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Author Topic: Hmmm? HT-37 hum  (Read 1957 times)
N6AF
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Posts: 56




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« on: August 21, 2017, 08:32:15 PM »

My question is directed at someone who has experienced and fixed the following specific issue. Please no "try this try that" responses-thanks!
My HT-37 has a hum level about 30 to 35 dB down from
peak output power when I connect a JT-30 or several other mics to the front panel Amphenol connector which is too high a hum level. But if I cable in the mic directly to the internal RCA connector on the modulator sub-assembly then the noise drops to perhaps 50 dB down. I confirmed the integrity of the internal coaxial cable that runs from the front panel connector to the RCA jack so at this point I'm not sure why using the front panel connection results in the relatively high hum level.
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G3RZP
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« Reply #1 on: August 22, 2017, 12:51:13 AM »

Is it 60 or 120 Hz hum?
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N3QE
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Posts: 5038




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« Reply #2 on: August 22, 2017, 05:41:50 AM »

My question is directed at someone who has experienced and fixed the following specific issue. Please no "try this try that" responses-thanks!
My HT-37 has a hum level about 30 to 35 dB down from
peak output power when I connect a JT-30 or several other mics to the front panel Amphenol connector which is too high a hum level. But if I cable in the mic directly to the internal RCA connector on the modulator sub-assembly then the noise drops to perhaps 50 dB down. I confirmed the integrity of the internal coaxial cable that runs from the front panel connector to the RCA jack so at this point I'm not sure why using the front panel connection results in the relatively high hum level.

Assuming you've already cleaned up grounds especially at the modulator sub-assembly. You want to be real sure power currents are going through the power cables/chassis and not the microphone cable.

Did you convert from tube rectifiers to solid state rectifiers?
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K1ZJH
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Posts: 3479




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« Reply #3 on: August 22, 2017, 09:37:59 AM »

As the previous poster noted, there is AC current on the cable shield. You could try opening the shield at the mike connector end, and leaving the shield connected at the SSB generator. 

Pete
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G3RZP
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Posts: 8241




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« Reply #4 on: August 22, 2017, 02:54:26 PM »

This is one of those cases where all you are likely to get is a 'try this' type answer. The shielding of cable at AF is pretty well purely electric, as the skin depth is so high that a parallel current will induce a signal on the inner conductor as well.

Lifting the outer conductor at one end MAY help - but may not. Should you wish to get an understanding of the possible mechanisms, you may well find that the book "Frequency Synthesisers, Theory and Practice", 2nd edition, Chapter 3, by Vadim Manssewitsch, Wiley and Sons, 1980, ISBN 0-471-07917-0 is helpful. Chapter 3 is entitled "Shielding".

vy 73

Peter G3RZP
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AC2EU
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« Reply #5 on: August 22, 2017, 03:00:39 PM »

This is one of those cases where all you are likely to get is a 'try this' type answer. The shielding of cable at AF is pretty well purely electric, as the skin depth is so high that a parallel current will induce a signal on the inner conductor as well.

Lifting the outer conductor at one end MAY help - but may not. Should you wish to get an understanding of the possible mechanisms, you may well find that the book "Frequency Synthesisers, Theory and Practice", 2nd edition, Chapter 3, by Vadim Manssewitsch, Wiley and Sons, 1980, ISBN 0-471-07917-0 is helpful. Chapter 3 is entitled "Shielding".

vy 73

Peter G3RZP

I didn't answer since he was looking for a mystical magic bullet answer, on the off chance that SOMEONE had the EXACT same problem...
...so why annoy him with "try this" troubleshooting? He doesn't want to hear it.
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KM1H
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Posts: 3111




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« Reply #6 on: August 22, 2017, 03:09:53 PM »

In the early 60's I used that setup with never a hum complaint.

Is there any hum at a full steady 100W carrier?

Id suspect age has created a poor ground somewhere that might require some "Try Its"

Also the elderly mike cable could still have a problem.

Carl
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N6AF
Member

Posts: 56




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« Reply #7 on: August 31, 2017, 11:11:06 PM »

Follow-up. Thanks for all the comments and suggestions. Always a useful forum. It was the shield connection gone bad at the modulator sub-assembly RCA style connector.
73 N6AF
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