[AMRadio] 6146A's Arrived (was DX-100B on Phone - bad audio distortion/ oscillation)


John Lyles jtml at losalamos.com
Wed Dec 7 17:53:15 EST 2011


The DX100 can indeed be neutralized, using the gimmick wire probe technique as used in other Heathkit transmitters. But it sounds like
yours isn't oscillating due to improper neutralization. As others have suggested, check all the grounding screws and terminal strips, and 
that the audio tubes and their sockets are tight. Using scope to probe along the chain like W5JO suggested is the way to go. Or if 
you have a spectrum analyzer, see what birdies are up on the screen. 
The screen bypass caps that Heath used for the screen grids were fine, unless yours are open. Ceramic disk of proper rating should work. 
Ceramic doorknobs are even better, but there isn't much space under the sockets to place them. 

One more thing I recall, if the copper box for the oscillator isn't tied down with tight nuts, it can give trouble, but not
so much straightout oscillation. Same thing for the audio input stage, it will get noisy if the screws are removed for the 
copper shield plate in front of it. I don't remember if you said the thing had problems with AM or only CW mode? 

73
John
K5PRO


> Message: 8
> Date: Wed, 07 Dec 2011 10:36:03 -0500
> From: Rick Poole <wa1rkt at arrl.net>
> Subject: Re: [AMRadio] 6146A's Arrived (was DX-100B on Phone - bad
> 	audio distortion / oscillation)
> To: amradio at mailman.qth.net
> Message-ID: <6.2.5.6.2.20111207095941.04f817a8 at rkt-tech.com>
> Content-Type: text/plain; charset="us-ascii"; format=flowed
> 
> At 09:44 AM 12/7/2011, Gerald Stockinger wrote:
> 
>  >>>>>
> >Rick, I have been following your tale of woe for a bit and this morning I
> >looked up a copy of the scheme of your rig. I could not find one for the
> >DX100B only the DX100. The first thing I see is the DX100 did not have
> >nuetralizing of the 6146's. That may have been added for the B version.
> <<<<<
> 
> Good morning, Jerry.
> 
> As best I can determine, neutralization was never a part of the 
> DX-100 or -B design.  There was an aftermarket modification that 
> involved some #12 wire run from the driver variable cap, through a 
> hole in the shield to the PA compartment, and then it ends hanging in 
> the breeze, and you bend it closer to or farther away from the plate 
> choke or tubes to vary the capacitance.  Mine has (had) that modification.
> 
> There are those who say that modification was only needed to make it 
> play nice with 6146B's or -W's and that with straight 6146's or 
> 6146A's it is not needed.  There are others who say it's not needed 
> at all.  I tried adjusting it through its entire range with the -B's 
> and again with the -A's and then ended up taking it out altogether, no help.
> 
>  >>>>>
> >The
> >thing I would try in any event is to replace the 1300pf screen bypass caps
> >for the finals. Use a good quality low inductance cap like a centralab door
> >knob no.850 type if you can get them.
> <<<<<
> 
> I will try that.  The previous owner (or maybe the owner previous to 
> him) had already done some of the recapping.  All of the 
> electrolytics have been replaced with modern equivalents, sometimes 
> in an interesting manner (the filter caps in the power supply were 
> replaced with radial lead electrolytics that were glued to the 
> underside of the chassis).  Some of the signal-level caps look 
> relatively new but I haven't gotten to things like searching for date 
> codes yet.
> 
> The original equipment screen bypass caps were disc ceramics.  Those 
> aren't good enough?
> 
> I guess the first thing I need to do is clean and retighten all the 
> grounds, then take it from there.
> 
> Rick WA1RKT



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