Fw: [ARC5] T-22 Capacitors

Wed, 6 Mar 2002 07:48:08 -0600

A slightly leaking capacitor effectively does nothing, but the question
is .......where do you draw the line. Must capacitor checkers vary
greatly on what causes the "eye" to close indicating leakage. The 
"eye" in an Eico 950 started to close at 2 microamps, on a Heathkit
IT-28 it started to close at 0.3 microamps. 

If it's a leaky screen bypass capacitor it will increase current flow
   through the screen resistor, causing the screen voltage to drop.
If its a leaky coupling/plate blocking capacitor between stages it
  will allow current through the grid resistor of the driven stage. One
  microamp of leakage across a 1 Meg resistor is one volt. This 
  will alter the bias level of that tube possibly causing distortion.
If it's a leaky capacitor in a resonant circuit, it will change resonance
  and probably affect circuit "Q"
If its a leaky capacitor in a cathode it will affect tube grid bias
levels.
If it's a leaky electrolytic in a power supply it will reduce it's
filtering
   effectiveness (cause hum). If it's real severe, it will overheat the 
   electrolytic causing additional internal failures possibly leading to 
   can rupture. Exceptable electrolytic leakage current is much higher
   (in milliamps) that for decoupling capacitors (in microamps).
If it's a power supply decoupler, it will cause a slight reduction in 
   voltage but mainly it's decoupling effectiveness will be reduced.
If it's in the RF out circuit (tank circuit) it consumes power, reducing 
   the rigs output and could lead to arcing/heating and failure.

For these "keep it original" rigs I'd leave it alone untill it causes a 
problem, not because a checker said it was "bad". For other rigs
I would replace any cap that showed any leakage. And if you are
real picky I know of people who carefully cut the cap open, remove/
drill out the internals, replace that with a small modern mylar 
capacitor and put the old ends back on on. That's purely to maintain
asthetics, allows a still original looking rig to be used on the air, and

looks great .......but I'm not that patient. 

And don't forget the resistors which, through moisture intrusion or 
mechanical problem, can change value "way outside" specified limits
and alter circuit characteristics or the mica in trimmers which 
deteriorates over time and can cause shorts. 

73s  Kees K5BCQ 

--------- Forwarded message ----------
From: "J. Forster" <[email protected]>
To: ARC-5 <[email protected]>
Date: Wed, 06 Mar 2002 01:48:40 -0500
Subject: [ARC5] T-22 Capacitors
Message-ID: <[email protected]>

Has anyone really checked to see what the impact of a slightly leaky
capacitor
is on the operation of a T-22 ?? There has been a lot of discussion about
replacing them, but WHAT DO THEY DO ??

Take a look at Figure 25 in AN 16-30ARC5-2. The capacitor in question
appears to
be C58

C58A is a bypass, isolated by a 20 ohn resistor. Probably little
consequences to
any leakage here, unless the cap overheats. This can be checked by
powering the
TX up gradually, with the filaments off.

C58B is part of the resonance indicator, which you probably do not use. A
leaky
resistor could upset the bias on this tube and might cause tube damage,
but I
have not analysed the circuit.

C58C is bypassed with R74, a15K ohm +/- 10%,  in the PA bias circuit. Is
the
leakage significant compared to this resistor?

IMO, unless the leakages are large, they are not really significant.

Flame shields up !!

-John

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