[AMPS] Blown TL922A... What to do?

[Phil Clements]

> My guess is that gold-sputtering is caused by caused by UHF.

> >Are you saying that during the sputtering no grid current flows?

> no.  The grid can rectify MF, HF, VHF, UHF.  My guess is UHF because of
> the typical pattern of missing gold on the grid.

All my 8877 amps have a grid-trip circuit similar to those found in the ARRL
Handbooks and Bill Orr's Handbooks.

The voltage drop across the grid meter shunt is sampled through a 10k ohm
pot that drives a TIP-120 transistor that latches up a relay to open the PTT
line.
An 8877 has no business having over 75 ma grid current flowing for any reason.
If this current level is reached from whatever cause; MF, HF, VHF, UHF, broken
antenna/feedline, bad T-R relay sequencing, wrong band, wrong antenna....
(reader can finish the list!) the relay latches when 75ma grid current is reached.
The same glitch diodes that protect the grid meter protect the grid-trip circuit.
This is faster and happens much earlier in a fault condition than fuses or resistors.

8877's are $700 + per pop (pun intended) now from RF Parts. The above is really
cheap insurance. Works good, lasts a long time.

(((73)))
Phil, K5PC





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