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DX-100B: need help in finding the source of the new hum on AM




 
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Author Topic: DX-100B: need help in finding the source of the new hum on AM  (Read 8398 times)
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WO4K
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« on: December 09, 2015, 10:40:19 AM »

My DX-100B usually gets great audio reports. This past weekend it developed a hum in the AM audio. I'm told it is not a 60 Hz hum. Mic connector is the original Amphenol 75. Tried several mics, ruling out a faulty mic. The transmitter was completely recapped a year ago and all resistors close to or beyond + or - 10% were replaced. All tubes are new and less than a year in service and the 12AX7 and 12BY7 in the audio circuit check good on my Hickok 600 tube tester. There are no obvious faults I can find in the modulation stage. Checked the ground for the transmitter's mic connector, it is OK. Modulation transformer? Low voltage tap from the power supply transformer? As a relative newbie to boat anchors I am wondering if there is a well-known or common source of this kind of hum in the DX-100 ...or other... transmitters...or if I am overlooking something obvious. I would appreciate some suggestions on where to look to stop the hum. Thanks in advance for the advice and assist.

73,
Frank W4FLN
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« Reply #1 on: December 09, 2015, 11:12:53 AM »

We need to determine if it is 60 Hz or 120Hz hum.

Beg, borrow or steal an oscilloscope and scope out all of the power supplies to determine if the waveform on the LV audio supply is a 60Hz or 120Hz waveform.

Some things to consider:

1. An increased load (lower resistance somewhere) can increase hum,

2. The 0.1 ufd caps for the 12AX7 filters should be 4.7 uFd electrolytic's,

3. DC filtering chokes can short,

4. filament to cathode shorts have occurred in 12AX7's, 12BY7's, and 1625's.

5. rarely, new DC filtering electrolytic's have failed.

Sometimes what we hear in the audio can be buzz from RF getting coupled into the  audio circuits.

Phil - AC0OB
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« Reply #2 on: December 09, 2015, 11:26:41 AM »


AHA! Phil, you are BRILLIANT! The culprit may well be the clothes dryer in the laundry room, about 10' from the DX 100B. I have noticed it generates RF into my Elecraft K3 on some HF bands, visible on the Panadpter. I will check that source out when I get home tonight.

Frank W4FLN
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« Reply #3 on: December 09, 2015, 11:31:36 AM »

Also, if the hum persists, disconnect the mike, turn the audio up a notch, and check for hum on the various stages. A scope is the key to trouble shooting.
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« Reply #4 on: December 10, 2015, 12:30:55 AM »


AHA! Phil, you are BRILLIANT! The culprit may well be the clothes dryer in the laundry room, about 10' from the DX 100B. I have noticed it generates RF into my Elecraft K3 on some HF bands, visible on the Panadpter. I will check that source out when I get home tonight.

Frank W4FLN

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Phil - AC0OB
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« Reply #5 on: December 10, 2015, 06:38:43 AM »

No joy on the clothes-dryer-as-culprit. The hum is unrelated to the dryer. Time to borrow or buy an oscilloscope. My one day of research has me leaning towards a Tektronix 465B. Any opinions on this or other scopes?
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« Reply #6 on: December 10, 2015, 07:55:12 AM »

My one day of research has me leaning towards a Tektronix 465B. Any opinions on this or other scopes?

I believe that Tektronix changed the design of 465 scopes during production ... one type were good and the other had intermittents ....I'm not sure which were which

as well the 22xx series did not seem to fare well in reliability....

I would check with Robert Garcia .... everything I bought from him has done well
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« Reply #7 on: December 10, 2015, 08:11:19 AM »

Time to borrow or buy an oscilloscope. My one day of research has me leaning towards a Tektronix 465B. Any opinions on this or other scopes?

I have this one, great bang for the buck:
 http://www.circuitspecialists.com/digital-storage-oscilloscope-dso5202b.html
As others have stated a scope will track down that hum very quickly.
73s  Nigel
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« Reply #8 on: December 10, 2015, 10:34:42 AM »

Maybe just pull the audio driver tube and see if it clears up.

Some of the digital scopes do not update fast enough to show active complex waveforms.
I have a very nice HP54601a scope, 4 trace 100 MHz, its useless to look at the 455 KHz IF of the receiver.
It maps dots, but not enough or fast enough to show the waveform of an AM signal at 455 KHz.
Not even close.

My 20 MHz scope shows it well...

Anyone want a nice digital scope cheap?
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« Reply #9 on: December 10, 2015, 11:52:12 AM »

Brett's suggestion to pull the low-level audio tubes was my first thought as well. If the hum disappears that will tell you if the culprit is in the first stage(s) of the audio chain. As previously stated, an oscilloscope can make finding the issue a very quick task. Whenever I go up to NearFest I always try to grab a scope or two. I picked up a Leader LBO-518 which was designed to be a military alternate to the Tektronix 465B for pocket change and an HP dual-trace 100 MHz scope for free. Both work fine. There's lots of that stuff sitting in the basements of many hams and often they will be happy to let you cart one away for free.

Rob W1AEX
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« Reply #10 on: December 10, 2015, 01:07:21 PM »

Good suggestions on checking each stage with a scope.

What I have found is that in some high gain audio stages (especially pentode stages) the grids will pick up EMI buzz from power supply rectifier switching, OR from chassis grounds in which the green safety ground is connected to the chassis, but mostly from the latter.

Do you have an Isolation transformer? If so, try that and see if the noise disappears.

Phil - AC0OB
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« Reply #11 on: December 10, 2015, 01:50:05 PM »

I've had a Tek 465 scope for the last 40 years. Has never failed me. Bottom line, get a scope. You can diddle all day long pulling things, poking things, or whatever, but if you're going to do trouble shooting or tracking down a hum culprit, a scope is an invaluable tool to have on the work bench. Of course, if you have no life, diddling, poking, prodding, etc. can keep you busy all day long.
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« Reply #12 on: December 10, 2015, 05:39:45 PM »

All -- thanks for the suggestions and assistance. I'm heading up to the Tampa Bay hamfest (Plant City) this weekend and will look for a scope. Phil, I do have an isolation transformer (from restoring All American 5's years ago) and will try that. Pete, I don't mind a little poking and prodding, unless its by a TSA agent...but I have always been told diddling will make you go blind.  Grin

Frank W4FLN
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« Reply #13 on: December 10, 2015, 06:06:46 PM »

I've had a Tek 465 scope for the last 40 years. Has never failed me. Bottom line, get a scope. You can diddle all day long pulling things, poking things, or whatever, but if you're going to do trouble shooting or tracking down a hum culprit, a scope is an invaluable tool to have on the work bench. Of course, if you have no life, diddling, poking, prodding, etc. can keep you busy all day long.

Yes, get a scope, a good one

Al
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« Reply #14 on: December 10, 2015, 08:43:49 PM »

Quote
I'm heading up to the Tampa Bay hamfest (Plant City) this weekend and will look for a scope.

Just don't diddle at the Hamfest.  Grin

Phil
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« Reply #15 on: December 11, 2015, 12:45:36 PM »

   Adding to the mix here. Realize that good probes are as important as having a good scope. The maximum input to a scope varies, but I'd worry about anything above 50v with a direct connection, such as with a 1X probe. With a 10X, worry about anything over 200V unless you are sure the probe and scope can take more. I like the 100X probes, and some that are entirely acceptable can be had for a reasonable price. I link to one at random on Ebay:

http://www.ebay.com/itm/100X-P4100-High-Voltage-2KV-2000V-Oscilloscope-Scope-Passive-Clip-Probe-100MHz-/251678142479?hash=item3a992fb00f:g:KyAAAOSwdsFUPUGS

  Also realize that those ratings are DC and not AC. The AC ratings are less, and as the frequency goes up the maximum RMS voltage rating drops. You don't want to be probing the plate caps of your 6146's/1625's on that DX-100, even if you have those 100x probes. For that, an AC compensated 1000X probe is best.

   When checking for ripple, you sometimes need to AC couple the scope. Here it is best to switch the input to GND on the scope front panel, then select say 500v/div (5X on scope X 100 if using a 100x probe), and then switch input to AC. The point is to minimize the scope input capacitor from dumping a charge pulse into the vertical amplifier input. Next you can increase the sensitivity to better magnify the ripple.

   If you have a DVM with true RMS AC capability, ripple can be checked by measuring the AC (start on the highest voltage range) seen on the B+ buss, and then measuring the DC level. The ripple percentage (%) is pretty close to (VAC/VDC) X 100. Beware that most hand held meters don't like much more than 300-400vdc. The low level audio stages should have < 1% ripple on the power buss. Some meters will overload when AC coupled and increasing the voltage sensitivity. For example, if you are on the AC 600v range, and the ripple is 2 vac, and going to the 60v range you should see 2.0 vac. If it jumps to something much higher, disregard that last reading, and go with 2 vac.

Jim
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« Reply #16 on: December 11, 2015, 05:33:10 PM »

Thinking more about this, I decided that buying a scope at a hamfest is like buying the proverbial pig-in-a-poke (not to be confused with the aforementioned poking). So take John's (W4BFS) advice I contacted Robert Garcia in Atlanta and ordered a rebuilt... and calibrated... Tektronix 465 with probes and manual. Now to learn how to use it properly...

Frank W4FLN
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« Reply #17 on: December 11, 2015, 09:06:44 PM »

Most fests have a place to plug something in to test it.
If it works, it works.

I do not use a scope for finding hum much, but pull parts, tubes, add caps.
If you hunt for hum in high voltage circuits, a cheap 20 MHz scope might be best...
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« Reply #18 on: December 12, 2015, 10:51:30 AM »


Frank,

    The Tek 465 is a very good scope if everything is working, and fully checked out. Realize though that those scopes are from over 40 years ago, and the supply of replacement parts has largely dried up. The folks restoring them often get parts from derelict old chassis's, where perhaps three scopes can be used to make one or two good ones. I had one for many years, and one day I left it on overnight with the trace intensity at maximum. The next morning I discovered the scope turned on and it was still OK. The next time I turned it on, the CRT filament was kaput. I got lucky, and found a 465 on Ebay that was missing modules, but had a CRT. It was cheap enough so I took a chance. That CRT was a little dim, but worked perfect.

    The 465 has a built in probe adjustment signal. It is a current loop for current probes, or a voltage source for voltage probes such as 10x, 100x, and 1000x probes. Get used to compensating the probes as that has a huge effect on the response of a fast pulse as seen on the CRT. Also be aware of the voltage limitations of whatever probe you have, and the scope input as well when direct coupling to the input.

Jim
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« Reply #19 on: December 16, 2015, 10:07:51 AM »

We haven't heard from Frank for a while........Wonder how he's making out with the DX100???
I hope he's not looking for the proper scope probe.

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« Reply #20 on: December 16, 2015, 11:37:51 AM »

Thinking more about this, I decided that buying a scope at a hamfest is like buying the proverbial pig-in-a-poke (not to be confused with the aforementioned poking). So take John's (W4BFS) advice I contacted Robert Garcia in Atlanta and ordered a rebuilt... and calibrated... Tektronix 465 with probes and manual. Now to learn how to use it properly...

Frank W4FLN

Frank - You made the right decision buying a scope that's been checked out rather than taking a chance that what you bring home from a hamfest is burned out junk.  I'm not familiar with the 465 but judging from the specs it should be fine for working on your DX-100B and other boatanchors. Read the manual first, especially the caution about avoiding shorts to ground through the test probe ground clip.

I have a DX-100B I need to get started on so I will be following your thread. Keep us posted.

Brad
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« Reply #21 on: December 16, 2015, 06:52:28 PM »

Hi, all

Thought I would check in. The scope arrives this week. One of the guys in my Fort Myers Amateur Radio Club is going to Elmer me on the proper use of the scope. That won't happen until after Christmas. I was on the Florida AM Net on Sunday and was told my signal no longer had a hum. Hmmmm. All I did was visually check the modulation circuits and test the 12AX7 and 12BY7 tubes.

Will post again after the holidays.

73,
Frank W4FLN
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« Reply #22 on: December 16, 2015, 08:37:32 PM »

Hi, all

Thought I would check in. The scope arrives this week. One of the guys in my Fort Myers Amateur Radio Club is going to Elmer me on the proper use of the scope. That won't happen until after Christmas. I was on the Florida AM Net on Sunday and was told my signal no longer had a hum. Hmmmm. All I did was visually check the modulation circuits and test the 12AX7 and 12BY7 tubes.

Will post again after the holidays.

73,
Frank W4FLN


Good news about the DX100 deciding to behave for you. Just shot gun the tubes. A tube tester is rarely accurate in anything we expect from one, UNLESS there is a really obvious badness.
Always been my experience with tube testers of any type. Usually either bad contacts of the pins to the tube socket or some component failure or bad solder joint.

Fred
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