DX 100 smokin'

[KB1AL]

Hello All

I'm Al,  KB1AL

&  I'd like to post this question on my DX 100.....wonder if anyone has been there!

With the idea of "improving" my audio, I hooked up an outboard EQ to the grids of the audio driver 12BY7 (thru a .1uf cap) and after hooking it up it seemed to work fine....until I tapped the mic...(sending in a loud pop into the circuit)...and bang!....something blew!!    and now....no audio is coming thru and it tunes weird....seems I can only tune it up with the meter switch in the "plate" position!   I get RF out OK when the meter is in this position...but nothing when the meter is in any other position....and again no audio comes thru now.

I went around with a VOM taking measurements and found nothing strange.....but there's evidence of residue indicating something went under the audio section...but I can't nail it down.

So...my plans are to pull the tubes in the audio section.....and disconnect the mod xformer and just run the B+ directly to the plates of the 6146's.....and see what happens.....   If I can get the rig to tune normally, I'll chalk up a burnt out mod xformer to the cost of a lesson of not pushing the audio of the DX 100 beyond its capabilities.   

I think the mod or interstage xformer popped......but why would the meter switch get involved and the rig can only be tuned when its set in the "plate" position???   More testing to follow!   Anyone have any ideas?

I'd appreciate it!!

Starting to enjoy boatanchors again ..  (good thing I have another working DX 100 !!)
Thanks,
Al
KB1AL   

[John K5PRO]

Al
Without pulling out my schematic, I would say you've hit the nail on the head with your observation that the plate voltmeter needs to be selected to get RF carrier out of it. Trace out what the common element is in the other sector switch positions for that meter, and maybe you'll see the culprit, a bypass cap or resistor in there. Or, as you suggested, a shorted audio transformer. Just don't know why it would only show up in the other metering positions.

[Jim KF2SY]

Al,

1.  Plug in a key and for fun see what you get in the CW position.
2.  Pull mod tubes and in phone, take readings for bias, screen and filament.

Very common to poof the audio driver xfmr.  Mine shorted, took out the 5V4 and other stuff,
was'nt pretty....

Its a start, kinda early here...

good luck
Jim

[WBear2GCR]

Wondering if anything was put on the EQ side of the cap? Otherwise that side of the cap can charge up to a high value, possibly. The output stage of the EQ, if it does not have a resistor to ground (it may) would then float up or down with negative bias, causing the output opamp to have an over-voltage condition, blowing the output of the EQ... possibly.

Otoh you can troublshoot the audio path without there being any RF present...

Probably something relatively simple, especially if there was a loud bang! 
Any smell? Smoke?

                _-_-bear

[KB1AL]

Probably something relatively simple, especially if there was a loud bang!  Wink
Any smell? Smoke>>

Yeah....the EQ is OK....

But...there was a bang....and something popped underneath the rig.....but just can't locate it....and yes...its probably something simple!!