Fixed Attenuator Repair / Mods

[joeqsmith]

I don't see any reason why testing with DC would be a problem.   Maybe just used a separate terminator rather than any test equipment to terminate the output.  If you have a way to measure temperature, that may be interesting to know as well.  Also mounting orientation and any sort of convection or forced air cooling.   If you know where you purchased them from and part numbers, this may also be helpful.

Should make for an interesting test. 

[Yansi]

RF current can flow only in thinner layer of the resistor and create more localized heating, than DC could, ... or... am I making this up?

I have just checked what I have ordered, an seems I have only ordered 10W 50ohm terminators (flangemount resistor with the copper foil strip on the side). Did not buy any of the 10W flangemount attenuators, yet   

[joeqsmith]

As long as you apply the same power level, I would doubt you could detect a difference in the temperature profile.   Actually, I doubt you could detect the profile just running it one way or the other.    The film is not very thick, even for a thick film part.  It would seem like if you ran some sort of extensive life test, you could maybe detect a difference in how you drive it.    I guess if the part is going to break down at the peak voltage, we could also see some improvement using DC.   A good study for your PHD  but I don't see any practical use in this case.   I would be far more concerned with how effective the heatsink is and want some general idea of the temperature. 

Too bad about the parts.  If you decide to buy some and run the test, please post the results.   

[joeqsmith]

The attenuator network is rated for 150W with proper mounting.  I let it run for a about an hour in the office basically the way I plan to use it.  Mini-Circuits VAT attached to the output is rated for 2W.   100W input power.   There is a measurable difference between the two case halves.   Majority of the VATs heating is conducted from the JFWs case. 

The heatsink is raised slightly but really this is pretty much a worse case condition.  I'm not comfortable with the temps and will go ahead and make a bracket for the cooling fans.   

[joeqsmith]

Made up the bracket for the fans and added a label.   Plan to run the fans from the small battery pack to the right.     

[Yansi]

Awesome work with that cooling bracket.   

But would have added a standard barrel jack for the power supply input to the fans. (I am not used to having USB power banks all around). Just a different personal choice. The bracket looks almost like factory made.

[joeqsmith]

Thanks.  Battery just meant less cables and the fans don't require much.  I have a few USB chargers and an extension cable if I need to run it off AC.   

[joeqsmith]

Old Motorola MRF421 home made power amplifer (note the old Intel 486 heatsink) and my cheap power meter attached to the attenuator.   30Watts without fans.

[joeqsmith]

Roughly 33 Watts is about it for this amplifier.   Brymen's T2 showing the amplifer's heatsink temp with the PC fan running, T1 is the attenuator's heatsink.   Attenuator's fans are running. 

[joeqsmith]

This amplifier was a kit I bought that was based on the MRF141G,  rated for around 300W.   Shown running at 150Watts.  This is what the attenuator network is rated for.     

You can see the kit here: 
https://www.eevblog.com/forum/projects/home-made-pa-and-load/msg635226/#msg635226

***
Poor watt meter.  Note the 0.5W MAX warning.  They shouldn't include a 2W range if they don't want you to use it. 

[joeqsmith]

Running at roughly 150W with the added attenuators (40dB total or 15mW output),  for about a half hour.   Temperatures are stable and the heatsink/fans are having no problem keeping up.    Idling along at 37MHz.   Actually, if you saw the thread where I built the amplifier, you will notice I did not use a spreader.   This is actually pretty much the limit of the amplifier as I built it.   

[joeqsmith]

Radiator and water pump off to the left.   Putting a bit over 160Watts into the new attenuator.   Seems to handle it just fine.