[ARC5] Interesting occurrence...

[Don Davis]

It might have been rewired for 12 volt filaments.  Do you tubes test ok? 
Bad habit to leave powered up when sleepinng.

73 de don ad6pb


----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: "ARC-5 List" <arc5 at mailman.qth.net>
Sent: Friday, April 01, 2011 3:28 PM
Subject: [ARC5] Interesting occurrence...


> Well, that R-25/ARC-5 I got working the other day has recently 
> exhibited an
> interesting (to me) phenomenon:
>
> First, an explanation: this unit had been rather badly hacked (among 
> other
> things, extra holes in the case) and all the remaining "flower-pots" 
> were bad.
> Some were missing.
>
> Overall, it didn't look all that bad, so I replaced the bad caps with 
> 0.05 mfd
> 1KV disk ceramics, cleaned everything cleanable, and plugged tubes 
> into it.
>
> The power supply that I am using was built back in the 1970s by 
> someone
> who did a pretty fair job. As it came, it output about 230 VDC and 24 
> VAC,
> and had separate switch to turn the filaments and the B+ on and off.
>
> However, the only DC filter was a triple 40 MFD at 450 VDC upright can 
> cap
> with all its sections in parallel. No choke or RC pi-filter.
>
> When connected to the receiver via an adapter made from a dynamotor
> base, the receiver exhibited quite a lot of hum, especially at low 
> audio levels.
>
> So, I added a 3K ohm 5 watt resistor between the input filter (2 
> sections of
> the can-cap in parallel) and the output cap. This brought the voltage 
> down to
> about 145 VDC, so I moved the screen tap to the high end of the 
> screen-
> dropping resistors.
>
> However, the hum was gone. I suppose I should reduce the value of the 
> filter
> resistor by 1/2 and restore the screen connection to the center-tap 
> point.
>
> I used the receiver for a couple of weeks, and was quite pleased with 
> its
> stability, excellent calibration, and sensitivity. I then replaced the 
> 12SK7s in
> the RF and 1st IF with 12SG7s. This dropped the noise down by about 
> 1/2
> and increased the sensitivity noticeably (unmeasured).
>
> When I would go to bed, I would turn off the B+ but leave the 
> filaments on.
>
> I started to notice that the overall audio level was slowly dropping, 
> and last
> night, it was almost down to zero.
>
> In fooling with it, I found that when the tuning reached about 2.7 
> Mhz, the HF
> oscillator quit completely.
>
> So, I replaced the original 12K8 with one I pulled from another 
> receiver.
> Although this revived the receiver, it couldn't be accurately 
> calibrated any
> longer, the low-end calibration cap being maxed out before reaching
> accurate calibration.
>
> Also, the antenna peaking cap was now way over against the left stop 
> for the
> peak over most of the tuning range.
>
> I am now wondering if leaving the filaments on, but no B+, for 
> extended
> periods has a deliterious effect on the tubes, or if some other 
> component is
> failing?
>
> Anyone have an idea?
>
> Eventually, I want to use this receiver to monitor 160 meters and WWV.
>
> Ken W7EKB
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