DX-60B No Audio Modulation

[ko4nrbs]

My voltage readings are highlighted in red.  
All take in the AM mode.
Both V4 and V5 were checked with my spare tubes.  No change.

Rig has a PTT mod in it.  Works well on CW and tunes up normally.  Traced a signal through V4 and V4 with no problem.  Mic keys the rig but no audio.

V4 Cathode Pin 9(0vdc)
V4 6DE7 Pin 2 (615vdc).
In AM mode (no audio) 6146 Screen voltage
 Pin 3 (0vdc).

Voltages on the speech amp 12AX7A plates
(pins 6 (81vdc) and 1 (185vdc).

The resistors and electrolytic capacitors have been replaced.


73,
Bill KO4NR

[Pete, WA2CWA]

My voltage readings are highlighted in red.   WHERE?? i ask
All take in the AM mode.
Both V4 and V5 were checked with my spare tubes.  No change.

Rig has a PTT mod in it.  Works well on CW and tunes up normally.  Traced a signal through V4 and V4 with no problem.  Mic keys the rig but no audio.

V4 Cathode Pin 9(0vdc)
V4 6DE7 Pin 2 (615vdc).
In AM mode (no audio) 6146 Screen voltage
 Pin 3 (0vdc).

Voltages on the speech amp 12AX7A plates
(pins 6 (81vdc) and 1 (185vdc).

The resistors and electrolytic capacitors have been replaced.
73,
Bill KO4NR

It also would be nice to point the readers to a schematic or attach a schematic.

[n8fvj]

What is 6146 screen voltage in CW. There is a problem with zero volts screen in AM.

[ko4nrbs]

My voltage readings are highlighted in red.   WHERE?? i ask
All take in the AM mode.
Both V4 and V5 were checked with my spare tubes.  No change.

Rig has a PTT mod in it.  Works well on CW and tunes up normally.  Traced a signal through V4 and V4 with no problem.  Mic keys the rig but no audio.

V4 Cathode Pin 9(0vdc)
V4 6DE7 Pin 2 (615vdc).
In AM mode (no audio) 6146 Screen voltage
 Pin 3 (0vdc).

Voltages on the speech amp 12AX7A plates
(pins 6 (81vdc) and 1 (185vdc).

The resistors and electrolytic capacitors have been replaced.
73,
Bill KO4NR

It also would be nice to point the readers to a schematic or attach a schematic.

Schematic attached.
Bill

[ko4nrbs]

What is 6146 screen voltage in CW. There is a problem with zero volts screen in AM.

335vdc in CW

V3 Pin 3 is listed on the schematic has having 65vdc when in AM mode.  I have 0vdc.  When the Function switch is switched to AM mode it connects V3 Pin 3 to Pin 9 on V4 6DE6.  There is 0vdc there as well.

I will check C36 tomorrow.

73,
Bill

[n8fvj]

I cannot find a DX-60B schematic. Heathkit archives will not download. Manuals only show 1/2 the schematic.

[ko4nrbs]

I cannot find a DX-60B schematic. Heathkit archives will not download. Manuals only show 1/2 the schematic.

It is attached here.
Bill

[DMOD]

V4 Cathode Pin 9(0vdc)
V4 6DE7 Pin 2 (615vdc).
In AM mode (no audio) 6146 Screen voltage
 Pin 3 (0vdc).

I think you are measuring the wrong pins.

Pin 1 should have 615 volts on it, Pin 2 should have about 25 volts on it.

If there is 615 volts on Pin 2 then R26  and C35 should be toast, as well as your 6146, because that much voltage on the grid should pull R26 up near 600 volts.

Remeasure please.

[ko4nrbs]

V4 Cathode Pin 9(0vdc)
V4 6DE7 Pin 2 (615vdc).
In AM mode (no audio) 6146 Screen voltage
 Pin 3 (0vdc).

I think you are measuring the wrong pins.

Pin 1 should have 615 volts on it, Pin 2 should have about 25 volts on it.

If there is 615 volts on Pin 2 then R26 and C35 should be toast, as well as your 6146, because that much voltage on the grid should pull R26 up near 600 volts.

Remeasure please.

Remeasured.
V4 6DE7
Pin 1-684vdc
Pin 2-622vdc
Pin 9- 0vdc

On my schematic R25 is 1 meg ohm.

I have had trouble with the R26 overheating and getting discolored.

I'm all ears as to what I should do to fix this.  I really appreciate you responding.

Bill

[N1BCG]

It's as if neither triode of the 6DE7 (V4) is conducting.

For example, pin 2 should be at 25V due to the first triode providing a load on R25 while pin 9 should be well in excess of 65V when connected via R27/C36 to the 6146 screen.

Is the heater (pins 4 & 5) getting power? The tube should be warm to the touch after a few minutes in standby.

Also, is there continuity between pin 8 on the V4 socket and the chassis? It should be a dead short.

[KD1SH]

   I've never experienced that problem with my DX-60B, but here are a few thoughts, for whatever they're worth:
The 2nd section of the 6DE7 is a cathode follower; the voltage across R26 is entirely dependent on the plate current. If you've got 622v on pin 2 (the grid), the tube would be in full conduction, and you'd be measuring nearly the full plate voltage across R26, at least until it blew itself open since it would be dissipating around 10 watts at that point. C35 might very well either blow open or short under those conditions, as well.
  I'm looking at your overheated R26—you mentioned that it was discolored and running hot—and the lack of voltage across it, as more symptom than cause, and so the question becomes, why do you have 622v on the grid? Looking at the 1st stage of the 6DE7, its plate current will cause a voltage drop across the 1M R25, which in effect becomes the grid voltage on the 2nd stage's pin 2. More 1st stage plate current equals less grid voltage on the 2nd stage. Thus the quiescent operating point of the first stage affects the grid voltage on the 2nd stage. Another measurement you might want to take: what is the voltage on pin 7, the grid of the 1st stage?

Edit: Clark's post came in while I was typing. Good point: open pin 8 to ground—cracked solder joint on the tube socket?—could definitely be a problem.

[ko4nrbs]

As soon as I get home I will review and reply.  Bad socket seems likely for sure.
Bill

[ko4nrbs]

  I've never experienced that problem with my DX-60B, but here are a few thoughts, for whatever they're worth:
The 2nd section of the 6DE7 is a cathode follower; the voltage across R26 is entirely dependent on the plate current. If you've got 622v on pin 2 (the grid), the tube would be in full conduction, and you'd be measuring nearly the full plate voltage across R26, at least until it blew itself open since it would be dissipating around 10 watts at that point. C35 might very well either blow open or short under those conditions, as well.
  I'm looking at your overheated R26—you mentioned that it was discolored and running hot—and the lack of voltage across it, as more symptom than cause, and so the question becomes, why do you have 622v on the grid? Looking at the 1st stage of the 6DE7, its plate current will cause a voltage drop across the 1M R25, which in effect becomes the grid voltage on the 2nd stage's pin 2. More 1st stage plate current equals less grid voltage on the 2nd stage. Thus the quiescent operating point of the first stage affects the grid voltage on the 2nd stage. Another measurement you might want to take: what is the voltage on pin 7, the grid of the 1st stage?

Edit: Clark's post came in while I was typing. Good point: open pin 8 to ground—cracked solder joint on the tube socket?—could definitely be a problem.

Voltage on Pin 7 is 0vdc.
Bill

[Opcom]

Sometimes the socket contacts get a loose fit on the tube pins. A little tweek with a small pointy PCB tool can fix that right up.

[KD1SH]

 Schematic says it should be -1v, but at least it's not some large positive value, and it shows that C34 isn't shorted. Check the value of R24, which should be 22M. There are no other DC paths, so it should measure accurately with an ohmmeter. Check R25, too, which should also read accurately. Power off for both readings, of course. And like Clark suggested, check that pin 8 is firmly grounded and that the heater is heating. It's a relatively straightforward circuit—not a whole lot of places for gremlins to hide—have you tried a new 6DE7? The diagram for the 6DE7 shows one heater, but in reality there are two, one for each distinct triode section; you can visually see one of them in operation through the top of the tube, but the other one—the smaller one—is often obscured by the getter and can't be easily viewed. I don't know if those heaters are internally connected in series or parallel—if they're in series, the opening of one would shut down the other, but if in parallel, you could theoretically have a tube with only one operating triode section, with the remaining heater still making the tube feel warm.
  

Voltage on Pin 7 is 0vdc.
Bill

[ko4nrbs]

It's as if neither triode of the 6DE7 (V4) is conducting.

For example, pin 2 should be at 25V due to the first triode providing a load on R25 while pin 9 should be well in excess of 65V when connected via R27/C36 to the 6146 screen.

Is the heater (pins 4 & 5) getting power? The tube should be warm to the touch after a few minutes in standby.

Also, is there continuity between pin 8 on the V4 socket and the chassis? It should be a dead short.

Pin 8 is grounded so yes there is a direct short to ground.

Pins 4&5 had a broken wire at V5 where they daisy changed the power over to V4.  It was pinched in under a resistor so very hard to see.  Replaced with some stranded Teflon wire.

Voltages are now:
V4
Pin-1- 674vdc
Pin 2- 65vdc
Pin 7- -2.9vdc
Pin 9- 163vdc

V3 Pin 3- 138vdc

Audio is back!!  Thanks fellas.
Bill

[KD1SH]

  Very glad to hear it. I would still suggest checking R24, R25, and especially R26—since you said it was getting hot and discolored—while you're in there. 65v still sounds a bit high for the grid of the 2nd section. Resistors are cheap, and now's the time to change them out if needed, while you're inside the rig.
  Good luck with your DX-60B; it's an excellent little transmitter.

Pin 8 is grounded so yes there is a direct short to ground.

Pins 4&5 had a broken wire at V5 where they daisy changed the power over to V4.  It was pinched in under a resistor so very hard to see.  Replaced with some stranded Teflon wire.

Voltages are now:
V4
Pin-1- 674vdc
Pin 2- 65vdc
Pin 7- -2.9vdc
Pin 9- 163vdc

V3 Pin 3- 138vdc

Audio is back!!  Thanks fellas.
Bill

[KD1SH]

Clark wins the troubleshooting prize. Good call.

Is the heater (pins 4 & 5) getting power? The tube should be warm to the touch after a few minutes in standby.

[ko4nrbs]

  Very glad to hear it. I would still suggest checking R24, R25, and especially R26—since you said it was getting hot and discolored—while you're in there. 65v still sounds a bit high for the grid of the 2nd section. Resistors are cheap, and now's the time to change them out if needed, while you're inside the rig.
  Good luck with your DX-60B; it's an excellent little transmitter.

Pin 8 is grounded so yes there is a direct short to ground.

Pins 4&5 had a broken wire at V5 where they daisy changed the power over to V4.  It was pinched in under a resistor so very hard to see.  Replaced with some stranded Teflon wire.

Voltages are now:
V4
Pin-1- 674vdc
Pin 2- 65vdc
Pin 7- -2.9vdc
Pin 9- 163vdc

V3 Pin 3- 138vdc

Audio is back!!  Thanks fellas.
Bill

I replaced R26, R25, and R24.   R24 22meg ohm was replaced with a 1 meg ohm for hum on the audio.  I found that recommendation on this forum but can't remember where.

All tuned up with VFO.  Can only get a little over 50ma Plate current in AM using the new D104 mike I bought.  Used to be higher than that with my old hand mic.  Did try adjusting the Audio gain.
Bill

[DMOD]

"I replaced R26, R25, and R24. R24 22meg ohm was replaced with a 1 meg ohm for hum on the audio.  I found that recommendation on this forum but can't remember where."

With R24 at 1Meg the grid leak bias on Pin 7 should be lower, so the Pin 6 voltage should be lower than 65 volts since the tube will be conducting more current. So your problems are not over.

With about 165V on the cathode of the Cathode Follower Pin 9, the voltage at the right end of the RC circuit (the Controlled Carrier time constant circuit) and the Final grid Pin 3 should be about 65V.

BTW, if you choose to upgrade the speech Amplifier and Modulator I have a circuit upgrade that has served me well.

Also, check the filament voltage across Pins 4 and 5 of the 6DE7. I have seen bad filament grounds at the tube socket. Check all grounds around the tube socket and resolder if necessary.

[ko4nrbs]

"I replaced R26, R25, and R24. R24 22meg ohm was replaced with a 1 meg ohm for hum on the audio.  I found that recommendation on this forum but can't remember where."

With R24 at 1Meg the grid leak bias on Pin 7 should be lower, so the Pin 6 voltage should be lower than 65 volts since the tube will be conducting more current. So your problems are not over.

With about 165V on the cathode of the Cathode Follower Pin 9, the voltage at the right end of the RC circuit (the Controlled Carrier time constant circuit) and the Final grid Pin 3 should be about 65V.

Also, check the filament voltage across Pins 4 and 5 of the 6DE7. I have seen bad filament grounds at the tube socket. Check all grounds around the tube socket and resolder if necessary.

Pin 1 is 670 vdc
Pin 7 is -2.9vdc
Pin 6 and Pin 2 is 66 vdc
Pin 9 is 164

V3 Pin 3 is 150vdc and bounces up and down some.

After resoldering the grounds and adding a jumper to another ground to ensure it is grounded here are the voltage readings.
Pin 1 is 670 vdc
Pin 7 is -2.9vdc
Pin 6 and Pin 2 is 65 vdc
Pin 9 is 163

V3 Pin 3 is 164 and is not bouncing around.  Stable.

Doesn't look like I changed much.  Filament voltage is ok.

On the schematic Pins 4 and 5 in the 12AX7 V5 are bonded on one end to Pin 9 which is ground.  On the tube socket Pins 4 and 5 are bonded together but no connection to Pin 9.  Since no connection is made to Pin 9 on the tube socket so I assume it must be done in the tube?

Thanks for your assistance.

Bill

[ko4nrbs]

R21 checked out ok.  Next is to check C36.  Can't think of much else to check at the moment.
Bill

[ko4nrbs]

R21 checked out ok.  Next is to check C36.  Can't think of much else to check at the moment.
Bill

C36 checked out as .09951mf.  On the schematic it is .1mf so pretty darn close.

Even tried a good tube but no change in voltage readings.  Need to get this problem resolved so I can focus on deer hunting.
Bill

[ko4nrbs]

  I've never experienced that problem with my DX-60B, but here are a few thoughts, for whatever they're worth:
The 2nd section of the 6DE7 is a cathode follower; the voltage across R26 is entirely dependent on the plate current. If you've got 622v on pin 2 (the grid), the tube would be in full conduction, and you'd be measuring nearly the full plate voltage across R26, at least until it blew itself open since it would be dissipating around 10 watts at that point. C35 might very well either blow open or short under those conditions, as well.
  I'm looking at your overheated R26—you mentioned that it was discolored and running hot—and the lack of voltage across it, as more symptom than cause, and so the question becomes, why do you have 622v on the grid? Looking at the 1st stage of the 6DE7, its plate current will cause a voltage drop across the 1M R25, which in effect becomes the grid voltage on the 2nd stage's pin 2. More 1st stage plate current equals less grid voltage on the 2nd stage. Thus the quiescent operating point of the first stage affects the grid voltage on the 2nd stage. Another measurement you might want to take: what is the voltage on pin 7, the grid of the 1st stage?

Edit: Clark's post came in while I was typing. Good point: open pin 8 to ground—cracked solder joint on the tube socket?—could definitely be a problem.

Thought about what you wrote yesterday. 
Looking at the 1st stage of the 6DE7, its plate current will cause a voltage drop across the 1M R25, which in effect becomes the grid voltage on the 2nd stage's pin 2. More 1st stage plate current equals less grid voltage on the 2nd stage. Thus, the quiescent operating point of the first stage affects the grid voltage on the 2nd stage.

I have 65vdc on V4 Pin 25vdc.  So, the 1st stage of the 6DE7 isn't pulling enough current to drop the voltage down to 25vdc?

Bill

[KD1SH]

  Yes, that's the idea. The 1st stage uses a grid-leak resistor, R24, to establish its operating point, using the grid current flowing from the grid to ground to create a voltage drop across R24, which is the bias voltage. In the 2nd stage the grid bias is determined, in effect, by the operating point of the 1st stage. Keep in mind that the actual value of grid bias on the 2nd stage is the voltage potential between its cathode—not ground—and its grid; the cathode is lifted above ground by the drop across R26.

I have 65vdc on V4 Pin 25vdc.  So, the 1st stage of the 6DE7 isn't pulling enough current to drop the voltage down to 25vdc?

Bill