What does a TAB Lucas CS-1 Mic PSU have to do with a Christmas Tree bulb?
Michael Wagener's TAB Lucas CS-1 Mic PSU was delivered to me a few weeks ago with a note saying it had died during a session. When I first hooked it up it seemed to be working AOK and after a few minutes I typed off an email to Michael saying it looked like all was well. No sooner than I had hit "send" I heard a "pop" and the mic went dead. I had to send an awkward second email to Michael saying that "I had spoken to soon" and found that I had already got a reply from my first message saying "it had to warm up for a while", and that it had been sent to Germany to TAB once before for the same problem. That time it turned out the problem had something to do with diodes in the power supply.
I removed two screws from the power supply ( PSU ) and pried open the incredibly tight case (my guess is they didn't account for the thickness of the paint) it was a trick to do without scratching the finish. Looked around and sure enough there were a couple of diodes that had obviously been replaced before and looked like they were running really hot - the PCB was burnt and charred and the traces were hanging on by a thread. I confidently patched and jumpered the traces, turned the power supply on and shot off a third email to Michael saying it was "fixed" then "pop" the mic was dead again - what?! - OK, no more emails until I know it is fixed!
Time for a schematic - my TAB folder had a bunch of files, but nothing on this... Logged on to the Brotherhood of Audio Service Technical Resources and Engineering Database (B.A.S.T.R.E.D for short) nothing there. Googled and Binged for half an hour - nothing but a lot of people looking for the same schematics. With that I began to draw my own schematic by tracing out the circuit board - this took a while. As the picture starts to form and you begin to understand the circuit, you inevitably have to re-draw the circuit once or twice so it doesn't look like a pile of spaghetti...
Why is it that when I finally get the big picture, I end up scratching my head and thinking "why did he do it that way?"... for what it is, it seems to be a lot more complicated than it needs to be. Well, maybe he did that on purpose to make it hard for guys like me to figure out... but probably the truth is the designer had a background steeped in tube technology and was designing with solid state in terms of tubes. Perhaps he wasn't familiar with all that was available in solid state power electronics. Or, maybe he had a surplus of parts that he wanted to use up...
Whatever the reason for the over-engineered power supply. It boiled down to a high voltage and a low voltage - one for the tube mic's plate and one for the tube's heater I assume. The heater supply also triggers a relay to switch on the microphone once the power supply was stable.
So I connect my meters and scopes and wait for the thing to fail. The first several times I had been connected to parts that turned out to be OK before and after the failure occurred. So I'd have to wait for the thing to cool off, then reconnect to the next part to see if it was the one failing and so on. One time I had to wait hours and hours, and finally turned it off to go home because it wasn't going to fail for me.
The next day it was ready to co-operate - I had worked my way to the suspect diodes, they all tested good, all the traces were connected properly. I connected the meters and waited for it to fail while I worked on other things... "pop" went the mic... I checked the meters and they were going crazy. The heater voltage would rise and then suddenly shut down, then rise again and shut down, over and over... One of the diodes was acting like a flashing Christmas tree light. Back in the days of incandescent bulbs, they would make a flashing bulb by building in a miniature thermostat... the bulb would get hot and click off, then cool down and click back on... but I have never seen a diode act like that in all my 30 odd years of doing audio repairs!
Well, ALL the diodes got replaced with a higher wattage version and the power supply worked like a champ! Remembering all those good people who were looking for the schematic, here it is for you!