Sb-220 power output at about half..

[Paul Blumhardt]

Hi John

	Did you notice the grid current and control setting before the
problem arose?

	Do you notice anything inside the PA getting excessively hot? Smell
hot?)

	What is grid current now with 100w drive? Are the Tune and Load
controls in basically the same place?

	Is the power down by the same proportion across all bands?

	At first blush, it looks like one of the tubes isn't conducting
(like it not getting drive), since the output power is a shade more than 3db
down. Answers to the questions above may help us point you in the right
direction.

	If grid current is in order, you may have something in the tank
circuit that's become lossy. 

Paul K5RT



-----Original Message-----
From: Heathkit Owners and Collectors List [mailto:HEATH at LISTSERV.TEMPE.GOV]
On Behalf Of John Godfrey
Sent: Thursday, May 24, 2007 6:33 PM
To: HEATH at LISTSERV.TEMPE.GOV
Subject: Sb-220 power output at about half..

I do hope you guys can help me. I have been in the process of restoring a
heath kit SB-220. I replaced all the filter caps - diodes in the doubler
etc. all with new parts including a new air variable for the plate tuning.
All was well and output was plenty acceptable on all bands. Then output just
went to zero after a power-on condition. Meaning it was fine when turned
off. Then after much investigation I found that the little bolt/nut that
holds the output tuning 10/15 meter coil to the plate tuning capacitor was
loose. After tightening and re-power up and tune. I just get half the output
as before. Both tube filaments light bright and voltage to the pins on the
filaments is 4.9 volts ac on my meter. Cleaned the sockets and pins in case
of any resistance but like I said voltage seems to be correct. Zener diode
seems to deliver exactly 5 volts DC through the center tap of the filament
transformer to the tubes at transmit and the 117 volts dc at receive mode.
Tubes do produce gain. With 40 watts in you get about 150 out. 100 watts in
gives 400 watts out. Again about half what I was getting. Both tubes EIMAC
3-400z l believe them to be full output because I tested them in another
amp. Tested the high voltage on SSB mode and I have about 3010 volts DC.
Input SWR is acceptable and I am driving it with tubes as well instead of
transistors. 

  

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