The AM Forum

Got the DX-100B out today and decided to start working on it. Got it tuned up making about 90 watts and put some audio to it. Audio sounds clean on a receiver but i only seem to be making about 75% positive, it will go fully negative though. Plate voltage is right at about 740 under load but sags to about 650ish with modulation, it does still have the tube rectifiers. 1625 screen voltage is about 325, bias is -40. I almost suspect some of that could be normal, the modulator in this thing does seem pretty anemic. The other more concerning issue is with the finals, key down after about 30 seconds i start seeing a red spot on the plates. I did have to replace the clamp adjustment pot and reset the clamp according to the manual, but I'm not sure how accurate the panel meter is and i might not be bringing it quite to zero, so I'm wondering if maybe i should try touching that up again or if maybe i might have something else going on back there. I have not checked any of the voltages to the 6146s yet.

were you using the internal VFO or a xtal?

VFO to check output, voltages. Just checked the 6146s, i got 225 on the screen and -60 on the grid. That seems too high for the screens, and not enough bias.

I think i figured out the glowing finals, that transmitter is very touchy with being off resonance, as long as the running is dead on it is happy. Looking on the scope i might be seeing a little bit of parasitics when the tuning is off, i might need to rebuild the suppressors. The tank circuit in this thing is a lot narrower than my homebrew 814 rig, i can pretty much go anywhere in the band with that one and be in tune, not the case with the heathkit.

Replace and then adjust the Clamp tube pot for about 175V on the final screens and then do some plate dips and loads and readjust.

Grid bias is a function of the condition of the 6AL5 and the RF drive from your 5763. Control Grid current should not exceed 5 mA.


Phil

So adjust the clamp tube by screen voltage and not by watching the plate current meter like the book says? Ok I'll try that. The pot was already replaced, the old one was seized up and i managed to break the stupid shaft off trying to free it up. Reading Timtron's mods for this transmitter made me realize everything I've found is something that is normal to the DX-100, so I've got some fixing to do to make it better. I'm kinda getting into this backwards, i designed and built a homebrew rig first and now I'm trying to rebuild and modify a commercially built vintage transmitter and realizing there's a lot here that doesn't make sense because its not how i did it in my own design.

I'm assuming that you're coupling an audio tone directly into the microphone connector to make your measurements. If you're speaking into a microphone, it may just be "out of phase".

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Wiring errors are always a possibility, especially with kit radios. I found an unsoldered filament wiring connection in my DX-100 that must have been there for over 50 years until it finally went open on me one day. It pays to give everything a good looking-over. Keep an eye out for sweaty resistors, bulging capacitors, cold solder joints, etc.

There's an outside chance that the modulation transformer secondary, which is tapped, is wired incorrectly. If the tap is wired, and not the full secondary, you'll never hit 100%. The step-down ratio is bad enough as it is, without having it wired wrong too. While this is unlikely, it's an example of a wiring error that could go unnoticed by the builder who, perhaps, didn't own an oscilloscope. My point being; take nothing for granted.

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You can get close to resonance of the output tank by tuning it in the "spot" mode while watching the S-meter on your receiver and tuning for maximum. It will then be close to resonance when you key-down and will require only a small adjustment. Red-plating the final tubes can quickly get expensive.

Yea i figured out the tune in spot mode pretty quickly. I was using a mic to test it but i did try swapping modulator plate leads around, it made a very slight difference. What i also need to check is to make sure the screen voltage is coming from modulated B+, on the scope it resembles what my 814 rig looked like when i hooked it up with three screen on a fixed supply and not the modulated plate voltage.

Well, I know that any rig I've owned hasn't left my hands in stock form. With a used rig, anything is possible. Pulling the screen voltage off the wrong end of the modulation transformer secondary is possible.

If you're hitting 100% negative easily it most likely is just a phasing issue.

No truer words spoken and this should be a "sticky."

And if this rig has had the Turbomod done to it, be careful since Tintron never produced a schematic showing the phasing verses the transformer wiring color codes.

We tried the Turbomod (from Opcom's schematic) on a friends DX-100 and it never worked properly so we went back to the original schematic transformer hookup.

I would recommend tracing out the wiring and then making a schematic of the "as is" and compare that to the original schematic.


Phil