Power Supply Repair

[Robert Shields]

I have an Astron RS-35M Power Supply that is out warranty and is blowing the glass fuse.  Does anyone know anybody local that can troubleshoot and repair the problem?  It’s only 1.5 years old.

Thanks in advance!

KC1RLS – Bob – Bedford, NH

[Hamilton]

Hello Bob,

I can easily do it, I’m down 101 a few miles in Amherst. Another option is Charlie Dunn, he is also a NARS member.

 

[Hamilton]

As a linear supply the RS-35(M if it has meters) is mostly a large heavy transformer with a small voltage regulator pcb and a few other components making it easier to troubleshoot and repair than a switching PSU. This can be seen in figure-1.

[attachment file=”astron psu.jpg”]

Here are the troubleshooting steps I used to repair Bob’s Astron RS-35 PSU;

1.) The first step is a quick test. I replaced the fuse, disconnect the PSU from other equipment, and turned on the power. The fuse promptly blew confirming the need for further testing.

2.) I removed the yellow wires from the transformer’s secondary to the bridge and the red wire to the large cap to isolate the primary bridge rectifier.

a.) The power supply turned on without blowing fuse.

b.) The secondary winding voltage was nominal at +34.24Vac. This is shown in Figure-2.  Note this is both sides of a grounded center tapped winding. The voltage is +17Vac  on each side when measured to ground.

[attachment file=”103361″]

c.) The meter lights illuminated, and the front panel volt meter read 13.8V suggesting the low voltage meter and control circuits are all ok.

d.) With the PSU unplugged the resistance to ground of the red wire disconnected from the bridge going to the large cap should be over 1MΩ to check the cap and NPN power transistors for possible shorts due to failures.

3.) With the PSU unplugged I measured one side of the bridge rectifier for junction voltage (or resistance if a diode setting is not available on your DMM). Vforward = 0.509V  is nominal for a high current bridge.   This is shown in figure-3. The low junction voltage is due to the low current available from the DMM. Under normal operating conditions a junction voltage of .9V to 1.1V is normal.

[attachment file=”103366″]

4.) I measured the other side of the bridge rectifier for the diode junction voltage (or resistance if a diode setting is not available on your DMM). The Vforward = 0.001V, this side of the bridge had failed shorted. With the two bridges in parallel it is unknown which bridge actually failed. No time was invested in reviewing this further as both bridges are always replaced.

The RS-35(M) is a solid well made PSU and is generally very reliable. I have seen a failed bridge several times before.  This is likely due to “current hogging” which can occur if the two parallel bridges are not closely matched for Vforward. The bridge with the lower junction voltage will conduct the majority of the current which will increase its junction temperature further lowering its junction voltage. The current sharing between the two bridge rectifiers will be reduced possibly leading to  diode failure.

I will replace the two bridges with one rated for 50A and a 1kV PIV. The other option is to place a 3mΩ 5 watt resistor in series with each bridge to ensure equal current sharing. These ultra low resistance power resistors are readily available from Digikey.<!–more–>

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[Hamilton]

Figure-1 did not attach to my prior post, here it is; [attachment file=”103473″]

A linear power supply has fewer parts than a typical switching supply. Astron has a good parts department and will ship most parts at a reasonable price within a day. This includes the complete regulator pcb.

The trade off between a linear PSU and a switching PSU with the same power capability  is the linear PSU is larger and heavier due to the large transformer, and its output has less to no RF noise which can be desirable for radios with a pan adapter including SDRs. Switching PSU are typically smaller, more efficient and generate less heat.

Personally I have both types of PSUs in the shack and even with the IC-7300 and a SDR I do not notice a significant difference between PSUs except when it is time to move the heavier linear PSU.

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