DX-100 audio problem

[Brian Wood]

Hi Dale,

Did the 12by7 turn blue/red before you took stuff out to check it? If not,
you may have accidentally shorted something out. That audio compartment is
pretty tight and there are some bare wires running around in there. You
might want to unscrew the copper plate and then move it so you can see in
there.

Under load, my "point M" (5v4->oa2 output that goes to the 12by7 plate
through a transformer) is 390V, so your plate voltage seems OK. If all
resistors seem OK, you might look at the 510 pf coupling cap that feeds the
audio signal to the 12by7. If you need to replace it, I'd suggest a .05uF
600V metalized polyester film. You can use another one from pin 6 of the
12ax7 to the gain pot. Those are two of several audio mods that look
worthwhile. See www.amwindow.org/tech/htm/dx100/dx100.htm for more.

You should also check the 2uF cap that's across the 680 ohm cathode resistor
of the 12by7. The mods recommend replacing this with a 100uF 35V
electrolytic and a 1 uF 35V tantalum.

GL

Brian, W0DZ


-----Original Message-----
From: Heathkit Owners and Collectors List
[mailto:HEATH at LISTSERV.TEMPE.GOV]On Behalf Of Dale KG4KZG
Sent: Wednesday, October 30, 2002 1:14 PM
To: HEATH at LISTSERV.TEMPE.GOV
Subject: DX-100 audio problem


Hi fellow greenies, Have no audio on my DX-100 I pulled the mod. trans. to
check potting and it seemed ok. No shorts to ground and the resistances
seemed with in limits. Also, checked the driver trans, found no shorts. Last
night  plugged it back in and the 12BY7 driver and it got very blue and
plate
started glowing a dull red then got brighter red to where  turned it off and
subbed it with a ok but marginal  tube and it did the same thing. Removing
the 5V4 LV rect. of course kept it from turning red again. The audio failed
during a 20m am qso about 6 months ago, no crispy critters no blown fuses.
I presume something in the screen circuit or cathode bias is the problem.
There are no voltages listed in the manual for the tube pin out.
Voltage on the plate is 375-380V and on pin 2, one of the screens its 175V.
Any ideas?
Can reply off list if you wish 73 and TIA
Dale
kg4kzg

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