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VK2ZRG > TECH 20.12.05 01:15l 33 Lines 1425 Bytes #999 (0) @ WW
BID : 1894_VK2ZRG
Read: DL1LCA GUEST OE7FMI
Subj: re: A puzzling fault
Path: DB0FHN<DB0FOR<DB0SIF<DB0EA<DB0RES<ON0AR<IK1ZNW<7M3TJZ<F6CDD<I0TVL<
VK2TGB<VK2IO<VK2WI
Sent: 051219/2246Z @:VK2WI.#SYD.NSW.AUS.OC #:1496 [SYDNEY] FBB7 $:1894_VK2ZRG
From: VK2ZRG@VK2WI.#SYD.NSW.AUS.OC
To : TECH@WW
VK2ZRG/TPK 1.83d Msg #:1894 Date:19-12-05 Time:22:44Z
Jerry K0CQ wrote in part :-
>Some production runs of other TNCs had a problem with the
>negative supply caused by the electrolytics in the negative
>supply being mounted with reversed polarity. Check those in your
>TNC too. Most electrolytics will accept a small reversed voltage
>but will act like a zener diode to limit that reversed voltage if
>there's impedance in the charging circuit.
Well the capacitors in the charge pump negative supply of my KPC-3 TNC
are 0.1 uF monolithic types. The charge pump frequency is 13.5 kHz, so
the circuit can supply only a limited amount of current...just enough
for a 14C88 RS 232 driver and load. Even then the O/C voltage drops
from -10 to -5 volts. When I replaced the damaged 14C88 with a TTL MC1488
the voltage was only -2.4 volts. So as an interim measure, I've raised
the frequency by around 6 times and now have -3.8 volts. The TNC is
working OK now and I'll get a 14C88 when it's convenient.
Note your comments on MAXFRAME Jerry. More often than not, there is
only one user at a time on the local BBS's. The BBS's seem to send
two packets at a time then wait for an ACK. So maybe I'll tACO41400VK2UEBalph VK2ZRG@VK2WI.#SYD.NSW.AUS.OC
/ack
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C Program run, C Program Crash...ReWrite in Pascal!
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