Garey, I love the idea of the diode, but I would guess 2 series diodes would be better. The TRany requires .5 volts at the TP. I seen to remember a couple times when diodes seemed to have lower than ~.6 volts drop. Maybe they were schotchy diodes or something. Just remember thinking they were suspect.
I also used a permanent marker to mark the min/max bias setting on the bias pot. Since I set the bias so infrequently, I can never remember which way to go. 73, Ron WD8SBB --- On Wed, 1/13/10, Garey Barrell <[email protected]> wrote: > From: Garey Barrell <[email protected]> > Subject: Re: [Drakelist] T4Xb meter shunt > To: "Drakelist" <[email protected]> > Date: Wednesday, January 13, 2010, 10:13 PM > Paul - > > Yes, that was one thing that should have been kept in the > AC-4. The > voltage at the test point shouldn't go over a half volt, > (0.35V to set > the Bias for the T-4(any) transmitters.) Unless the > resistor opens up! > I think if I was going to bring the TP outside the cabinet > I'd use a BNC > connector, and make a BNC shorting plug to keep over > it. Probably > wouldn't hurt to put a power diode (1N5408,) across the > jack as well, so > the most voltage that could appear across the jack would be > ~0.6VDC. > > 73, Garey - K4OAH > Glen Allen, VA > > Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement > CDs > <www.k4oah.com> > > > Ron wrote: > > Garey, > > I took a different approach to setting the bias. > I installed TP of the original AC3 power supply into my AC4 > (actually my AC4R). It was an easy modification and > now I don't care how accurate the meter is, or how difficult > the hash marks are to see on the meter. > > > > The original AC3 had a resistor in the ground side of > the HV diode string. It also had a shorting jumper to > remove the resistors after you set the bias. The bias > adjustment on my AC4 is now a simple meter reading made with > any good VOM. > > > > The only negative I see with this modification is that > you are exposed to the 650 volt B+ high voltage while doing > the bias adjustment, so care MUST be exercised. You > could actually bring the TP outside the AC4 "cabinet" if you > wanted and avoid the HV exposure risks. > > > > I sent Ron WB4HFN information to post, if he wants, > when he gets the chance. The modification was more > difficult to describe accurately than I thought it would > be. Hopefully the pictures and AC3 schematic will > help. > > > > 73, > > Ron WD8SBB > > > > --- On Wed, 1/13/10, Garey Barrell<[email protected]> > wrote: > > > > > >> From: Garey Barrell<[email protected]> > >> Subject: Re: [Drakelist] T4Xb meter shunt > >> To: "Drakelist"<[email protected]> > >> Date: Wednesday, January 13, 2010, 2:20 PM > >> Paul - > >> > >> The resistor is ok, but you need to calibrate > (with R38) to > >> get the BIAS > >> setting of 0.07A correct on the meter. > Obviously the > >> idle current needs > >> to be set correctly for linearity. The 1/4W > is a > >> little small, but as > >> long as you keep the final dipped it may be > >> ok. Typical plate current > >> of 0.340A is just a little over a quarter watt in > the > >> resistor. The > >> concern is that as the resistor is overheated it > will go up > >> in value. > >> You never know where you are! > >> > >> I like to use 1W or, even 2W if I can find them, > >> "flameproof" metal > >> oxide resistors. They don't change value so > once > >> calibrated you don't > >> have to worry about it any more. The finals > can > >> easily draw an amp or > >> more of current off-resonance, which would leave > 2.2W in > >> your resistor! > >> Even a half amp is over a half watt in the > resistor. > >> > >> But at least now you know HOW to fix it > >> > >> 73, Garey - K4OAH > >> Glen Allen, VA > >> > >> Drake 2-B, 4-B, > C-Line& TR-4/C Service Supplement > >> CDs > >> <www.k4oah.com> > >> > >> > >> Paul Gerhardt wrote: > >> > >>> Garey > >>> Thanks for the help on this one. The > meter shunt > >>> > >> on the vertical > >> > >>> board behind the three power resistors was > open! > >>> > >> Yea... > >> > >>> I replaced it with another 2.2 it is probably > too > >>> > >> small at 1/4 but it > >> > >>> seems fine for now and I can order a higher > wattage > >>> > >> one for later. > >> > >>> May still need to get the resistor on the > slide switch > >>> > >> changed as I > >> > >>> think the meter is now reading a bit low but > the > >>> > >> output is way up from > >> > >>> yesterday. I was able to load it up to > 100 watts > >>> > >> out and it may go a > >> > >>> bit so it still needs work but yesteray it was > 7-8 > >>> > >> watts so it is up a > >> > >>> LOT! > >>> > >>> Will try to get a pix of the 'bad' part up on > my > >>> > >> blog. > >> > >>> 73's > >>> > >>> Paul K3PG > >>> > >>> If I screw the bias back I see up to 80 watts > with the > >>> > >> plate meter > >> > >>> pegged hard over. > >>> > >>> With the bias set 'properly' I only see power > output > >>> > >> if I turn the RF > >> > >>> Gain up to where the meter is again pegged to > the > >>> > >> right. When it hits > >> > >>> the pin I am getting 5 - 7 watts. > >>> > >>> When the modulation 'looks' right on the meter > I am > >>> > >> getting 5-10 watts > >> > >>> pep out but the plate meter is kicking up all > the > >>> > >> way. > >> > >>> I am hearing anti vox coming out the speaker > while in > >>> > >> SSB is this > >> > >>> normal? I would think not? > >>> > >>> The R4B seems to be working well. > >>> > >>> The two VFO are transceiving using either or > both with > >>> > >> the inj cable connected. > >> > >>> I am going to start 'dusting, cleaning and > visual > >>> > >> inspection' > >> > >>> Paul Gerhardt > >>> K3PG > >>> http://pgerhardt.blogspot.com > >>> QRP ARCI 6674 > >>> FP 274 > >>> > > > > _______________________________________________ > Drakelist mailing list > [email protected] > http://mailman.zerobeat.net/mailman/listinfo/drakelist > _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
