Tektronix 466 restoration (II)

JOSE V. GAVILA (EB5AGV/EC5AAU) <eb5agv@...>


After the third 3/4 A fuse failure (remember, we have 220VAC here in
Spain), I have gone back again to my Tek 475 manual, as it shares lots of
things with the 466, apart of front panel. I wanted to read about the LV
power supply description. Everything I had looked on the 466 so far was
right: all rectifier bridges, filter capacitors, resistance of each voltage
supply to ground and so on. I had also checked the power supply with the HV
supply disconnected (by means of a fuse removal) and this was the third 3/4
A fuse casualty... So I was starting to find myself without ideas :-(. And
local time was about 23:30...

So, I went back again to read the manual. Then I found a comment about
'unused power transformer connections... except for Option 7' (DC supply;
my unit has not) 'and DM-44'... which my unit has!. Yes, there are several
wires going from the power transformer to another board, labelled as 'POWER
SUPPLY' which, in turn, go to the (now removed) DM-44... what if that board
is faulty?. No, it would be too easy!. But, well, nothing to loose (but
some sleeping time ;-) ). So, armed with my soldering iron, I have
disconnected all wires going to that PS board, just in case...

To make things worse, I had just two 3/4 A fuses left... and I did not want
to ruin another one. So I have put a 1 A fuse (yes, I know this is not on
the safest side but I was confident what was causing the 3/4 A to break
would also break a 1 A fuse). Expecting to see some nice fuse flashing, I
have applied power to the oscilloscope and then, a noise came from it!...
Yes, the fan was running!. And there were some lights on the front
panel!!!. And then, suddenly... a green line on the screen!!!!!!

Fastly checking the PS voltages, all were within specs. So, I powered it
down and connected a signal source to CH2 (CH1 is knob-less due to
transport damage, as I commented in previous message). So, with a nice 1MHz
signal coming from my HP-8640B, I powered on the 466 again. And I got no
smoke ;-) but a nice trace. Hooray!!!!

I have checked most of the options, including storage and, so far,
everything works fine :-). And trace is NOT distorted so the tube seems in
good shape!

Now I will attempt the small PS board repair but, trust me, I am HAPPY!!!

Go back to the workbench... a _working_ scope is waiting for me ;-)

Regards from a really happy camper!

La Canyada - Valencia (SPAIN)

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