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 <k4...@mindspring.com> wrote: > From: Garey Barrell <k4...@mindspring.com> > Subject: Re: [Drakelist] T4Xb meter shunt > To: "Drakelist" <drakelist@zerobeat.net> > 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 > Drakelist@zerobeat.net > http://mailman.zerobeat.net/mailman/listinfo/drakelist > _______________________________________________ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
