Putting the SB-614 back into service

[Doug_Hensley at GENERALHEALTH.ORG]

Thanks Jim,

I am taking the liberty of sharing your thoughts with the list as I think there
will be other SB614 users out there that would enjoy the thread.  Since the
zeners are warming from their idling current, I'll look for a suitable
replacement.  Will also check C201 and C202 as you suggest.  I am working
without a manual at present so your input is very much appreciated.  Will also
let you know how it turns out.  Very 73, Doug

____________________Reply Separator____________________
Subject:    SB-614
Author: "Jim Cunningham" <marijimc at axs.net>
Date:       2/20/99 3:40 PM

ZD101 and ZD102 are an attempt to achieve regulation of a 180 volt supply.
A rather poor attempt indeed as no 180 volt zener was available apparantly
so two were used - 110 and 68 - in series.  Nothing magical about these.
Power handling is the big issue.  if, as reported in the schematic, the
source is 290, it must be dropped to 180 (plus a smidgen) under minimum
conditions.  2.1K is used so current must be 52 ma to drop the voltage.  The
180 volt line feeds the horizontal deflection, vertical deflection, and the
astig grid.  The astig grid is a relatively dry circuit with only the 150K
pot to dissipate heat. Obviously, prolonged heavy deflection currents might
generate some heat.  Boards in the vicinity of power resistors, diodes, etc,
will generally show signs of heat but the discoloration isn't a real failure
or threat of one.  Those components should be mounted up, off the board to
allow cooling air to engulf them while, mounted flush on the board, one of 4
sides will be prevented from radiating any heat.

The 1.5K, 2W, if R201, is directly accross the horizontal deflection plates
of the CRT (pin 9 to pin 10 of CRT).  Since input is capacitavly coupled,
the overheating there is hard to understand.  Barring the failure of a
coupling capacitor on the input, the resistor is really from 180 to 180
(plate to plate) with 100 pf capacitors blocking the DC.  Resistor would
have to have reacted to RF coming in I would expect - in a negative way.
But I doubt the difference between 1.5K and 2.0K is significant.  1.5K +-
20% would be 1.8K anyway.  If mounted up, off the board, that resistor
shouldn't hurt you.  Should one of the coupling (or blocking) capacitors
between the resistor and deflection plates leak a lot of current, that could
account for a lot of dissipation since there is only a coil (choke) to
ground.  DC current to the tune of about 120 ma, >21 watts, would zoom right
on through if C201 or C202 is shorted (er.....high leakage).

The transformer isn't very special.  6.3 VAC, half wave, doubler to get 9
volts, 210, full wave bridge, zener regulated to get 180, 600 VAC,
rectifier/doubler to get 1400 for CRT anode, and a straightforward 6.3 VAC
for CRT filaments  You don't have to retain a 210 volt source just to get
180 regulated (290 after filtering).  Transformer heat rise would be less if
that zener current were diminished (so also the zener diodes).  And the 9
volt supply could be derived from an 8 or 9 volt AC source, 12, or just
about anything.  Adding a seperate filament transformer for CRT filaments
and 9 volt supply would reduce the heat rise.  You've a lot of flexibility
on that one.

That's about all I know of your problem.  Hope it isn't too much of a
problem for you to resolve.  (BTW - use a 50 watt resistor if you want!  Not
going to shift the problem anywhere!  Just wouldn't get as hot!).  Jim




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From: "Jim Cunningham" <marijimc at axs.net>
To: <Doug_Hensley at generalhealth.org>
Cc: "DavidC" <davidc at bit-net.com>
Subject: SB-614
Date: Sat, 20 Feb 1999 15:40:30 -0600
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