[ARC5] Update on first IF mod - screen voltage...

[J Mcvey]

Hello Ken

My radios are running with the original dynamotor (24V, 240V )to have that "authentic experience".

However I fixed up a R27 for a buddy that I found at a Hamfest with the worst construction power supply of power supply that I had ever seen. After tearing it down and rebuilding , it only put out about 155V plate voltage. The radio , as you say, seriously lacked sensitivity. Removing the ground from the divider brought the supply up to about 170V and brought up the screen voltages too, but I think it was a bit too high so I put a zener string there to adjust it to about 105V , if memory serves. The only iIl effect was a little less audio power due to the reduced B+...

I came to that solution by looking at the 12SK7 data sheet where it shows that it wants 100V on the screen whether you run 100V or 250 on the plate!  Who am I to argue?




 

    On Tuesday, April 24, 2018, 2:27:47 PM EDT, Kenneth G. Gordon <kgordon2006 at frontier.com> wrote:  
 
 Have you checked the screen voltage on your RF and IF tubes? According to specifications, 
that should be the same as the plate voltage. As I remember it, the screen voltage is 
normally less than 1/2 of that.

Since the screen voltage comes from the center-tap of those two large black resistors in the 
rear, if you are using "lower" (than 250 VDC) plate voltage, you really should "adjust" the 
screen voltage upwards to compensate.

Those of us who have used 24 VDC as the plate voltage have found that moving the tap for 
the screen voltage to the +HV input to those two resistors makes a HUGE difference in 
sensistivity.

At my favorite of +170 VDC plate voltage, moving the screen voltage lead to that same point 
makes a big improvement in how well the receiver works too.

And it is easily reversible.

Ken W7EKB


On 23 Apr 2018 at 14:50, J Mcvey via ARC5 wrote:

> 
> I played a little more with the R-27 selectivity mod yesterday.
> To compensate for the looser coupling loss from increasing the distance between 
> the coils, I jumpered R-9 (1st IF cathode resistor) with an 820 ohm resistor.
> The radio seems to have the same sensitivity as a before now.
> 
> Ironically , when I compared the two IFs ( one modified, one not) on the spectrum 
> analyzer, they didn't look all that different. However, I think that may have had 
> more to do with the low levels I was using to drive the cans and impedance 
> matching which probably affected the overall Q in both tests.
> 
> In practice, there is a noticeable improvement in selectivity which is , of coarse, 
> most noticeable in the CW area.
> 
> 



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