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Wednesday, February 14, 2018

Beomaster 8000: Power Testing the Receiver

Time to catch up with the status of the Beomaster 8000 restoration. I left off with the Beomaster reassembled and ready for its initial power up test.

Plugging the Beomaster into the AC power outlet for the first time is always filled with a little nervous excitement but it was all anticlimactic. Nothing visually happened. Mainly there was no red dot on the display board showing that the Beomaster was in standby mode. As I unplugged the Beomaster I did hear one of the power relays click so that was good to hear.

When this sort of thing happens the first thing to investigate is what is going on with the power supply. The 120 VAC, 60 Hz line voltage was there at the transformer (my house is actually 125 VAC). The 5 VDC regulator was measuring 5 volts. The ±15 VDC regulators and the ±55 VDC rail voltages were not present...but that is to be expected on initial start up if the unit never gets past the standby mode.

I opened the lid to the processor board and verified +5 VDC to the processor chips. That wasn't so welcoming of news. It means a problem on the microcomputer board.

I have three spare Beomaster 8000 microcomputer boards for this type of scenario. I swapped in the first one and tried power on the receiver again.

This time the standby LED illuminated.

Now I could start exercising some Beomaster 8000 operations. A recheck of the voltages showed me the ±15 VDC on the +15 and -15 voltage regulators. The large reservoir capacitors for the 55 VDC rails measure ±56 VDC.

During those above tests I also tried to increase the volume level but it was stuck on zero.
The problem with that could be anything from the rotary volume wheel sensor (or cable) and the microcomputer IC.

I tried a second spare microcomputer board with the same result so the problem is most likely the sensor or sensor cable. I have a spare sensor to swap with so I tried it.

There it is. The original rotary volume wheel sensor assembly has a problem. The replacement sensor assembly allows the microcomputer board to adjust the volume level up and down.

That left me with some decisions to make regarding this Beomaster microcomputer board. I have spare boards ready to go but I also have one set of Beomaster 8000 master and slave processor chips. The set is used and I don't know 100% if they are good but I decided I might as well exhaust all of the options I have. I installed the spare processor ICs in the original microcomputer board I took out of this Beomaster and gave it a try.

What do you know, the replacement processors bring the Beomaster into standby mode and then to on mode. The volume control works but now there are missing segments from the display. I know the display board itself is fine as I ran a 24 hour burn-in test on those display modules earlier.  So is the segment problem due to the processor chips or are there additional problems on this microcomputer board?

I decided to reflow the solder joints of the signal paths for the display segment drivers along with trying a couple replacement driver ICs (SN74247) I have for IC1 and IC2. The results were not successful and the problem got worse.

Eventually I got to a point where the microcomputer board would not function again. Not totally surprising as I wasn't confident that the replacement ICs I have available for testing this microcomputer board were good so I decided to go ahead and use one of my spare, working microcomputer boards as a replacement for this project. It is the safest and more reliable direction to go. I considered moving parts from the spare board to the original board out of this Beomaster but that would be an unwise risk. The less handling of the processor chips the better (even with wearing anti-static straps).

Spare board installed...These are much better results.

Moving on I found another problem with the Beomaster.  After the original bench testing of the output amplifier boards where I set the initial no-load idle current, I rechecked the left and right output amplifiers again now that they are re-installed.

The idle current check still measures good so I moved on to the DC offset voltage adjustments (for the left and right channels).  The right channel easily adjusted to the specificied tolerances of the service manual : 0.0 ± 5mVDC.  I am at 0.0 - 0.6mVDC on the right channel.

On the left channel adjustment I can only adjust the DC offset down to around 0.0 ± 50mVDC.

It isn't a high enough value to prevent me from test playing the Beomaster. I have been playing the radio and an ipod connected to TP2 for a few hours now.  However, it is not satisfactory to leave it like this. I will have to pull out the left channel output board and investigate the problem with the offset.

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