While we are on the subject of DX-40s, I have one with a problem
that's driving me nuts. In AM mode, just sitting at idle, the plate
current pulses up about every second or so. The scope shows a pulse
on the plate of the second 12AX7 audio stage. It's acting like some
sort of relaxation oscillator. I've rebuilt the audio section to no
avail. Anyone have an idea what the heck is going on?

73, Roger





Remember the USS Liberty (AGTR-5)
http://ussliberty.org/

Roger D Johnson wrote:
While we are on the subject of DX-40s, I have one with a problem
that's driving me nuts. In AM mode, just sitting at idle, the plate
current pulses up about every second or so. The scope shows a pulse
on the plate of the second 12AX7 audio stage. It's acting like some
sort of relaxation oscillator. I've rebuilt the audio section to no
avail. Anyone have an idea what the heck is going on?

Have you tried replacing the 12AX7?

-Chuck

Typically, this sort of thing is a form of motorboating. There are
a couple of ways it comes about. One is through power supply sagging
as current in some stage starts increasing. The supply sags until
a certain point is reached where the current in the affected stage
decreases, and the cycle continues. It is very low frequency because
the power supply filters are the resonant element.

Another way it can happen is through grid leakage. Many tube stages
of the low power sort have a resistor from the grid to ground that
is supposed to keep the grid at a reasonable negative bias value. If
that resistor is open circuit, or if the tube develops excessive
gas, the grid, the leakage, and the coupling capacitor of the
previous stage can get a relaxation oscillation thing going. A leaky
coupling capacitor, such as C27, or C25 can do the same thing. Look at
pin 7 of V4. Try paralleling a 1M resistor on that stage and see if
it changes the rate of the motorboating. If it doesn't, try the grids
on the 12AX7... and, of course, replace C27 and C25...

-Chuck

Chuck Harris wrote:
Roger D Johnson wrote:
While we are on the subject of DX-40s, I have one with a problem
that's driving me nuts. In AM mode, just sitting at idle, the plate
current pulses up about every second or so. The scope shows a pulse
on the plate of the second 12AX7 audio stage. It's acting like some
sort of relaxation oscillator. I've rebuilt the audio section to no
avail. Anyone have an idea what the heck is going on?

Have you tried replacing the 12AX7?

-Chuck

Chuck Harris wrote:
Roger D Johnson wrote:
While we are on the subject of DX-40s, I have one with a problem
that's driving me nuts. In AM mode, just sitting at idle, the plate
current pulses up about every second or so. The scope shows a pulse
on the plate of the second 12AX7 audio stage. It's acting like some
sort of relaxation oscillator. I've rebuilt the audio section to no
avail. Anyone have an idea what the heck is going on?

Have you tried replacing the 12AX7?

-Chuck

Yes....several times! I don't have a spare 6DE7 however.

Roger

--


Remember the USS Liberty (AGTR-5)
http://ussliberty.org/

In article ,
Roger D Johnson wrote:
While we are on the subject of DX-40s, I have one with a problem
that's driving me nuts. In AM mode, just sitting at idle, the plate
current pulses up about every second or so. The scope shows a pulse
on the plate of the second 12AX7 audio stage. It's acting like some
sort of relaxation oscillator. I've rebuilt the audio section to no
avail. Anyone have an idea what the heck is going on?

Bad caps.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

If you check the screen voltage of the 6146 I expect you will see it pulsing
from 70 +/-10 volts to over 100 volts, possibly to 160 volts. Seventy volts is
normal screen bias in the AM mode. With modulation it follows voice peaks to
~150 volts.

The time constant of the 6DE7 to 6146 RC circuit is about 0.025 seconds. So, I
would exclude the RC between the 6DE7 and the 6146 screen.

I would check pin 6 of the 6DE7. It should be stable in the AM mode. The input
circuit to the 6DE7 has a 0.1 second RC time constant. If the input is stable
[pin 7] and the output is pulsing the replace the 6DE7. If the input [pin 7] is
pulsing then check the RC from the 12AX7.

/s/ DD

PS: let us know what you find out.
+ + +

Roger D Johnson wrote:
While we are on the subject of DX-40s, I have one with a problem
that's driving me nuts. In AM mode, just sitting at idle, the plate
current pulses up about every second or so. The scope shows a pulse
on the plate of the second 12AX7 audio stage. It's acting like some
sort of relaxation oscillator. I've rebuilt the audio section to no
avail. Anyone have an idea what the heck is going on?

73, Roger





Remember the USS Liberty (AGTR-5)
http://ussliberty.org/

Scott Dorsey wrote:

In article ,
Roger D Johnson wrote:

While we are on the subject of DX-40s, I have one with a problem
that's driving me nuts. In AM mode, just sitting at idle, the plate
current pulses up about every second or so. The scope shows a pulse
on the plate of the second 12AX7 audio stage. It's acting like some
sort of relaxation oscillator. I've rebuilt the audio section to no
avail. Anyone have an idea what the heck is going on?


Bad caps.
--scott

I assumed the caps had been replaced. But, replacing them is a GREAT idea for
several reasons if you have not yet done so.

There are four electrolytics. The two 40 uF in the high voltage supply. Mine
warmed up immediately on power application and started to leak. They were
replaced with aluminum electrolytics for about $9.

There is a 20 uF 450 vdc cap on the audio, 12AX7, plate line with a one second
time constant. Check that capacitor. The time constant matches your symptons.
That makes it a good place to start.

The final electrolytic is a 2 uF connected between the 6DE7 output and the 6146
screen. I replaced mine on principle ... I had no problems in this part of the
circuit.

On Thu, 19 Apr 2007, Roger D Johnson wrote:

Chuck Harris wrote:
Roger D Johnson wrote:
While we are on the subject of DX-40s, I have one with a problem
that's driving me nuts. In AM mode, just sitting at idle, the plate
current pulses up about every second or so. The scope shows a pulse
on the plate of the second 12AX7 audio stage. It's acting like some
sort of relaxation oscillator. I've rebuilt the audio section to no
avail. Anyone have an idea what the heck is going on?

Have you tried replacing the 12AX7?

-Chuck

Yes....several times! I don't have a spare 6DE7 however.

You ought to get one. Tubes can sometimes have weird "heat-up-short"
cycles and if everything else is _new_, look at what you have yet to
not yet replaced.

You might try "bridging" in -- with aligator clips--external electrolytics
at those locations in the circuit where an on-board electrolytic might be
going through some kind of "charge-up-discharge" cycle to cause the spike.
One of the guys who answered this had a good idea about that.

Did your rebuild the audio section with all new capacitors? Sometimes even
new components can include a run of defects in a given lot.

What else is the second plate of the 12ax7 connected to?

Are the working voltages of the new capacitors you put in all higher than
any voltage you expect on those circuits?

Roger

--


Remember the USS Liberty (AGTR-5)
http://ussliberty.org/

"Dave" wrote in message
. ..
I have just re-capped the power supply capacitors in a DX-40 with 40uf 450
VDC Aluminum Electrolytics from NTE Electronics in New Jersey.

I have noticeable hum on the carrier. Measuring the ripple voltage with a
10 Megohm ac coupled meter shows 2.3+ volts [average/rms ...
non-sinusoidal] on the 600 vdc line and 1.2 volts [average/rms ...
non-sinusoidal] on the 300 vdc line.

It appears the RS of the capacitors is way too high.

What manufacturer or brand are others using with success? Does MOUSER
carry suitable replacements? If so, what brand/style?

Other sources?

TIA

DD, W1MCE


FIRST,

Mike Bryce, WB8VGE [The Heathkit Shop]
http://www.theheathkitshop.com/
and Jeff Weinberg, W8CQ [Harbach Electronics]
http://www.harbachelectronics.com/

have been offering replacement capacitors and boards for Heathkit products.
An ideal solution for the inexperienced newcomer -- who does not wish to
guess about replacements or repair parts.

Here are the "Major" distributors for new electronic components.

DigiKey
http://www.digikey.com

Mouser
http://www.mouser.com/

Allied
http://www.alliedelec.com

Newark
http://www.newark.com/

MCM Electronics (a favorite for radio-TV shops)
http://www.mcmelectronics.com/

all have ample selections of Vishay/Sprague; CDE/Mallory and Panasonic
models.

Start with 2 of the major mfg. for specifications and selection.

CDE - base in South Carolina (USA)
http://www.cde.com/

CDE Selection Guide
http://www.cde.com/catalogs/aesg.pdf

CDE Application Guide
http://www.cde.com/catalogs/AEappGUIDE.pdf

and Panasonic (Japanese) - available from DigiKey
**Panasonic has been slowly discontinuing some of their screw terminal
models -- favoring the "snap in models"
http://www.panasonic.com/industrial/...p_lcanelec.htm

Panasonic Computer Grade
http://dkc3.digikey.com/PDF/T071/1283.pdf

Panasonic Snap-In
http://dkc3.digikey.com/PDF/T071/1276-1282.pdf