Fellow enthusiasts, I've been buying parts from RF Parts for some number of years now, going back to when their "catalog" was something like a 6 page flyer, not the 96 page one they now have.? Have bought from them at the Dayton Hamvention, via online, and via phone.? Without going into details, my most recent experience indicates that these are some of the most reputable people you will find.? Good products, excellent customer service, and friendly.? Hopefully, this is true for everybody that deals with them.? I hope they keep up the good work for a long, long time.
73, Evan -----Original Message----- From: Garey Barrell <[EMAIL PROTECTED]> To: [EMAIL PROTECTED] Sent: Fri, 30 Nov 2007 2:59 pm Subject: Re: [drakelist] AC3 Bias 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? ----------------------------------------------------------------------? ________________________________________________________________________ More new features than ever. Check out the new AOL Mail ! - http://o.aolcdn.com/cdn.webmail.aol.com/mailtour/aol/en-us/text.htm?ncid=aolcmp00050000000003
