band. The manual power control modification should work just as good on these radios as it does on MaxTracs and Radiuses, but I don't know what else will happen if the temp sensor tells the microprocessor that it's getting too hot. Obviously the power can't be controlled by it if a pot has been installed, but it could still stop transmitting if it felt like it.

Bob M.

Maxtracs do to have a hardware temperature sensor on the RF board and this is or'd with the microprocessors timed variable. If you look in the manual it is U176, it uses the diode CR176 as the temperature sense element. The radio uses the timed variables that the tx is keyed and unkeyed along with the RF board hardware mentioned above. This is the last line of protection for the PA.

This is all explained in the maxtrac manual under the RF Power amplifier section, 1.7.4 OVER-TEMP PROTECTION. Since the Temperature sense circutry on the RF board is shown near the Reference ox/Synthesizer of the schematic, a lot of people may have assumed it was only used for TCXO/synthesizer temp tracking which it is not.

Why they didn't put this circuit on the RF power amplifer board is unknown.

Mike

I'll agree that the manual says all that. But have you looked inside the radio and noticed that the temp sensor diode and op-amp are located under the VCO shield of the RF board, quite close to the loudspeaker, and about as far away as possible from the PA? If this is trying to sense the PA temperature, it's gonna take quite a lot of moving air to get heat from the PA into the radio chassis and into the reference oscillator area, where it can finally be sensed.

On the 900 MHz MaxTracs, the temp sensor is in the middle of the logic board. At least it's a bit more out in the open.

I believe that this circuit has a lot more to do with temperature stability of the operating frequency than it does in protecting the PA. Maybe when the entire radio gets hot enough to fry eggs, the temp sensor will cause the power to be cut back, but I fear that if the environment gets that hot, the operator would have unkeyed long before the PA kills itself.

Bob M.

From what I can tell the thermal hardware is definitely calibrated to the heat flow on the RF board which is at a lower temp than that of the POWER AMP board IE the PA board is at a higher temp than the RF board when the cutback occurs. This is what was driving me nuts until I got ahold of the service manual, I had performed the power mod's and after several minutes of RPTR service the radio was cutting back to about a watt on RF out.

Prior to doing the Power control mod it would cutback in power after about 2 minutes no matter how hot or cold the radio was.

I can take a heatgun and heat up either the PA board or the RF board and when I key the radio up it's instantly into cutback, but I have to get the power amp board hotter than The RF board to do this. If I had a calibrated thermal probe I could tell what the 2 temps, Rf board cutback temp & POWER AMP board cutback temp, are.


Mike

Then they're relying on conduction of heat from the heatsink through the chassis and up into the RF board. There can't be much heat to be sensed by convection or radiation.

I was going to try removing the covers from one of my MaxTracs so air could flow over the temp sensor to see if that had any effect on lengthening the transmit time. Right now, I have one fan blowing on the heatsink from the rear of the radio, and I get about 45 minutes of transmit time before it cuts back. Also, the radio is only running at about 7 watts out as it's feeding a Henry repeater amp which makes all the heat it wants to.

If the temp sensor needs to be kept cool, and at my repeater I'd say the entire radio is probably at 78F after 45 minutes, the first thing to do would be to open the cover and shields and cool it off.

By chance is your environment hotter than that when the radio shuts down after two minutes? That's awfully quick for a mobile rig. Are you sure you're not drawing too much PA current and that's shutting it down? A 45w UHF rig usually draws 9-10 amps. A 12w 900 MHz rig draws about 5.

Bob M.

Yep my location is very hot this time of year. The rptr is in my garage, even here in Northern Az at 5800 feet elevatin it has been 85 to 95 in the shade and well over 98F in the garage with the doors closed. With all the other rptrs and remote bases running in the garage it is uncomfortable to even work for a few minutes on the rptr.


Mike

Just a quick question on this, when you mentioned conventional firmware for the Maxtrac to make it a 900Mhz receiver, do you mean the 5.34 version for a 32ch full feature Maxtrac, or is there something else in particular that should be used?

Just wondering because i got an 800Mhz radio, and new filters installed in the RF board, and it has a 16 pin logic board, all ready except for programming. The logic board has been blanked but not initialized yet.

What should i initialize it as, do you just set up as the stock model number and then program in the out of band frequencies, and ideally which firmware should be used?

If you are going to use it as just a receiver, I don't think you want to go to all the extraordinary effort to make it 32 channels....your decision of course....you can use ordinary firmware...just blank and initialize it as an 800 mhz radio, cause that is what it is. Program in the Out of band freq's afterward, in addition to the transistor mod. Done quite a few of these...piece of cake. Personally, after I have done the blank, I have reinitialized them using Maxtrac 7.02 version SW with no trouble at all
Dave N1OFJ

Ok, great! Just wanted to check what version of firmware i should use before trying it.

Been meaning to build a 900Mhz receiver for a while now.

I was waiting for Dave's comments before I added my own.

I have a radio, D35MQA..., and the version of software I was using to blank and initialize it would only let me choose from several D35MJA... model numbers. I probably got one that worked, but don't ask me what settings I chose, and I'll be doing it again with a newer version.

On one attempt I tried making it a high signalling radio, since the logic board seems to be able to support it (the radio is a 2ch unit). Well, after sending 320 codeplug blocks to the radio, it did strange things. Eventually I chose a different model and options and got a 64-block codeplug into it, and it's working fine on 851 and 870 (I haven't done the RF board mods to it yet).

Bob M.