[AMPS] Dentron MLA-2500 Project

[measures]

>
>I picked up a Dentron MLA-2500 a couple of months ago and have been going
>thru it.  The PS filter caps were a mix of Spragues and CDEs.  I pulled the
>old caps and ordered 8 Panasonic 220 MF 450 volt electrolytics.  I got
>creative with my art work and ecthed a board for the Caps and equalizing
>resistors (100K 3 watt MOF).  The caps were much smaller physically then
the
>originals.  The board was mounted on .5 inch insulated stand offs.  I also
>replaced the rectifier diodes with 1N5408s.
>
>I put a new power cord on the amplifier.  The old one looked fine on the
>outside, but when I went to change the plug for switching from 110 to 220
>volt, I found that the black rubber insulation on one of the conductors was
>brittle and cracked clear through the cord.  When I pulled out the cord, I
>found a big burn mark on the terminal strip where the cord enters the
>chassis.  The black conductor had shorted to the mounting screw.
>
>I fired up the amp to give it the smoke test.  No leaks so far.  The meter
>is showing about 2350 volts on the B+ with no load, and about 2250 volts
>under load.  I hooked the amp up to my FPM-300 transciever and am seeing
>about 1000 watts out on 50 watts drive.  I was glad to see that the 8875s
>are still working.  The plate current was about 650 ma.
>
>Someone on the list had mentioned changing out the grid current multiplier
>to get some more swing on the meter, so I changed out the grid current
>multiplier to a 5 K resistor (the original is a 510 ohm job).

Increasing the R of this resistor makes the less grid-current meter less
sensitive.

> I did not see
>much movement on the meter when it was switched to grid current position.
I
>will need to check out the circuit a little closer to see what is going on.
>
>Still need to speed the fan up abit.

A little oil injected in the fan bearings is also a good idea.  To do
this you need a syringe and some 5w oil.  The neoprene seal that covers
the bearings is hidden behind the paper label.  By pressing on the label,
you can locate the seal.  Inject c. 2/100 cc of oil every 5 years or so.
>
>A question for the group.  Any special percautions on using exciters that
do
>not have an ALC output (other than keeping the drive down to a clean
level)?

More important is that you have an exciter with adjustable output.  100w
of drive is way too much for the cathodes of a pair of 8875s.  Key-down
tuneup is esp. stressful to the cathodes.  'Tis better to tuneup 8875s
with high speed cw dits or a tuning pulser - such as the one shown on my
Web site.  //  There is a mod for improving the input match if you are
interested.
>
cheers, Jim

-  Rich..., 805.386.3734, www.vcnet.com/measures.
end


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