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Thursday, February 4, 2021

A Pair of Beomaster 1900 Receivers for Restoration: Unit #1 Power Restored

The first of two Beomaster 1900 units sent for restoration is completed and ready for some testing.
To begin testing a restored Beomaster the work always begins with plugging it in to line power, turning it on and checking the power supply voltages.

With this Beomaster I was not getting any power.

In the previous post I suspected the +15 VDC power supply fuse (a 2A fuse) and the fuse holders that had burned spots on them. I also looked at the Beomaster 1900 mains power switch and found a wire exposed through the insulation.

I fixed the wiring on the main power switch.

The Beomaster 1900 has a nice way of securing the wire to the line voltage. It holds securely but it is easy to open up and work on.

There is a plastic wire holding clip that just slides out to loosen the wire.
























I fed more wire into the Beomaster so I could cut off the bad section that had the exposed copper wire.
Then I wired the main power switch back up and put everything back in place.


























That looks much better.

Next I changed the fuse holder for the +15VDC supply.

However, there was still no power. I could see now that I was getting proper voltage from the transformer to the bridge rectifier but no DC voltage out of the device. 

The reason I changed the bridge rectifier was because the Beomaster arrived with a pieced together diode bridge and I could see the charred remains of some past event involving power on this receiver.
There is nothing wrong with making a bridge rectifier that way but I wanted to return this Beomaster back to the way it originally was.

Here is the original state of the Beomaster 1900 +15 VDC power supply as I received it.
















Here is the Beomaster 1900 with new parts: fuse holder, bridge rectifier and 2200uf smoothing filter capacitor.

















There is no way the new bridge rectifier could be bad so I investigated my installation of it again.
Checking continuity of the signal paths on the trace side of the main circuit board I discovered that the solder pads for the bridge rectifier leads had two damaged pads. They didn't reveal their damage when I installed the new bridge rectifier but underneath they were not making contact with the trace on the board.

Here I show what happened when I pulled one of the AC voltage connections to the bridge rectifier.
The +VDC pad was also not making connection even though you could not visually see it.

















I reworked the bridge rectifier connections and now it looks like this.



























You will note that I also had to place a jumper on one side of the new fuse holder leads on the trace side.
This was required because the original two-pin holder clamp previously makes that jumper.
At first I thought that was the source to my power supply problem but after making that addition I still had no +15 VDC. 
There were still the two bridge rectifier solder pads that required repair.
As often happens in these repairs there can be multiple issues affecting one function. :-)

...and now there is +15VDC power on the Beomaster.
The transformer sends around 18 VRMS to the bridge rectifier whose DC output is around 23 VDC as shown on the meter below as measured across the smoothing capacitor.
That voltage then goes through a +15 VDC regulator.




























Now I can finally move on to testing the Beomaster 1900 power supplies and make some service manual adjustments.

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