Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Alan -
The AC-3, (with the exception of a very few early units,) is
functionally identical with the AC-4.
The fault that makes an AC-3 "cook a set of finals" is exactly the same
fault that can cause an AC-4 to do the same! This "fault" is a failure
of the bias portion of the power supply, which is identical in both
supplies. The bias supply is by far the most "critical" part of the
AC-3/4. It is used to keep the T/R relay in the standby condition, and
limits the idle current drawn by the final tubes in transmit, among
other things.
The failure mode is this..... One or both of the filter capacitors in
the bias supply of the AC-3/4 dries out. The "early warning", if you're
lucky, is erratic VOX operation, the VOX taking longer to drop out than
it should, or increasing idle current in the final. If you're NOT
lucky, like most of us, one of the filter caps will short, killing the
bias supply completely. Even if the transmitter is in standby, while
you have gone to the kitchen for a snack, or to a hamfest, when the bias
supply fails the T-R relay will close, putting the transmitter in
transmit. Since there is no bias, the final tubes will draw as much
current as they can until either one of the final tubes melts, shorting
the HV supply, and hopefully blowing the power supply fuse, (assuming
you or a previous owner hasn't replaced it with a piece of #000 copper
wire,) or burning up the transmitter, power supply and/or house!
Alarmist? Yeah..... You wanna take the chance? Didn't think so.......
At the very least, with a "new" or unknown, or even a supply you've had
"for years", replace the filter caps on the bias supply NOW if they
haven't already been replaced within the last 10 years. Better yet,
upgrade your AC-3/4 with the AC-4R kit from Mike at The Heathkit Shop,
and forget about it for another 30 years or so. Also, check that the
fuse is either 5 or 6A Slo-Blo. Early supplies are marked 5A, but
tended to blow from the turn-on surge and so was increased to 6A.
Finally, (at last,) the answer to your question.... Yes, the supplies
are fully interchangeable across the 4 Line. The AC-3 doesn't have the
VOX connector to key a linear amplifier, since the AC-3 was for the TR-3
and very few people wanted to put a linear behind a transmitter that put
out 200 W. The little RCA jack pigtail coming out of the back of the PS
connector was originally wired for the speaker with the TR-3 or TR-4.
If you need an external VOX jack, the easiest way is to add another RCA
pigtail in the Cinch connector.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B & C-Line Service Supplement CDs
<www.k4oah.com>
[EMAIL PROTECTED] wrote:
[EMAIL PROTECTED] made an utterance to the drakelist gang
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I was trying to find out if the AC3 power supply can be used with the T-4
series transmitters or whether the AC4 is necessary. Searching the Drake
archives I stumbled across one of the few postings addressing the subject:
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POSTING---------------------------------------------
Does anyone know if there is a differnce between the Drake AC-4 and
AC-3 power supplies electrical wise in the connector? I have both and
the AC-4 works fine but the AC-3 seemed to cause various problems including
cooking a set of finals in a T4XB. Seems like it causes the T4XB and T4XC
to go into transmit part of the time.
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I was hoping to get a clarification from this list.
Thank you
Alan KB7MBI
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