On Wed, Aug 19, 2020 at 10:47 PM, Eric wrote:

Due to the difficulty of being able to source a tube if I needed one.

That's been my excuse for having two or, better even, three copies of all models for a few years now. But we're treading on very different ground here...

Raymond

AFAIK, the 7904A does *not* use and MCP CRT. The 7104 and 2467 do use MCP CRTs.

DaveD

On Aug 19, 2020, at 15:24, Eric <ericsp@gmail.com> wrote:

Looks like my frame needs a bit of a tune up then. Ill be working on this once I get the current plugin done. I have shied away from the MCP CRT as read some where that those tubes were all on borrowed time as even at very low levels on intensity there was some slight phosphor damage. Not sure if this turned out to be the case.

Eric

On 8/19/2020 8:48 AM, Chuck Harris wrote:
Your description is pretty accurate.

The more modern scopes have compensation circuitry
that changes the focus voltage with the instantaneous
intensity setting.... and changes the instantaneous
intensity settings for the different beam uses...

So, the intensity is separately controllable for the A
horizontal plugin, the B horizontal plugin, the readout,
and in some cases, the "intensified" horizontal A vs B
settings... but FOCUS, and ASTIGMATISM, each have only
one user control.

Each of these settings result in a different beam current,
which require a different focus/astigmatism voltage.

For the 7904A, which was one of the earlier adopters of
this enhancement, they have a circuit called Auto Focus,
and in the Performance check and Adjustment procedure,
page 5-12 of my manual, they explain how it gets adjusted.

The do the main focus adjustment with the beam very dim,
and then they adjust the Auto Focus circuit with the beam
at full intensity control position.

-Chuck Harris

Roger Evans via groups.io wrote:

I am not an expert on this, trained in physics not electronics. When you look at a low rep rate signal at 500ps/div the beam is on a very small fraction of the time, about 5ns every msec so one part in 200000. The readout is being triggered automatically and runs around 1kHz or so, I don't remember the actual clock speeds. So for the same brightness the signal trace has to carry maybe 1000 times the current compared with the readout, this means a higher density of electrons and the space charge can affect the beam focussing compared with a low current beam.

I tried putting a fast rise square wave into my 7904 at 1kHz and qualitatively I see what you see. I have to turn up the brightness a lot to see the trace at 1ns/div and it does defocus compared with the readout. I can improve the focus slightly and that worsens the focus of the readout significantly.

Maybe if we get some more comments we can decide who is unrealistic and who is easily satisfied. I have no idea what the factory specifications were.

Regards,

Roger

On Aug 19, 2020, at 15:24, Eric <ericsp@gmail.com> wrote:

Looks like my frame needs a bit of a tune up then. Ill be working on this once I get the current plugin done. I have shied away from the MCP CRT as read some where that those tubes were all on borrowed time as even at very low levels on intensity there was some slight phosphor damage. Not sure if this turned out to be the case.

Eric

On 8/19/2020 8:48 AM, Chuck Harris wrote:
Your description is pretty accurate.

The more modern scopes have compensation circuitry
that changes the focus voltage with the instantaneous
intensity setting.... and changes the instantaneous
intensity settings for the different beam uses...

So, the intensity is separately controllable for the A
horizontal plugin, the B horizontal plugin, the readout,
and in some cases, the "intensified" horizontal A vs B
settings... but FOCUS, and ASTIGMATISM, each have only
one user control.

Each of these settings result in a different beam current,
which require a different focus/astigmatism voltage.

For the 7904A, which was one of the earlier adopters of
this enhancement, they have a circuit called Auto Focus,
and in the Performance check and Adjustment procedure,
page 5-12 of my manual, they explain how it gets adjusted.

The do the main focus adjustment with the beam very dim,
and then they adjust the Auto Focus circuit with the beam
at full intensity control position.

-Chuck Harris

Roger Evans via groups.io wrote:

I am not an expert on this, trained in physics not electronics. When you look at a low rep rate signal at 500ps/div the beam is on a very small fraction of the time, about 5ns every msec so one part in 200000. The readout is being triggered automatically and runs around 1kHz or so, I don't remember the actual clock speeds. So for the same brightness the signal trace has to carry maybe 1000 times the current compared with the readout, this means a higher density of electrons and the space charge can affect the beam focussing compared with a low current beam.

I tried putting a fast rise square wave into my 7904 at 1kHz and qualitatively I see what you see. I have to turn up the brightness a lot to see the trace at 1ns/div and it does defocus compared with the readout. I can improve the focus slightly and that worsens the focus of the readout significantly.

Maybe if we get some more comments we can decide who is unrealistic and who is easily satisfied. I have no idea what the factory specifications were.

Regards,

Roger

Your description is pretty accurate.

The more modern scopes have compensation circuitry
that changes the focus voltage with the instantaneous
intensity setting.... and changes the instantaneous
intensity settings for the different beam uses...

So, the intensity is separately controllable for the A
horizontal plugin, the B horizontal plugin, the readout,
and in some cases, the "intensified" horizontal A vs B
settings... but FOCUS, and ASTIGMATISM, each have only
one user control.

Each of these settings result in a different beam current,
which require a different focus/astigmatism voltage.

For the 7904A, which was one of the earlier adopters of
this enhancement, they have a circuit called Auto Focus,
and in the Performance check and Adjustment procedure,
page 5-12 of my manual, they explain how it gets adjusted.

The do the main focus adjustment with the beam very dim,
and then they adjust the Auto Focus circuit with the beam
at full intensity control position.

-Chuck Harris

Roger Evans via groups.io wrote:

I am not an expert on this, trained in physics not electronics. When you look at a low rep rate signal at 500ps/div the beam is on a very small fraction of the time, about 5ns every msec so one part in 200000. The readout is being triggered automatically and runs around 1kHz or so, I don't remember the actual clock speeds. So for the same brightness the signal trace has to carry maybe 1000 times the current compared with the readout, this means a higher density of electrons and the space charge can affect the beam focussing compared with a low current beam.

I tried putting a fast rise square wave into my 7904 at 1kHz and qualitatively I see what you see. I have to turn up the brightness a lot to see the trace at 1ns/div and it does defocus compared with the readout. I can improve the focus slightly and that worsens the focus of the readout significantly.

Maybe if we get some more comments we can decide who is unrealistic and who is easily satisfied. I have no idea what the factory specifications were.

Regards,

Roger

Hi!

Some time ago I got a used 2246 oscilloscope which was working quite well
until recently. First it began to show a single point on the screen and
then it would return, or you just had to turn it off and turn it on again.

Soon after, it started to cycle on and off and sometimes I needed to turn
it off for a moment and turn it on again to get it working again.

Now it kept turning on and off cyclically without stabilizing. So far, I
have changed things like some electrolytic capacitors (I read right here
that it is a common fault), however I have not found all. I have also
changed zener diodes and some transistors without success.

With the service manual I have tried to measure some points however since
it turns on and off the voltage is erratic. I have not yet found a similar
power supply for sale so at the moment it is not an option and I would like
to be able to repair it.

Any suggestion?

On 19 Aug 2020, at 19:21, Dave Wise <david_wise@phoenix.com> wrote:



Do you think [oscillation] could be an issue in the low power supply section?

It's a much-talked-about complaint. In this particular case, it's unlikely since the transistors in question only operate in unusual cases - power off and overload, respectively.
But since we're doing Component Engineering figuring out a suitable retrofit part, why not put in 100% effort? Do work you can be proud of.

Dave Wise
________________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Stephen via groups.io <stephen.nabet=gmail.com@groups.io>
Sent: Wednesday, August 19, 2020 10:00 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] SC502 With Display Issues

On Wed, Aug 19, 2020 at 05:48 AM, Dave Wise wrote:


To replace 2N3565 in the SC502 power supply, BC639 is better than 2N2222A,
100MHz vs 300MHz. Under a dollar at Mouser. But the best choice is KSC1008.
It's only 30MHz - perfect to avoid oscillation - and even cheaper. Mind the
pinout.

Ooh ok. Now I understand. You were talking about oscillation. This is the thing I didn’t grasp before from your previous post. Thank you for clearing that out. Do you think that could bean issue in the low power supply section?

On a side note, I’m trying to find parts that are readily available at my local shops because it’s just a 20min ride from my place.
And I know that if something fails, I can easily go and get them without having to order, and wait for (sometimes) a week to 10 days to get them. Yes, it’s terrible, I know.
BC639 is available. Not the KSC1008. I’ll try that.