Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
----------------------------------------------------------------------
Thomas -
Yeah, there are mighty few AC-3 power supplies still running with their
original electrolytics!
First of all, the Final idle current is a DC circuit that requires
nothing more than Plate, Screen and Filament voltage, and that the
Cathode is at ground potential. These conditions will result in the
Final tubes drawing literally Amps of plate current for as long as the
power supply, its fuse, the tube plates and/or the wiring will last!!
To control the idle current we apply a Negative voltage to the Grids of
the final tubes, and somewhere around -50 to -60 VDC at the Grids, the
final plate current will be around 0.1A for a trio of good tubes. If
the Grid voltage is More negative, say -70 VDC, the idle current will
_decrease_ and if the Grid voltage is Less negative, -40 VDC the idle
current will _increase_ . So either range you are seeing on your Bias
supply is sufficient to hold the idle current to 0.1A.
Your problem is that your Bias supply is dropping to zero volts, which
results in maximum Final Plate current. You don't say where you were
checking the Bias voltage when it was at Zero, but I suspect you have
either a bad solder joint or broken wire in the Bias supply. You can
work on the supply with it disconnected from the transceiver, by
shorting Pins 1 and 2 of the connector together, which applies AC to the
power supply. Don't forget that there is still 300 and 700 VDC floating
around on the other pins of that connector when it is ON, and for a
brief period after it is turned off until the bleeder resistors can drop
that voltage back near zero!!
It's also possible that an intermittent open of either R6 or R7 is
occurring, or that the pot wiper is losing contact with the resistive
element. An "open" in either of these elements will take the Bias
voltage to zero. An open in R5 will take the Bias voltage to maximum.
A pot cleaner/lube such as CAIG CaiLube can often clean a dirty pot, and
sometimes just rotating the pot through it's range several times with
the power off will clear it. Occasionally a pot element will open up
where the wiper has been sitting for a while, and the circuit will
operate as long as the wiper is "bridging" the break.
From what you wrote, I'd suspect a problem with R5 and/or its connections.
I'd suggest carefully examining the Bias supply wiring, and then with a
meter across the output while disconnected from the TR-4 push and prod
the connections and components will an insulated stick. Again, be aware
that there are serious, dangerous voltages floating around in there any
time the power supply is on.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B & C-Line Service Supplement CDs
<www.k4oah.com>
Thomas Wright wrote:
Maybe you guys can direct me a little. My AC3 has been showing signs
of bad caps, especially in the bias section ( one of the bias caps
measured at 700MFD). I replaced all the caps in all sections, using
33MFD 160 volt caps in the bias section, and while I was there did the
diodes. After reassembly the bias was adjustable from 41 or so to 80
volts. TR-4C was running well. Today after operating for an hour or
so, the transmitter started to show very high plate current, so I
checked the bias and found that it was either zero or above .4 amps,
without much adjustment between the two, and of course when higher the
xmit relay was engaging. Reducing xmit gain would not reduce the
plate current. I measured the voltage at the plug and the bias is now
only adjustable from 52 to 94 volts, which I think is too high.
There's not much else to replace in the AC3 except the bias pot and
two resistors, both of which measure within tolerance statically. Any
thoughts?
------------------------------------------------------------------------
No virus found in this incoming message.
Checked by AVG Free Edition.
Version: 7.5.503 / Virus Database: 269.16.10/1160 - Release Date: 11/29/2007 8:32 PM
----------------------------------------------------------------------
Submissions: [EMAIL PROTECTED]
Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body
Hopelessly Lost: [EMAIL PROTECTED] - help in body of message
Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net
----------------------------------------------------------------------
