VK Powermaster fails after more than 25 years


On Monday I put out a CQ SSTV on 20m and when the transmission was finished I looked over to the receive screen and saw the waterfall display was empty. No signal was being received. Not even noise.

I went out to the shack and sure enough the FT757 was lifeless. No lights, no noise, nothing.

I have been working in the radio maintenance game for most of my life and when I don’t see lights where I should, I instinctively look at the power source, in this case, the VK Powermaster.

My VK Powermaster has no voltage meters or output indicators on it, just an illuminated mains power switch. This switch was dark so I knew there was no mains power to the unit. A quick check of the other 240 V appliances in the same power point showed the mains was still present. I unplugged the VK Powermaster from the wall and checked the fuse. One 3 Amp fuse – blown.

I replaced the fuse and the second one blew straight away. Hmmm. This is looking serious.

I built the VK Powermaster over 25 years ago and it has powered many rigs along the journey and has given me faultless service until now. The only modification I have made is a reset switch that releases the over current protection circuit in the event of shut down. I did this because the original design required the mains power be turned off long enough for the filter capacitors to discharge. From memory this took about 2 minutes. I got sick of waiting after each shut down so I fitted a momentary switch that removes the wait. I can’t recall exactly how this works but once I find a circuit diagram I will post details.

So with fuses blowing at switch on I took the cover off and started working through the circuit from the mains input toward the regulator circuit. The VK Powermaster is a linear regulator typical of the 1980s era having an 18 Volt AC transformer, a four way diode bridge rectifier, four big electrolytic capacitors, and four 2N3055 pass transistors as the major parts of the circuit.

A visual inspection showed the wiring in and out of the mains switch and the transformer was good so I disconnected the bridge rectifier from the transformer, replaced the fuse and switched on again. This time the fuse survived. This told me the fault was between the bridge rectifier and the DC output.

Since the rectifier has Faston terminals it was easy to remove it from circuit and test it out. With the diode tester on my Fluke DMM I measured between the terminals and found two diodes were good and two diodes were short circuit. The two shorted diodes were effectively placing a short across the output of the transformer. Hence the large current flow and the fuse failure.

A trip to Jaycar yesterday saw a new 400V 35A bridge rectifier fitted and the VK Powermaster lives again.

73

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3 Responses to VK Powermaster fails after more than 25 years

  1. Eraina & Richard Jenkins says:

    My Powermaster suffered from shutdown at turn-ON from the beginning. I think that I must have a super sensitive SCR. It’s an issue with several Yaesu rigs … I saw a hint suggesting that the 0.01 cap on the gate be replaced with a 2.2uF non-polarised electro to address this problem. Also suggested was to adjust the resistor chain by changing the 3k3 resistor with a 1k5 resistor. I have done this second mod … and it gives me no trouble at switch-ON. This PS is 30 + years old now but still gives great service. Any other comments about the Powermaster??

    Richard
    VK1RJ

    • Peter Fraser says:

      Thanks for your comments. The MK2 version changed the capacitor on the SCR gate to be 4.7 uF, added a reset switch across the Anode and Cathode of the SCR and an ammeter. For my first VK power master I didn’t change the capacitor but it hadn’t given me start up problems with the loads I was using.

      A couple of years ago I bought a second MK 1 very cheap and dead. I changed out the bridge rectifier and rebuilt it as the internal wiring between the board and everything else was poor. I also added an ammeter and reset switch to this one as well. I don’t know its history but wherever it had been it was doing lots of heavy work as the transformer has a significant hum when the unit is loaded for any length of time.

  2. Geoff Armstrong says:

    I have seen problems with these supplies, caused by the crowbar circuit not triggering the scr. I believe the problem lies in the fact that the basic powermaster is only capable of 8 amps continuous. The voltage required to trigger the circuit is not provided at these lower currents. Thank you for your interesting report on this good old power supply.

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