Re: Type 503 Oscilloscope Issues

Dave Casey

Evan -

I would not bother trying to re-connect to T601, as that is where you
problem lies.
The fix is to add a small filament transformer to your scope to take the
place of the bad winding in T601. The important characteristics are 6.3VAC
output for 120VAC input, and an isolation rating better than the 3kV that
will be floating on it.

Hopefully someone has a part number handy that they have used in the past,
but some quick searching tells me that the Triad F-313X will suit your
needs. This is available from either Mouser (553-F-313X
or Digikey (237-1965-ND
for about $10. It shows isolation to 2500V on the datasheet, Digikey says
this is Vrms, which would be ~3500Vpk and thus safe for use in your

You will need some small screws and nuts to mount the transformer in the
scope case (you can probably use the vent holes in the bottom of the case
instead of having to drill new ones). Once mounted, you would connect the
primary of your new transformer in parallel to the primary of T601, and
then connect the wires that used to go to pins 6 and 7 of T601 to the
secondary of your new transformer. I can draw you a schematic later if that

Dave Casey

On Wed, Apr 19, 2017 at 2:34 PM, enchanter464@... [TekScopes] <
TekScopes@...> wrote:

So I resoldered back in V692 and desoldered Pins 6 and 7 from T601. V692
had a faint orange glow, and all of the other tubes were glowing normally
(no sporadic flickering). No arcing was visible or audible, and no burning
smell came from V620.
I retook the voltages on the analog meter just to confirm again:
-100 V bus measured at ~70 VDC
+250 V bus measured at ~300 VDC
+100 V bus measured at ~117 VDC
+12.6 V bus measured at ~14 VDC
+500 V bus measured at ~480 VDC
+85 V bus measured at ~87 VDC

The values were similar to those from yesterday with V692 removed, but I
still lack the means to measure the -3kV.

At this point, should I resolder back the -3000 V wiring to Pins 6 and 7
on T601, since that transformer is most likely at fault? I know Mike had
mentioned a fix using a 6.3 VAC filament transformer, so what does this
involve? (I can try to get the part in over the weekend).

- Evan

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