[Amps] TL922 AMP PROBLEMS

[Kim Elmore]

A quick note about using DC brush-less fans:  At least some types have 
little DC-to-AC inverters in them, and the fan motor really runs on AC (a 
few kHz, I think).  In my case, I found the inverter was subject to RFI on 
some frequencies causing the fan to stop.  I solved the problem by placing 
grounded 1/4" hardware cloth (essentially very heavy galvanized screen with 
1/4" square holes) in front of the fan, between the fan and the tube (my 
experience is with an Ameritron AL-80A). This provided enough isolation 
between the RF deck and the fan so that the fan works perfectly.  I don't 
know if this is a problem unique to the brand I used or if the problem is 
general.

I now must keep an eye on the screen, to make sure it doesn't collect so 
much dust that the airflow is reduced.

Kim Elmore, N5OP

At 03:44 PM 10/8/2002 -0700, Rich wrote:
> >Hi Rich
> >
> >OK about the fan speed and, yes, the black baffle was in place. The amp
> >has not been used on RTTY
> >but I suspect it has been pushed to the limit in CW and SSB contests.
>
>thanks.  A DC brushless fan operated from a rectifier/dc-filter connected
>to the 11Vrms fil winding would give more cooling for berserko
>contesting.
>
>cheerz
> >
> >
> >----- Original Message -----
> >From: Rich <2@vc.net>
> >To: Brian Miller <brianmiller@xtra.co.nz>; AMPS <amps@contesting.com>
> >Sent: Wednesday, October 09, 2002 12:07 AM
> >Subject: Re: [Amps] TL922 AMP PROBLEMS
> >
> >
> >>Hi all
> >>
> >>The ZL6QH team is using a TL922 amplifier which has developed some
> >>problems over the past 9 months.
> >>
> >>Here is a chronological summary of the events. Note that I have not
> >>personally observed all of the
> >>events - the following is a combination of what I have been told and what
> >>I have seen.
> >>
> >>Jan 2002 - New RF Parts tubes installed and then one of the grid chokes
> >>burnt out on start-up. The
> >>choke was repaired and the amplifier appeared to work normally again.
> >>
> >>Sep 2002 - One of the new tubes failed with an open circuit filament half
> >>way through a contest. An
> >>old "used" tube was installed as a temporary replacement.
> >>
> >>Oct 2002 - The second new tube failed, again with an open filament. This
> >>time it failed on start-up,
> >>along with several "popping" noises. At this stage I also noted that the
> >>enamel wire on the L3
> >>filament choke was discoloured - indicating that it may have been
> >>overheating.
> >>
> >>Oct 2002 - Installed two good "used" Eimac tubes. The amplifier now
> >>appears to work normally in the
> >>CW 2 KV position but is not stable in the SSB 3 KV "Operate" position.
> >>After switching to "Operate",
> >>the IP current (indicated on the front panel meter) gradually increases
> >>and then suddenly starts to
> >>intermittently jump to full scale - along with "popping"noises.
> >>
> >>It is likely that parasitic oscillations are contributing to the above
> >>situation. We will have
> >>careful look at the relevant components in the anode, grid and bias 
> circuits.
> >>
> >>I understand that the amplifier does NOT have a history of instability -
> >>the above problems are a
> >>recent development. So something has changed!
> >>
> >>Two questions
> >>
> >>1. Have other owners of TL922 amplifiers observed a similar sequence of
> >>events. If so, then what was
> >>the root cause and how was it fixed?
> >>
> >>2. Any thoughts on why the filaments in both tubes should go open circuit?
> >
> >50Hz reduces the fan speed compared to 60Hz.  Was the perforated black
> >baffle next to the  tubes in place?  Was the 922 used on RTTY?
> >
> >>
> >>I did notice that the
> >>filament voltage was running around 5% high. We have fixed this by
> >>changing the line voltage setting
> >>from 220V to 240V and reducing the external line voltage to the amplifier.
> >>
> >>Note. I HAVE read the earlier threads about parasitic suppression -
> >>including Richard's web pages.
> >>No
> >>need to repeat this information please.
> >>
> >>Thanks and 73
> >>
> >>Brian Miller ZL1AZE
> >>Registered Electrical Engineer
> >>67 Hornsey Rd
> >>Melrose
> >>Wellington
> >>New Zealand
> >>
> >>Tel 027 4446614
> >>Email: brianmiller@xtra.co.nz
> >>
> >>
> >>_______________________________________________
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> >>http://lists.contesting.com/mailman/listinfo/amps
> >>
> >
> >
> >
> >
> >**
> >
> >
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                           Kim Elmore, Ph.D.
                        University of Oklahoma
         Cooperative Institute for Mesoscale Meteorological Studies
"All of weather is divided into three parts: Yes, No, and Maybe. The
greatest of these is Maybe" The original Latin appears to be garbled.