[ARC5] ARC5 Digest, Vol 169, Issue 69
Jerome Wysocki
jeromewysocki48 at gmail.com
Mon Feb 26 10:00:44 EST 2018
I would second the statement of running a separate power supply for the oscillator. This should eliminate possible voltage sagging issues caused by modulation. I read a discussion on this same topic a number of years ago, where separate power supplies, not just VR tubes, were thus recommended as a preventative measure when operating on AM phone. Good luck and let us know what finally fixes the problem.
73s
Jerry Wysocki, KC9JXE
Sent from my iPhone
> On Feb 25, 2018, at 3:36 PM, arc5-request at mailman.qth.net wrote:
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> Today's Topics:
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> 1. Re: Lopsided modulation (Richard Knoppow)
> 2. Re: Lopsided modulation (Richard Knoppow)
> 3. Re: Lopsided modulation (Fuqua, Bill L)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Sun, 25 Feb 2018 13:16:26 -0800
> From: Richard Knoppow <1oldlens1 at ix.netcom.com>
> To: arc5 at mailman.qth.net
> Subject: Re: [ARC5] Lopsided modulation
> Message-ID: <474978e7-75d7-ac83-04de-a8796f19a6b2 at ix.netcom.com>
> Content-Type: text/plain; charset=windows-1252; format=flowed
>
> Is this a plate modulated rig, transformer makes it sound
> like one. I still think you could hear it if the oscillator is
> FMing. I think you will be on the wrong track going after the
> audio. You might also see if the B+ for the oscillator is varying
> with the modulation. A scope is the test tool for this but a VOM
> will show variation on a steady tone to the microphone. The
> amount of FM will depend on the particular oscillator circuit.
> Even though the final amp is running AB1 the plate current is
> still varying with the modulation so it could affect the plate
> supply. Another experiment would be to run the oscillator from an
> independent supply.
> Also, you will not see the relative strength of the
> sidebands on a scope anyway. A spectrum analyzer is best but you
> can use a selective receiver, say one with a crystal filter in
> it, by modulating with a steady tone. You can separate the
> carrier and the two sidebands with that setup.
> BTW, someone posted that the carrier never went away in
> either AM or FM, this is true of AM but not FM where the carrier
> definitely nulls at certain combinations of modulating
> frequencies and modulation indices. In fact, carrier nulling is a
> method of calibrating the modulation index.
>
>> On 2/25/2018 11:46 AM, AKLDGUY . wrote:
>> Modulator is homebrew:
>> * Dynamic mic salvaged from Kenwood TR-9130, with input circuitry
>> closely adhering to the Kenwood schematic -->
>> * 3 stages of MPF-102 FET amplification to 50k pot for level
>> setting -->
>> * LM-380 power amp driving the 8 ohm winding of a small audio
>> line transformer connected backwards (Jaycar MM-1900). The 20k
>> and common taps feed the modulator grids, with 5k tap used as
>> center tap -->
>> * pair of 2E26s in AB1, screens regulated at 150V, bias via
>> center tap from 3x9 volt batteries decoupled with 47 uF 35V
>> electrolytic -->
>> * Thordarson 75 watt modulation transformer T-11M75 set for 8k:8k
>> ratio, appears to be a NOS item.
>>
>> I have had a report that my audio is good, but want to fix the
>> unequal sideband level. Checking with CRO is not an option, a
>> multimeter is all I have.
>>
>> Neil ZL1ANM
>
> --
> Richard Knoppow
> 1oldlens1 at ix.netcom.com
> WB6KBL
>
>
> ------------------------------
>
> Message: 2
> Date: Sun, 25 Feb 2018 13:20:24 -0800
> From: Richard Knoppow <1oldlens1 at ix.netcom.com>
> To: arc5 at mailman.qth.net
> Subject: Re: [ARC5] Lopsided modulation
> Message-ID: <a219d0d7-26b1-da4b-0672-a235a664889a at ix.netcom.com>
> Content-Type: text/plain; charset=windows-1252; format=flowed
>
> I wonder if your 1001A also had hum modulation at higher
> frequencies. Mine does and it seems to be from the magnetic field
> of the power transformer affecting the amplifier. GR made these
> things for a couple of decades. Very definitely has asymmetrical
> sidebands. It was a cheaper generator (and lighter) than the
> 805B/C which has excellent modulation but weighs over 100 lbs.
>
>> On 2/25/2018 12:15 PM, Fuqua, Bill L wrote:
>> ? Lopsided sidebands are not unusual in older AM test equipment.
>> A classic is the GR-1001a signal generator. M
> --
> Richard Knoppow
> 1oldlens1 at ix.netcom.com
> WB6KBL
>
>
> ------------------------------
>
> Message: 3
> Date: Sun, 25 Feb 2018 21:36:05 +0000
> From: "Fuqua, Bill L" <wlfuqu00 at uky.edu>
> To: Richard Knoppow <1oldlens1 at ix.netcom.com>, "arc5 at mailman.qth.net"
> <arc5 at mailman.qth.net>
> Subject: Re: [ARC5] Lopsided modulation
> Message-ID:
> <DM2PR0301MB0703EA4579D49ACEE6DE920FCBC20 at DM2PR0301MB0703.namprd03.prod.outlook.com>
>
> Content-Type: text/plain; charset="iso-8859-1"
>
> The first carrier null occurs at when the deviation is 2.4 times the audio frequency.
>
> This is wideband FM and also has multiple sidebands. But Narrow band FM with a modulation index of less than 1 the carrier is not greatly affected. I did not notice the actual sideband levels were mentioned. You should be able to calculate the modulation index. I will have to think about the math but suspect the modulation index is less than 0.5. At that level of FM the carrier would be reduced by about 1dB.
>
> The only solution in the transmitter is to add a buffer between the VFO and the amplifier.
>
> I would not worry about it. It did not seem become a problem until it was noticed. I suggest that others check their AM arc5 tx.
>
> My GR1001A had some 120 Hz sidebands. The SB12a was the first SA that I had that had the resolution to see them. Since the best resolution was 100 Hz I could not see 60 Hz sidebands if they were any to notice.
>
> 73
>
> Bill wa4lav
>
>
> ________________________________
> From: arc5-bounces at mailman.qth.net <arc5-bounces at mailman.qth.net> on behalf of Richard Knoppow <1oldlens1 at ix.netcom.com>
> Sent: Sunday, February 25, 2018 4:16 PM
> To: arc5 at mailman.qth.net
> Subject: Re: [ARC5] Lopsided modulation
>
> Is this a plate modulated rig, transformer makes it sound
> like one. I still think you could hear it if the oscillator is
> FMing. I think you will be on the wrong track going after the
> audio. You might also see if the B+ for the oscillator is varying
> with the modulation. A scope is the test tool for this but a VOM
> will show variation on a steady tone to the microphone. The
> amount of FM will depend on the particular oscillator circuit.
> Even though the final amp is running AB1 the plate current is
> still varying with the modulation so it could affect the plate
> supply. Another experiment would be to run the oscillator from an
> independent supply.
> Also, you will not see the relative strength of the
> sidebands on a scope anyway. A spectrum analyzer is best but you
> can use a selective receiver, say one with a crystal filter in
> it, by modulating with a steady tone. You can separate the
> carrier and the two sidebands with that setup.
> BTW, someone posted that the carrier never went away in
> either AM or FM, this is true of AM but not FM where the carrier
> definitely nulls at certain combinations of modulating
> frequencies and modulation indices. In fact, carrier nulling is a
> method of calibrating the modulation index.
>
>> On 2/25/2018 11:46 AM, AKLDGUY . wrote:
>> Modulator is homebrew:
>> * Dynamic mic salvaged from Kenwood TR-9130, with input circuitry
>> closely adhering to the Kenwood schematic -->
>> * 3 stages of MPF-102 FET amplification to 50k pot for level
>> setting -->
>> * LM-380 power amp driving the 8 ohm winding of a small audio
>> line transformer connected backwards (Jaycar MM-1900). The 20k
>> and common taps feed the modulator grids, with 5k tap used as
>> center tap -->
>> * pair of 2E26s in AB1, screens regulated at 150V, bias via
>> center tap from 3x9 volt batteries decoupled with 47 uF 35V
>> electrolytic -->
>> * Thordarson 75 watt modulation transformer T-11M75 set for 8k:8k
>> ratio, appears to be a NOS item.
>>
>> I have had a report that my audio is good, but want to fix the
>> unequal sideband level. Checking with CRO is not an option, a
>> multimeter is all I have.
>>
>> Neil ZL1ANM
>
> --
> Richard Knoppow
> 1oldlens1 at ix.netcom.com
> WB6KBL
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