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Re: [Amps] follow-up to Titan 425 problems

To: David J Windisch <davidjw@cinci.rr.com>
Subject: Re: [Amps] follow-up to Titan 425 problems
From: Joe Giacobello <k2xx@swva.net>
Reply-to: k2xx@swva.net
Date: Mon, 15 Oct 2007 09:39:58 -0400
List-post: <mailto:amps@contesting.com>
Dave, I'll wager that the grid current readout is pegging because the 
4.7 ohm resistor, R17, on the bias switch board is open. I believe this 
is a 1% resistor.   If you call John Maples at TT, he will send you 
one.  It's an easy repair.

I would also strongly recommend that you bite the bullet and just 
replace all the filter capacitors in the amp.  Most likely, all the 
original capacitors will start going west in a relatively short time.  
They're available from Mouser and or Digi-key at half the TT price.

73,

Joe
K2XX

David J Windisch wrote:
> Hi, all concerned:
>
> This amp was built in 1994 or 1995, according to TT's service dep't.  It sat 
> apparently unused for years: no wear spots, no dings, no scratches, no 
> finger jam anywhere, no tracks on bandswitch contacts, no silvery pilot 
> lamps, and so on.
>
> When I fired it up for testing, the QSK was inoperative.  This first repair 
> effort was my initial contact with TT manuals and sorta-schematics*.  The 
> schematics omit parts and leave out nomenclature on some of the parts they 
> do show.  It took an effort to trace the QSK circuit out, going from board 
> to board, and another effort to figure out just how the QSK works, even with 
> the sorta-explanation in the manual.   The eventual solution was to unwind 
> an end-turn on an opened RF choke, and splice it.
>
> Second attempt at testing produced a bodacious BANG, from the amp deck, I 
> think, and the primary fuses were toast.  Didn't figure out what caused that 
> bang.  Replaced the fuses, and powered up without further incident.
>
> For the first year or so, I operated the amp on c-w at the low-voltage 
> setting with 1500W out, and then, after another bodacious BANG followed by 
> RF power drop-off to about half, I went to the high-voltage setting to get 
> back to 1500W out again.  Didn't find anything amiss -- at that time, 
> anyway.
>
> There happened later another bodacious BANG, which turned out to be the 
> traces to the step-start resistor in the power supply simply vaporizing.  I 
> replaced them with #12 jumpers.
>
> Last week, the power supply produced a couple more bodacious BANGs.
>
> I pulled it apart, and found a popped filter cap, which I think is C6.  I 
> noticed that one of the two resistors in parallel with C6 is 150 ohms, not 
> 150K like all the others.  The step-start resistor was also burned open, and 
> the soft-start fuse was blown.
>
> TT quoted $35 a pop for the filter caps.  They're available at better prices 
> elsewhere.
>
> I removed the cap and jumpered its connections.  Fired it up again, got HV 
> OK, but the meter pegs in the grid-current position and the red OVERDRIVE 
> lamp illuminates.  Same after removing the tubes.
>
> While looking into that, I noticed what may have produced the bodacious BANG 
> followed by power dropping, mentioned earlier: there had been an arc and 
> carbonization where the plate RF choke plate-side lead was run (too) near 
> the middle of the choke body.  Wire-brushed the choke body windings, and 
> they looks OK under magnification.
>
> Any one out there have any ideas about the grid meter problem?   I've 
> searched the archives on "titan or 425" and seen nothing except that it 
> happens.
>
> TIA & 73, Dave,
> N3HE
>
>
>
> *Afaik, I have the matching manual. 
>
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>   

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