And here is a question (no answer yet).....
Does anyone think that hot switching an amp (perhaps by using the tune
switch as described below) can TRIGGER a VHF parasitic ??
I have no trouble recognizing the parasitic or its aftermath, but I have
serious problems understanding the triggering.
I know that many amps are quite susceptible to VHF parasitics without mods,
yet even those amps will go years and years without trouble. Then,
suddenly, BANG (literally).
73 de Gary, AA2IZ
----- Original Message -----
From: "Rick Denney" <rick@rickdenney.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Monday, March 31, 2008 11:56 PM
Subject: Re: [TenTec] Ameritron amp issues and looking for a Centaur
> Bob McGraw - K4TAX writes...
>
> > Your initial problem was caused by a VHF parasitic oscillation that
> > caused failure of the 811A's. Common faults occurring after a
> > massive VHF oscillation can likely be switch damage on the input
> > section or output section of the band switch. Likely you should see
> > evidence of burn marks or melted contacts or maybe contacts that are
> > no longer present.
>
> The band switches all look good and test good, with both DC and RF (at
> low levels from my MFJ-259) continuity through all bands. The problem
> behavior is the same no matter what the position of the band switch.
>
> > Although the 572B's have a higher plate dissipation, I don't suggest
> > replacing 811A's with 572B's as there is no advantage due to power
supply
> > voltage and current limitations.
>
> I didn't do it to get more power--I know that the amp is constrained
> by the power supply. I did it to get graphite plates instead of
> sheet-metal plates with those soft, shiny bits in the middle where I
> cooked them, heh, heh.
>
> > If you are using the N.O. relay in the Omni to place the amp into
transmit,
> > yes, you will experience hot switching in the amp. This is not a QSK
amp
> > and thus the transceiver should not be operated in the QSK mode. You
will
> > have to use manual PTT to accomplish RX to TX switching.
>
> I thought of this later, and that was the reason for my question. The
> only time I see the hot switching as a problem is when I use the
> "Tune" button. I'm normally using the SSB mode, and as long as I don't
> start talking until everything's keyed up, there shouldn't be much
> signal making heat when it switches. But I was doing a lot with the
> tune button during my testing. Once I get it working again, I will use
> the amp-control switch on my foot switch to key the amp.
>
> I would have thought that hot-switching would be primarily visible
> with burned contacts on the T/R relay, but there's no evidence of that
> and the relay works perfectly. It might have taken out a component,
> but dang it there aren't that many components in an AL-811, and I've
> tested many of them.
>
> Keeps those cards and letters coming.
>
> 73, 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
>
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>
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