Are Dummy Loads Repairable

[nokones]

I just learned that my Dummy Load took a dump and I am wondering if they are repairable and worth repairing? The casing/housing appears to be assembled with two halves and the N Female fitting has 4 screws attaching the fitting to the housing.

I didn't realize that my EMR Corp Model 1650 0-1GHz 125 watt Dummy Load failed on me. At first, I thought that the three radios I was measuring the RF output with the Dummy Load was in a dire need of an alignment and tune since I was measuring only 7 watts of output power. I measured the three radios with three different in-line Watt meters and one of the meters and slugs were calibrated last year by Bird.

So, I packed up the three radios and headed to a Motorola Service Shop in Tucson. As I was about to leave, the technician came out to my Jeep and told me the first radio is putting out 50 watts and is on freq. I said no way. He quickly checked the second radio with the same results, and the third radio was transmitting 40 Watts, but was a tad off freq and needed an alignment. I said do it.

The first radio was my Jeep radio so I put the radio back in my Jeep and remeasured the RF and it was at 48 watts. After the Tech tuned the third radio, I headed home and when I got home, my first task was to recheck the radios again. I checked two of three radios on the bench since they were loose, and they again only put out 7 watts.

So, I started replacing cables, cords, adapter connectors, ensured the power supply was putting out 14 volts. I bypassed my Anderson Distribution Block and directly connected the radio and controlhead to the power supply, only 2 watts were drawn from the power supply. So, I tried another power supply and still only two watts being pulled from the second power supply.

The only thing that I haven't changed was the Dummy load. My second Dummy load was only an UHF Dummy load and these three radios were VHF mobiles so I couldn't use the UHF Dummy load. However, I have NMO magmounts and a couple VHF mobile antennae so I hooked up the antenna to the radio in place of the dummy load. When I keyed the radio, 48 watts, hot damn. I reconnected the Dummy load, 7 watts, hooked the antenna back up, 48 watts.

I decided to get out my ohm meter and check the Dummy load resistance and it measured "OL". Although, I wasted a whole day going to Tucson with two perfectly good working radios, I did get one radio aligned and I did discover that the Dummy load failed and my meters and radios were not the cause of measuring 7 watts.

 

[prcguy]

Simple open connections like right at the connector might be repairable but if the resistor itself has changed value or cracked open its a lost cause. I've accidently burned up more than one load over the years and was able to purchase an exact replacement resistor and repaired them good as new.

 

[cavmedic]

Had a big one go bad in a Sinclair hybrid combiner. Whole shelf was replaced, intercepted it before it hit the dumpster, turned out the resistor had a cold solder joint and opened up. Unfortunately the two isolators bit the dust as a result, but now have a spare heavy duty load now along with the smaller ones from the circulators and some harmonic filters cluttering up the bench.

 

[nokones]

Simple open connections like right at the connector might be repairable but if the resistor itself has changed value or cracked open its a lost cause. I've accidently burned up more than one load over the years and was able to purchase an exact replacement resistor and repaired them good as new.

I found a brand new EMR Corp 1650, still in the box, for sale on eBay for $150, so I went for it, in the interest of time. When I get a boring moment looking for something to do, I'll try to crack the bad unit open and see if I can try to fix it. It'll be my first attempt of playing around with a resistor. I'm the type that will try to make an attempt to repair something before it gets thrown away in the trash. The problem with things these days is obtaining the parts for repairing.

 

[ramal121]

My second Dummy load was only an UHF Dummy load and these three radios were VHF mobiles so I couldn't use the UHF Dummy load.

I've never seen a band specific dummy load. Where can I get one?

 

[W2JGA]

I've never seen a band specific dummy load. Where can I get one?

I was wondering the same.

 

[prcguy]

This type of resistor is in many loads, some rated to 2GHz.

I came across a pair of 100 ohm 800w versions and paralleled them to make a 50 ohm 1600w load on a forced air tunnel heatsink. I really need to put a copper spreader under the loads as the load to aluminum junction cant pull the heat away fast enough.

 

[W2JGA]

This type of resistor is in many loads, some rated to 2GHz.

I came across a pair of 100 ohm 800w versions and paralleled them to make a 50 ohm 1600w load on a forced air tunnel heatsink. I really need to put a copper spreader under the loads as the load to aluminum junction cant pull the heat away fast enough.

View attachment 176288

Use liquid nitrogen..

 

[prcguy]

Use liquid nitrogen..

Maybe with my next amplifier.

 

[Project25_MASTR]

Reminds me of the 30 dB power attenuator I acquired that was labeled as "BAD"...just a bad solder joint.

 

[nokones]

Ok, I gave up trying to slit open the defective dummy load, so I tossed it. Also, I found a brand new EMR Corp, still in the box, #1650A, which is a 150-watt unit, for $150.00 as opposed to $275.00 - $330.00 on the open market.

The purpose of this post since, I am not an expert by any stretch of anyone's imagination, I measured the new dummy load at 50.3 Ohms, which I assume that it can't much better than that and it shouldn't affect the accuracy of measuring RF output power too much, but what I was wondering is with my cheap Chinese 50-watt dummy load, that measures at 49.1 Ohms. Of course, I kinda figure that this is not ideal and since it is a POS Chinese dummy load that was given to me some time ago, is .9 of an ohm that significant in regard to the accuracy in measuring RF power?

I thought about scrapping it just because it is a Chinese POS, but it is kinda handy and a little itty bitty unit and a backup if need in the future. I am just curious if the .9 ohm is significant enough in regard to the accuracy of measuring RF power.

 

[prcguy]

These 250 watt dummy loads show up now and then for as low as $40 shipped and they have a sample port that is about 30 to 40dB down depending on frequency. It has a real 250w rated chip resistor inside and easily handles 250w or even maybe 350w for a brief period. Its probably the best deal in loads and the upper frequency range is at least 1GHz.

Celwave Dummy Load Termination EMC Sample Port N Female Sweep 6040980 BNC | eBay

Celwave Dummy Load Termination EMC Sample Port N Female Sweep 6040980 BNC.

www.ebay.com

 

[nokones]

These 250 watt dummy loads show up now and then for as low as $40 shipped and they have a sample port that is about 30 to 40dB down depending on frequency. It has a real 250w rated chip resistor inside and easily handles 250w or even maybe 350w for a brief period. Its probably the best deal in loads and the upper frequency range is at least 1GHz.

Celwave Dummy Load Termination EMC Sample Port N Female Sweep 6040980 BNC | eBay

Celwave Dummy Load Termination EMC Sample Port N Female Sweep 6040980 BNC.

www.ebay.com

I remember seeing that one and I did consider it for a brief second, but I was unable to determine what was the power rating for that unit. I had a hunch that the power rating may be more than 100 watts based on its size, but I ended going with the brand new unit I ended up purchasing.

 

[prcguy]

I remember seeing that one and I did consider it for a brief second, but I was unable to determine what was the power rating for that unit. I had a hunch that the power rating may be more than 100 watts based on its size, but I ended going with the brand new unit I ended up purchasing.

I use it often with my service monitor when dealing with 100w repeaters. It will handle 100w forever and the sample port is handy for generating signals back into the repeater for various testing.

 

[davidgcet]

a little off 50 is fine, a simple vswr sweep or forward/reverse power read will tell you if the load is ok. they are not band specific either, that is the point of the load is they provide a true load, band independent. i used to have a cantenna that worked great from low band all the way to 800 band, though it was slightly of at 800 due to the SO239 connector but still not enough to cause issues. once i did boil it over because i ran 650 watts in it too long, but adding more mineral oil had it back to normal.