Transmitter troubleshooting

[n9iz]

Hello.  I've checked this website for info in years past but just now got around to actually joining.  Hoping someone here can help me out.  I'm something of a novice to the whole boatanchor radio modes.  I've owned a TS-830 and FTdx-570 in the past so I'm comfortable around high voltage and tubes.  I got a really good deal on a Heathkit DX-60A transmitter.  It's very clean and appears to be well-built from the soldering and routing of the wires.  I replaced a bad resistor I found while cleaning it up.  I brought the voltage up in stages by placing a light bulb in series with the hot wire (I don't have a variac) start at about 40VAC and up to 105VAC before going with full line voltage.  We see around 122VAC here usually.  I only have one crystal (7184kc) and no VFO so I plugged it in and proceeded to set the grid drive per instruction to 2.5mA.  Switched the meter to plate current and started with the final tuning.  I reach my max way before I even get close to 150mA as per the instructions.  I'm somewhere around 70mA or so.  I do see power out of 6W into the dummy load measured with my WaveNode meter.  It's handy and shows PEP and avg. at the same time on my computer screen.

Found that the I'm only getting 252VDC to the top of the 6146 tube.  Seems like I should get at least 600VDC from my experience with the TS-830 (it had 6146 finals).  Starting with the transformer here are my measurements:
1.  272VAC at the transformer
2.  272VAC through the switch
3.  272VAC up to the diodes
4.  737VDC after the diodes
5.  737VDC from C41 to ground
6.  253VDC from C39 to ground

This is where I figure I have my problem since I already replace R35--the 270 ohm resistor mentioned above.  The original was cracked and definitely no good so I suppose it could've taken out the capacitor?  Only other thing I've measured is the bias of -157VDC.  Looking for some suggestions to get this radio transmitting with some decent power.  I figuring the caps need replaced from my limited knowledge.  Hope I'm on the right track.  Maybe I can find some parts or a decent VFO at Dayton in a couple weeks.  Thanks for your suggestions.

Jeff

[KA2DZT]

Check to see if someone has not set up the rig to run at low power output to drive a linear amp.  Running the final off the lower B+ circuit (usually around 300V) instead of the the 600V supply line.


Welcome to the AM Forum,

Fred

[KC2ZFA]

the dx-60 uses a voltage doubler to get its HV...maybe there's trouble there,
or it was never built according to the schematic.

[n9iz]

Okay, I'll double check the schematic again.  I have a full size schematic that I used from last night to go through the power supply circuit.  I don't see anything that would indicate a mod for lower voltage.  While taking the measurements the voltage definitely goes down from 700+V down to 250+V.  Maybe I'll see something I missed earlier.

Jeff

[K1JJ]

Here's a schematic of the DX-60B for everyone to refer to. Hopefully the "A" is similar.

http://www.kb2ljj.com/manuals/37-Heath%20Kit/DX-60%20Schematic.pdf


Jeff,  if you have a capacitor checker, verify that all the electrolytics in the doubler are close to correct values.  Measure all the associated resistors too.

There are some great mods to make that rig hi-fi, put out by WA1QIX -  once you get it running right.


73,

Tom, K1JJ

[n9iz]

Okay new info.  Looks like it's time to make an order to Mouser.  Thanks for posting the schematic.  Yes, it's the same for the PS.  I started unsoldering one leg of the resistors to take some measurements.  The 100K resistors are good.  I do know the caps charge and discharge when I take measurements withe the ohm meter as that was a check for initial tuneup in the manual.  I haven't checked them for value, though.  They may be okay?  I found that the 100R 7W wirewound resistor is open.  That's definitely a problem.  The new 270R 2W resistor I just installed is now up to 325R.  That's way out of spec for a 5% resistor.  My spare I bought is actually 294R which is within 10% and is technically out of spec per the schematic.  I don't know how critical this value really is.  I suspect that the open wirewound caused too much current through the 270R 2W?  The original was blown which is why I replaced it.  Now it's value has gone up considerably since I installed it.  I've installed original carbon type resistors the first time because I wanted it to look original.  Now I'm wondering if I should replace with a newer type resistor of same value and precision that would be more hearty?  Maybe someone can comment on a better type of resistor to use for the 270R.  I'll use a wirewound for the 100R 7W as was the original.  Thanks for the continuing comments.

Jeff

[K1JJ]

From the symptoms and schematic, I would say that R34 and R35 need to be replaced with fat power resistors to handle several times the calculated load.  Power resistors take a lot of abuse and should be rated overkill.

Assuming the caps are of correct value and not leaky, you should be OK from there.  Order the power resistors from Mouser at X2 the wattage as speced by the schematic to be sure.  That's probably why they went in the first place.


R36 and R37 are bleeders and will not show the symptoms you saw. Make sure they are working for safety, however.



T