[TMC] SBE-3 & SBE-6 on AM

[Duncan Brown]

The AWA Museum has a SBT-1KV transmitter with a SBE-6 (similar to the 
SBE-3) exciter.  I tried to set it up for AM for the AWA QSO Party a few 
weeks ago and the AM signal looked very distorted. I pulled the SBE-6 
and brought it home to work on, but have not been able to find the problem.

Basically, in AM, I can get 100% positive modulation, but only about 50% 
negative modulation.  Played around with audio levels and carrier 
insertion, changed tubes, used external oscillators and realigned 
everything, but to no avail. I wonder if others have had the same problem???

The problem seems to be in the 250Khz mixer/IF area.  The only 
conclusion that I have come to is that perhaps there is something wrong 
with the way the 250Khz is reinserted with the sidebands. The SBE-3 
manual never talks about 100% carrier AM (only 10-50%).

In the later SBE-8,9,10, there is an added 90 degree phase shift circuit 
(that is not in the SBE-3 or -6) between the 250KHz oscillator and the 
carrier insertion point (V126-1).  The SBE-8,9,10 manual says that this 
is to provide a "true AM signal".  Implying that previous designs did 
_not_ provide a "true AM signal" ???   (It is well within the realm of 
possibilities that the SBE-3 was originally designed mainly for SSB and 
that full AM (by turning up the Carrier Insert control all the way) was 
just thrown in as an "extra".  Then later on (after the SBE-6 was in 
production) some customer wanted to use the SBE-x for real AM and the 
TMC engineers realized that they had to correct the phase of the carrier 
inset, so it was put into the SBE-8,9,10 version.)

Has anyone with a SBE-3 or -6 had this problem??


Another problem I found with the SBE-6 is a lot of 60 hz hum that seems 
to be getting into the signal in the 250Khz section.  It is about -40 to 
-50dbc on the output, but audible.  I spent a lot of time trying to 
track down the hum source thinking it might be connected to the 
distortion, but never found a smoking gun. Probably due to a bad ground 
somewhere and the fact that they run the tube heater returns through the 
chassis.  I tightened up all the tube socket screws (they provide all 
the ground points for each tube) but didn't make much difference.  Is 
this a common problem?? (I'm thinking about running separate wires for 
the filament returns in this section.)

Thanks for any help/comments,


Duncan Brown, K2OEQ

Antique Wireless Association Museum Asst. Curator, Commercial Equipment
(also Chief TTY operator & repairman)
http://www.antiquewireless.org/



---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com