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7904 problem

simoniep
 

Hello

I would like to bring back to life a 7904 scope and before any action I need some advices.

At power up, there is a very bright unfocused spot on the screen with absolutely no reaction at any front panel command.

I suppose a power supply problem, but how can I switch of the spot to protect the CRT during troubleshooting?

And what to test first?

Thanks in advance for reading and helping.

Pascal

Merchison Burke
 

Do you have a vertical and a horizontal plug-ins installed?

On 2016-Sep-16 12:37 PM, fcbtt3.nina@... [TekScopes] wrote:
Hello

I would like to bring back to life a 7904 scope and before any action I need some advices.

At power up, there is a very bright unfocused spot on the screen with absolutely no reaction at any front panel command.

I suppose a power supply problem, but how can I switch of the spot to protect the CRT during troubleshooting?

And what to test first?

Thanks in advance for reading and helping.

Pascal









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Posted by: fcbtt3.nina@...
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simoniep
 

Yes

From left to right: 7A19, 7A12, free, 7B71.

John Griessen
 

On 09/16/2016 12:33 PM, fcbtt3.nina@... [TekScopes] wrote:
From left to right: 7A19, 7A12, free, 7B71.
Horiz mode should be in B to use the 7b71.

Does intensity knob have no effect on the brightness?

If not I wonder if the z axis input might still work to control brightness?

Merchison Burke
 

Have you tried the intensity control? Does it work?

On 2016-Sep-16 12:37 PM, fcbtt3.nina@... [TekScopes] wrote:
Hello

I would like to bring back to life a 7904 scope and before any action I need some advices.

At power up, there is a very bright unfocused spot on the screen with absolutely no reaction at any front panel command.

I suppose a power supply problem, but how can I switch of the spot to protect the CRT during troubleshooting?

And what to test first?

Thanks in advance for reading and helping.

Pascal









------------------------------------
Posted by: fcbtt3.nina@...
------------------------------------


------------------------------------

Yahoo Groups Links



simoniep
 

The intensity knob have no effect on brightness.

Merchison Burke
 

What about the focus control, any effect?

Since the intensity control has no effect, there is the possibility that the HV circuit is defective.

I would look at the schematic for a fuse for the high voltage (HV) circuit. If there is one and you remove it, this should kill the bright spot so that you can troubleshoot the low voltage (LV) power supply without burning the screen.

On 2016-Sep-16 1:52 PM, fcbtt3.nina@... [TekScopes] wrote:
The intensity knob have no effect on brightness.

simoniep
 

No effect of focus control.

I am searching on the schematic how to stop HV degeration. Seems to be difficult because HV and LV are generated thought the same transformer (T1310).

 

Can you de-focus it ?
 HankC, Boston WA1HOS

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

 

That is weird that the focus control has no effect. Based on the
schematic, it looks like that could happen if some combination of the
-50 and -130 volt low voltage power supply outputs are missing.

Instead of disabling the high voltages, what about disconnecting the
CRT socket?

On 16 Sep 2016 11:37:44 -0700, you wrote:

No effect of focus control.

I am searching on the schematic how to stop HV degeration. Seems to be difficult because HV and LV are generated thought the same transformer (T1310).

simoniep
 

No, no action possible of the spot.

simoniep
 

3 solutions:1 disconnecting the CRT, 2 disconnecting P1710, 3 desoldering R1603 and/or R1887.

I try to deternine the more convenient

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
heater.
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.
Brgrds,
Fabio

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]


Ed Breya
 

From the discussion so far, it seems to me that the +130V supply may be gone. As I recall, there's a small fuse protecting it on the power supply regulator board. If plug-ins are installed and timebase set up for a proper free-running sweep, that should spread the beam out to a workable non-damaging level. If the sweep doesn't happen on-screen even though it should, then the 130V is probably out. In that case, I'd take David's suggestion and unplug the CRT socket or the anode connection to protect the CRT while servicing.

Ed

simoniep
 

It seems to be very easy to disconnect the CRT. 2 plugs on the Z axis board and 1 plug on the +21KV lead.

David DiGiacomo
 

On Fri, Sep 16, 2016 at 2:52 PM, <fcbtt3.nina@...> wrote:

It seems to be very easy to disconnect the CRT. 2 plugs on the Z axis board and 1 plug on the +21KV lead.
It's better to leave the CRT anode lead connected.

You didn't mention whether the horizontal and vertical position
controls allow you to move the spot.

If they do, you have the common HV DC restorer failure.

If not, you probably have that plus some other problem.

simoniep
 

No action possible on the spot with vertical and horizontal controls.

simoniep
 

With CRT disconnected, a tic tic is now audible and 2 neon lamps flash on the Z axis board.

 

The +130 volt supply comes directly from the off-line power supply
which uses it for regulation and I see no fuse in the schematics
although there is a 10 ohm series resistor before the CRT circuits. If
the +130 volt output was completely missing, then the switching power
supply would shut down.

On 16 Sep 2016 13:22:58 -0700, you wrote:

From the discussion so far, it seems to me that the +130V supply may be gone. As I recall, there's a small fuse protecting it on the power supply regulator board. If plug-ins are installed and timebase set up for a proper free-running sweep, that should spread the beam out to a workable non-damaging level. If the sweep doesn't happen on-screen even though it should, then the 130V is probably out. In that case, I'd take David's suggestion and unplug the CRT socket or the anode connection to protect the CRT while servicing.

Ed

Göran Krusell
 

What I did was to disconnect the HV cable to the CRT. With extreme care and
thick rubber gloves.
Göran


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