[ARC5] SCR-274N Update

Wed Oct 2 14:21:04 EDT 2013

  Modified ARC-5 transmitters had been used for very sable VFO's for Central Electronics Exciters
with no issues. They were operated at much lower voltage and power. I used them for many years but not
lately. The problem may be more complex than capacitor contacts. The important issue with operating class C is
that it is maintains an impedance that the modulator can handle for AM operation.

73
Bill wa4lav

________________________________________
From: arc5-bounces at mailman.qth.net [arc5-bounces at mailman.qth.net] on behalf of Ian Wilson [ianmwilson73 at gmail.com]
Sent: Tuesday, October 01, 2013 7:18 PM
To: ARC5
Subject: Re: [ARC5] SCR-274N Update

I had a slightly hacked 7Mc ARC-5 transmitter that had all sorts
of problems with the oscillator. It would warble, the frequency would
jump, etc. At one stage I removed and cleaned the main tuning
capacitor. This made things magically OK for a while. Then the
problem returned. I still suspect a dirty contact somewhere in
the tuning capacitor area (or perhaps the padder capacitor). This
(a high-resistance path in the oscillator) could cause the sort of
problem you are seeing with low output.

Have you checked the grid current on the problem TX? If the
drive from the oscillator is low, the tubes won't be in Class C,
and the matching network might not stretch to the impedance that
the plates want to see.

73, ian K3IMW

On Tue, Oct 1, 2013 at 3:44 PM, Mark K3MSB <mark.k3msb at gmail.com> wrote:

> Hi Gang
>
> Got to have some radio time yesterday and did some experimentation to try
> and characterize my ARC-5 “issues” related in the previous email.
>
> To save typing:
>
> 40M TX/RX is BC-459-A / BC-455-B
>
> 80M TX/RX is BC-696-A / BC-454-B
>
> Receiver drift tests were to allow 30 minutes warm-up then measure drift
> after 30 minutes.
>
> 40M RX:  1.92 KHz / hr.  Dynamotor used.  RX works well.
>
> 80M RX:  This is the receiver with the nasty hum on the audio.   Drift was
> 0.98 KHz / hr with the dynamotor.   The tone from my ICOM was T7 or T8.   I
> then removed the dynamotor and used my HP-23 for the B+ (250).  Drift was
> less than ½ KHz / hr, and the ICOM tone was a nice T9.      I tried
> swapping the dynamotor with the 40M RX last week and the hum on the audio
> persisted, so the hum looks to be an RX problem related to the
> dynamotor.  Perhaps
> L15.   All capacitors, except C5, were replaced (forgot that one…)
>
>
>
> Transmitter testing:  30 minute warm-up then test after 30 minute
> intervals.
>
> 80M TX:  almost no drift, nice tone heard on ICOM RX.  TX works well.
>
> Plate: 555V, Screen 308V,  Osc 225V,  Power out: 30W @190 mA Ip
>
> Here’s an on-air recording my friend made:
>
> http://www.k3msb.com/scr274n/k3msb_80m_on_air.wav
>
>
>
> 40M TX: (The problem child….)
>
> Plate: 550V, Screen  296V,  Osc 198V,  Power out 20W @ about the same Ip.
>
> Drift:  sometimes none, sometimes about 1 KHz over a 60 second interval.
>
> Tone was rough (T7 or T8) on my ICOM as well as received by my friend:
>
> Here’s an on the air recording:
>
>
>
> http://www.k3msb.com/scr274n/k3msb_40m_on_air.wav
>
>
>
> I swapped the Oscillator and PA tubes from my 80M TX and put them into the
> 40M TX, and the results were slightly better, but only very, very slightly:
>
>
>
> http://www.k3msb.com/scr274n/k3msb_40m_new_tubes_on_air.wav
>
>
> I then returned the original tubes into the 40M TX.
>
> As a final test, I removed the PA tubes and listened only to the Oscillator
> signal on my ICOM—the oscillator was rough!!  So, it looks like a bad
> component in the BC-459A oscillator circuit.   All caps were replaced in
> the transmitter.  No resistors were replaced; some were slightly out of
> spec but not by much.
>
> A final observation – regardless of the series capacitor I use on 40 or 80,
> the TX coupling control is always at max for maximum power, which seems
> strange.  I’m going through a 4:1 UNUN after the series cap.
>
> 73 Mark K3MSB
>
>
>
>
>
>
> On Thu, Sep 26, 2013 at 7:54 PM, Bill Cromwell <wrcromwell at gmail.com>
> wrote:
>
> > On 09/26/2013 01:29 PM, Mark K3MSB wrote:
> >
> >> Both the 40 and 80M transmitters have a slight chirp which doesn’t
> bother
> >> me in the least.
> >>
> >> The stability and tone on the 80M transmitter are fine, but the 40M
> >> transmitter drifts and the tone is about T8, maybe T7.  The drift is
> >> noticeable as I sometimes have to slightly retune during a transmission.
> >>
> >> The power supply for both transmitters is the same.  It’s a hefty 28V
> >> power
> >> transformer with a full wave bridge rectifier and a 38000 uF cap (with
> >> bleeder) as the filter.  The HV/OSC/Screen supply is completely
> separate.
> >>
> >> 73 Mark K3MSB
> >>
> >>  Hi Mark,
> >
> > A slight amount of chirp gives a signal some "character". On a busy
> > contest weekend when the bands are filled wall to wall with sterile,
> > computer generated code a fist and some "character" stands out better
> than
> > any sliver-slicing filter. Large amounts of 'drift' that take a
> transmitter
> > out of the distant receiver's passband (and onto a different QSO in
> > progress) is a problem. Good to get rid of *that*.
> >
> > 73,
> >
> > Bill  KU8H
> >
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