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

[L L bahr]

Rick,

I have no idea what your problem may be at this point but I do know in the long run you need to check every resistor and make sure they are in tolerance and to change those out of tolerence. (I use modern 1 watt jobs for the old 1/2 watt resistors and 2 watt new resistors in place of old 1 watt resistors as the sizes better match one another and a little more heat tolerence is a good thing anyway as some old resistors were stretching their heat capabilities).  You need to change out all the paper capacitors with new poly ones.  You need to change out all the electrolytics. (Don't go crazy here and put in too much bigger ones then original.  A little bigger cap wise is ok, but too much bigger will stress the rectifiers and transformers which you do not want to do.)  (I would not consider using solid state rectifiers as the already increased line voltage we have today will mean an increase of around 60 volts AC plus the solid state rectifiers will also add another 20 to 25 volts plus Heathkit has already pushed the voltage limits on the tubes way back when before the above two things have or could occure.  You add all these threee things together and you will really be stressing the transmitters circuits.  Don't solid state the rectifiers).  You need to check every ground lug and make sure there is no oxidation between the lug and chassis. This includes the ground lugs under tube socket screws too.  You need to do the same thing with every ground shield plate. (I'm not talking about just a physical inspection).  All these things need to be done with any of these old transmitters anyway for a multitude of reasons.  Doing the above usually solves many of the problems and since they needed to be done anyway, the process saves a lot of time just hunting down one bad part.  This is not to say you won't still have a problem.  From there on you have to start hunting down weird stuff like bad tube sockets, bad blocking capacitor, etc.  So if you haven't done the above, you really need to do all these things before you go on.  This could save you a lot of time in the end and maybe even solve your present problem.

As for that added neutralization wire, leave it in.  It is not your problem and will do good things for your transmitter when it is working properly.  As others have mentioned,  I would make new parasitic supressors.

Those resistors near the plate caps of the 6146 tubes get a lot of heat and over time they tend to drift up in frequency.  Just wind new coils as on the old resistors for your new resistors.  Since carbon resistors are hard to find these days and still within tolerence, go buy some good Ohmite OY resistors and use these in your supressors.  They have been proven to work well in these applications.  I know both Mouser and Allied Electronics carry the Ohmite OX and OY resistors.  Use the OY as they are 2 watts and OX are 1 watt resistors.  Since the new resistors are a little smaller then the old carbon resistors, use the old resistors as a coil winding forms for your new ones, so the coils have approximately the same inductance as the old coils.  Winding new coils is easier then trying to salvage the old ones.  One last thing, I can't stress enough how important it is to make sure every ground connection (lug/bolt/nut/chassis)are working properly.  The DX-100 is a wonderful transmitter.  Thousands were built.  Once you got the radio restored, it will offer many hours of enjoyment and few electronic problems. 

Once you have done the above, you may have to track down some small weird problems such as jumping of the VFO which could be ground problems or even a bad mica capacitor which are known to have failed in this VFO.  Be careful and don't replace existing negative coeffecient caps in the VFO unless they have failed as you will upset the alchemy used by Heathkit to stabilize the VFO from drift.  I'm sure they used some "trial and error" to come up with the temperature sensitive caps they decided to use in this kit.  If a negative coeffecient cap would go bad, you would have to attempt using modern COG or NP0 caps in their place.

This is just a shear guess, but I would bet when the problem is finally solved it will turn out to be bad grounds and/or bad parasetic supressors. Once the rig is starting to work properly you can fine tune it by neutralizing the final.  Bye the way,  I bet you will find your old 6146B tubes will work just fine in your rig once it is working ptroperly.  I know the above is all a lot of "pain in the but" work, but once done, it will give you a superior rig to most out there today.

Once working, you might consider adding a 3-wire line cord with an internal fuse block to the rig.  Also,  maybe use green LED replacement lamps for the VFO and meter to reduce heat and thus reduce heat.  The reduced heat will help keep your Green VFO lens from warping. 

One last thing, the audio section is hard to get to, so when you are replacing all those audio caps, that would be the time to consider what mods you want to do in the audio section by changing values to your liking.  Don't go crazy as you have enough problems to resolve now without adding new ones.  If you modify the audio use tried and proven mods already worked out.  You don't want to be adding more problems to the rig.

So the above is what I would do with your transmitter at this point if I owned it.  I am in the process myself of restoring a really basket case DX-100.  It is so bad,  I am stripping it down to individual parts, having the chassis replated and then using new resistors and mostly all new caps rebuild the transmitter as a new kit using new hardware.  I'm even going to install a newly made wiring harness. I built a new DX-100 as a 13 year old kid and it worked first time.  Now I'll see if I can do it again 60 years later. 
 
Keep us all posted on your project. 

Lee, w0vt
Houston, Texas


----- Original Message -----
From: "Rick Poole" <wa1rkt at arrl.net>
To: heathkit at mailman.qth.net
Sent: Tuesday, December 6, 2011 11:02:37 PM
Subject: [Heathkit] 6146A's Arrived (was DX-100B on Phone - bad audio distortion / oscillation)

Well, the 6146A's arrived today, to replace the 6146B's in my DX-100 
which are suspected of causing my spur / self-oscillation problem.

I bought four of them (I could use a couple of spares for my KWM-2A 
anyway) and took the two that matched most closely on my TV-7D and 
put those in.

No joy... self-oscillation is as bad as ever.  Tried several 
positionings of the neutralization wand and then removed the 
neutralization wand altogether... still no joy.

So, I got the XYL to help me wrestle the thing over to the workbench 
(feed me the line cord through the back of the cabinet and then move 
the cabinet out of the way), whereupon my Simpson 260 fell on her 
foot, so she is not happy... :-(

But it's now out of the cabinet and ready for me to start going over 
it from front to back and try to figure out what's happening.  Stay tuned...

Rick WA1RKT