Ed writes...
> You need to measure this for forward/reverse resistance, not continuity.
> This diode acts as a fuse to protect the metering circuit. With no drive
> applied, you should see no grid current.
Bingo! D16 was a dead short. I replaced it, and the amp now seems to
work fine.
> If I understand it correctly there were/are 2 versions of this amp.
> Would you take the time and look here to see if this schematic agrees
> with yours ?
> http://www.ameritron.com/man/pdf/AL-811.pdf
Mine is older--doesn't have the transistor switching of the T/R relay;
doesn't have the diode string in the filament circuit; used a
full-wave bridge rectifier in the low-voltage circuit that powers the
dial lamps and T/R relay, etc.
> Another thing that bothers me is the ALC circuit. again an old and
> proven design. However it picks off some of the RF output, rectifies
> it and sends it back to the input circuit. If it were me I would
> take this out of the RF path, especially since TT does not provide
> ALC.
If I end up with a Centaur, this will be my second operating position
when I install a new antenna later this year, and that radio (a
Kenwood) has ALC.
> Keep in mind that TT stopped producing the Centaur, not because it
> didn't sell, but they could not find acceptable tubes.
What is meant by "acceptable"? Does this mean that the Centaur has
different requirements than the Ameritron in terms of tubes? I figured
I could just order the Chinese-made Taylor Select tubes from RF Parts,
and not drive them as hard.
> I'm not going to comment on the parasitic discussion other than to say
> that most catastrophic events in an amp are caused by component failure
> and/or operator error.
When tuning on 10 meters, I overdrove the grid and plate currents. I
was looking at the wrong meter (the wattmeter) and overdrove the amp
during tuning. Also, for a while now I've been tuning the amp using
the Tune control, and switching the amp with the Relay out from the
Omni V. This combination apparently hot-switches the amp. I suspect it
was the overdriving that caused the problem. I wonder if the old
suppressor design's inductor wasn't bypassing all that excessive 28
MHz RF around the resistor, causing it to overheat and blow. For all I
know it may have already been blown, waiting for me to use it on 10
(which I had not done before).
Anything I should look at carefully before declaring victory?
> **** KEEP IN MIND HV CAN AND WILL KILL YOU **** BE CAREFUL ****
Oh, yes.
Thanks for your excellent help.
Rick, KR9D
---
Richard W. Denney, Jr. PE|Iteris, Inc.
Associate Vice President |107 Carpenter Dr. Ste 230 | 703.925.3819
rwd@iteris.com |Sterling, VA 20164 |Fax 703.471.1757
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|