The AM Forum

I think for some reason I'm just not getting any RF drive when the mode switch is in the "phone" position.

At the same swap that I got the 4-1000 amp, I also got a funky old, orange-translucent plastic Pomona Electronics 15kv high voltage probe. (I had fun this morning jamming it under the anode cap of a computer monitor that got scrapped... way over 15kv!)

Anyway, I put this probe on one of the 6146 plate caps, and its still getting HV (through my mod tranny) in the phone position... But I'm not getting any rf out with the plate switch up, and the mode switch to Phone.... thats on my meter connected to the output, and from the speaker of a nearby ricebox.

What did I screw up?

Thanks,
Jason kf6pqt

Perhaps the drive pot crapped out?

http://www.amwindow.org/tech/htm/drivepot.htm (http://www.amwindow.org/tech/htm/drivepot.htm)

Although since we have the empirical evidence of your DX-100 operating on the air before your rebuild of the mod section, I would first suspect something amiss in any of the circuits you laid hands to.

Eagerly awaiting DX-100 veterans to chime in here.

Mee no expert on DX-100s, never owned one.

But, I'd take my scope, and DVM then first check for B+ being present on the RF section's plates. Assuming B+, then take the scope and check for the VFO operating, then follow the RF through the sequential stages and see where it stops.

Likely either the function switch or that drive pot being high on the list.

You're getting RF in CW position??
If so, then there is no circuit defect there.

Probably the fault will show up after this, if not, then the fault is elsewhere!  :D

          _-_-bear

What is your grid current meter showing in the phone position?  IF you are getting sufficient grid drive then check your final screen voltage also.

Rodger WQ9E

Rodger said:
I'm with Rodger on this one. Also check that 'goose tube' as Tim, WA1HLR likes to refer to the 6AQ5.

I am with Mike on the 6AQ5 and if you find that your screen voltage is low/MIA then the clamper circuit is a prime suspect.  In my Ranger 1 the similar circuit was responsible for some really exceptional hum on the carrier due to heater/cathode leakage in the 6AQ5.

73, Rodger WQ9E

I had thought about the clamp tube... is this behavior consistent tho, working fine on cw, but not phone?

Hi Jason,

Before we go after the clamp tube, what are the readings for driver plate and final grid current when you are in AM mode with the transmitter active?  Compare these to what you see in CW mode under transmit conditions.  Without excitation to the final there will be no output and that is when the clamp tube is operative to "clamp" the screen voltage and prevent excessive final plate current when excitation is lost since part of the total operating bias for the final in class "C" mode is generated by the grid drive.  The presence or absence of proper grid drive will help point you in the right direction for troubleshooting.

73, Rodger WQ9E

Jason, couple of things.  Check the mode switch.  It puts a ground on the cathode of the VFO for keying when switched to Phone.  The other contact goes to the plate switch and when you put the plate switch up, it grounds the VFO through the mode switch.

I didn't change anything in that area.  I did however, have to re-locate the filament lead of the 5763.... Are any filaments switched by the mode switch? Don't have the schematic in front of me at the moment, but it didnt strike me that any were.

Ok, printed a new schematic and went out and had a look.

Using a crystal, not the vfo...

Just like normal tuneup on cw... set the grid current to 5 ma. Flip the plate switch, tune and dip at what my wattmeter says is 100 watts (it can do more, but I like to run it at 100) Comes out to 175ma @100w.

Now, going back to the grid current... I does go away (to zero) when I move the mode switch to phone, irregardless of whether or not I've got the plate voltage turned on or off.

With the plate voltage on, turning it to phone causes the plate current to drop to about 80ma and then move up top about 150 ma.

Ok, the 6au6 cathode wiring checks out good. 22k ohm to ground in all positions. However thats the "blue" wire.  ;) Also connects to the mod tube cathodes, and position "G" on the meter switch, modulation.

Thats not right, is it?

I tried disconnecting this wire from the mod cathodes, at point "G" on the schematic, on the right leg of the .1 ohm resistor.

No change.

I am not familiar with the DX-100; I have a Viking II.

Trace the VFO signal in Phone mode. First, is the VFO running at all?  Then, trace the drive through the driver stages in Phone mode, comparing against CW mode at each point.  Either the VFO is not oscillating, or one of the driver stages is not working, or the final PA is somehow being held back in Phone mode.  With no grid drive, the "clamp" tube on the final screens should pull the screen voltage down to 150V or less.  (It would try to go to 600V otherwise, hence the clamp tube is there in the first place.)

Plate switch off, pin 1 to ground is zero ohms... Plate switch on, zero ohms in cw, open in fone.

I greatly appreciate the help Mack!

-Jason

Er... I got those results in the xtal position...???

So, I should be able to put a clip-lead from pin 1 (cathode) of the 12BY7 to ground, and expect hear my osc running (and heck, get full rf out with the Plate sw on) with the mode switch set to Fone, correct?

Thanks again,
Jason

Just FWIW, I have repaired a lot of gear over the years and kit built stuff quite often turns out to be some of the worst nightmares.

In days gone by many tried to beat the system and save some money by buying a piece of gear as a kit and assembling it themselves to save the money. Many who did this found themselves in way over their heads and floundering. Many were not up to the skill level needed to do the assembly and did some seriously piss-poor jobs.

Cold (or no) solder joints, poor lead dress, parts improperly installed, stuff miswired, crappy wiring, part damage, and poor construction practices are some of the many maladies one can find repairing kit built equipment.

My Valiant was so bad that It could have NEVER worked at all. I completely gutted the chassis and started from scratch. I now often find myself doing the same thing to just about every piece of kit gear I get. It makes for a much neater and definately more reliable job. Take poor construction and add years of additional JS's and you can quite often end up with a nightmare. Quite often it actually takes less time to gut and reassemble than to "become one" with it the way that it currently exists. Also then you know what you have.

When looking at what may be the next "jewel" for your collection laying there on a hamfest tailgaters table, look for some of the tell tale signs of factory construction before making your final offer. Look for signs like "wired and tested" stamps on the chassis, riveted in tube sockets, neat wiring and other tell tale signs that it was factiry built. If not make your offer figuring that you WILL be doing a gut and reassemble job on it.

If you dont feel that you are up to that kind of challenge, leave it there for someone who is (or thinks they are) up for that kind of job. If I buy something from a hamfest vendor I will always make an offer figuring that it doesnt work unless the vendor can demonstrate otherwise.
                                                                             The Slab Bacon

I have never had a Heath that didn't have problems because of construction practices.  However, that said, the circuits are basic and simple in any of the DX series radios.  One can find most of the stage diagrams in old handbooks. 

If someone purchases one of these, it is excellent training for troubleshooting.  Fix a few and read as you go and you will develop a sixth sense about trouble when you have it.

The big thing is don't give up and take your time.

Ok, I've performed the clip lead test, and the transmitter works... Only the speech amp sounds like cat butt, but I've got a plan for that.

Will track down the open connection, and try out an idea I have for an interim speech amp while the stocker gets ripped out and re-reworked.

Thanks again for all the help and advice!

Congrats! You're a DX-100 VETERAN now!

Oh yeah, and when improvising in the speech amp, make sure there's no place RF can sneak into the audio signal path below the modulator.  I bet the ole rig is gonna sound GREAT.

It'll probably still be a while, Joe, before the monster is dragged back inside and put back on the air!

My amp project is at the stage where I'm still setting up the table saw and the sheet-metal brake.

Ok, we're not out of the woods yet...

My task today was to find out whats keeping the pin 1 of the 12by7 oscillator from getting grounded when  the plate switch is turned on and the mode switch is set to FONE.  Recall I clip-lead'ed it to ground and we had good RF.

Guess what, the thing DOES go to ground on its own. I'm flummoxed.  Could something be "pulling it up" elsewhere?  These wires disappear into the VFO box, is it possible to get inside that box without tearing the whole transmitter down?

Thanks,
Jason kf6pqt

Jason that wire that goes into the VFO will key it on pin 7 of the 6AU6.  It is routed from pin 7 to the Crystal/VFO switch.  Do you have that switch in the VFO position?  Is the knob in the wrong position?  It has 5 positions, 4 for crystals and one for VFO. 

When in the VFO position, the cathode of the VFO tube is in parallel with the 12BY7 cathode.  So if the Crystal switch is not in the right position, the 12BY7 could be going to ground, but not the VFO. 

Although you say when you cliplead pin 1 of the 12BY7 it works.  You must have an open wire either at the Crystal/VFO switch or over to the mode switch, or over to the Plate switch.

This should be easy to check with an ohmmeter.  Put ohmmeter from pin 7 of the VFO tube (from the top of the tube socket, you need not get into the box) to ground with the rig turned off and the AC cord pulled.  The put the Plate Switch in the on position and you should see very low resistance.  The only thing in the circuit is the RF choke in the cathode lead of the 6AU6.  If you don't see the meter read very low, then you have an open, which is causing the trouble for you.

Ok, I'll check this out... Recall that I have the dx-100B, so i only have a DPST switch and one xtal socket.

Kind of a pain only having the plain-dx100 schematic, but I've made it so far this way. ;)

Thanks,
Jason kf6pqt

This is only good when the switch is in the vfo position. OPen when in the xtal position, but that is correct, correct?

Lets review here:

It ain't:

The plate switch
mode switch
xtal switch.

What else could be "pulling up" the osc cathodes?

Jason, if the cathodes of the osc and buffer tube go to ground with the mode switch in phone and the VFO/Xtal switch in VFO and the plate switch on, then there are only few thing that could cause no output.

1. The oscillator is not working, no plate voltage or screen voltage or the filaments are not on.  Or one/both of the510 pf. caps in the divider circuit are bad.
Do you have regulated voltage on the VFO when the VFO is selected?

2.  The clamp tube circuit is not right.

3.  Are you sure the bandswitch is right for the band you are selecting. 

4.  The 5763 stage is giving trouble.

There just isn't a lot to go wrong.  I would suggest you use a scope and follow the signal from the grid of the 12BY7 to the grids of the 6146s.  If you don't have signal on the grid of the 12BY7 and the clamp adjustment is correct, it may be time to go into the VFO box.

Ok, the osc IS working in the fone position... IF I ground its cathode. works fine and normal in CW.

Could something be "pulling up" this line????

Ok, now trace that wire and be sure it attaches to the same point the cathode wire on the 12BY7 is.  You say when you put a clip lead on the 12BY7, everything works.  That lead us to say there is a wiring mistake on the CW-Phone switch terminal that goes to the plate switch.

Ok..... (deep breath)

A previous owner, NOT the guy I bought it from, had replaced the 1625 modulators with 6146's, and did a bunch of other hokey mods to it. I bought it. It worked. It sounded kind of crappy, and was getting worse, so I decided to rip out the modulator section and rebuild it as stock. Which I think I did.

Now, I get no RF (no oscillator, nuthin)  with the mode selector in Fone position with the plate switch on.

Its normal to get no oscillator at all in fone position with plate switch off.

It IS normal to get the oscillator running, and final grid drive, in CW position with plate switch off.

IT DOES THIS.

And of course, it seems to work just fine in CW mode, plate switch on.


I'm using the xtal, not the vfo, as the vfo dial has come loose and is waaaaaay of frequency.  I'm pretty certain back in early July when I was put this thing back together, I had the same problem using the vfo. I'll check this.


When I put a cliplead from the 12by7 cathode to ground, I get the oscillator, regardless of the mode switch.

The mode switch, the xtal switch, and the plate switch all seem to be doing what they are supposed to.

Now... where to go from here?  I want this stupid thing off of my work bench, its keeping me from starting some more interesting projects!


This wiring has not been touched. I only worked on the modulator and speech amp sections. The thing USED TO work, GFH (among others) heard it.

clarification... the hokey mods were in the modulator section only... rest of the tx seems bone stock.

FIXED.

All roads returned to the mode switch. But it seems to be working!!! Aaaargh!

So, I went and bought the smallest tube of Deoxit I could find, and just now swabbed some on the mode switch (wafer rotary) contacts, and cleaned 'em up best I could.

WORKING!!!!!  ;D

Thanks everyone for your help!

Now, on to fix the audio...

-Jason kf6pqt

Well good.  My only issue with Deoxit is that it will eventually wear off.  If you can, turn the contacts so you can get to the wiper and use a pencil eraser to clean that part of the switch.  Usually that is where the problem is.   

Use a magnifier and be sure the contacts are in line where they should be.  Sometimes the little rivet will loosen over time.  If that should be the case, you will have to remove the contacts with a Dremmel then put the contacts back with a 2-56 screw and nut.  Then solder the nut to the screw once everything is aligned.

Another thing is those contacts can be reformed by heating them with a soldering gun or iron.  They will return the almost normal shape with heat application.

Hope everything keeps working