Hi Sam...
Gary and I almost never disagree, so there is probably something here that I
am missing.
But:
The transistors are graded into beta (gain) groups as they are manufactured.
The transistor cases are marked either with letters or color dots to
indicate their gain range. Naturally, when replacing a pair of transistors
in an 8-device amplifier, one should use transistors in the same gain family
as the rest, and I should have mentioned this in my earlier note.
However, as long as the two being replaced are in the same gain range as the
rest in the amplifier, I do not consider it important or econically feasible
to replace them (8) all. In fact, someone who isn't experienced in changing
out these devices can very easily do more harm than good. I only would
change the two on that particular amplifier board, and pay close attention
to matching voltage and quiescent current on all four amps.
Motorola uses alphabetic designations for beta groups. Toshiba and others
use color dots.
Here are some of them:
Beta Range Color Code Letter Code
48-63 Green F
57-78 Blue G
72-93 Violet H
I think RF Parts has a complete list on their website but I don't have the
URL handy.
If I decided to change all eight of the transistors in my Herc II, it would
be to put 2SC2879s in it, a much more robust device than the 454 or 458.
73
Phil
VFO Stabilizers and PEP adapters for wattmeters...Check out
http://home.comcast.net/~k4dpk
----- Original Message -----
From: "Sam Bridges" <radiobridge@InfoAve.Net>
To: <tentec@contesting.com>
Sent: Monday, November 14, 2005 6:10 PM
Subject: [TenTec] repair of Herc II amp RE Phil (k4dpk)
> Hello Phil:
> I wondered about replacing all eight trans myself, but Gary at TT says
that is the only safe way. I gather the reason is that (I am told) the trans
must be matched in current drain or trouble will results. Secondly, it
appears that RF trans in or out of circuit can appear to test good when in
fact they will not produce full output. This seems to be the result of the
physical make up of the trans.,viz. part of it will be working but the rest
will not. As a result the trans will not fail a continuity test but be bad.
I am new to RF Trans so I could be wrong.
> Anyone know what the resistance between the collector and the base
should be of a MRF454 out of circuit?
> Tnx de KJ4ZM Sam
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