[Heathkit] Question on SB-200 meter circuit, replacement

[wb8jkr at juno.com]

 Dennis,

 All that's involved here is basic ohms law, the meter
reads the voltage across the 15K ohm resistor R4
it, in series with the three 4.7 meg ohm resistors
form a resistive divider across the HV supply output.
 the meter is a 1400 ohm 200 ua movement which means
it will read about three volts full scale, put that in the back
of your mind for a moment. The three 4.7 meg resistors
R3 and the 15K ohm resistor R4 total 14,115,000 ohms,
the meter is marked to read 3000 volts full scale, if 3000
volts were applied to the R3/R4 divider approximately
.0002125 amps would flow through the divider, in a 
series circuit the current is the same through all the 
resistors (R3/R4) but the voltage drop will differ. The
amount of voltage dropped across the 15K resistor (R4)
would be 15,000X.0002125 or slightly over 3 volts, thus
causing the meter to indicate 3000 full scale. In actual
operation there is only about 2350 v on the HV supply
output so the meter reads accordingly.
 If the original meter isn't reading right, either the supply
output is wrong (doubtful) or the divider resistors 
have changed value. Usually meter movements work, or they don't.

73,  Mark  WB8JKR

On Thu, 23 Sep 2004 12:05:26 -0400 "Dennis Berry"
<dennisberry at worldnet.att.net> writes:
> I'm trying to understand the metering circuit on the SB-200 
> amplifier.  The meter I have seems to be off on the high voltage 
> reading, and I'm just trying to understand the circuit operation 
> better.  But, with me and understanding meter circuits and their 
> multiplier resistors, well it's like me and statistics.  My brain 
> sometimes just gets locked up and comes to an abrupt halt.
> 
> I've got a decent 200 ua meter that will fit in the existing space 
> fine, but it's internal resistance is 450 ohms.  So I need to 
> understand how the original circuit uses the resistors chosen for 
> grid, plate and HV in order to adjust the values for this meter.
> 
> If someone has done this and would care to pass on some knowledge, 
> that would be great.  You can contact me off list if you will, as I 
> may have some follow up questions as I work my way through this.
> 
> I know this is simple stuff, but it's just one of those areas where 
> it doesn't always make sense to me, and I'm sure with some good 
> clear descriptions it will make sense.
> 
> Thanks in advance to anyone who responds, 


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