dx-100 modulator grid voltage and cur-- What current?? ZZZT...

[kf6pqt]

OK, here's the setup. Dx-100, 1625 mod tubes, with the stock 1k grid resistors, (they're good) stock driver transformer... and thats it. Speech amp is removed still.

So, finally doing what I was instructed to do eons ago, I have measured the grid bias voltage, at the center tap of the driver transformer.

Resting, no modulation, as none is possible at the moment:
standby, phone:  -39.5v
standby, cw:       -39.5v

transmitting, phone  -37.2v
transmitting, cw      -37.4v

Then, I unsoldered the center tap, and put a meter in series... make that two meters, my usual $3 halbol flate DVM, and a tankerly Triplet model 630-NA... and I cannot detect any current flowing none.



Lemme know what you think of this...



Here's the 1625 plate voltage:


#1                                   #2  (at each plate cap, to ground)

standby
cw   2.06v                         2.01v
ph    2.96v                         2.94v

Transmitting
cw    795v                         792v
ph     789v                         789v


Then, as I re-read my instructions, I lastly checked the screen voltage, between the screens to ground... and it got exciting.


Screen voltage:

Standby:
cw: nil   phone 1 volt

transmitting
cw 4.17v   Phone:   went up to 240, and then started dropping slowly. At about 236v, something arced, and I shut off the plate voltage.

Looking at it do this again ( still didn't see where the arc came from) I noticed that my aft 1625 has the cool blue glow, and the plate is starting to heat up good. Short?

I detected a tiny bit of blue on the front 1625, and I'm pretty sure its not a reflection from the back one. Is it still good, or too gassy as well?

Hmm, will be digging out the box of 1625's.

Here's the instructions I was going by:

WA1GFZ:
Measure the bias voltage at the CT of the driver transformer. It should be around -36 volts and the resting current around 50 ma. report your reading and get plenty of advice. Also check the screen voltage and report. remove the driver tube before you read the numbers to make sure nothing is driving the modulators.

Thanks and 73,
Jason kf6pqt

[KI4YAN]

The resting current is measured from the cathode of the 1625 to ground, and should be 50mA. The bias shouldn't be drawing much, if any, current.

That screen voltage seems strange. I'd check the screen supply, and get a pair of known good 1625 to test with, those you have sound a bit questionable.

[W1EUJ]

Had a filament-to-plate short in a rectifier for a S-100 reciever. Very pretty blue glow, melted away 1/4 the plate! I was amazed that the transformer didn't go. Liked it so much I made an 866 nightlight.

A couple simple things here:

If you don't already have one, get yourself a good and simple tube tester. Check all of your tubes. Something like a Heathkit IT-17 or IT-21. Doesn't have to have transconductance measurement, just need a check on emission and shorts/leakage. It will be money very well spent, and those Heathkits can test most anything (even oddballs, if you make an adaptor - they tell you how to calculate the settings from the tube datasheet). All you need to do with them is replace a couple caps if they are past a certain age.

Also, I have been fooled with surplus DVMs, where somebody has blown the current fuse for me (a.k.a myself). Make sure it actually measures current right, easy to miss.

Just a couple other things to check.

BTW, congrats on getting back on the DX-100 horse. I eagerly await the news that another one is back in good health.

David Goncalves
W1EUJ

[WBear2GCR]

blue glow doesn't mean the tube is good or bad.

check the modulator current across the cathode resistor - or if it lacks one, put one there for checking.

the screen voltage is not out of the question - check the online specs for the tube in AB2 P-P audio service - the screen current has to not exceed some maximum value, so there is usually a series resistor at the screen to limit the current to a reasonable figure. I don't have any tube books handy at the moment, but the specs are online.

I don't recall how the DX-100 develops the screen voltage - a divider from the B+??
check the resistor values and the current through the divider, that will make certain that the resistors are ok.

             _-_-bear

[W1EUJ]

TUBEDATA.ORG is the master source!

http://www.mif.pg.gda.pl/homepages/frank/sheets/049/1/1625.pdf

http://www.mif.pg.gda.pl/homepages/frank/sheets/141/g/GL807.pdf

The first is the datasheet for the 1625, the second is for the 807, (a 1625 with different filament voltage).

[w4bfs]

blue glow outside the plate MAY indicate parasitic osc....sniff CAREFULLY with NE2 on insulated stick.....blue glow inside plate structure MAY indicate gas....blue glow with extra heating not good ...  hope this helps ...73   John

[N3DRB The Derb]

have you replaced the main filter caps? DX-100's are almost always in some stage of bad as found. the slowly dropping screen voltage is where it's at. When a voltage starts slowly going down ( or up ) it usually means cap or resistor trouble. Please note that the DX 100 caps go back to B- not chassis ground!

[kf6pqt]

Thanks everybody. Yep, I have a couple of tube testers.

Have the ne-2 setup, will check for oscillations, that thought had occurred to me.

I'll see about changing the filter caps... I recall declaring them good, at one point, but I could have been mistaken.

Will definitely investigate the screen voltage supply next.

73,
Jason kf6pqt

[W1EUJ]

Any news?