[ARC5] SCR-274N Dynamotor Question

Mark K3MSB mark.k3msb at gmail.com
Sat Feb 5 10:36:04 EST 2022


GM Gents

Your comments are much appreciated.   I will attempt to answer all
questions in a single response.

Dynamotor rebuild:  Yes, I rebuilt it when I put the dynamotor into service
about 10 years ago.
All caps were replaced in the BC-456 modulator.

Float Power Supply:
This is what I'm using now, and it's not working out well.   I measure
26.8V power off.
Power On Key Up:  23.8V    Power On Key Down:  23.5V.   This gives me an Ep
of only 480V.
The above voltages are measured directly on the battery terminals.

New Power Supply:

Wire Sizes:  This could be an issue and I realized it early on.  I'm using
18 AWG doubled up.
That means each connection uses two wires in parallel.
Not optimal, but a single 18 AWG wire wouldn't do.
Changing to a larger wire size isn't a problem (I just have to get some).
The dynamotor is spec'd at 5.8 A @ 28VDC so I figured paralleled 18 gauge
wire should work.

28-0-28V 50A.   Using a full wave bridge rectifier.   Positive lead goes to
0.5 ohm resistor for current inrush.
That's connected to a 20,000 uF filter capacitor.  Using a 680 ohm 2W
bleeder resistor.
I would prefer to use a 1 ohm, but didn't have one of sufficient wattage.

Using a IM-11 VTVM for all the following measurements (unless otherwise
indicated)
I have panel meters for voltage and current in circuit -- voltages agree
with VTVM readings.

With the Panel Meter set for 28 VDC:
Voltages / current measured at the filter capacitor terminals:
Key up:  28V    4A
Key Down:  24V  7.5 A.   510V Ep   30W output

Voltages measured at the bridge rectifier output:
Key up:  29 V DC
Key Down:  26 VDC

Voltages measured at the transformer output (using DMM)
Key up:  28.34 VAC
Key Down:  27.41 VAC

If I increase the voltage to 30 VDC measured at the filter capacitor output:
Key up:  30V  4A
Key Down:  25V  7.5A    Ep 550V (which is what it should be)  40W output

Current spike on startup: Yup it is !!   As expected.

73 Mark K3MSB



























On Sat, Feb 5, 2022 at 2:29 AM Robert Downs <wa5cab at cs.com> wrote:

> No.  Set it for 28VDC key up.  32 VDC would put 16 VDC on each of the 12.6
> V rated tube filaments which will significantly shorten their service life
>
>
>
> Robert Downs
>
>
>
> *From:* arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net]
> *On Behalf Of *Mark K3MSB
> *Sent:* Friday, February 4, 2022 19:38
> *To:* ARC5
> *Subject:* [ARC5] SCR-274N Dynamotor Question
>
>
>
> I'm working on a new 28V power supply for my SCR-274N system as well as
> other projects.
>
> I'm using a 28-0-28 V 50A Transformer with a bridge rectifier (so I'm not
> using the CT).   I'm using a 220V 16A Variac to set the primary voltage.
>
> When the setting set for 28V key up,  I'm getting about a 4 to 5 volt drop
> key down.
>
> I didn't expect that much voltage drop as the transformer is pretty stout
> for an SCR-274N.
>
> Can I run the system on 30 to 32V key up without damaging the dynamotor?
>
> This would be a short term fix as I suspect I'll need a VR circuit.
>
> 73 Mark K3MSB
>
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