DX-60B audio circuit levels

[W3JKO]

I meant to say C9 not C39 as being good.  Is C9 a known weak spot?  The plate spacing seems wide enough. 

I could change R35 as well. I probably have that value in my stash.

I did go so far as to lift the ground side of the filament winding and floated all filaments to check if that was the source but it proved not to be. 

Having installed a 3 prong power cord I can say the hum is much stronger with the chassis grounded than if I lift the ground wire. That's what took me down the path of lifting the filaments off ground.

[DMOD]

I meant to say C9 not C39 as being good.  Is C9 a known weak spot?  The plate spacing seems wide enough.  

I could change R35 as well. I probably have that value in my stash.

C9 is not known to be a weak spot if untouched. What happens occasionally is that one bumps C9 with a tool, such as a soldering iron, bending the plates, or accidently dropping a small glob of solder across one or more set of plates, or accidently touching the plates with a screwdriver while powered up, causing a short to ground and increasing current through L2 and R9.

In any case, it appears a short occurred in or around the V2 stage taking out various V2 and PS components. The added .005 1kV in series with C9 will prevent the failure of L2 and R9

AC0OB

[W3JKO]

I had the diodes in my batch of parts.   I replaced all of them with no improvement.  Also increased the cap values in the HV section and the -130V section with no noticeable change in audible hum and only a slight decrease in ripple on the scope. This isn't a good solution long term due to arcing in the line side contacts of the function switch when powering the rig off.  The DC at C33 is clean. 

Since it occurs in CW mode also the modulator is out of circuit.  I went so far as to pull the 12AX7 and 6DE7 to confirm this also.

So far all components test ok around the V2 circuit. Swapping V1 and V2 doesnt change anything.

Not many things left to eliminate except the transformer or trying another 6146.

I've tackled hum issues on many other tube rigs but this one has me stumped.   

[DMOD]

The DC at C33 is clean. 

Not many things left to eliminate except the transformer or trying another 6146.

I've tackled hum issues on many other tube rigs but this one has me stumped.   

Ok, if the voltage at R21-C33 is clean, and you pulled the modulator tube, then the hum/ripple is not coming from the speech amp/modulator.

If you have a portable radio, listen to your signal on it and see if you still hear hum in it.

Q1: How much max power at 150 mA plate current do you get with 3 mA of grid current when properly tuned into a dummy load?

If you still have hum, then something is loading the power supply excessively or the transformer T1 has some shorted turns, maybe even to ground.

Q3: What are the measured values of R36 and R37? Are there any solder globs reaching down to ground from those lugs?

AC0OB

[W3JKO]

Ok, if the voltage at R21-C33 is clean, and you pulled the modulator tube, then the hum/ripple is not coming from the speech amp/modulator.

If you have a portable radio, listen to your signal on it and see if you still hear hum in it.

Q1: How much max power at 150 mA plate current do you get with 3 mA of grid current when properly tuned into a dummy load?

If you still have hum, then something is loading the power supply excessively or the transformer T1 has some shorted turns, maybe even to ground.

Q3: What are the measured values of R36 and R37? Are there any solder globs reaching down to ground from those lugs?

AC0OB

My hum issue may be self-inflicted.  I do not hear what I consider objectionable hum on a portable receiver at full volume or not nearly as bad as my base station rig. If I touch the chassis or coax or move the portable around I can pick up hum.  The only ground the DX60 has is through the line cord.  Im thinking its the lack of a hard ground which I cant do on the workbench so maybe I need to button it up and get it over on the radio bench and try it.

As far as power out at 150mA and 3mA grid.  Wattmeter shows 10W AM, about 40W CW into the dummy load with my cheap MFJ wattmeter. 

R36 and R37 are within the tolerance of 100k.

[DMOD]

"My hum issue may be self-inflicted.  I do not hear what I consider objectionable hum on a portable receiver at full volume or not nearly as bad as my base station rig. If I touch the chassis or coax or move the portable around I can pick up hum.  The only ground the DX60 has is through the line cord.  Im thinking its the lack of a hard ground which I cant do on the workbench so maybe I need to button it up and get it over on the radio bench and try it.

As far as power out at 150mA and 3mA grid.  Wattmeter shows 10W AM, about 40W CW into the dummy load with my cheap MFJ wattmeter. 

R36 and R37 are within the tolerance of 100k."

After I have upgraded or modified circuits, I put in say, an 80 or 40m crystal or VFO signal and set the scope to a 1mS sweep rate.

I sample the RF across the dummy load or the coax output and tune it up into the dummy load.

I then Keydown and look at the waveform. The waveform should be straight across the top and bottom. If there is any real power supply ripple on the carrier, it will show up as a wavy line on the top and bottom with an approx. 8.3 mS period.

The DX-60 engineers designed the Full-Wave Voltage Doubler power supply with an "economic" mindset and minimal values of capacitive filtering. With increased power supply capacitance, I would not expect any such ripple on the carrier.

One other item: A ten ohm 10W resistor (R**) in series between the junction of D1 and D2 and the transformer secondary lead limits turn-on surge current.

AC0OB