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.
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
>
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|