Puzzled and Stumped

[WD4TC]

  A question for the pros.. My homebrew transmitter has dev. a problem . I can tune up to full out and everything is Good on the 3-400s finals.. Idle current on the 572 B modulators is OK. I can put  abt 20% audio to it and things are OK. I can bump up the audio to abt 40% and it trips the PA plate supply breaker . .I have done  a bunch of test  and checked for bad /shorted tubes and caps ect.. Any Suggestions where to look Next..?? And thanks in Advance....Tim....WD4TC..

[steve_qix]

Hmm... Don't you hate those kinds of intermittents?

Anyway, on your circuit toplogy - common power supply or seperate?

If seperate, does the modulator power continue chugging along?

Do you notice any of the current meters pin for a moment?  Plate current?  Modulator current?

If you load the plate input of the final to equal the sum of the power input of the modulator at 50% modulation and the RF amp at normal power, does the breaker still trip?

Does tuning the final off resonance, so you're pulling a LOT of plate current, still trip the breaker?

Are the rectifiers solid state or tubes?

Oh well, sorry for all the questions.... just trying to understand the design to assist in the analysis :-)

Intermittents of this nature are, to say the least, annoying.  Burn out or work but don't be intermittent!

Regards,

Steve

[The Slab Bacon]

how close to the edge is the plate supply breaker?? If the plate supply breaker is right on the edge of tripping at resting carrier, the extra current on mod peaks may be enough to push it over the edge. (not enough headroom on the breaker).

try sticking an amprobe on the primary of the plate supply transformer and see what it is actually drawing, watch what it does when you modulate it. You may just have a weak breaker, or one that is just a tad too small.

It is somewhat difficult to pinpoint your problem without more information. Assuming that all else is ok and your toobz arent gassy, this would be a good starting point.

                                                                       The Slab Bacon

[WD4TC]

 Thanks Steve and Slab  For the reply. Steve first it has seperate supplies for the mod and finals . It trips the plate supply for the finals so quick I don't see any unusal readings until it trips, other than a rise in the mod current slowly as mod is increased.And you are 100% right I had rather see a puff of smoke or a spark or a Bang and you know where to go!. And thanks Slab Bacon,I just assumed the 3-400 in the final are OK, after 30+ yrs they might be getting tired. I will put 2 new ones in and see what happens.. I hate moving things all this started after moving to the New building abt 75 feet from the old Radio room. Thanks agn I am gone problem looking.. Tim....WD4TC...

[steve_qix]

Interesting... Is the plate current meter before or after the modulation transformer (and/or modulation reactor)?  That will make a big difference.  Often in tube rigs, the meter is AFTER the transformer/reactor.

If so, there may be some kind of odd arc occurring in one of the modulation components, OR a feed-through capacitor that's before the plate meter (have had this very thing happen to me!) or some other component ahead of the plate meter, but after the mod components.  Could even be the meter itself (have had THIS happen, too!).

It may be worth inserting a resistance in series with the primary of the plate transformer, to allow the problem to happen without kicking out the breaker.  Do you have a powerful electric heater?  This may do the job.  It will have some voltage drop under normal operation, but hopefully not too much.  Then, when the fault happens, the heater will absorb most of the energy, allowing you to debug the problem without the breaker.  If you have an extra current meter (or more than one), these could also be inserted in various points along the line, providing further indicators as to the location of the fault.

Hopefully, you have sufficient rectifiers, etc. to withstand whatever power your heater is.  Are you using solid state or mercury rectifiers (or Xenon - forgot about that!).

Regards,

Steve

[k4kyv]

Try temporarily inserting a 1/4" glass fuse of appropriate current rating in series with the HV line, between transformer and rectifier, rectifier and filter components (depending on amount of filter capacitance), power supply and rf  final, power supply and modulator, etc.  The fuse, not rated for high voltage, will blow up like a firecracker with the intermittent, but serve as a sure-fire indicator where the current surge is taking place.  Actually, to save on fuses, this should be done in reverse order from the above description, so that the power transformer-rectifier link is the last one.

I have isolated intermittent breakdowns numerous times this way.

My stuff rarely ever has a classic textbook malfunction.  It can be counted on to be nearly always intermittent and/or a totally wierd problem.

The one that takes the cake was the time I had an exciter unit that went into self-oscillation with the DC power completely shut off, with nothing on except the filaments.  I'll leave the details for another time,  for those who have not heard me describe it over the air.

[WD4TC]

Thanks Don for the tip. I am in the middle of taking the transmitter down a deck at a time and checking everything . I have Not found anything that is Sparked ,Burned, arked, been hot, or out of tolerence,Nothing, I am thinking Now I will find the problem In the wiring Harness of it buried deep ,.as eack deck will run OK on the bench. O well it could use a good cleaning and connection tighting anyway after the move..  Tim..

[steve_qix]

Hey, nice construction job!!  Beautiful  :-)

I wish I could build like that.. It's a work of art.

Keep us informed..

Regards,

Steve

[W2VW]

Ceramic chassis feedthroughs have a bad habit of zorching out only with the added Voltage under modulation. Check 'em if you have 'em.

[WD4TC]

 Well after a short break and back to it, The problem with the homebrew transmitter has been found. After  a hour or so of digging in the RF deck  a small but vital Ceramic feedthur was after all the problem. Fine under Carrier but when the voltage was up during modulation (Not to Good) ! Such a simple but pain in the A@% to find and replace. Thanks to ALL for the suggestions  and help in Solving the problem..Tim---WD4TC 

[k4kyv]

I recently had a similar problem.  I could load my Gates up to full carrier power on 160, and no problem.  But as soon as I applied modulation, it would kick the O/L  relay.  I could run it into the dummy load and modulate 120% with no problem.

Turns out that an insect had crawled between the plates of the L-network tuning cap in the network at the base of the tower.  I could detect the pungent aroma of burnt flesh as soon as I opened the antenna tuner enclosure.  I stuck a stiff wire between the plates and cleaned out the crud and now it works ok.

Another time when I had a similar problem, it was due to breakdown of the insulation inside a barrel coax connector at the same tuning network.

Before that, I once had a mouse that insisted in building a nest on top of one of the air variables and the straw it was dragging in kept falling between the plates.  Every time I would clean it out the thing would just build a new nest right back.  I tried a mouse trap but that didn't work.  Finally I crammed a piece of styrofoam over the cap in the space where it was building the nest and it never returned.

I would estimate that ever since I first started dabbling with radio as a little JN about 50 years ago, well over 90% of my malfunctions have always been intermittent in nature.  Of the non-intermittent ones, very few of those have ever been textbook problem-solutions.

[WBear2GCR]

Fyi, to take a really sharp ultra closeup picture, take a magnifying glass, put it flush up to the lens of ur digital camera, and watch the LCD while moving the glass+camera together back and forth until it is sharp, then snap!

 

Works ok, fine!

          _-_-WBear2GCR

[wa2zdy]

I'll second Steve's comment; that rig is a great looking piece of work.  Glad you found the fault.  Good luck with it.

[WA3VJB]

Pretty transmitter !
Makes it nice to work on, the few times ever likely.
Glad you found it, but I would check your others too, and perhaps make some expansion pads, washers really, between the ceramic and the chassis. I cut up an old bicycle inner tube.
--background --
I have similar ceramic feed throughs in the T-368.
There are cork washers that allow expansion and contraction as the chassis heats up.
When they get compressed for many years there's no wiggle room anymore.
I've had ceramics crack and allow a zorch INSIDE the column between threaded shaft and the chassis, not visible from the outside. Nice mystery, eh ?

As long as you're in there ...

[WD4TC]

 I think I came up with a good ideal to completely do away with that problem. I took a 1 inch square hole stamp and punched ALL High Voltage feed thur and put in 1/8 thick plexiglass and drilled that for the feed thur and mounted it and there should never be anymore problem with shorting or arking..

[KL7OF]

Nice fix on the feed thrus.  I have had trouble with them several times..The expansion pad/washers that Paul mentioned are absolutley necessary.. Also never use a bolt or screw with a head on a feed thru insulator.. Use threaded rod or cut the head off the screw and double nut them... The first nut secures the feed thru(not too tight) and the outer nut holds the wire terminal on..  Very nice rig... Steve