The AM Forum

I have a DX-100 that came from N3IBXsk yrs ago. Have had a couple on the air over the yrs but this needs to be serviced before it hits the airwaves.
The big question is how do you get to get to the parts the are connected to the 12AX7 & 12BY7? Can that shelf be removed for easy access? Or loosen the screws up shove it back a bit?

Tnx for your sage advise

Terry N3GTE

Terry, is this a DX-100 or a DX-100B? I thought Joe used the B version. I have his Taliban-100 complete with extra 6146 and 814 modulators. I'm trying to picture the underside of it. The big Achilles heel in both the DX-100 and the Apache is that tiny driver transformer.

No it's not the Taliban rig. I remember that one too. This is as described a 1956 DX-100 build by W2MYQ over in Brooklyn,NY. As far as I could find he fell off the face of the earth around in the mid 70's.

The VK I & II have the awful driver transformers. They make the audio sound like a soup can and a string. On the Benton Harbor Kilowatts it's the 510mmfd coupling caps that don't do bassy guys justice.

I did service one back in the '90s but I've been thru a lot since then and I just don't remember how I did it. The rest of it seems pretty straight forward. Both p/s and it has an issue w/ no heater voltage at the vfo. Those are obvious things and I'll go from there.

Terry     

Terry,

I have the DX-100B. If you're referring to the RF shield for the speech amp and it's like the one in the B model, it can be loosened and tilted somewhat or removed.

Brad K4RT

I would suspect on the B it's aluminum but the same. I read the construction manual and that section is put together before top of the chassis is screwed to the lower section. So it's take on screws till the the shield comes out. And then have a dike fest...

Tnx Much!
Terry   

I looked at the tight spacing in the low level audio section of the DX-100 I have  here and quickly decided not to waste my time trying to work in there.  Did a quick and not so dirty end run around the low level stages suggested to me by WA1HLR.

1. Pull the 12BY7 audio driver tube, it won’t have anything to do’anymore,

2.  Bridge 600 ohm balanced audio from an external processed audio chain onto the grids of the 1625 modulators.  The secondary of the 12BY7 to PP 1625 modulators stays in place to supply grid bias to the 1625s and provide phase shift to drive them in push pull.

                                        CAUTION

Be sure that neither side of the balanced 600 ohm feed that’s being bridged onto the grids of the 1625s is grounded to prevent shorting out the nominal -33 V fixed bias that is fed in via the center tap of the original driver transformer.  If your incoming outboard audio is unbalanced with the low side grounded you will need to add an isolation transformer so the balanced audio is floated.  This transformer also allows a comvienent point to flip the audio phase for best peak symmetry.

3. Jumper out the original circuit’s 1K series resistors between the
secondary of the driver transformers and the 1625 grids.

4.  Remove the “splatter filter” cap across the secondary of the mod transformer.

I originally used an RCA 1940s vintage broadcast equipment line Type 82C monitor/program amplifier as the driver.  It had a single ended grounded output which was easlily changed to a floating output by disconnecting the ground conmection to the low side of the output transformer secondary.  The 82C uses a pair of 6L6s in the output stage which was overkill to drive the 1625s.  I’m
currently using a small GenRad Type 1206B lab type wide band direct coupled audio amplifier which can make 3 watts into a 600 ohm load.  It has a single ended grounded output so
an isolation transformer is used between it and the grids of the 1625s.

Here is a circuit that I use that retains the 12BY7A:

It takes the DC off the driver transformer so as to prevent saturation of the driver's core.

Phil -- AC0OB

I bought one of those A-53C's for my Viking I transmitter. They work very good for $16 but regardless it is more beefier than that puny Xformer in the DX-100. BTW Terry, I do have Joe's Taliban-100. When he knew the 'end' was nearing he asked me what I would like from his estate. That was the first rig I asked about because it was ugly as sin but fairly reliable. I have it covered up in the garage where it is waiting for me to give it an operating space!

Phil, skip those back to back electrolytics!
Use a nice Mylar or Polypropylene cap. Looks like you call for ~10ufd, these are easy to
find in over 400volt ratings...

Since ur driving  7k ohm primary, do you actually need or want 10ufd?
A queeck calculation says that 10ufd and a 7kohm load gets a -3dB frequency of
 2.2747952684 Hz!
So 1 ufd will get 22.27479...Hz.
Which, imho is still to low, given the inherent core limitations of the mod iron!
Something between that 1ufd, and ~0.5ufd would in theory put the -3dB around
30-40Hz, which is pretty darn good.(if it works as the calculation holds, then 0.6ufd will get
38Hz as the -3dB point, which BTW will still have a fair amount of energy an octave below!)
 And, will help avoid inadvertently swamping the mod iron.

Not to mention that the driver iron isn't going down to 2Hz or 20Hz... fwiw.

Testing in practice to find the actual -3DB point is a good plan.
For easy testing, half voltage is 6dB down.

                              _-_-bear

Please lift up the cover and shoot some pix??
Thanks...

it sounds as if you modeled it as a high impedance, RC coupled circuit.

I modeled it as a low impedance series resonant circuit, with a primary inductance of 6.4H, which gives us a low frequency starting point.

Phil - AC0OB