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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: 369
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.

Image

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

Image


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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: 1788
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: 948
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: 4780
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 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 369
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: 789
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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