[Johnson] Viking I Modulation troubles

[Gary Schafer]

Hi Miles,

I don't know if anyone has gotten back to you on the Viking 1 yet?
How much output power are you loading the transmitter to? If you load it to
maximum output, over 100 watts, you may not be able to get 100% modulation
as the modulation transformer in those rigs do not have the optimum ratio.
They under load the audio tubes a bit so that they go into limiting before
you reach 100%. It acts as a high level clipper.

I haven't played with a Viking 1 but assume that it is similar to the Viking
11 in operation.

73
Gary  K4FMX

> -----Original Message-----
> From: johnson-bounces at mailman.qth.net [mailto:johnson-
> bounces at mailman.qth.net] On Behalf Of k2cby
> Sent: Wednesday, March 28, 2012 5:29 PM
> To: Johnson at mailman.qth.net
> Subject: [Johnson] Viking I Modulation troubles
> 
>        I'm new to the list, and I am having trouble getting an ancient
> Viking I back on the air.
>        I seem to have two problems:
>        First: I am unable to get better than about 50% AM modulation.
>        I rebuilt the speech amplifier as per the factory release titled
> "Modification B for Viking II Transmitter" (courtesy of the BAMA
> website)
> replacing all the paper and electrolytic capacitors in the process.
>        Feeding 1000 cycles into the microphone connector, my scope shows
> 7
> volts peak-to-peak on the plate of V1, the 6AU6 speech amplifier.
>        By the time I get to V2, the 6AU6 audio driver, I'm showing 1.1 V
> p-p
> on grid 1 and 28 volts p-p of audio on the plate.
>        The plates of the 807 modulators and the 4D32 PA carry 720 volts.
>        The 807s show 55 V p-p audio to ground on each grid #1 with a
> bias of
> -38V dc. The 807 screens show 260 Volts with R13 adjusted to produce a
> no-signal idling current of 70 mA (as per the manual). The audio looks
> pretty much undistorted on the 'scope, even though the 807s are
> obviously
> being driven into the grid current region. I can run the audio drive up
> to
> 80 V p-p before the modulators start flat-topping.
> 
>        This is the second problem.
>        High voltage B+ is supposed to be 660 volts. I measure 770 volts
> in
> the CW mode and 720 in phone, which is about 17% high. The ac line
> voltage
> at the time these measurements were taken was 123 volts, which is only
> about
> 12% high. This, by the way, is with the original 5R4 rectifiers - not
> solid
> state replacements. There is also more hum on the signal than I would
> like.
> I originally thought that the high voltage choke L1 might be shorted
> (converting this to a capacitor-input power supply) but its dc
> resistance
> tallies with the manual.
>        Oddly, enough, the medium voltage (+300 volts) and bias (-75
> volts)
> power supplies are pretty close to spot on.
>        Questions:
>        1. Do I have a problem in the HV supply, or is it just a question
> of
> higher line voltage?
>        2. Will bucking down the High Voltage B+ to nominal 660 improve
> the
> modulation percentage?
>        3. Will replacing the 807s do anything for me?
>        4. Should I increase the negative bias on the 807s and push more
> audio drive into them?
>        5. Should I consider replacing the 4D32 with two 6146s and adding
> a
> clamp tube a la the Viking II?
>        What say some of you AM-ers who have had more experience than I?
> 
>        Miles B. Anderson, K2CBY
>        16 Round Pond Lane
>        Sag Harbor, NY 11963
>        Tel.: (631) 725-4400
>        Fax.: (631) 725-2223
>        e-mail: k2cby at optonline.net
> 
> ______________________________________________________________
> Johnson mailing list
> Home: http://mailman.qth.net/mailman/listinfo/johnson
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Johnson at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html