[R-390] Lankford Filter Mod

[Bob Camp]

Hi

Not to pick to many nits here, but ….

The power transformer as designed should have been capable of running the heaters on the crystals and VFO in the radio. That’s not an insignificant amount of power. Anybody running one of these radios with those heaters turned on should turn them off. The have no benefit for a radio running at rational temperatures. Once you have turned them off there should be a few watts to spare off of the filament winding. I suspect that relatively few people are running 26Z5W’s rather than solid state diodes. That gives you a couple more watts.  Yes indeed you need to be careful about how you turn the AC into DC. 

There are indeed good reasons not to mess with a 390A.  Not having a couple of watts to spare on the 25V winding for this or that would not be one that I would worry much about. 

Bob

On Aug 30, 2014, at 6:45 PM, Charles Steinmetz <csteinmetz at yandex.com> wrote:

> Perrier wrote:
> 
>> I would like to see that with your skill set you would design a plug in DBM first mixer replacement using the LM 1496 for the *A*.  I think that it would be a quantum leap improvement.
> 
> I'm not sure a "plug-in" replacement is practical.  The first challenge with adding any SS circuitry to a 390/390A is getting a "ground" reference into it and providing low-voltage DC power for it.  We have seen any number of designs that create LVDC supplies from the resources already available inside the radio, but none of these is a solution I'd adopt for various reasons (hum, excessive loading of power transformer, etc.).
> 
> I would specify an additional power transformer to run the DBM, making it not strictly a "plug-in" replacement.  There is also the question whether it could be packaged (complete with any necessary heatsink) in the space available over and around the mixer tube socket.  The power and ground could go to a connector at the top of the unit, so if it would physically fit the available space it could be "plug-in plus."
> 
> There would still be things that could be done better if some further mods to the radio were allowed.
> 
> Note that high dynamic range active mixers and post-mixer amplifiers draw quite a bit of current and can be sensitive to noise on the power supply, so the power supply needs to be beefy, well regulated, and low-noise.
> 
> I would certainly not use an LM1496 as the mixer -- the best you could do with that at the levels required would be at least 20dB noisier than any good design.  A passive diode DBM looks attractive, but it would need a post-mixer amplifier, so we're back to the "not really plug-in" issue.  Adding a diode DBM and using the existing mixer tube just for post-mixer gain is a possibility, but it's not a "plug-in" solution.
> 
>> Please remember, the roofing filter mod was never stated as a cure for other R390A problems. It maybe that to achieve its best performance enhancement other mods need to be implemented.
> 
> My concern is that it doesn't cure ANY problem that simply using the 4kHz filter doesn't cure (or replacing the 8 or 16kHz filter with a narrower filter).
> 
>> There are over 10 authors that I'm aware of whom over the last 20 years who have made design improvements.  Some major, some minor.
> 
> Well, changes, yes.  Not all "improvements," by any means (judging by the random selection I've seen, not very many are net improvements).  Anyone contemplating making such changes should do their own careful design evaluation to see if the mod being considered is likely to solve any real problem(s) without creating any worse problems.  But not everyone who plays with 390s has the engineering skills to do this, so when I see a mod that, based on my design evaluation, (i) would likely solve no real problem, (ii) would likely cause worse problems than any real issue it may help, or (iii) is not the best way to attack the problem, I think it is appropriate to speak up.
> 
>> I have already released a two part schematic that incorporates most of those upgrades for those who want to join the dark side of excellent performance.
> 
> Again, anyone contemplating making changes should do their own careful design evaluation to see if the mod being considered is likely to solve any real problem(s) without creating any worse problems.  If they have questions about the wisdom of any mod that they cannot determine for themselves, they should feel free to ask on the list.  Making a change and observing how it works "on the air" is a useful process, but it takes thorough and systematic testing to see what other aspects of performance may have been compromised or degraded by the change -- things one could easily miss just by using the radio on the air in average conditions.
> 
> 
> Best regards,
> 
> Charles
> 
> 
> 
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