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Beolover SyncDrive: DC Platter Motor Replacement for Beogram 4002 and 4004 (Type 551x and 552x)

Late Beogram 4002 and the 4004 (Types 551x and 552x), which have DC platter motors instead of the earlier synchronous AC motors usually suff...

Thursday, February 22, 2018

Beomaster 6000 (2702) restoration: finishing re-assembling without chassis + testing

All the PCB's have been reconnected and time to check if all is working. I know, it looks a bit messy, but at least I got it all together on my piece of wood. I checked and rechecked all the wiring, looked for possible short-circuits, etc. 

Let's see if we can bring this mess to life ! Plugged it into my variac and slowly brought up the power. I saw lights coming on.........and the power relay kicking in. Yes !!!! No smoke or bad smell either :-)

I was a  happy man, but not for long sadly enough........Two major events happened. The first one: I could not select anything. The volume/tone/balance control was working. Motor and clutches running. I also did not get any bias current on the output stages (I had connected my multimeter before powering up to check). Hmmm.. No 60V DC rail either. I had checked the power supply before and all was working fine with the correct voltages. The 20V and 21,5V DC for the electronic switch are derived from the 60V DC, so that is why the switch is not working and no selection can be made (FM presets and input selection). Turned out the power relay did not make good contact to bring the 60V DC to the PCB's! The other voltages from the power supply PCB (18V DC, -5V DC and 180V DC) are not going through the power relay. I took the relay out and cleaned the contacts (again). But then decided to put in a spare relay that I knew was working fine. Powered up again and YES, not only lights but also switch board working and bias current. Measured over the output darlington emitter resistors and got about 4mV. Should be 14.4mV DC but I had set the multi-turn trimmers to almost minimum to be safe ! Plugged in 4 speakers and got music on all 4 channels! Time to relax a bit and take a drink...

And now the second event: when I came back a few minutes later (with my drink...) the music was still playing and I wanted to raise the display a bit because I could not see it very well. A big electrostatic discharge between my fingers and the display and 2 channels out with big hum. Disaster arrived....It's freezing cold over here, heating turned up, dry air, rubber shoes, synthetic sweater, chair with synthetics and rubber wheels,...The perfect "ingredients" to get my body fully charged with static electricity!! Should have known better. Half an hour later and 5 transistors replaced (all in the output stage) and everything was back to normal.

Now it was time to finish my drink.....

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