On Sat, 23 Jun 2018 20:52:35 -0700, you wrote:
Harvey, Thanks for your reply. I referred to the manual suggested here by other members, 465 Late, as my sn is the later model. On pg 215 and for the next several pages it explains (to someone with more savy than I have re these circuits) troubleshooting the lv circuits. All voltages seems dependent on the 55v working properly. I wonder if I need to first bump the 55v up from its out of spec 46.9v first? I have very limited experience with transistors and their proper function in this circuit. But i did notice most seem to be plug and play connections instead of soldered like l am familiar with in old tube amps. That makes the plug in 40ish yr old connections throughout this scope suspect imho. I hope I'm wrong.
You'll find that plug and play is a good thing. In old tube amps, you
could removed the individual tubes (which generally fail with low
emission), to make sure that the tubes are not shorted (that's
something I personally have not seen).
With the plug connections, you'll find that they may fail with a
higher resistance, but that is not generally going to lower the power
supply voltage at the supply itself. So you can unplug several
sections to see what is going on.
It is possible for a power supply to be dragged down by another
supply. One way is a reference problem. If the 55 volt supply is
used as a direct reference (no zeners in another supply), then
changing it changes the other supply. If there's a reference in that
second supply and all the 55 volt supply does is power the reference,
then the 55 volt supply has to change significantly to affect the
other supply. (9-10 volts is significant, but not here).
A second way is that whatever is supplying the bulk voltage to both
supplies (an inverter, a power transformer, or a common winding)
simply has too much current drawn from it and resistive losses do the
dirty deed.
Now, what I remember on the 15 volt supply readings you've provided is
that the voltage is zero. Zero is a pretty significant number here.
I'd start at the bulk supply input for the 15 volt supply, that's the
unregulated input. (you may want to look for some of my other posts
on power supply theory, if you are interested, they may help a bit.)
Generally, in these supplies, there's a large transistor used as a
variable resistor. The supply is effectively a high power voltage
divider with the transistor being the top resistor and the rest of the
circuitry being the lower resistor.
These supplies are frequently current limited as well.
So, look for the bulk voltage, you'll find it to be a bit low. Now
look at the voltage drop on the pass transistor (that's the variable
resistor function). If there's a short on the output (I'd suspect
one), then the transistor drops the whole bulk supply voltage and
should be rather warm. There's a current sense resistor in there, say
0.1 to 2 ohms or so, generally with a transistor base/emitter junction
across it. Look for .5 to .7 volts across this resistor. If so, then
the supply is trying to current limit to protect itself. Again, look
at power supply theory and then look to current limited supplies,
series regulator style.
Now, for power off tests, which you can do first. Unplug as many
leads as possible from the power supply, you're interested in the
lines going to the various boards. Make sure the power supply
capacitors are discharged, then measure the resistance from ground to
each of the power supply output leads. Those should be within spec.
If they are not, then at least one possible fault lies on the power
supply board itself.
If those readings are ok, then measure the resistance at each of the
power supply inputs to each board, those should be within spec.
Remember that a 1 ohm reading on the 15 volt line will try to draw 15
amps, so that's completely unreasonable. (I'd guess that the
resistance would be at least 15 ohms and perhaps 30 or more). Don't
worry about the low resistance due to capacitor charging, the final
value is important here.
Some people will power up the scope with the supply disconnected
(check to see if Tek says that's ok), and then check voltages. Then
power off, and plug in boards one by one until something fails. That
works, too, but I'd suggest resistance readings first.
Generally, you're likely to find one of those little epoxy drop
tantalum capacitors shorted. Also, with the boards as exposed as
possible, look for evidence of overheated parts, darkened paint,
discolored PC board, or ashes in what's left of a resistor (which, if
open, disconnects the short. Look anyway....)
Hopefully, this gives you a start. We've all been where you are, but
for some of us, it's a little longer ago.
Harvey
Any coaching with specific instructions for a guy with a pretty well equipped lab but limited sources here in Mexico for technical help. In the late 60's I was able to fix a Perkin Elemer Atomic Absorption Spectrophotometer in a lab I ran in Alaska with phone coaching from PE saving perhaps months of down time during a very short productive prospecting season. I have much better repair tools than I did then, so l am confident with the diagnostic veterans here helping, that i can get this otherwise clean old scope working again.
Thx, Russ
