Problems with my Downeast Microwave 2M75LDPA...

Roy Laufer

I have an early model Downeast Microwave 2M75LDPA (#138), to use my 6700 for 2 Meter operation.

Although it is 'rated for 75W, these days the best I can get out of it is 40W when it is at room temperature...

Unfortunately, it doesn't continue to output at 40W - instead over minutes of FM transmission the RF will drop to below 18W?

No doubt, if I tried this test longer, I could get even a lower power output.

The chassis wasn't warm, even at the 18W output stage.

I don't believe it is my power supply, since everything else has its usual full duty cycle capabilities, and I don't think it is my 6700's drive power, since if I adjust the 6700 to drive it with all the LDPA's LED bar light glowing, they continue to all be lit as the output power declines.

Downeast Microwave sold this model off to Q5Signal, and I have contacted them. They suggest I ship it down to them, to take a look for what might be going on.

Anyone with any similar experiences, or good ideas (I don't have any test equipment at my QTH and getting parts may not be as easy as in 'calmer days')?

Stay safe!
Roy AC2GS

John WA7UAR

Roy, is there a potentiometer or something on the bottom side of the 2M75? I seem to recall that it might tweak your power out levels? I have the same device but never tweaked that element myself. Just trying to remember and make a helpful contribution here. Good luck!

Roy Laufer

Thanks John,

If it was just a matter of sub-optimal initial output levels, I would be game to 'tweak' an internal '****' (except that it might introduce distortions into the signal at a higher gain rate).

My present problem would appear to be a thermal issue - as the device warms up, tis RF output steadily diminishes. I assume it is a fault with one or more discrete components and if I had a proper repair setup I could 'cold spray' different components, find the errant parts and order them online.

Q5Signal supplies full schematics and parts lists...

I was just hoping for some design flaw that someone has already discovered and found a quick and easy fix for it.

Stay safe,
Roy AC2GS

John Kludt

Roy,

You can fiddle with it for a while or just send it to Cory at Q5.  He does great work at very reasonable prices and he is really quick about turnaround.  If you want to work on it then of course go ahead.  But don't worry about using Q5 Signal.

John