The AM Forum

Good morning.  I have been using my Ranger to drive my Desk KW for several years.  Recently I've received reports of hum in my audio.  Here is what I have found:
1.  OSC meter at half scale in Zero Pos
     Pegged in VFO or XTAL pos
2. With or without a mic plugged in I get the following
1.4vac at C78+, 300v line
V7B pin 7 no noise or hum with audio gain pot at min or max.
V7B pin 6 .1vac hum with audio pot at min.  As gain is increased a really nasty, almost sine wave appears.  .3v with pot at mid range.  .8v distortion at V9 and V10 pin 5.
All electrolytic caps have been replaced and I can't find anything obvious.
Any help is greatly appreciated.
73 de K5OG

Is the carrier clean with no audio?  Listen to your spotting signal because if the hum is from the B+ supply it will show up on the spotting signal; no audio needed.  I had multiple Ranger issues causing hum with my Desk KW.  If the carrier is dirty, you need to address that first.

If the spotting signal is clean, have you tried substituting a different 12AX7A?  It could be a simple case of heater cathode leakage.  Are decoupling caps C51 and C55 OK?

The Ranger I use with the Desk KW just about drove me to throw it in the garbage in frustration.  Shortly after I set them up I noticed a slight hum on the carrier which was seemingly cured by changing the clamp tube.  A few months later hum was back and this time replacing the keyer tube made it go away.  A few months later more hum and this time I replaced the VFO tube.  Next we were back to replacing the keyer tube.  No, this did not make sense!  I finally found that the previous owner had replaced the low voltage choke with one which was the same physical size and looked original but had a different part number and much lower inductance.  So the B+ supply ripple was marginal and anything that slightly exacerbated hum (such as slight heater cathode leakage in any tube) made the hum obvious.  Once I replaced the choke (2 years ago) with the proper value it has been hum free.  Hum on the audio was never an issue so from my experience any ripple on the LV supply will create problems with the RF side (VFO in particular) long before it becomes noticeable in the audio.

Good luck and I hope your Ranger is far less frustrating than mine!

Rodger WQ9E

Hi Rodger.  Thanks for the quick reply.  Swapped the 12AX7A.  Didn't help.  The caps are good.
I can't hear the hum in the carrier using the demod on my HP spectrum analyzer.  I've attached a pix I took with my iPhone.  Shows the mess at V7 pin 6.  Choke looks to be the original one.
Where V13 and V14 should be on the small PC board there is only a terminal strip with some caps, resistors and two diodes.  Don't think this has anything to do with this issue since the Ranger has worked for years.  Just thought I'd mention it.
While I do have a lot of vintage gear this is the only Ranger I have and it has actually worked quite well till now so I don't have a lot of experience on this rig.
Gonna keep plugging away. 73 de K5OG

I am going through similiar issue with a Reciever.  What TONE is the hum?  60, 120 or higher?  The reason I ask is if its in the audio or speach amp, It might be a higher tone like you would hear on a PA system with a Ground loop.  This helped me locate my problem.

When this hum appeared, have you noticed whether the overall audio system gain has changed?   A shorted cathode bypass cap on either audio stage will cause a gain increase and a change in operating point leading to increased susceptibility to hum pickup.  Check your cathode to ground resistance on both pins 3 and 5.

I am a bit perplexed that you are not seeing any problem at the grid (pin 7) but the audio gain control does impact the hum level. 

In your scope photo, what is the sensitivity and time per division for the displayed waveform?

It's .3v p-p and 5ms on the sweep.
And yes it is strange that pin 7 has no noise at all no matter where the pot is set.

If the scope is set for 5 ms / div, that trace is approx 16.7 ms the time period for 60 Hz.  Being almost a square  wave it has high harmonic content, typical of hum pickup from a unshielded pickup of line AC

Al

Still trying to find something wrong.  All grounds seem to be fine.
Two questions:
1.  Any thoughts on the OSC meter being pegged when VFO or XTAL are selected?
2.  That 1.4v 60 cycle ripple on the 300v line seems troubling.  Is it really an issue and should I isolate the 300v supply and use an external supply for testing purposes?  Will that present any issues if I do that?  Guess that's really 3 questions. HI
Thanks for all the replies to my post.
73 de K5OG

I am currently doing a radical Ranger mod ....

1. almost every meter shunt resistor in EFJ products are not stable over time ... of a dozen or so shunts removed from 4 or 5 different transmitters ALL were at least 40% high in value and some double (checked out of ckt with Fluke dvm)  .... start here

2. just pull the lv rectifier tube and connect in your bench supply .... expect 50 to 100 mA draw ... since load is removed from power transformer, check other voltages to ensure within limits ...

finally ....most Rangers were kits .... ALWAYS check the simple stuff .... good solder joints,  good mechanical connections ... 73 ...John

Fred,

Here are some more thoughts about your Ranger problem.  Once I see something that does not appear correct, I never go to step 2 before step 1 is resolved.  Lets break your issues with the rig down into 2 buckets. 

Bucket 1 (Osc Plate Current)

Lets tackle the wrong oscillator plate current first.  I just checked my Ranger and my value matches with the book.  You should be reading around 23 MA of Oscillator  plate current while in the VFO position.  If not, verify that your meter shunt resistor is correct (3 ohm).  If that is correct, then check the screen voltage on the 6CL6 buffer stage.  You should have approximately 95 volts on the screen.  If not, check the screen dropping resistor to make sure its 68K.  If all of that looks good then check the tube.  I doubt its the tube though.  Get that working and then go back to the hum problem. 

Bucket 2 (Hum problem)

1.4V of ripple that your measuring on the LV Filter cap translates to .46% of 120 Hz ripple on the LV rail.  I believe earlier it was determined that you noise is at 60 Hz.  I doubt if you have a LV filter issue based on the amplitude measured besides the ripple frequencies does not match you waveform on the plate of V7B.  I think you put aside a power supply issue.

Its always good to measure the DC operating points around the stage that your having a problem with.  This gives good insight if you have any issues with any of the components around the tube which control its bias point.  Things such as plate resistors and cathode resistors along with the integrity of the cathode bypass capacitor which may be DC leaky which will shunt the cathode resistor and shift the bias point on the tube.  You should measure 135V on the plate of V7B and 1.5V from the cathode resistor to ground.  Noticed I said to ground, since that will give you a first insight into those chassis grounds that I will mention further down on this trouble shooting guideline.         

The Ranger uses the chassis for one side of the filament connections.  So you have current flowing through the chassis with a ripple frequency of 60 Hz.  You frequency measured was indeed right around 60 Hz so that's a big clue.  Since the rig use to work, we can put aside the bad design principle in running filament current through the chassis.  Next you want to look at each rivet or tie point where the filament is tied to the chassis. Examine the tube socket connection to the chassis.   Any small amount of resistance will cause additional voltage drop.  Since grid resistor returns and cathode resistor returns are referenced to this same chassis they can AC bias these return points with 60 Hz AC.  Not a good thing to say the least.  Rivets can be worse than screws in this area.  I am not sure if yours is a kit or factory wired unit but regardless, trace each and make sure its making solid contact with the chassis.  Again since yours did work, something may be broke in that area.  I sometimes use a small constant current power supply to force current through those rivets and measure the voltage drop.  That's a great way to see if you have any connection problems.

Check you continuity of the gain pot.  Make sure the carbon element is indeed at ground and not floating.  An ohm meter with one probe on the wiper and the other connection on chassis ground will verify that all ok.  Additionally as a test you could quickly parallel a 500K resistor from grid to ground on pin 7. 

Hopefully this will prove to be helpful and please let me know how you make out solving this issue.

Regards & Good Luck!
Joe, W3GMS   
 
               

Thanks for the great info Joe.
I've been checking all the ground points but perhaps should give them a closer look.  I can't find any bad components so something mechanical may indeed be involved here.  One other thing to mention on the VFO is that it is about mid scale in the Zero position but pegs in all other positions.  I'm at work right now so I don't have the exact reading on the meter but it is about mid scale.
I'll dig back into it tonight.
73 de K5OG

Does your ranger have the keying tube and 6al5 above the rectfiers?  Or is it an early version that does not have this?

C

It does not have the tubes

The "Oscillator" meter position actually displays cathode current in the buffer/xtal osc. When in the VFO position, negative bias on Grid 1 of the buffer should result in zero cathode current. When in the spot position the voltage should be near zero, as the switch shorts the key line to ground. Excessive cathode current means that the tube is not cut off.

Measure the Grid 1 voltage in VFO position with a high impedance meter. Should be some negative voltage. C22 could be leaking some +dc to the grid. This assumes rest of keying tube circuitry is working correctly.

Does the meter read zero in the "buffer" position when Oscillator is in VFO position? Buffer position of meter actually reads multiplier/driver cathode current. IF the cut off bias circuitry and voltages are good the same negative bias on grid 1 of the driver is applied through a divider of R44 and R45 to about 3/4 the value and then applied through R7 to the grid of the buffer. So, if the driver is cut off (buffer current is zero), then the voltage is ok and the path to Grid 1 of the buffer is the problem (or C22).

Very interesting.  This radio does not have V13 and 14.  Instead there are two diodes, two caps and three resistors on a terminal strip.  This feeds -28 mod bias.  The 100k resistors to V1, 3 and 4 grid 1 are all tied to ground.
The more I get into this poor radio the more I am amazed it has worked as well as it has for so long.  I cannot find any schematics that indicate the 100K resistors should be tied to ground.  Any thoughts on this?

This was the source of ALL my trouble..  My original ranger did not have those two tubes. My later one does.  Start looking at how that Front mode switch is wired. Mine was clearly wrong and this made the osc peg out.

C

Well, it was wired wrong.  The meter resistor was wired neg side to pin 10 on the switch and not to ground.  25ma current in OSC position now.
Still no joy on the audio hum. Gain pot is good and I've checked all ground screws and soldering.  I'm still concerned about that ac on the 300v line.  I know it's very low but........
Adding more cap does nothing. 
Anyway, one issue solved and one to go.
Thanks again for all the helpful suggestions.
73 de K5OG

Three wire plug or two?

C

The old "widow maker" two wire plug.
Did notice something while testing.  No hum/buzz with the audio gain down.  Raising the gain brings up the noise.  So what I'm seeing on the scope is what I'm hearing for sure.  Not a pure 60 cycle hum but certainly full of square wave artifacts.

This is usually the first test to make when you have hum on your transmitted audio. Since the hum goes away when you turn down the audio pot... the hum is either coming from the very first audio stage or the microphone.

I suggest that you try removing the microphone from the jack, and use some other means (carefully) to bring the transmitter into transmit mode (if you have some kind of push-to-talk installed).

If the hum is absent, even when you turn up the audio pot... then you should check for such things as a loose ground in the microphone cable.

If the hum is present, even with the microphone unplugged, the the problem is in the first audio amplifier (1/2 of the first audio stage tube... V7A, the 12AX7). Try grounding the microphone input lead at the microphone jack. If the hum goes away, check the shielding that Johnson used on the cable that goes between the microphone jack and the input of the 1st audio stage.

If the hum doesn't go away when you ground the microphone input lead at the microphone jack, the check to make sure that C50A, the 10uF cathode bypass (to ground) is ok (tack a 10uF or greater cap across it... observing the polarity)

Finally, try swapping out the 12AX7. It could have a short between the cathode (pin 3) and the filament (pins 9,4, and 5).

Good luck

Stu

Flip the plug around.

C

There is another thing to check:

C51 (0.1 uF to ground) and R20 (220k ohms between the 300 volt line and C51) form a filter... which will reduce noise on the plate voltage supply for the first audio amplifier... at frequencies above 7 Hz (the 3dB rolloff frequency of this RC filter).

It is particularly important to minimize any low frequency hum on the B+ supply for the first audio stage, because any noise on the audio that is introduced by the B+ supply will be amplified by the next stages.

Check to make sure that R20 is in place (its value is not critical, as long as its resistance is above 150k ohms), and try tacking an extra capacitor across C51 (.1uF or greater).

Stu

This Ranger is a real challenge.  I build radio and television broadcast stations and repair Signal Ones and Alpha amps all day long BUT this one has got me.
Here's a recap of everything so far.
Radio worked in the past so something changed/went Tango Uniform
Swapped mics - no change
Swapped tube - no change
Reversed AC plug - no change
Resoldered all gnd points on mic cable
Jumped another cap at C50A - no change.  All caps check good with ESR tester.
I agree it is in the first audio stage..  The distortion is present even in standby.
Grounding mic input removes the distortion from the waveform but the low level ac is still there on pin 6 and varies with pot setting.
Attached are some pix
Pix1  R23 B+ side .1v/div 5ms.  Pot up or down
Pix2  V7 pin 1 50mv/div 5ms.  Pot up or down
Pin 7 has the same distorted waveform but at a very low level
Pix3  pin 6 .1v/div 5ms.  Pot down
Pix4  pin 6 .1v/div 5ms.  Pot up
V7 voltages pin 1 140v, pin 2 0v, pin 3 1.3v, pin 6 156v, pin 7 0v, pin 8 1.4v
Again, thanks for all the input.  I read every one and check out the suggestions.
73 de K5OG

Here's pix 4

Good morning Stu.  I agree that B+ hum is an issue with the gain involved here.
I'll check that circuit very carefully and tack in the extra cap.  Think I did  that already but it sure won't hurt to try it again. HI
Update:  Resistor reads 240K.   A bit high but should be ok.  Tacked in another .1 and the distortion did go way down but the low level ac hum is still there.  Think I'll just rework that entire circuit and see what happens.
73 de K5OG

I'm surprised that you would see such a large 120Hz component on the B+ side of R23.

Correction... I'm not surprised, because the 120 Hz component of the 300V B+ supply would probably be around 1% (peak) = 3 volts peak.

Are you sure that this is not pickup from the probe? Try using a 0.1 uF capacitor to ground the probe (simulating the measurement at R23, but not actually touching R23). See if the 120 Hz sine wave goes away.

If you really have that much hum at 120 Hz on the B+ side of R23, see how much 120 Hz hum you have on the B+ side of R24.  Since R24 is only 47k ohms, the combination of C55 (0.1 uF) and R24 isn't really that good a filter at 120Hz... although it should provide some attenuation (about a factor of 4)... since its 3dB roll off frequency is 34 Hz.

If the 120 Hz signal you are seeing on the B+ side of R23 is real, y You might want to increase the capacitance of C55. That will remove reduce the 120 Hz signal on pin 6.

However, this does not explain why the hum becomes audible when the audio gain control is turned up.That still points to a problem in the first audio stage.

Since the "distorted" waveform goes away when you ground the mike input (still leaving 4.7k ohms between the grid and ground), check to see if R17 is still 1M ohm. If the grid leak resistor (R17) were open, that would mess up the biasing of the tube... but grounding the mike input would restore the biasing to a reasonable level.

Also try this (sort of a work-around).

Using a microphone that does not have to look into a very high impedance (i.e., almost anything except a D-104), put a 10k ohm resistor from mike input to ground (DC coupled) or (if easier) from pin 2 to ground. See if that fixes the hum problem (by lowering the impedance, at low frequencies, between the grid and ground)

Stu

Good morning.
Checked the 1 meg resistor and it is high at 1.5 meg.  I'm feeding audio to the Ranger with a Daiwa RF-440 so impedance is not an issue.  Installed the 10K resistor and the noise is gone (I think).  I can't hear anything on the demod on my HP spectrum analyzer or on a receiver.  The waveform still has a very low level of AC (far less than 1%) but the highly distorted waveform is gone.  No time for air check until Thursday night but hopefully the reports will be good.
Thanks to everyone for all the helpful suggestions.
73 de K5OG
Fred
www.k5og.com