Beomaster 1100
This one came from Germany.
The owner said it had stopped working.
And right he was. It didn't work.
Covers off and I soon discovered a warm area and a burned hole around a solder pad for power resistor 5R1.
Under normal conditions this component gets about handwarm, so this one definitely faced a higher current flow than usual.
A brief look at the schematics and it was clear, that this could only come from a shorted voltage regulator 5TR1.
And quite right, it had a dead E-C short, allowing a good 45 Volts onto the 33V rail.
Luckily most of the circuits on that rail is fairly forgiving and nothing had blown so I chose to replace
the transistor and fitted a new diode for 5D1 for good measure. After all that poor diode saw the same current flow as the resistor.
With the burned hole and broken copper traces tidied, the radio was back to life.
I decided to replace the idle current trimmers and adjust them. The original parts looked better than average but they will fail sooner or later.
I also replaced a good handful of capacitors now I had it on the bench.
I want to take this opportunity to answer many questions regarding the blobs of thermal paste on the driver transistors.
I have seen cases where owners clean it off and that could spell disaster; The thing is, that the driver transistors
amplification factor (hFe) depends greatly on temperature. So does the output stage transistors - and even more so because
they are darlingtons and in effect "two transistors in one", one after the other.
As the output stage heats up in use, the amplification increases, that in turn increases the temperature and if no means for
regulating this was incorporated the whole thing would do a thermal runaway until something burns or blows.
(These repairs are often quite involved).
To counter for this situation, the driver transistors are placed physically near an output stage transistor and thermally connected
to its collector pin (center pin in this case), which representes the "spine" material of the output stage transistor, which is also
the surface that is bolted towards the cooling fin. All copper so very good thermal leads.
The thermal connection is done by placing the driver transistor in a coil of quite heavy gauged tinned copper wire, often soldered
to the output stage collector, with a good blob of thermal paste added between the driver transistor and the coil.
This allows the driver transistor to "sense" the temperature of the output stage and regulate the idle current accordingly.
So whenever you see this, don't clean it off, unless it has hardened and started flaking off in which case it must be renewed.
(Always use the good old white thermal compound, - the modern grey stuff sold for computer processors will dry to an almost
solid mass in a matter of weeks = no good for amplifier purpose).
A fresh set of lamps were fitted and with the Beomaster now playing along nicely, I noticed the pushbutton for the power switch
had a nasty scratch.
I can't send this otherwise well-preserved and loved set back with that unsightly button.
The dungeons provided a nice replacement.
It played the whole next evening while I worked on an unusually stubborn Beogram CD50 that had
numerous "mods" and "hacks" done to it - all of which I had to first figure out and then reverse to make it play again.
Another Beomaster back in daily use.
They are nice sets. Too bad some countries chose to settle for DAB. FM provides a much better sound quality, being a technology refined
through decades.
BTW - there's no such thing as "digital broadcast". You cannot send 0s and 1s (read: morse) "fast enough" unless modulated into
something else. Just like data on magnetic media, hard disk drives etc.
It's all FM modulated.
Did I hear a laugh?..
Martin
