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High SWR on Messenger M250

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High SWR on Messenger M250

Post by ou812 » Friday, 20 April 2007, 1:55 AM

My Messenger M250 heater I had got used about 15+ years ago. Its max dead key was about 80-100 watts.
I figured the finals were bad and installed two new Motorola MRF454's.
I finally got a 60 Amp DC power supply and tried the Heater with new finals on my aluminum 5\8 wave Hustler antenna.
My first concern was 2+ watts on the meter on Low side??
Then when keyed on Medium and High power it maxed at about 80 watts and the SWR was cranked into the red when in High mode.
The radio (Cobra 142 GTL) with 4 watt dead key with swing to 15 watts will show a SWR of 1.3:1 on channel 19 and close to that on 26.815 upto 28.085 Mhz with new Solarcon antenna.
Max power on High side had the SWR well over 3.1:1 with about 100 watts dead key and reverse swing with mod.
Short test durations were used during these tests.
So at some point I marked the only tunable pot and then turned it (Bias control??) and before I new it I had lost count by turning it more than full circle!!
The max temp reached so far (using Snap-On test gun) on the Finals was 133F on the rear Final and about 10 degree's less on the front Final??
Voltage on the DC power supply Maintained 13.76 volts during transmit.
Long ago I had changed the power feed cables to decent a size wire to carry the 20 amp load.
Radio=Cobra 142 GTL with 4 watt dead key swing to 15 watts with full mod.
Heater = Messenger M250 Mobile w/ 2 MRF454 Finals (Input 10 Watts CW) Output: 140-150 watts average and 225-250 watts PEP as marked on rear of unit.
All Coax=RG8/U 95 % sheilded Foam USA
3' Jumpers total of Approx 75' to Antenna.
Amphenol Connectors
Meter= 3-Way- Watts, Modulation,SWR Palomar PW-5000
Low Pass Filter= Gold Line 0-30Mhz Low Pass TVI 1000 Watt Max 50 ohm.
Antenna mounted approx. 36' feet to bottom of antenna.
Antenna replaced with Solarcon IMax 2000 and GPK.
Tried with Low pass filter between radio and amp and then after the amp.
(Noticed better TV picture with Low pass filter mounted after Heater).


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Post by Mountaineer » Friday, 20 April 2007, 11:42 AM

It sound to me that your Reflected and Forward pot (if there is one, otherwise its a bad resistor) needs to be tuned. This idication is done by the following.

Testing
After setting low SWR's, Dead Key your amplifier and look at where the meter reads on the power setting. This indicates signal without modualation. While holding down the key, say the word "audio". This should cause the needle to swing forward, hence your forward power reading. If the needle is swinging back wards it indicates that the power is getting back into the transistors and will burn them out.

Fix
Find the pot that allows you to adjust this setting. Key down and turn it clockwise until the needle moves forward while saying "Audio" in intervals. When you get to the point that the needle stops moving while you are saying audio you have set the amp up for perfect forward power. It can swing a little forward of dead key but get this movement as close to each other as possible.

This all needs to be done on RMS setting of the meter.

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Post by mopar2ya » Friday, 20 April 2007, 12:07 PM

You have a number of potential problems.

1. Input tuning
2. Output tuning
3. Bias adjustment (The base of the transistors should see about .6 - .7 volts with the amplifier in transmit mode, with no drive applied.)
4. Dirty amplifier controls. (i.e. the low/med/hi switch or variable potentiometer. You did say this was an old amp. Could this be your problem?)

The high SWR problem that you are referring to.... Is this with the meter between the radio and amp, or between the amp and antenna?
Is the MRF454 the device that was in the amp originally? If it was, then potential problem numbers 1 and 2 -probably- don't apply.
Does the SWR problem only occur when the high setting of the amplifier is used?
Is there any chance that something was dislodged during one of the repairs? Fingers and eyes make wonderful diagnostic tools! Look around. Gently push on wires and components to make sure the connections are secure.

Good luck,

Mopar

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Update Messenger M250

Post by ou812 » Saturday, 21 April 2007, 23:42 PM

Upon looking at the Finals I see that I apparently installed only one Final MRF454 thinking only one was bad since it did have half power in High mode at approx 80-100 watt dead key. That didn't help. Later I installed an NTE335 and then set the amp aside for a few more years until now. I don't know when the high SWR became an issue.
I have checked the SWR at the radio and at the Palomar PW-5000 Meter:
Hookup=Cobra 142 GTL - M250 - LowPass Filter - PW-5000 Meter
1) Heater off Cobra 142 SWR = 1.1 and PW-5000 = 1.2
2) Heater on Low power Cobra 142 SWR now reads 3 and PW-5000 SWR = 1.3
3) Heater on Medium - Cobra 142 SWR now at 4 and PW-5000 SWR = over 2
4) Heater on High Cobra 142 SWR now at 2.5 and PW-5000 SWR = 1.5

I am surprised to see the SWR drop when in High mode on the Heater and also showing the highest AVG power deadkey at 100 watts???

Voltage check at Base leg of Finals:
1)Heater on Low Power - Rear Final Base Voltage = .250 (MRF454)
Heater on Low Power - Front Final Base Voltage = .250 (NTE335)
2)Heater on Medium Power - Rear Final Base Voltage = .420 (MRF454)
Heater on Medium Power - Front Final Base Voltage = .420 (NTE335)
3) Heater on High Power - Rear Final Base Voltage = .400 (MRF454)
Heater on High Power - Front Final Base Voltage = .420 (NTE335)
I got this unit used over 15+ years ago and never have really used it. It had MRF454's when I got the amp.
I don't remember it having this SWR problem back then so it must be something I have done...Finals???
When I first got it (used) the power output was near half of normal so it has been wrapped in plastic mostly and once in a while I have worked on it i.e. replace Finals and power feed wires.
The unit is very clean and much like new in most respects.
The 3 - position Low-Med-High switch seems ok, but I will clean with some tuner spray.

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Post by mopar2ya » Sunday, 22 April 2007, 9:08 AM

First, the mismatched set of transistors has to be dealt with in order to properly fix this amplifier. In a class AB amplifier, you need to have a matched set. Yours are not only not matched, they aren’t even the same transistor. This is a portion of your problem. RF Parts sells matched transistors. The other alternative is to eliminate the AB bias. Alter the amp to operate class C and you might be able to get away with the mismatched transistors. I wouldn’t do this if I were you. I would replace the transistors with a matched set.

You can -see- the effect that the mismatched transistor compliment is having on amplifier operation in the different voltage and heat measured on the transistors, even though the voltage measurement you took doesn't mean much. You most likely attempted to read an RF AC voltage with a DC volt meter. Those transistors should be identical in every measurement.

Second, in order to read bias voltage, you need to measure this with the amp in transmit mode, with –no drive applied-. Usually this is accomplished by grounding the ground side of the relay coil. In most amplifiers the bias is only –on- when the amplifier is keyed. This amp may be an exception. If there is voltage present at the base of the transistors when the amp isn’t transmitting, then just measure it without keying the amp. I doubt this will work though. Bias voltage will be measured with the volt meter in the DC setting. It should be between .6 and .7 volts. This can’t be measured while the amplifier is driven by a radio.

Looking at the cost of the MRF454, you may choose to either not fix the amp, or choose a different transistor to replace the MRF454’s. If you do choose a different transistor, you might want to consult someone else on the best cost effective alternative transistor. Once again, it will have to be a matched set and you will probably want a very similar transistor to keep the change as simple as possible. I think most folks here would gravitate towards the 2SC2879. This may not be a bad choice as it is probably the most well known RF transistor on the forum.

Crusher can probably help you with tuning as the change will require some minor tuning to make it work right. In reading his posts, I think he has probably done more experimenting with amplifiers than most of us here. He may even tell you that the 2SC2879 is a bad choice as a replacement. Maybe he will chime in.

RF parts pricing for matched pair.
MRF454 $72.90
2SC2879 $51.90

In reading your follow-up post, I don't believe the input swamping network is your problem.

Good luck,

Mopar

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Post by Mountaineer » Monday, 23 April 2007, 6:48 AM

THis is true. Crusher could get you hooked up. They guy once sent me pictures of an amp he built and installed variable capacitors on every place there was a capacitor. This was funny looking yet yeilded him a design that worked great for him. Me... I take schematics that have been around forever and build tried and proven circuits. Works for me and my amps. It also works for every other amp on the market. Fatboy, EastCoast, Powerplate, davemade, and now Mountaineer. Why fix what aint broken.

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Messensger M250 it deserves better!

Post by ou812 » Wednesday, 25 April 2007, 4:03 AM

Mopar & Mountainer,
Thank you for the information. I have heard of matched transistors and how important they are...but I have been reluctant to believe that different manufacturers let alone matched sets from said manufacturer are that different.
I guess the meaning linear is much more serious than I had taken it for. Apparently I have taken this to lightly.
I like to make things work, that's what I do, but I am a heavy equipment mechanic not a radio tech. With help I have been able to fix a few transceivers but only with the absolute guidance from techs on the forums.
My Cobra 142 GTL finally has the mod I have longed for and my Mckinley is working just like the 142 base (PC385 boards)
My 29 LTD works awesome brought back from the dead (a bad electrolytic cap no RX/TX) all from techs giving me guidance.
I suppose this Messenger M250 should be easier than a transceiver...but I am lost!
If I understand Mopar right the mismatched Finals are causing spurious radiation on my SWR meter. This needs to be addressed first and foremost and also as Mountaineer says I could be getting feedback from burned out component's, resistors? ...maybe the mismatched finals themselves?
I like the idea of going to the 2SC2879 if this is possible...even with a few component changes should be worth it as the data sheet looks real good to me and the price!
I still wonder about the drop in carrier when on Low side and lack of full calibration on the SWR and Mod meter, yet on Medium Power the SWR is jacked as well as jacked SWR at the radio itself (3.0:1) and about 50 watts dead key yet 1.4:1 swr on High setting on the external meter and also lower swr on the radio itself?
On high side with its decent swr I also have my best power out at 140 watts avg dead key...spurious radiation???
I have tuned the only tunable coil to get to this point and no-matter what...I have downward modulation?
I guess I would like to know if these problems on Medium and Low side could be from the finals also or could it be something else that needs fixed before I mess up a matched set of new finals??
I also tried to drive the amp with 2 watts from my RCI 2950 but it has an intermittent no RX/TX that usually goes away when it warms up? Anyway I gave that up as I didn't want to mess up the RCI as I bought it new years back and would cry if i hurt it.
I believe I found the proper ground on the relay as Mopar said and it keyed up with no Xmtr attached but i got no reading on the voltmeter DC setting when connected to the base of the MRF454's? ( went back to dual MRF454's although they still are not a matched set.) I was surprised that there was not some noticeable difference in my problems after this change though?
Do I need a RF voltmeter?? Or build the RF adapter for my meter? I have a fair amount of basic components here on tap.
The only way I was able to see anything on the Base leg of the MRF454's was to have the radio inject a signal to the AMP and with that I was reading about .250 on Low side, .450 on medium and .650 on High side...these are approx. as all were floating slightly in their respective range.
I just received Lou Franklin's book Understanding CB Radios and although I understand a mere 2% it is very interesting to read. It's way over my head...but maybe I will catch on to some of it in due time.
I would really like to see this Amp work properly and I like the idea of the 2SC2879 matched set, but I it seems like there is something going on with the Low/Medium output that also needs addressed...I have no idea!

All comments are welcome but this amp if far to clean cool looking to **Censored** can!

ou812

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Post by Mountaineer » Wednesday, 25 April 2007, 8:34 AM

looks like you have been doing some tests. Now is the time to get some more manuals and maybe a schematic of a similar amp and start experimenting! I bet you could fix the amp to your likiing, you just dont give yourself enough credit. Its not as hard as it looks if you have the right tools and the right knowledge. The tools can be had for cheap and the knowledge can, for the most part, be found on the internet. This is a great time to be living, you have unlimited resources and you live in America where verything is affordable.

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Post by mopar2ya » Wednesday, 25 April 2007, 10:06 AM

I suppose this Messenger M250 should be easier than a transceiver...but I am lost!
If I understand Mopar right the mismatched Finals are causing spurious radiation on my SWR meter. This needs to be addressed first and foremost and also as Mountaineer says I could be getting feedback from burned out component's, resistors? ...maybe the mismatched finals themselves?

Output power reduction and distortion/spurious transmissions will be the result of mismatched transistors in a Class AB circuit. Your amplifier may be oscillating as well.

I like the idea of going to the 2SC2879 if this is possible...even with a few component changes should be worth it as the data sheet looks real good to me and the price!

This is probably the route I would go.

I still wonder about the drop in carrier when on Low side and lack of full calibration on the SWR and Mod meter, yet on Medium Power the SWR is jacked as well as jacked SWR at the radio itself (3.0:1) and about 50 watts dead key yet 1.4:1 swr on High setting on the external meter and also lower swr on the radio itself?
On high side with its decent swr I also have my best power out at 140 watts avg dead key...spurious radiation???
I have tuned the only tunable coil to get to this point and no-matter what...I have downward modulation?
I guess I would like to know if these problems on Medium and Low side could be from the finals also or could it be something else that needs fixed before I mess up a matched set of new finals??

I don’t know Why you are seeing what you are. It would be very helpful to see the output on a scope. I might ask you this. What do you mean “tunable coil” What is this device? Is it an inductor, capacitor, or variable resistor? What led you to believe it was part of the bias circuitry?

I also tried to drive the amp with 2 watts from my RCI 2950 but it has an intermittent no RX/TX that usually goes away when it warms up? Anyway I gave that up as I didn't want to mess up the RCI as I bought it new years back and would cry if i hurt it.
I believe I found the proper ground on the relay as Mopar said and it keyed up with no Xmtr attached but i got no reading on the voltmeter DC setting when connected to the base of the MRF454's? ( went back to dual MRF454's although they still are not a matched set.) I was surprised that there was not some noticeable difference in my problems after this change though?
Do I need a RF voltmeter?? Or build the RF adapter for my meter? I have a fair amount of basic components here on tap.
The only way I was able to see anything on the Base leg of the MRF454's was to have the radio inject a signal to the AMP and with that I was reading about .250 on Low side, .450 on medium and .650 on High side...these are approx. as all were floating slightly in their respective range.

I am not familiar with that particular amplifier. It could be that there is more than one relay in that amp to actuate the transmit circuitry. There could be one for the amplification circuit, one for the bias circuit, and maybe another for the pre-amp. You need to figure out how to actuate the bias circuitry, if it is still operational. Bias is DC. You won't need any special meter other than a standard volt meter to measure it.

Mopar

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Messenger M250 Build date ?

Post by ou812 » Wednesday, 25 April 2007, 22:42 PM

First of all I would like to thank Mopar and Mountaineer for responding to my posts.

RF parts lists the 2SC2879A-MP $47.90 as well as the 2SC2879-MP $51.90
Should I order the 2SC2879A -MP $47.90 RFParts site says the difference between the 2SC2879 and the 2SC2879A is that the “A” version is RoHS compliant, and uses an Aluminum Oxide insulator (color gray) on the header vs. Berillium Oxide I guess the A version is a better part?

I don’t know Why you are seeing what you are. It would be very helpful to see the output on a scope. I might ask you this. What do you mean “tunable coil” What is this device? Is it an inductor, capacitor, or variable resistor? What led you to believe it was part of the bias circuitry?
As you know I don't have a scope...yet. I did recently buy a new 7 digit bench top frequency counter 0-2.4Ghz. I have no intent on working on anybody's stuff but my own, I just want my equipment clean and on freq. I would like to get a new scope at some point and a signal generator but I don't want to get to deep.
As far as the adjustment pot on the M250...I guess I associated the need for bias adjustment with the driver/final bias adjustments of AM/SSB radio's? I thought that the need to fine tune the input / output made perfect sense but I have looked at a lot of Amps on the net at Amateur Amplifiers.com. I was surprised not to see some sort of trimming pots on most any of the amps as near as I could see. I thought it would be mandatory much like my old D&A MDX-200 had 4 front panel controls for loading and tuning. I know tube amps and solid state is apples to oranges but it made sense to me.
I don't know what this control is and I have scanned the Internet for info but I it is hard when I don't know for sure what it is.
I thought it may be tuning capacitance...I don't' know. It is ceramic I believe with a a metal flap with an air gap (adjustable)
it has about 4-6 turns top to bottom and connected to ground on one leg and the other leg is tied to a large loose wound coil and what appears to be T2 a 4 turn 1 inch long transformer?? The numbers on the pot are GM A 80400.
I have searched for those numbers with no results on the Internet.
I can say that turning coil will change the SWR readings seen at the radios SWR meter as well as the external SWR mounted last in the chain. It also changes the output power seen on the watt meter to a max of 140+ watts AVG which I gather is the spurious radiation.
I closet schematics I have found are the Messenger schematics at Amateur Amplifiers.com possibly the AB1 schematic but the tolerances on the AB1 are different for the most part and I do not see the tunable pot on any of the schematics either or the small relay that I am able to at least partially key the AMP (TX light comes on and the other large relay/relays also seem to snap in when I ground the terminal on the small relay?? Also my M250 does not have SSB capability as seen on the Messenger 250 AB1 which I would also like to add later if possible.
Most of the 2 pill schematics look very similar but the pics I have seen don't seem to have the 2 large relays and small relay or the tunable pot? I would like to purchase a schematic for this exact layout if possible. The REC amp does fire at least one of the large relays when I turn it on and it seems to make a big difference on the receive.
I have looked the board over top and bottom in search of some PCB numbers but there is nothing.
I would like to take some pics and email them if that would be okay?

ou812

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Post by mopar2ya » Thursday, 26 April 2007, 4:17 AM

First of all I would like to thank Mopar and Mountaineer for responding to my posts.

Your welcome!

RF parts lists the 2SC2879A-MP $47.90 as well as the 2SC2879-MP $51.90
Should I order the 2SC2879A -MP $47.90 RFParts site says the difference between the 2SC2879 and the 2SC2879A is that the “A” version is RoHS compliant, and uses an Aluminum Oxide insulator (color gray) on the header vs. Berillium Oxide I guess the A version is a better part?

Either one will work fine. There has been some speculation on the forums that the new part (A) would not be able to dissipate heat as well as the original one. I don’t know what the consensus is now that the new one is in regular production. Maybe you could ask Mud Duck and see what he has to say.

I thought it may be tuning capacitance...I don't' know. It is ceramic I believe with a a metal flap with an air gap (adjustable)

Ding! Ding! Ding! You get a prize. It is a capacitor known as a compression mica trimmer. But, it is NOT for bias, it is for output tuning. Bias adjustment would be a variable resistor if the bias on the amp is adjustable. Many are not. If it were bias, the circuit the trimmer was attached to would eventually lead to the primary winding on the input transformer. What you described is on the secondary winding of the output transformer. This is definitely output tuning.

it has about 4-6 turns top to bottom and connected to ground on one leg and the other leg is tied to a large loose wound coil and what appears to be T2 a 4 turn 1 inch long transformer??

This is the output transformer as you have also guessed. Nice job.

The numbers on the pot are GM A 80400.
I have searched for those numbers with no results on the Internet.
I can say that turning coil will change the SWR readings seen at the radios SWR meter as well as the external SWR mounted last in the chain. It also changes the output power seen on the watt meter to a max of 140+ watts AVG which I gather is the spurious radiation.

Normally you would tune this compression mica trimmer to maximum output. Max output and lowest SWR usually occur fairly close to one another. A happy medium between the two is what most builders go for. Something enters my mind though. What are you tuning this into? Please say a dummy load. While it is possible to tune an amplifier into an antenna, it would be problematic if the antenna is highly reactive, and unless you have an antenna analyzer, you have no way of knowing if that low SWR (If a low SWR is what you have.) is purely resistive, or partially reactive. This could be some of your problem. Forgive me if I am wrong about the antenna thing. You may have already mentioned it and I missed it.

I closet schematics I have found are the Messenger schematics at Amateur Amplifiers.com possibly the AB1 schematic but the tolerances on the AB1 are different for the most part and I do not see the tunable pot on any of the schematics either or the small relay that I am able to at least partially key the AMP (TX light comes on and the other large relay/relays also seem to snap in when I ground the terminal on the small relay?? Also my M250 does not have SSB capability as seen on the Messenger 250 AB1 which I would also like to add later if possible.
Most of the 2 pill schematics look very similar but the pics I have seen don't seem to have the 2 large relays and small relay or the tunable pot? I would like to purchase a schematic for this exact layout if possible. The REC amp does fire at least one of the large relays when I turn it on and it seems to make a big difference on the receive.
I have looked the board over top and bottom in search of some PCB numbers but there is nothing.

Is it possible that this amp is already class C?? Hmm. Is this possibly the reason that you have not been able to measure any voltage on the bases of the transistors? Look at the input transformer (The smaller transformer.). On the primary winding, (The body of it, not the wire.) on the side away from the power transistors (MRF454) it will either have a path to the bias circuit, or it will be grounded with a couple of wraps of wire through a ferrite bead. If it is grounded through a ferrite bead to the same foil trace as the emitters of the MRF454's, it is class C, and you –will not- have to match the transistors. If the primary on the input transformer doesn't appear to be grounded, look on the board for a large white ceramic/cement looking resistor. It will be approximately 1/4” x 1/4” x 1 1/2” and rated for approximately 20 ohms at 10w or so. If you see this resistor, chances are it –is- AB biased.

I would like to take some pics and email them if that would be okay?

Absolutely. Send ‘em and I will help you identify everything. Click on my profile for the email link. I re-read what I wrote and realized some of it might be a little confusing. If you send the pics, I will label them and send 'em back. This will make it much easier for you.

Mopar

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Post by Mountaineer » Thursday, 26 April 2007, 7:30 AM

Thanks OU812. I am just getting started in amp building and have much to learn. Mopar hit the nail on the head. The guy knows his stuff.

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Messenger M250

Post by ou812 » Friday, 27 April 2007, 1:50 AM

Well I guess this is where I have to admit that I do not have a dummy load and there for I have been dumping into the antenna.
I need to build a simple dummy load to handle 300 watts or so as I don't plan on pushing any more than this 250 watt amp will ever do. I have wanted to build one for a long time. I did see Mountaineer's dummy load and would like to do something more along that line as opposed to the oil filled paint can although it to seems to have its merits.
As for the Amp the only ferrite bead (I think) is near the far end (away from the finals) of what should be T2.

Either soldered side of T1 has 7.5 ohms between it and ground. Both of the soldered sides of T1 show 0 ohms to the Base legs of the MRF454's. The wire winding of T1 is direct to ground on one end and the other end is soldered to the center lead of a mini coax that goes to the Low-Medium-High switch/resistor board and then returns back threw another mini coax
to one of the large relays that is near the xmtr SO-239.
Next to T1 were the wire connects to the board is a 2200uf 16v cap as well as a choke and a 35ohm 10% 5w ceramic resistor, a small diode and a 104M 50v Disc capacitor (.1 Micro Farad?).
Resistance between T1's small wire threw the L-M-H switch and back to the rear relay is 41.5 ohms on Low, 15 ohms on Medium and 0 ohms on High.
I think what I have written here is correct but if something looks out of line let me know and I will recheck.
I guess my handle should be Dummy Load...LOL
By the way Mountaineer, nice job on the amp and dummy load...I spent almost 2 hours reading the posts and looking at the pics. It has drawn a lot of attention with over 6200+ hits!
Mopar pm'd my email in hopes that you will return it so I can send pics direct. I have seen no quick and dirty way to get there from here??
Thanx to all again!

ou812

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Post by mopar2ya » Friday, 27 April 2007, 4:25 AM

I will respond to your last post after I see the pics. I am having trouble with your email addy. It keeps coming back. Send pics to mopar2ya2@yahoo.com.

Mopar

wan•na•boo (wah-nah-boo) Informal n. Derived of wan-na-bee 1. One who aspires to be a
Peakaboo. 2. One who imitates the behavior, customs, or dress of a Peakaboo. 3. A product
designed to imitate the qualities or characteristics of something built by a Peakaboo.
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Re: Messenger M250

Post by Mountaineer » Friday, 27 April 2007, 15:18 PM

ou812 wrote:I have wanted to build one for a long time. I did see Mountaineer's dummy load and would like to do something more along that line as opposed to the oil filled paint can although it to seems to have its merits.

By the way Mountaineer, nice job on the amp and dummy load...I spent almost 2 hours reading the posts and looking at the pics. It has drawn a lot of attention with over 6200+ hits!
ou812
Thanks. It works out great and as a matter a fact that dummy load is happier at or around 300 watts as opposed to the 600 originally stated. It gets hot with the small heatsink so I installed a fan to cool it down on long keys. It works great but I want something a little beefier. I am gonna get a 75 dollar MFJ dummy load for 1.5KW, that way I know it can handle bigger amps down the road.

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Dummy Load for Messenger M250

Post by ou812 » Friday, 27 April 2007, 22:04 PM

Well I always thought I would build a dummy load someday. And I have been nickel dime'n these radios lately but with all the help and a Lil money I am very happy, meters swinging, forward mod what more could I ask for!
It appears that I could get close to a 300 watt dummy load for around $40-$60.
Maybe that's better than trying to copy and build one.
I have seen some wild stuff online in my recent google's. One was 3 100 watt incandescent light bulbs good for 300 watts I think? The other said to use a portable heater.... I got all that stuff just laying around here!

Quote #1 "i use a old oil electric heater, it has a 2400 watt element with the already
inside. Just rip off the power connector and install SO239 socket and feed
into an antenna tuner, a instant 3kw cheap dummy load."


Quote #2
"300 Watt Dummy Load For Under $10.00

The Dummy Load Resistor is 3 light Bulbs of 100 Watts each. They are wired as shown in PARALLEL. This makes a cool 48 Ohms of Resistance at 300 Watts of Power!

1. Bases - porcelain light sockets made for home use.

2. Wire in PARALLEL as shown.

3. On the left is a PL-259 RF PLUG. The Center Wire goes to the Copper colored center contacts on the Bases --- The Shield/Ground goes to Silver Screws on Bases --- Make wires between bulbs as short as possible and mount the three bases on a small board using wood screws. Mount them close to each other.

4. Connect a piece of REG-58U or REG-8 Coax to the wires coming from the Bulbs, to run to your Transmitter."

oi812

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Post by Mountaineer » Friday, 27 April 2007, 22:15 PM

That is some good ole boy rigging there!!! :shock:

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Dummy Load for Messenger M250 Amp

Post by ou812 » Friday, 27 April 2007, 23:38 PM

Hey Mountaineer,

I was looking at your pics on your dummy load and it looks real professional. I thought maybe I could copy your design.
I did a search on the resistors you used and did not come up with a match? It appears that the resistors could be a bunch and I kinda wanted to buy local and get this thing built right away.
Do you think it is feasible to build some hing comparable for under $40.
I do have a few large aluminum finned CPU heat sinks and 3"x2 1/2"x 2" tall with high speed 12v dc fans and a SO-239
chassis mount as well as misc large wall outlet 14v (.900 ma) transformers for the fans and a chassis mount female plug for the transformer to plug into.
If this would be enough heat sink/cooling for 250 pep then I would only need the necessary resistors and a project box??
I am good for any fab work and mounting the components but I am not sure of the recommended resistors and layout.
The heat sink you used looked like the the cats meow. Was that one expensive? If I could get by with the CPU heat sinks then that would be cheaper yet.

ou812

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Post by Mountaineer » Saturday, 28 April 2007, 22:04 PM

I have about 10 of those things laying around. If you want a few just PM me and I could sell you a few of them, cheap. The resistors are hard to comeby sometimes. You have to check ebay or google the part number on the resistor. Dont recall right now.

You will need several drill bit sizes and standoffs to mount to the enclosure, time consuming but works.

Let me know M.


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