[Heathkit] SB-230 grid current meter is wrong

[Kurt Fitzner]

Hi Miles et al,

Thanks so much for your reply. It's nice to know I wasn't seeing things 
on the schematic or in my math.

I do disagree on the instructions though.  I have verified that on my 
SB-230 there is indeed a 47 ohm resistor in series in the grid meter 
circuit as described on the schematic. I have also verified the 
instructions clearly install it thus.

R25, the 1.5 ohm grid current measurement resistor is installed on 
terminal strip TC1 as instructed on pg 33. The instructions on pg 35 
take one side of RFC 1 and wire it into R25 and also place R33, the 
offending series 47 ohm resistor, on the same terminal strip.  RFC1 is 
where the grid connects into the two grid meter circuit resistors.  The 
schematic shows the connection as longish lines, but electrically they 
all connect at one point, which is how they are wired. On pg 52 is where 
a white/blue wire starts on the grid position of SW2 and on page 73 is 
where it terminates at the other side of R33.  Also, the circuit 
description on pg 108 also has R33 wired the same way in the simplified 
meter schematic.

In short, it seems like it's not only the schematic.  Which means as far 
as I can tell by design every SB-230 reads 25% low on the grid current.  
That's a pretty significant error, considering how susceptible 8873 
tubes are to excessive grid current.  Figure 6 on pg 95 clearly states 
the meter is intended to show grid current as 0-80ma, then the 
instructions go one to give warnings never to exceed 40mA grid current.

Is there someone on the list familiar with the SB-230 and its history 
who can shed some light on this?

      Kurt

On 2017-08-20 12:12, k2cby wrote:

> Kurt,
> 
> Although I don't have an SB-230, your query intrigued me, so I took it 
> upon myself to investigate.
> 
> First, your math is impeccable, as is your analysis of the schematic.
> 
> I decided to look a little further.
> 
> I first noted that there is only one 47 Ohm resistor in the parts list, 
> described as a ½ watt ten percenter. That didn't sound right for a 
> metering circuit.
> 
> I then took a look at the assembly diagram.
> 
> R33 appears on page 35. It is connected to pins 1 and 3 of terminal 
> strip TC in series with RFC1.
> 
> I wasn't able to trace the circuit further in the assembly 
> instructions.
> 
> In particular, I couldn't find the page where R16 is installed.), but 
> it seems obvious that R33 is outside the grid metering circuit, which 
> is limited to R26 and the meter itself.
> 
> In short, the schematic IS wrong.
> 
> 73,
> 
> /Miles
> 
> Miles B. Anderson, K2CBY
> 16 Round Pond Ln.
> Sag Harbor, NY 11963
> 
> k2cby at optonline.net


-- 
KURT FITZNER (VA1DER)
YOU DON'T KNOW THE QRO OF THE DARK SIDE!