Paul -
There were at least four different versions of the AC-3.
The first "rare!" version had a filter choke in the 250V supply. It
didn't last very long before Drake realized that a resistor worked good
enough and dropped the extra cost and weight.
The next version (s/n 2501) has the TP.
Next (s/n 11501) also has the TP.
Then (s/n 20622) does not have the TP.
None of the AC-4 supplies had the test point.
The resistor is 5 ohms, 2W. I would strongly suggest using a ceramic
composition or at least wirewound resistor for this function. Carbon
comps, especially at the very low resistances, are prone to changing
value over time. We want it to BE 5 ohms.
The easiest way to select R38 is to tack in a pot. If you have a 10
turn trimpot of 500 or 1000 ohms, set it to make the meter read 0.70A,
then remove it and measure. Select the closest 5% resistor and solder
it in there! Here a carbon film resistor would be a good choice. They
are stable over time, and are typically very close to marked value. If
you really want to go nuts, you could select a resistor from a batch to
find one "exactly" right! :-) Again, we're talking a $1 meter, so ...
No, your efficiency is not that good. About 55% to 60% is about the
best you can hope for. The full load voltage of the AC-4 is 650V. Your
plate meter is just way out of calibration because of the 2.2 ohm
shunt. This makes the meter, at best, read 20% LOW. Also, don't forget
that your "2.2" ohm resistor is probably +/- 10%, (assuming carbon
comp,) IF it hasn't been overloaded at some time, or been is humid
conditions. So your "0.2A" is perhaps 240 mA. With 650V, your input
power "should" be 155 W. Your measured 110 W output would be 70%
efficiency, which is not realistic. I suspect your wattmeter is reading
high, and/or your dummy load is not a perfect 50 ohms. Even a GOOD
wattmeter (Bird?) with a 250 W slug is spec'd at +/- 5% _of FULL scale_
(250W) so even a Bird could read 100W as anything from 87W to 113W and
be "in spec". Lots of ways to go astray! :-)
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
Paul Gerhardt wrote:
Garey and Ron
Thanks for the additional info on bias setting. I still have the
cover off the T4 and the AC4 so I need to continure with the meter
calibration/bias setting. The T4 seems to be working well except the
output is a bit low. I am seeing 110W out on 80 and 40M at this
point. The meter is reading low now so I need to get a more permanant
2.2ohm (i.e. higher wattage) and set the R38 value after setting the
bias to the proper level. I have a DMM and an old Heath V-6a to
measure with as well as a HV probe for the V6.
Anybody remember the value of the AC3 TP resistor (s)?
What is the best way to determine R38 value?
With 110 watts out I am only measuring a bit over .2 amps on the
meter. No load plate V in the AC4 is 704V so that is ok and
regulation seems to be within limits. .2 X 700 is 140 W and with 70%
that should give 98Watts. Well that is in the ball park I guess to
what I am measuring but is efficiency really that high? I will put it
on the 'real' dummy load (i.e. Heath Cantenna) and try again.
I did work a Virginia Beach station on 75 for about 40 min with only a
wire out the window with a decent report though and seemed to be
working well (185) miles or so at 9pm local time.
Put a pix of the bad shunt resistor and the T4 on the blog yesterday.
Paul K3PG
Paul Gerhardt
K3PG
http://pgerhardt.blogspot.com
QRP ARCI 6674
FP 274
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