[TMC] SBE-9 question

Tue Jan 24 19:31:16 EST 2012

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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