DX100B no modulator current draw or audio

[kb3ouk]

So while testing out my DX100, while talking i hear a loud pop from inside the rig and now have no audio and zero modulator current. So far I've checked plate screen and bias voltage on the 1625s, the windings of the mod transformer, and the driver secondary. Everything there checks ok as far as i can see. I would think that if it were anything ahead of the modulators i would still see resting current, correct? What else could i be overlooking?

[WBear2GCR]

A loud pop usually means something blew up.
Ought to be visible.
Or smell.

Also did you check the continuity of the primary winding and the secondary?
The continuity of the B+ line feeding the mod iron?

For B+ on the plates?

No B+ = no current, no output.

Don't forget the shunt that reads the modulator current... does it modulate and
you show no meter reading, or does it simply not modulate and no meter reading?

[kb3ouk]

Nothing visible blown up, that was the first thing i checked. B+ is good, i'm getting voltage at the plates of the 1625s, and continuity with the rig unplugged and just checking the wiring itself. shunts are good. There is no audio on the scope or on a receiver when i try to speak. The windings appear ok, i was measuring them in the circuit but if i have to i can unsolder everything and try again. One thing in going to try this morning is remove the cap from the secondary of the mod iron and try it, I'm wondering if maybe it arced through internally and is shorting out the secondary.

[W3SLK]

I agree with Bear. Something let out all of its smoke in one shot. Lack of current means voltage isn't getting to where its supposed to. I would take a close look around the electrolytics. Sometimes you can identify a bad one by a slight bulge or they may have a small hole in them. Also use your 'God-given' logic probe, aka your nose. Its more than likely the failed component left behind a trace of foul air when it retired!

[kb3ouk]

The only thing i have figured out is there is no path to ground from the cathode of the 1625s. From what i can see on the schematic, it goes through a .1 ohm shunt resistor and a second .1 to ground, both of which themselves are ok, but from pin 6 on the 1625s to the grounded end of the .1 is open, so somewhere there's an open wire in the harness.

[kb3ouk]

Well the lost ground turned out to be the whole problem. Took a clip lead fron the cathode pin of the 1625s to ground and had audio, so just soldered a new wire straight to the closest ground point. Somewhere in that harness it must've burned open, but i looked all over and couldn't visibly see where, there are places where that wire is buried in the middle of the bundle and you can't see. The way they loop ground connections all over the chassis is ridiculous.

[w8khk]

Well the lost ground turned out to be the whole problem. Took a clip lead fron the cathode pin of the 1625s to ground and had audio, so just soldered a new wire straight to the closest ground point. Somewhere in that harness it must've burned open, but i looked all over and couldn't visibly see where, there are places where that wire is buried in the middle of the bundle and you can't see. The way they loop ground connections all over the chassis is ridiculous.

If the cathodes of the two 1625 modulator tubes are directly grounded, then they will have the same path to ground as the 6146 finals.

The original design grounded the finals, and measured the final plate (cathode) current across the shunt between chassis ground and the plate transformer center tap (the shunt nearest the plate power transformer in the schematic diagram).  The modulator plate current was measured via a separate shunt connected from the modulator cathodes directly to the plate transformer center tap.

While the modulator will function properly if the modulator cathodes are connected to the chassis ground, the modulator plate current will read ZERO when the meter is switched to the modulator position, but in the final plate current switch position it will read the combined final amplifier and modulator plate current.

Now that the fault is defined, if the wiring harness is defective, it is suggested that the modulator meter shunt could be disconnected from the harness, and a new wire run from the modulator meter shunt to the center tap of the plate transformer secondary, thus restoring proper meter functionality.

A snippet of the associated section of the DX-100 schematic is attached.

[WBear2GCR]

Good advice.

Also carefully check the shunt resistor, or replace if possible.

[kb3ouk]

The shunt is still in place, just the ground side goes straight to ground to the closest ground instead of through the entire chassis harness. I'm still able to read modulator current on the meter, for now when tuning i switch to cw since there is no current draw from the modulators. The shunts are good, i've already checked them probably 4 times now. Next time i open it up for work I'll probably run a new wire up to the original ground point.

[w8khk]

The shunt is still in place, just the ground side goes straight to ground to the closest ground instead of through the entire chassis harness. I'm still able to read modulator current on the meter, for now when tuning i switch to cw since there is no current draw from the modulators. The shunts are good, i've already checked them probably 4 times now. Next time i open it up for work I'll probably run a new wire up to the original ground point.

That sounds like a good plan, Shelby.  Your method gives you the ability to measure the current draw on the modulator and final, individually.  As heavy as my DX-100 and Valiant are, I get a backache just thinking about moving them.   I do not even look at the Collins 32V2 anymore!   That wire can wait!