[R-390] B+ Dropping Resistance (was Drill a Hole...)

[Drew Papanek]

Bill Hawkins wrote (not in the order I quote):

  "Anybody nostalgic for a 535/545 series scope? I have 5 of them."

That's a little too much nostalgia for me (I already have a 535).  The Tek 
535 is a fine scope, works pretty well even by today's standards and is a 
must-have for boatanchor enthusiasts.  If it does nothing else, with its 
half a kilowatt power consumption and big cooling fan it makes a good space 
heater.

" Well, Tek used a number of regulated supplies and all were based on the
   value of the -150 volt regulator [Warning - Do not adjust -150 unless you
   are prepared to realign the entire scope]. This was reason enough to 
delay
   the application of high voltage."

Drift of -150 supply (and moving those other regulated voltages along with 
it) and failure of a decoupling resistor or two is all the trouble I've had 
with it over the past 15 years or so.  Use a DVM to set -150 and then the 
adjustment will be repeatable.  If you don't have silver solder for the 
ceramic terminal strips then cut leads close to defective part and then 
splice to replacement part.

  "Another reason is that the no-load voltage
   across filter caps (and wiring) is higher than the loaded voltage. I 
don't
   think that either concern applies to our favorite receivers."

My R-390A has silicon rectifiers and a 220 ohm series resistor. Audio output 
tube screens measure 240 volts at turn on.  As it warms up it drops to 190 
volts.

Some have experienced RF deck coil insulation breakdown which they have 
attributed to elevated B+ before warmup, but I doubt that to be the cause.   
Also, that voltage surge forms electrolytic filter capacitors to a voltage 
higher than that encountered in operation and so might improve filter cap 
reliability in the R-390A.

Drew

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