>I would start by measuring the RFC's at the grid pin of each tube. If one
>is open, or the pi's slammed together there was a short in that tube.
>
I used to wonder what damages the RFCs, so I tried simulating a grid/fil.
shorted tube with a clip lead to see what happens to the RFC. The
result: the ZSAC (zero sig. anode current) increases (because the bias
changes from c. 7v to 0v)--but the RFCs were not damaged. ........
Something else damages the RFC. .
>Also check the bias zener, it is located on a small bracket bolted to the
>bottom of the filament xfmr. When that shorts it often takes the small
>electrolytic on the terminal strip right next to the zener.
>When this happens it often takes out the 100VDC supply which may take out
>the filament/bias xfmr if you keep it on too long. There is a decided
>domino effect with the 922.
>
I agree, Carl. IMO, the first domino is an increase in grid current when
the 3-500Zs occasionally oscillate at 130MHz. Since VHF energy can not
pass through the HF tank, there is virtually no load on the 3-500Zs, so
high grid current results. The grid current is apparently large enough
to implode the grid RFCs.
- All of the grid current passes through the Zener diode. Apparently,
the surge in grid current during the 130MHz oscillation is sufficient to
sometimes cause a short in the Zener. .... ..... To check out whether an
intermittent VHF parasitic oscillation took place or not, measure the
resistance of the resistors in the 922's VHF parasitic suppressors.
(note: this is not easy because one end of the suppressor resistors
needs to be unsoldered from the silver-plated strap inductor that they
are in parallel with) - If the VHF suppressor resistors increased in
resistance above their upper tolerance limit, a VHF parasite was quite
likely the reason. IMO, the reason for the problem is that the 922 uses
an illogical, although esthetically-elegant, VHF suppressor
design--i.e., one which presents a relatively high VHF-Rp to the anodes
of the 3-500Zs. Since VHF voltage amplification is proportional to
Mu(VHF-Rp), more VHF-Rp is not good news unless one is trying to build a
VHF oscillator.
.........for a comparison between a Rauchian suppressor, made from
copper-wire, and a low VHF-Rp suppressor made from resistance-wire, see
Figure 12 at:
http://www.vcnet.com/measures
>Next, power up with both tubes removed...making sure there are no
>fireworks, etc.
>
>Install one tube at a time, looking for unwanted plate or grid current.
>If all is OK, close the relay line and look for some idle current; about
>50ma in CW position.
The filaments are in series in a TL-922. The above test puts 14a through
the cathode CT DC return choke - which is obviously designed to carry a
lesser amount of current, so tarry not.
- Another reason not to tarry is that the 922 uses voltage cutoff bias,
so if a tube has a grid to filament short, a short is automatically
placed on the 110VDC cutoff bias supply, which will destroy its unfused
transformer in short order. If the 922 is converted from voltage-cutoff
bias to resistor-cutoff bias, the transformer can not be crispy-crittered
by a shorted 3-500z. ........ SB-220s suffer from the same design
weakness.
N.B.: There is apparently no such thing as a perfect amplifier, so it
might not be a really great idea to toss out your soldering iron.
Rich---
R. L. Measures, 805-386-3734, AG6K
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