>
>I just got a used AL1500 for testing before a posible
>purchase. The amp was bought in 96 in the USA and,
>according to the seller, has been used only for a
>couple of hours.
>
A used amplifier that has been used very little not infrequently has a
little problem.
>After making the necessary conections I find out that
>something is wrong. I follow the procedure in the
>manual for cw tunning but things don't go as they
>should (I don't get beyod basic manual procedures
>cause I know nothing about amps):
>1. HV on standby reads about 3750 volts (manual says
>absolutely no more than 3700)
>2. when I key the exciter (no drive) the plate current
>on the 10000mA scale remains flat- manual says should
>read 75mA. When doing the same but on the SSB position
>the same thing happens (manual says plate current
>sohuld read 250mA).
>3. whatever drive I apply, the grid current remains
>flat.
>4. The SWR when I turn the switch from standby to OPR
>goes from 1:1 to abour 3:1. I don't use any antenna
>tuner (I don't need it). This seems odd to me.
>Of course, after seeing these things happening I
>rechecked everything (cables, conections, switches,
>etc.) but found nothing that could explain this. Fan
>works, the 3 minute delay for the plate meter to light
>works, etc.
>
>
>The amp looks clean both outside and inside.
>
>Any ideas why this might be happening?
AL-1500s have a reputation for frequent 8877 replacement. I know of one
AL-1500 that has had seven replacement tubes installed. Thus, it might
be a good idea to find out if the 8877 is ok before trying to
troubleshoot the amplifier. A suitable test can be performed with a
high-potential leakage tester. Remove the 8877. With positive 8kV on
the anode and common on the grid, measure the leakage current. If the
leakage is more than 10uA, retest with reverse polarity. If the
anode/grid leakage decreases with negative on the anode,
gold-sputtering/evaporation is indicated. To confirm the presence of
loose gold meltballs, tap on the top of the anode with a small hammer.
If the anode/grid leakage current decreases after tapping, loose gold is
the reason. .(vibration causes gold meltballs to fall into the base if
the tube is held upright during tapping)
. On my Web site, there is an item called Eimac Letter,
<http://www.vcnet.com/measures/EimacF.html>
wherein the cause of gold evaporation is discussed by an Eimac
spokesperson. There are some photographs of tubes with gold evaporation
damage in the September 1990 *QST*.
.
>The amp has a great price so I would like to give it a
>final chance before returning it. Thanks in advance.
>Ciao,
>
- good luck, Fernando
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|