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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 17th Jan 2019, 5:40 pm   #1
g4wim_tim
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Default Carrier PSU hum question

Hi guys,

as some of you may know I've been building a 160 / 80 / 60 mtr AM tx from scratch.

It's very nearly finished and I'll post picture shortly - however I have come across a rather odd problem.

At full power on 80 and 60 mtrs the 100Hz PSU ripple is about -40dBc perfectly adequate but on 160 mtrs it's significantly higher at -25dbc.

I have the same problem with the modulator disabled and the drive signal into the 6164's is clean so it appears to be RF PA related but only on 160Mtrs.

I suspect it maybe RF feedback related as th VFO, buffer and driver would all be on 160 mtrs - but no fiddling with the wiring seems to affect the carrier hum.

Anyone else come across and figured out a cure ?

Granted it's not a big problem and in real life operation probably wouldn't be noticed - but I'd like to get to the bottom of it anyway !

Regards Tim
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Old 17th Jan 2019, 6:56 pm   #2
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Default Re: Carrier PSU hum question

I'd be looking at RF-getting-back-into-the-VFO. RF-getting-into-the-VFO-when-operating-on-the-VFO's-fundamental has always been an issue - the answer is either:

1] Decouple, Decouple, Decouple - and add a zero-gain buffer-amp immediately after the VFO to prevent RF from the PA going-backwards through the chain and causing problems.

or

2] Arrange for the VFO to never be on the RF PA's output-frequency or any of its subharmonics [meaning VFO on something like 200-500KHz and then up-mixing with crystal-derived step-frequencies to generate the transmitted frequency].

**In the days when I did this sort of thing professionally I remember seeing a prototype HF transmitter designed by one of my big-bucks competitors. its PLL VFO ran at the frequency they were wanting to transmit on. It had horrible issues regarding time-taken-to-lock, because of leakage from the Kilowatt-or-so of PA output back into the VFO.
I didn't win that particular gig but was happy to see my competitors spend £0.5Million trying to get theirs to meet the specs.

Also, does your modulator have a *hard* cutoff of audio-response below 300Hz? I've seen at least one old-style AM transmitter where the modulator's response went down unnecessarily-low and hum picked-up on the low-level AF stages was still being amplfied, meaning the [slightly-unbalanced] P-P modulator re-introduced this hum to the whole power-supply rail.

Does the wobbliness continue if you short-circuit the modulation-transformer *PRIMARY* ends??

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Old 17th Jan 2019, 7:43 pm   #3
g4wim_tim
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Default Re: Carrier PSU hum question

Thanks for the suggestions which line up with my thoughts.

The VFO is well screened and filtered but will add more filtering plus there is one short wire which is exposed from the VFO output to the first buffer. I'll screen it.

The hum is always there even at reduced power but not quite so bad.

I haven't shorted out any of the windings on the mod tranny yet but have removed the KT88's so pretty sure it's RF feedback related much as you suggest.

The mod response is from 340Hz to 3.4kHz.

I'll do some more tests and experiments then report back.

Interesting to hear about your experiences - but a mixer VFO is not possible due to space.

Regards Tim
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Old 17th Jan 2019, 7:55 pm   #4
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Default Re: Carrier PSU hum question

How are you stabilising the VFO's HT? A simple purple-glow-valve is OK, but remember that these present a varying impedance when the upstream HT supply has any kind of ripple - and that varying impedance can be 'reflected' into the downstream stages.
It's hard to decouple the varying impedance of a glow-regulator without risking it turning into a relaxation-oscillator.

Also, look at adding decoupling to the VFO's heater-circuit, specially if the VFO design has an indirectly-heated cathode that's 'live' to RF rather than being grounded. Add a couple of RF chokes to the heater-feed to make sure no RF can find its way back from the PA heater to the VFO. If this were the best of all possible worlds, you were Doctor Pangloss and had unlimited funds you'd be feeding the VFO heater from a separate LT transformer/power-pack to avoid such coupling.
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Old 17th Jan 2019, 8:23 pm   #5
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Default Re: Carrier PSU hum question

Hi again, looks like I may have found the problem - but answer your questions first the 85V VFO supply comes from a well filtered and stabilised 85A2 with no noticeable ripple.

The VFO uses an EF91 and just for kicks I powered its heaters from an external 6.3V DC supply and low and behold 100Hz ripple modulation of the carrier is now very nearly as good as it is at 3.5 and 5MHz.

So going to focus on filtering the VFO heater supply and see what happens as clearly its a potential feed back path from the 6146's to the VFO.

At present the VFO heaters are powered via a couple of filter con feed thru' capacitors into the VFO diecast box - but the capacity of the feed thru' is likely insufficient.

Regards Tim
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Old 17th Jan 2019, 10:06 pm   #6
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Default Re: Carrier PSU hum question

All sorted now, 100Hz PSU carrier hum now greater than -45dBc on all bands and all power levels.

Fitted a balanced pi filter to the VFO heater circuit and all good now - that simple !

Thanks for the guidance !

Regards Tim
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Old 17th Jan 2019, 11:00 pm   #7
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Default Re: Carrier PSU hum question

Result!!

It's always fascinating how these things work out when you get down to the nitty-gritty of volts and amps and not-immediately-obvious resistance/capacitance-paths. Sometimes the solution's not at all complex, but is obvious when you look at it in hindsight.

If you want to try your design out on-air drop me a PM: I can do 25W of AM on all HF bands from my FT897D and 7.5W on 80/60M with the PRC320 [which reportedly sounds a lot louder than the 897 because the PRC320 has battle-ready speech-processing courtesy of Plessey's 1970s series of SL600 VOGAD chips]
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Old 18th Jan 2019, 3:55 pm   #8
g4wim_tim
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Default Re: Carrier PSU hum question

Yes very satisfying when you fix a problem like this.

Pictures of final assembly now in the homebrew section - time for some air testing !

Regards Tim
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