On Sun, 7 Dec 97 17:31:47 -0800 Rich Measures <measures@vc.net> writes:
>>
>>On Sun, 7 Dec 1997 11:26:57 EST JW KIMBALL <JWKIMBALL@aol.com>
>writes:
>>>Well the old saying of keep the smoke inside or it will quit working
>
>>>seems to
>>>be true.
>>>
>>>My TL922 smelled so I turned it off. Upon opening I found:
>>>L7 on V1 smoked
>>>R22 on V1 smoked
>>>C33 and C32 on V2 smoked
>...
>...snip...
>>Sounds like a simple case of internal arcing in the tube. A 10-20
>Ohm
>>resistor in the HV lead would possibly have minimized the damage.
>>The grid chokes are also way too large in current capacity. Replace
>with
>>150-200 ma types which have both extra resistance to provide a bit of
>>degeneration and will act better as current absorbers.
>>
>Chokes make flaky fuses. Ordinary carbon film resistors are easier to
>
>blow than chokes, and they are cheaper. Fast-acting 0.25A 250v fuses
>would probably work.
Are you suggesting he actually drill 2 holes on the rear apron for fuse
holders ???????
Seems you were bitchin' about a similar suggestion last week.
In any cases fuses should be in some easily and quickly accessible
location.
Personally, I prefer the chokes since as current absorbers of minor arcs
from gas, etc they will not blow or collapse. Once the event has passed
the amp works as before.
>>In extreme cases when the tube arcs or shorts you will also take out
>the
>>Zener D-2, and Bias diode D-1 and caps C-3 and C-26.
>
>If a gassy tube arced between the anode and the grounded grid, how
>could
>current flow in the cathode bias Zener?
>- With zero volts bias between the grid and the cathode, the tubes
>draw
>around 300mA of cathode current. I don't see how 300mA could blow the
>
>cathode bias Zener, Carl.
Which goes first...the grid choke or the zener? And if C3, C26 and D-1
also blow would that not assume a rather high voltage thru that path?
What is the frequency spectrum generated from an internal gas arc Rich?
>>If the amp failed while on 10/12M best to check PC-1 and PC-2 R
>values
>>also. Replace with metal oxide.
>>
>Good advice, Carl, except that I would not be as hasty to replace the
>resistors. 10m heating produces changes in the external appearance of
>
>the resistors.
Only if heated excessively. Long time exposure to lower dissapation will
still change resistor values and show no outward change in appearance. At
DC, radiated heat or RF.
A pair of 2 or 3W metal oxides in parallel will give long term
reliability if the amp will be used a lot on 10/12M.
73 Carl KM1H
> If there is no external sign of overheating, and the
>resistance has increased substantially, it is unlikely that 10m or 12m
>RF
>did the damage.
>Rich...
>
>R. L. Measures, 805-386-3734, AG6K
>
>
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