Re: 7904 problem

Fabio Trevisan

Hello Pascal,
I don't know the innards of the 7xxx mainframe to know if what I'm about to
suggest is easy or not to achieve...
In order to protect the CRT and at the same time not changing whatever else
where you will want to troubleshoot, the easiest way is to disconnect the
Just make sure you disconnect both pins because in many CRT circuits,
usually one of the sides of the heater is also connected to the cathode and
then I'm afraid that cathode voltage may find its way through the heater
into something else. (As I speak I don't even know if what I'm saying makes
any sense, but it's better to be safe then sorry.

With no heater, there's no emission and therefore no damage to the tube.
Since there won't be any HV current flowing, I think that it's expected
that the Post acceleration anode may be higher than spec'd, since it's non
regulated (the highest voltage which, in the case of the 7xxx series is
probably around 16kV) but I don't think that it will cause any damage and
it will not affect any other voltage, since the cathode voltage is
regulated and no matter if current is flowing or not it should maintain the
voltage according to the regulation circuit.
This will allow you to check all the CRT biasing with peace of mind that
you aren't burning your CRT.

On Sep 16, 2016 4:38 PM, "HankC @HankC [TekScopes]" <
TekScopes@...> wrote:

Can you de-focus it ?
HankC, Boston WA1HOS

[Non-text portions of this message have been removed]

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