[TMC] SBE-9 question

Wed Jan 25 02:50:55 EST 2012

John,

thanks for the commentary.

Sheldon

On 1/25/2012 3:31 AM, John Vendely wrote:
> Interesting observations.  I've never seen these changes before.  TMC
> seems to have had a bad habit of neglecting to provide a detailed
> explanation of many of their equipment modifications.  But it looks to
> me like this is just a mod to provide high line input impedance.  The
> original design was for the usual 600 ohm balanced line operation, and
> the use of 560 ohm resistors (R101 and R102) across the audio input
> transformer secondaries suggests the impedance ratio of these
> transformers is 1:1.  The 5K value of the gain adjust pots is consistent
> with this--a standard value about 10X higher than the termination
> resistors, so as not to significantly affect input impedance.  The grid
> impedance of the audio input stages is of course enormous, and of no
> significant effect.  Assuming transformer mutual inductance isn't the
> limiting factor, increasing the termination resistors to 22K would
> increase the exciter's audio input impedance to about this value.
> Naturally, it would have been necessary to increase the gain adjust pot
> resistance as well, hence the change to 250K--roughly 10X the value of
> R101 and R102, just as before.  There seems little point in upping
> R101/R102 to 2.2M, so perhaps the reference to this resistor value on an
> adjacent page is simply a  mistake or typo.
>
> The existence of the carrier phase shift network on the SBE-10 is
> interesting, another feature of which I was unaware, and which was not
> present on the SBE-3.  This would certainly improve AM operation, which
> was truly dismal on the SBE-2 and SBE-3.  However, even with this
> improvement I would not expect great AM, due to differential delays
> across the USB and LSB filter passbands which will upset the
> all-important phase relationships between the two sidebands.  This is
> certainly no way to produce high quality DSB-AM.  However, TMC was,
> after all, a "sideband company" and they undoubtedly considered AM to be
> very "passe", when these exciters came out.  The SBE series exciters
> were really aimed at the more important sideband market.
> Interestingly, TMC did AM "right" in the MMX-2 series exciters, which
> featured a separate DSB AM modulator to avoid this problem...
>
> 73,
>
> John K9WT
>
>
> On 1/24/2012 11:59 AM, Nick England wrote:
>    
>> Thanks, Sheldon - This weekend I did notice those other resistor
>> changes which as you say should work out to be a wash on the line
>> input levels. And I'll confess to having skipped right over the Change
>> 1 pages that listed the new parts values (aside - I sure prefer Field
>> Changes that include some note of *why* things changed!).
>>
>> The 0.05v (-23dBm) input is for the 600 ohm line inputs, not the mike
>> input (-50 dBm input). And I see -20dBm to +10 dBm input range in the
>> SBE-2,3 spec sheets&   manuals as well as the SBE-9 ones.
>>
>> Maybe some day we'll come across a note in K4OZY's pile o' paper that
>> explains this change.
>>
>> And no, somehow I can't see TMC acting like WRL and changing parts
>> values simply because they got a good deal at the surplus house!
>>
>> Progress report - I went through all the alignment on Saturday and it
>> seemed to be working OK. On the scope I saw good SSB, DSB, and AM
>> waveforms - but later I went back and re-aligned some stuff, and now
>> the AM output looks hinky - I must have twiddled something the wrong
>> way. After looking at the schematic today, when I get home I'll check
>> the carrier re-insertion phase shift control R280 which is critical to
>> AM. I'm writing stuff like this partly to remind myself and also in
>> case it will help someone else some day who is resurrecting one of
>> these gadgets.
>>
>> Man, getting one of these things tuned up is a far cry from your
>> average Drake or Collins ham rig...great fun!
>>
>> Cheers,
>> Nick K4NYW
>>
>> On Tue, Jan 24, 2012 at 9:10 AM, Sheldon Daitch<sdaitch at kuw.ibb.gov>   wrote:
>>      
>>> Nick,
>>>
>>> The change adding that voltage divider appears also to be associated with
>>> the change in the T101 and T102 secondary loading resistors from 560 ohms to
>>> 22K ohms.  Interestingly enough, sheet 2 of the change notice calls for R101
>>> and
>>> R102 to be 2.2 megs, while the schematic calls for 22K ohms at those points.
>>>
>>> Also note that the two gain pots (R168 and R169) were changed from 5000 ohms
>>> to 250K ohms.
>>>
>>> This change might not have any impact on the line input levels needed to
>>> produce
>>> a certain RF output in the SSB mode, but the audio test procedure using the
>>> 0.05V
>>> calls for the test audio into the mike input, right?.
>>>
>>> The higher resistance pots would not load down the output from the V101 mike
>>> amplifier and that would require require some attenuation at the grids of
>>> V122A
>>> and V123A, to keep all the levels about right.
>>>
>>> Looks like with the changes in R101/R102, R168/R168 and the addition of the
>>> R289/R290 and related R291/R292 voltage divider, it would seem the line
>>> levels
>>> would pretty much remain the same for a given SSB RF output, but it
>>> certainly
>>> seems to change the way the mike input levels would work.
>>>
>>> As to why the change?  Maybe a bargain on 250K pots?  The sales volume of
>>> exciters
>>> doesn't seem to warrant the addition of four resistors - the trade off in
>>> the cost of the
>>> pots vs the additional resistors, so economics might not be the explanation.
>>>
>>> Another comparison is in the specs between the SBE-9 specs and the SBE-3
>>> audio line input levels is that the -3 just has the lower limit (-20dBm) for
>>> full RF
>>> while the spec with the -9 also has a high input spec (+10 dBm), so perhaps
>>> the
>>> changes were associated with this spec change in some way.
>>>
>>> What do you think?
>>>
>>> 73
>>> Sheldon
>>>
>>>
>>>
>>>
>>>
>>> On 1/21/2012 1:58 AM, Nick England wrote:
>>>        
>>>> Hi gang -
>>>> I've just started to work on getting my SBT-1K operational. I have an
>>>> SBE-9 exciter for it and today had a little head-scratching fun
>>>> tracking down some hum in the USB audio path (re-tightening ground lug
>>>> screws fixed 95% of it). Note to self: do this throughout these TMC
>>>> units.
>>>>
>>>> But in the process I noticed a difference between my unit and the
>>>> schematic I was using -
>>>> On the inputs to audio amps V-122A and V-123A there evidently was a
>>>> field change to add a 25:1 voltage divider (8200 ohm series&     330 ohm
>>>>
>>>> shunt to ground) . It does not show up in the Army AN/URA-28 TM or in
>>>> the original SBE-3 and SBE-9 manuals on K4OZY's web site, but it does
>>>> show up in the field changes in the front of his SBE-9 manual. Well,
>>>> it shows up on a conveniently undated schematic there but not in the
>>>> Field Change instruction sheets.
>>>>
>>>> In my SBE-9 someone didn't like the audio gain reduction this gives
>>>> (or there was a later FC I don't know about) and they cut the 330 ohm
>>>> ground lead, essentially bypasssing the voltage divider.
>>>>
>>>> Does anyone know why this mod was done or have any other info on it?
>>>> There is something odd about such a drastic change in AF input level.
>>>> The other thing I have noticed so far is that the alinement (it's an
>>>> Army TM) procedure I'm using calls for a 0.05v audio line input
>>>> signal, but that really overdrove the USB and LSB sections. Like maybe
>>>> that 330 resistor needs to be hooked back up. But that manual's
>>>> schematic doesn't show the voltage divider so maybe the field change
>>>> did two things - gave more stage gain somewhere else but attenuated
>>>> the input signal.
>>>>
>>>> I'm just guessing in the dark here and it really won't matter to
>>>> getting this rig running, but I always like to understand why/what is
>>>> going on when I come across modifications.
>>>>
>>>> OK, ramble mode off - if you were here, I'd just point underneath the
>>>> chassis and show you the scope.
>>>> cheers,
>>>> Nick K4NYW
>>>> www.navy-radio.com
>>>>          
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