[ARC5] SCR-274N Update

[Mark K3MSB]

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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