View Full Version : Trouble with 40 meter 20 watt amp
08-02-2006, 09:21 PM
I am having difficulty with the bias adjustment (R4) on this kit. I am supposed to adjust R4 to get 250ma of total amplifier current with no RF input when the collector of Q3 is shorted to ground. (Manual, page 12 and 13).
I start with R4 totally CCW, and the current starts at 52ma. I gradually nudge R4 CW so carefully that the current only increments about 1ma at a time. Once I get to somewhere between 72ma and 74ma, the next nudge gives 2.5a and I turn off the power because the manual says not to let the current exceed 500ma.
Any ideas as to the cause of this behavior/what I've done wrong? I'm a pretty experienced kit builder and have gone over the board a couple of times and don't see an obvious answer ...
Thanks for any help!
08-03-2006, 04:26 PM
The first thing you should do is verify that all the components are in the right place, there are no solder bridges on the board and all solder connections are good. Then check that the voltage on TP1 is in the range indicated in he instructions. Generally setting it to something around the mid point indicated is OK. Once you verify everything above if you still have problems or questions you should email us at the address given in the instructions or call our tech support line. The techs who work on those units are busy and often don't see this forum or reply. Directly emailing them at 'email@example.com' is the best way but the first thing they will ask you to do is perform the above steps first, so it will save you time if you have the information before you call. If you email your question make sure you provide as much information about the problem and what you have checked so far.
You should also have the revision of the circuit board available so they will know if a particular problem is common with that board. It will be near the "Ramsey Electronics" text and read "Rev xxx", usually printed on the top of the board and/or etched on the bottom.
08-03-2006, 06:12 PM
Could the pot be bad?
Perhaps the element is burned open somewhere (right where the current jumps from ~75ma to 2.5A on it's rotation.) or the wiper is lifting,and not making good contact?
Double check everything first.
Then,If you have a DMM/VOM you can take that pot back out and test it to see if it's open or not making good contact,etc.
08-03-2006, 10:58 PM
Thanks DigitalJunkie and RDEngineer!
I replaced R4 with a very precise multi-turn trimmer that I had verified to be gradual throughout its range - and got the same problem.
It's a Rev 1.5 board, and I checked the usual suspects - i.e. semiconductors in the right direction, etc. Components that could be verified in-circuit (caps, resistors, Q3, Q4, D2, D3) all seem fine. Of course, there are a number that are harder to verify in-circuit. The most likely location for a solder bridge is C2, but there seem to be no bridges.
SO I'll need to call or write. Apparently my version of the kit is pretty old because while the Copyright 1994(!) manual lists the website (ramseykits.com) it doesn't give an email address.
What is the email address?
Thanks! And I'm open to any and all suggestions that don't require 120VAC. (*grin*)
08-04-2006, 08:13 AM
The older manuals don't alsays have an email address for tech support but if you check my previous post you'll see it is 'firstname.lastname@example.org'. Don't assume there are no bridges by checking visually. Use an ohmeter to check between traces, etc. You will not harm anything this way. Bridges are not always obvious and it could also be a fault on the etch of the board, (rare but it does happen).
08-09-2006, 03:57 PM
Great News! Believe it or not ... there was absolutely nothing wrong.
Here is what was happening. R4 is a voltage bias adjustment for the Mosfets. If that gets turned up too high - the Mosfets start conducting, and that is what was happening to me.
Meanwhile - there are two instructions in that step. Yes - the manual instructs to keep turning R4 clockwise until you get 250ma; but it also instructs to test the voltage at TP1 and it should be between 3.2 and 3.5 volts.
It just so happens that I got a kit that, out of the box and properly adjusted, has a current draw of only 53ma until the transmitter is keyed.
I adjusted R4 to give me 3.48volts at TP1 without the mosfets conducting - and then fired it up.
It works perfectly! 1.5 watts in = 20 watts out.
It may be that the manual is wrong or I just got a wierd kit - but it works fine.
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