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 Post subject: Heathkit Apache TX-1 Hum
PostPosted: Dec Tue 05, 2017 3:50 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 378
Location: Ringoes, New Jersey, USA
I am just about finished (or so I thought) with the restoration of this TX-1 and another problem has popped-up. When transmitting there is a fairly prominent hum present. It can be seen on the carrier in the scope photo below.

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If I pull the 6CA7 modulator tubes the hum disappears so I started investigating the audio circuits. I injected a 1000hz signal at the MIC input and followed the signal with a scope through the stages. The signal remains clean all the way to the 6CA7 grids (pin 5).

Image

Interestingly, the 6CA7 side of the modulation transformer measures only 362 ohms, quite different from the 11,000 ohms the schematic specifies. However, I have another modulation transformer from an Apache and it measures very similar. I can modulate the carrier and listening on an adjacent receiver the audio is quite decent, just accompanied by an annoying hum. Also, 6CA7 voltages measure very close to schematic.

Any thoughts on how to proceed would be appreciated.

Matt

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 Post subject: Re: Heathkit Apache TX-1 Hum
PostPosted: Dec Tue 05, 2017 4:32 am 
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Joined: Feb Tue 07, 2012 9:32 pm
Posts: 1789
Location: Middlesex,NJ
HI MATT:

DID YOU SCOPE THE PRIMARY CENTER TAP?

I WOULD TRACE THE B+ LINE BACK TO THE RECAPPED FILTERS IN THE POWER SUPPLY AND RECHECH YOUR WIREING. I HAD ISSUES THERE WHEN I DID MINE.

ARE THE PRIMARY & SECONDARY WINDINGS REVERSED?

SECOND:ARE THOSE 6CA7'S BALANCED.IF OUT OF BALANCE THAT COULD ALSO CAUSE SOME HUM.

BEST 73

WALTER-W2WIQ


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 Post subject: Re: Heathkit Apache TX-1 Hum
PostPosted: Dec Tue 05, 2017 6:01 am 
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Joined: May Fri 01, 2015 2:33 am
Posts: 1124
Location: Roslyn,Pa 19001
Hi Matt,
> The signal remains clean all the way to the 6CA7 grids (pin 5).
Look for hum coming the the bias supply going to the ct on the driver transformer. If you have a clean carrier w/o modulation I would suspect that the HV is hum free. The next thing might be that the 6CA7 has some cathode to grid short enough to couple some 60cy hum from the filament supply.
>Interestingly, the 6CA7 side of the modulation transformer measures only 362 ohms, quite different from the 11,000 ohms
That's because the 362 ohms is the resistance of the winding and 11K is the impedance hence the Z sign next to the 11K.

GL
Terry N3GTE


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 Post subject: Re: Heathkit Apache TX-1 Hum
PostPosted: Dec Tue 05, 2017 12:46 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 5420
As Terry suggested the bias supply could easily be the source of the hum but don't rule out it appearing in early audio stages just yet. The injected signal you used to drive the audio could be greatly in excess of the hum amount causing it not to be obvious on your scope trace.

If the bias supply ripple is as it should be then pull the 12BY7 audio driver and see if the hum disappears. If so you will need to trace back through the audio amp and I would start with seeing whether the hum is gone when the mic is disconnected.

Rodger WQ9E


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 Post subject: Re: Heathkit Apache TX-1 Hum
PostPosted: Dec Wed 06, 2017 12:45 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 378
Location: Ringoes, New Jersey, USA
Hi Guys-

Thank you for your replies. I will continue along with your suggestions.

Walter: I did not scope the CT. I will check it out and follow your other suggestions. Not sure on the 6CA7's. I think I may have another pair I could try.
Terry: I will check the bias supply and thanks for the clarification on the modulation transformer measurement - I missed the "Z"!
Rodger: Thanks for your input. Hum is still present with no mic connected. Will check bias supply.

I will report my findings.
Thanks again,
Matt


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 Post subject: Re: Heathkit Apache TX-1 Hum
PostPosted: Dec Wed 06, 2017 4:20 am 
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Joined: Jul Sat 23, 2011 9:33 pm
Posts: 810
Location: Long Beach Ms. USA 39560
Please remember to check the ground connections in the audio area.
A poor ground used for filament return and audio cap or resistor will induce hum.
Tighten the loose screws but not the adjustments!
Good hunting.

_________________
Pat W5THT
Unhappy tubes blush while unhappy power FETs scatter plastic


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 Post subject: Re: Heathkit Apache TX-1 Hum
PostPosted: Dec Wed 13, 2017 3:27 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 378
Location: Ringoes, New Jersey, USA
Hi Guys - Sorry for the delay, not much radio time lately.

If I pull the 12BY7 the hum is gone, so I re-inserted the 12BY7 and the hum returned. I then pulled the 12AU7 and the hum again disappeared. Worked my way to the 12AX7 with similar results, hum is gone with the 12AX7 pulled.

I subbed different tubes for everything up to and including the 6CA7's and the hum is still present. Shorting the MIC connector center pin to ground has no effect on the hum. C67 is tubular ceramic and tests OK for value and leakage. I still need to check C66, disc ceramic.

Have also been checking grounds as pehamel suggested (thanks), so far so good.

Will continue the hunt...

Thanks again,

Matt


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 Post subject: Re: Heathkit Apache TX-1 Hum
PostPosted: Dec Wed 13, 2017 3:57 am 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 5420
Check the wiring around the gain control; that is often a good place for hum to originate.

Since it disappears when the 12AX7 is pulled the hum is originating in that stage or prior to it and since grounding the input pin didn't stop the hum there is a good chance it is originating between the output of the first audio stage and the input to the second which is why I suspect the wiring around the pot but take a close look at everything in the 12AX7 stage area.

If you need to try to narrow it down further you can try sequentially shorting control grid to ground for the first and then second audio stage. Install the temporary short with the power off and then power up, normally you could just short the grid to ground but if you do this with it powered up (or if the connection is poor) you could generate large transients, particularly with the second audio stage.

Rodger WQ9E


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 Post subject: Re: Heathkit Apache TX-1 Hum
PostPosted: Dec Thu 14, 2017 1:10 am 
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Joined: Jul Sun 15, 2007 9:34 pm
Posts: 212
Location: PA
Looks like you found the low level audio stages to be a source of hum. I had a similar problem with an Apache, and traced the problem to filament wiring dress in the low level audio stages. The first thing is to try a different 12AX7, or check the currently used tube for heater-to-cathode leakage.

You may find, as others suggested, poorly dressed low level audio leads, bad solder joints to circuit common, AC ripple in the bias supply, or bad shielding from the mic connector to the first 12AX7 stage at fault. For me, re-dressing the filament wires helped reduce, but not remove the hum.

The front panel gain control on my Apache actually controlled the clipper stage; the pot accessed through the key jack controlled voltage gain. It seemed to have been built that way. I've seen some Apaches wired like that, and other ones wired with controls reversed. Must have been a kit builder option / modification. In any event, that 6AL5 stage performs poorly, and is the source of the transmitter being known as "scratchy Apache". ...I bypassed mine with shielded cable.

My permanent fix to remove hum was to run the 12AX7 filaments from a DC supply. First, I half-wave rectified and filtered the 6VAC supply to that section. The resulting DC was smoothed and regulated using a common LM7806 3 pin voltage regulator (6 VDC,TO-220 case, 1A) circuit pulled from a data sheet. The heating factor of DC is different than AC, but the tube didn't seem to mind the difference. I ran the transmitter this way for ten years, the problem did not return, nor did tube life seem to suffer.

-O.B.


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 Post subject: Re: Heathkit Apache TX-1 Hum
PostPosted: Dec Mon 18, 2017 12:57 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 378
Location: Ringoes, New Jersey, USA
Hi Guys-
It seems as if the hum is indeed coming from the 12AX7. Gently re-positioning wires along with re-soldering connections and tightening grounds has seemed to eliminate about 75% of the hum. Swapped between a few different tubes with no change. There is still a slight hum present. Grounding either control grid seems to almost completely eliminate the hum. Checked all components around this circuit and can't find any fault.

OB - Perhaps it is originating from the filament supply as your hum did. I like your idea of using a DC supply for the 12AX7 filament. Any thoughts on the importance of the LM7806 - is it necessary to regulate the supply? Also, as to the gain control, I believe stock Apaches front panel GAIN control were wired to the clipper - that's how it is shown in the manual. It seems to be a popular mod to remove the clipper and rewire the mic gain control to the front panel.

Still poking around to see if there is room for improvement...

Thanks again for all the help,
Matt

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 Post subject: Re: Heathkit Apache TX-1 Hum
PostPosted: Dec Mon 18, 2017 6:09 am 
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Joined: Nov Sat 07, 2009 11:37 pm
Posts: 1863
Location: Sayreville, NJ 08872
I built my Apache in 1960. Even after 57 years, I still have no need to run my audio tube filaments, or any filaments, off of a DC supply.
The front panel gain control of all designed Apaches is a combined gain and clipping control. The audio control behind the key jack is used to set the 100% modulation level. If you remove the clipper and its circuitry, you probably should also remove the low pass filter circuitry. To optimize performance after removal of stuff, some component value changes would also be in order. A good quality scope is also very useful in trouble shooting hum-related issues.

_________________
Pete, WA2CWA - "A cluttered desk is a sign of genius"
http://www.classicradiomanuals.com


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 Post subject: Re: Heathkit Apache TX-1 Hum
PostPosted: Dec Mon 18, 2017 12:48 pm 
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Joined: Feb Tue 07, 2012 9:32 pm
Posts: 1789
Location: Middlesex,NJ
HI GUYS:

DECADES AGO THE PROPER WAY TO WIRE FILAMENTS WAS TO USE A TWISTED PAIR FROM THE TRANSFORMER AND CONTINUE ON WITH TWISTED PAIR TO EACH TUBE.NO GROUNDINDING OF THE FILAMENTS AT ANY POINT.THAT ELIMINATED A LOT OF HUM PROBLEMS.
ALL TRANSFORER PAIRS WERE TWISTED TO THEIR DESTINATION INCLUDEING THE AC INPUT PAIR.

WALTER-W2WIQ


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 Post subject: Re: Heathkit Apache TX-1 Hum
PostPosted: Dec Sun 31, 2017 11:38 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 378
Location: Ringoes, New Jersey, USA
Hi Guys-

Sorry for the long delay. Investigating with a scope I find that there is a 60 cycle AC signal present throughout the audio section. I can't detect any AC on pin 2 of the first half of the 12AX7 (no mic connected) but on pin 1 (plate) there is 100mv. Pin 7 of the 12AX7 (second half grid) there is no detectable hum but on pin 6 there is again about 100mv. Pin 1 of the 12AU7 there is about 1v, pin 7 250mv and pin 6 there is about 3.5v.

With the scope at pin 2 of the 12BY7, this 60 cycle ripple is still present, although reduced when either of the 12AX7 or 12AU7 tubes are removed.

There is also about 250mv ripple on the bias supply driver transformer CT (T4, point J on schematic). Adding an additional filter capacitor eliminates the ripple on the supply but does not eliminate the ripple present throughout the audio circuits as described above.

I assume that this hum is being induced by the filament line. I suspected that there are plenty of Apache's that don't suffer from this problem (and Pete confirmed). I could always go through the assembly manual for the audio section to see if I can find anything suspect.

Thanks again for all the help and suggestions and Happy New Year to all.

Matt

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 Post subject: Re: Heathkit Apache TX-1 Hum
PostPosted: Feb Tue 20, 2018 12:15 am 
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Joined: Dec Wed 24, 2014 7:34 pm
Posts: 1968
Location: Weimar, Texas
I was just wondering what the outcome of this was. Hopefully she sings and doesn't hum anymore

_________________
It's hard to solve an equation if every term is an unknown.

73
NE5U

Mike


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 Post subject: Re: Heathkit Apache TX-1 Hum
PostPosted: Feb Fri 23, 2018 3:52 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 378
Location: Ringoes, New Jersey, USA
Hi Mike-
It turned out that the hum was being induced by the bias supply. It was originating from the wire that fed AC to the selenium rectifier M3 (see left most portion of the schematic above), the DC from the bias supply was OK. The wire was run in a main wiring harness that runs through the audio amplifier compartment and induced hum into the 12AX7 tube. I replaced the wire with a piece of coax using the center conductor and grounding the braid.

I finally found the problem as Walter suggested I use an oscilloscope as a "sniffer" to find the problem. Just moving the probe around in the audio compartment (not directly connected to anything) quickly showed a very strong AC signal present and it didn't take long to determine that it was strongest at the above mentioned wire.

Hope this helps,
Matt


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 Post subject: Re: Heathkit Apache TX-1 Hum
PostPosted: Feb Sat 24, 2018 11:34 pm 
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Joined: Dec Wed 24, 2014 7:34 pm
Posts: 1968
Location: Weimar, Texas
Interesting. I've made a note of this post for the future. My TX-1 is waiting for my attention. Thank you

_________________
It's hard to solve an equation if every term is an unknown.

73
NE5U

Mike


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