Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped various
points in the sweep generator circuit. When in X-Y mode the sweep gen is
still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem as
they're not used in all settings. There is also a little cap which must
be part of the timing cct, but even if that was duff, why should it have
a huge effect on the varying speed when in a slower setting as its
capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the
top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.

On 15/12/2020 08:22, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped various
points in the sweep generator circuit. When in X-Y mode the sweep gen is
still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem as
they're not used in all settings. There is also a little cap which must
be part of the timing cct, but even if that was duff, why should it have
a huge effect on the varying speed when in a slower setting as its
capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the
top when you remove the cover.

There is often a potentiometer somewhere as well with the timebase only
being strictly calibrated when it is at one end of its range. My first
guess would be a dry joint causing trouble.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.

How much is the timebase shifting around by?

--
Regards,
Martin Brown

I'm assuming there is no pot controlling the timebase speed that may be
somewhat knackered in its ability to connect to bits of itself, ie the noisy
volume control syndrome?


I also assume this is a semiconductor design, not valve. In that regard is
there a way it can be synchronised, would not be much of a scope without
such a feature, so maybe that part of the circuit is up the spout.

What about internal connectors between boards etc? Those can create some
interesting faults, from memory, many are just crimped or idc ribbon types.
Brian

--

This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please
Note this Signature is meaningless.!
"Grumps" wrote in message ...
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly, sometimes
speeding up, sometimes slowing down. And it does this to the same degree
in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped various
points in the sweep generator circuit. When in X-Y mode the sweep gen is
still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem as
they're not used in all settings. There is also a little cap which must be
part of the timing cct, but even if that was duff, why should it have a
huge effect on the varying speed when in a slower setting as its
capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the top
when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.

On 15/12/2020 08:22, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped various
points in the sweep generator circuit. When in X-Y mode the sweep gen is
still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem as
they're not used in all settings. There is also a little cap which must
be part of the timing cct, but even if that was duff, why should it have
a huge effect on the varying speed when in a slower setting as its
capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the
top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.
Is there a pot involved? Most scopes have a fine tune pot for s3eep
speed...if that is scratchy...

--
€œBut what a weak barrier is truth when it stands in the way of an
hypothesis!€�

Mary Wollstonecraft

On 15/12/2020 08:22, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped various
points in the sweep generator circuit. When in X-Y mode the sweep gen is
still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem as
they're not used in all settings. There is also a little cap which must
be part of the timing cct, but even if that was duff, why should it have
a huge effect on the varying speed when in a slower setting as its
capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the
top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.

Perhaps look for electrolytic capacitors that are bulging or leaking?

(Not to be confused with the manufacturer Bulgin. I remember 50
years ago that we in the G3UOE radio club wound up the electronics
dept store manager by enquiring as to whether he had 4 rubber
feet and a Bulgin knob! :-) )

73 de Gareth G4SDW

On Tue, 15 Dec 2020 08:54:36 +0000, The Natural Philosopher wrote:

So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to
the
same degree in ALL timebase settings except the X-Y mode.

Is there a pot involved? Most scopes have a fine tune pot for s3eep
speed...if that is scratchy...

That would tend to produce step changes in the time base rather than,
as how I read the OP, smooth variation in the rate. Also implied by
looking at the timing capacitors, I've not known capacitors to step
randomly step change. Of course if the variations are step changes
then dirty/worn pot or switch or other connection is the likely
source.

--
Cheers
Dave.

On 15/12/2020 09:18, Dave Liquorice wrote:
On Tue, 15 Dec 2020 08:54:36 +0000, The Natural Philosopher wrote:

So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to
the
same degree in ALL timebase settings except the X-Y mode.

Is there a pot involved? Most scopes have a fine tune pot for s3eep
speed...if that is scratchy...

That would tend to produce step changes in the time base rather than,
as how I read the OP, smooth variation in the rate. Also implied by
looking at the timing capacitors, I've not known capacitors to step
randomly step change. Of course if the variations are step changes
then dirty/worn pot or switch or other connection is the likely
source.

Another possibility is poor voltage regulation somewhere


--
The urge to save humanity is almost always a false front for the urge to
rule.
€“ H. L. Mencken, American journalist, 1880-1956

On 15/12/2020 08:22, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped various
points in the sweep generator circuit. When in X-Y mode the sweep gen is
still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem as
they're not used in all settings. There is also a little cap which must
be part of the timing cct, but even if that was duff, why should it have
a huge effect on the varying speed when in a slower setting as its
capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the
top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.
I don't know that model but the I had a Tektronix scope "back in the
day" that developed dirty timebase switch contacts - might be worth
checking if your scope has the usual stacked wafer switches.
Alternatively (and I don't have the circuit diagram) might the output
stages be wandering, or there be a problem with the power supply. If
it's an old'ish scope, and without full test gear, some useful steps
would be to clean all switches and connectors, look for pcb cracks,
re-flow and dodgy-looking joints and replace any electrolytics.

On 15/12/2020 10:02, wrote:
On 15/12/2020 08:22, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped
various points in the sweep generator circuit. When in X-Y mode the
sweep gen is still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem
as they're not used in all settings. There is also a little cap which
must be part of the timing cct, but even if that was duff, why should
it have a huge effect on the varying speed when in a slower setting as
its capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the
top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.
I don't know that model but the I had a Tektronix scope "back in the
day" that developed dirty timebase switch contacts - might be worth
checking if your scope has the usual stacked wafer switches.
Alternatively (and I don't have the circuit diagram) might the output
stages be wandering, or there be a problem with the power supply. If
it's an old'ish scope, and without full test gear, some useful steps
would be to clean all switches and connectors, look for pcb cracks,
re-flow and dodgy-looking joints and replace any electrolytics.
(Apologies for the typos; I hope it still makes sense)

On 15/12/2020 08:22, Grumps wrote:

A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I take it you get a stable Lissajous trace in X-Y mode?

--
Spike

On 15/12/2020 09:10:30, gareth evans wrote:
On 15/12/2020 08:22, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped various
points in the sweep generator circuit. When in X-Y mode the sweep gen is
still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem as
they're not used in all settings. There is also a little cap which must
be part of the timing cct, but even if that was duff, why should it have
a huge effect on the varying speed when in a slower setting as its
capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the
top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.

Perhaps look for electrolytic capacitors that are bulging or leaking?

(Not to be confused with the manufacturer Bulgin. I remember 50
years ago that we in the G3UOE radio club wound up the electronics
dept store manager by enquiring as to whether he had 4 rubber
feet and a Bulgin knob!Â*Â* :-)Â*Â* )

In which case I would leave it running for an hour or so to see if the
problem cleared up. It's more likely that same electrolytics will need
reforming.

On 15/12/2020 08:22:41, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped various
points in the sweep generator circuit. When in X-Y mode the sweep gen is
still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem as
they're not used in all settings. There is also a little cap which must
be part of the timing cct, but even if that was duff, why should it have
a huge effect on the varying speed when in a slower setting as its
capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the
top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.

I would be searching for the manual. Apart from leaving the 'scope
turned on for a while to see if settles down finding the fix is like
shooting in the dark.

This might help:

https://elektrotanya.com/goldstar_os.../download.html

You will also need DjVuLibre or similar app to view the file.

As others have said it could be a noisy pot, but then resistors can
become noisy with age. Look at the wiper on VR601 on page 56 of the
manual, and check how stable the voltage is.

Good luck.

The Natural Philosopher used his keyboard to write :
On 15/12/2020 08:22, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly, sometimes
speeding up, sometimes slowing down. And it does this to the same degree in
ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped various
points in the sweep generator circuit. When in X-Y mode the sweep gen is
still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem as
they're not used in all settings. There is also a little cap which must be
part of the timing cct, but even if that was duff, why should it have a
huge effect on the varying speed when in a slower setting as its
capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the top
when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.
Is there a pot involved? Most scopes have a fine tune pot for s3eep
speed...if that is scratchy...

Check the timebase voltage regulation.

In article ,
Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped various
points in the sweep generator circuit. When in X-Y mode the sweep gen is
still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem as
they're not used in all settings. There is also a little cap which must
be part of the timing cct, but even if that was duff, why should it have
a huge effect on the varying speed when in a slower setting as its
capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the
top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.

I'd first check for dirty/worn out switches.

--
*Great groups from little icons grow *

Dave Plowman London SW
To e-mail, change noise into sound.

On Tue, 15 Dec 2020 08:41:45 -0000, "Brian Gaff \(Sofa\)"
wrote:

I'm assuming there is no pot controlling the timebase speed that may be
somewhat knackered in its ability to connect to bits of itself, ie the noisy
volume control syndrome?


I also assume this is a semiconductor design, not valve. In that regard is
there a way it can be synchronised, would not be much of a scope without
such a feature, so maybe that part of the circuit is up the spout.

The idea of syncing a free-running scope T/B went out decades ago.
Modern scopes have "trigger level" control which determines the point
on the i/p waveform at which the sweep begins. This also enables
single-shot operation, although you would have needed a storage scope
or a camera to make much use of this feature before digital scopes
came along.
(BTDTGTTS)

On 15/12/2020 08:37, Martin Brown wrote:
On 15/12/2020 08:22, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped
various points in the sweep generator circuit. When in X-Y mode the
sweep gen is still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem
as they're not used in all settings. There is also a little cap which
must be part of the timing cct, but even if that was duff, why should
it have a huge effect on the varying speed when in a slower setting as
its capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the
top when you remove the cover.

There is often a potentiometer somewhere as well with the timebase only
being strictly calibrated when it is at one end of its range. My first
guess would be a dry joint causing trouble.

That particular pot doesn't have the range that I remember when I last
used a scope like this. It does vary the TB a bit though.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.

How much is the timebase shifting around by?

About +/- 25%.

On 15/12/2020 08:54, The Natural Philosopher wrote:
On 15/12/2020 08:22, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped
various points in the sweep generator circuit. When in X-Y mode the
sweep gen is still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem
as they're not used in all settings. There is also a little cap which
must be part of the timing cct, but even if that was duff, why should
it have a huge effect on the varying speed when in a slower setting as
its capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the
top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.
Is there a pot involved? Most scopes have a fine tune pot for s3eep
speed...if that is scratchy...

There is a pot like that. It has little effect on the TB (not as much as
I recall this type of adjustment as having).
I'll check it out.

Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped various
points in the sweep generator circuit. When in X-Y mode the sweep gen is
still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem as
they're not used in all settings. There is also a little cap which must
be part of the timing cct, but even if that was duff, why should it have
a huge effect on the varying speed when in a slower setting as its
capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the
top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.

OK, so how do you make a sweep ? What principle is involved ?

One way to do it, is take a capacitor and run a constant current source
into it. The voltage potential across the cap, gives you a constantly-rising
waveform. Then, when it hits a threshold voltage, you short out the cap
(with a bit of resistance so the current flow level isn't ruinous) and
that falling voltage interval (with beam gated off) is the retrace. In
my sample picture below, the falling edge should be steep, but not
exactly vertical. It takes some time for retrace to happen.

The result is a sawtooth waveform, with a slowly rising front half, and
a quickly falling back half.

/|
/ |
_____/ |_____ (Apply voltage to horizontal deflection)

__
| |
_____| |_____ Beam gate (turn off beam during retrace)

A varying current flow, instead of a constant current flow, on
the leading edge, makes the sweep rate non-linear. Speeding up
or slowing down in proportion to the deviation from ideality.

If you have a schematic, look for the sweep circuit and see
if you can fit some notion of that to the components present.

Unless there is conductive dirt near the capacitor, I'd be
most interested in the health of the tube providing a constant
current.

In the digital era, you could take a binary counter, and run it
into a DAC, to make a ramp. But of course nobody does that,
because the clock rate needed into the binary counter
would be pretty high.

They also made waveform generator chips at one time, which
is just the embodiment of various waveform generation tricks,
into a single chip. The one I got came from TI, but I believe
it was soon canceled.

Paul

On 15/12/2020 09:18, Dave Liquorice wrote:
On Tue, 15 Dec 2020 08:54:36 +0000, The Natural Philosopher wrote:

So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to
the
same degree in ALL timebase settings except the X-Y mode.

Is there a pot involved? Most scopes have a fine tune pot for s3eep
speed...if that is scratchy...

That would tend to produce step changes in the time base rather than,
as how I read the OP, smooth variation in the rate. Also implied by
looking at the timing capacitors, I've not known capacitors to step
randomly step change. Of course if the variations are step changes
then dirty/worn pot or switch or other connection is the likely
source.

It's not a step change. It's smooth.
It can be steady, then fall in frequency, and the next moment it can
rise in frequency - all within the space of several seconds.

On 15/12/2020 12:10, Harry Bloomfield wrote:
The Natural Philosopher used his keyboard to write :
On 15/12/2020 08:22, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to
the same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped
various points in the sweep generator circuit. When in X-Y mode the
sweep gen is still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem
as they're not used in all settings. There is also a little cap which
must be part of the timing cct, but even if that was duff, why should
it have a huge effect on the varying speed when in a slower setting
as its capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on
the top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are
more relevant forums available to post to.
Ta very much.
Is there a pot involved? Most scopes have a fine tune pot for s3eep
speed...if that is scratchy...

Check the timebase voltage regulation.

I did quickly check it, and it looked OK at 12V.

On 15/12/2020 09:10, gareth evans wrote:
On 15/12/2020 08:22, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped various
points in the sweep generator circuit. When in X-Y mode the sweep gen is
still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem as
they're not used in all settings. There is also a little cap which must
be part of the timing cct, but even if that was duff, why should it have
a huge effect on the varying speed when in a slower setting as its
capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the
top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.

Perhaps look for electrolytic capacitors that are bulging or leaking?

(Not to be confused with the manufacturer Bulgin. I remember 50
years ago that we in the G3UOE radio club wound up the electronics
dept store manager by enquiring as to whether he had 4 rubber
feet and a Bulgin knob!Â*Â* :-)Â*Â* )

73 de Gareth G4SDW

None physically look bad.
Next step would be to remove the timing board and then test each cap.
But as much as I'd hate to give up on this, there is only so much reward
for the effort employed.

On 15/12/2020 10:02, wrote:
On 15/12/2020 08:22, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped
various points in the sweep generator circuit. When in X-Y mode the
sweep gen is still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem
as they're not used in all settings. There is also a little cap which
must be part of the timing cct, but even if that was duff, why should
it have a huge effect on the varying speed when in a slower setting as
its capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the
top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.

I don't know that model but the I had a Tektronix scope "back in the
day" that developed dirty timebase switch contacts - might be worth
checking if your scope has the usual stacked wafer switches.
Alternatively (and I don't have the circuit diagram) might the output
stages be wandering, or there be a problem with the power supply. If
it's an old'ish scope, and without full test gear, some useful steps
would be to clean all switches and connectors, look for pcb cracks,
re-flow and dodgy-looking joints and replace any electrolytics.

These Goldstar model is supposed to be reliable.
I have my own scope and multimeters, and switch cleaner.
Yes, it has the usual stacked wafer switch for TB setting.

On 15/12/2020 11:26, Spike wrote:
On 15/12/2020 08:22, Grumps wrote:

A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I take it you get a stable Lissajous trace in X-Y mode?

I get a stable dot that doesn't wander when I have it set to X-Y mode
and no inputs.
I'm pretty sure the problem is in the sweep generator and not the
horizontal amp.

On Tuesday, 15 December 2020 at 15:58:34 UTC, Grumps wrote:
On 15/12/2020 09:10, gareth evans wrote:
On 15/12/2020 08:22, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped various
points in the sweep generator circuit. When in X-Y mode the sweep gen is
still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem as
they're not used in all settings. There is also a little cap which must
be part of the timing cct, but even if that was duff, why should it have
a huge effect on the varying speed when in a slower setting as its
capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the
top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.

Perhaps look for electrolytic capacitors that are bulging or leaking?

(Not to be confused with the manufacturer Bulgin. I remember 50
years ago that we in the G3UOE radio club wound up the electronics
dept store manager by enquiring as to whether he had 4 rubber
feet and a Bulgin knob! :-) )

73 de Gareth G4SDW
None physically look bad.
Next step would be to remove the timing board and then test each cap.
But as much as I'd hate to give up on this, there is only so much reward
for the effort employed.

I'd bet on it being dirty/oxidised switch contacts or a dirty pot contact. Or some other iffy contact. Other factors are possible but a lot less likely.


NT

On 15/12/2020 11:49, Fredxx wrote:
On 15/12/2020 08:22:41, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped
various points in the sweep generator circuit. When in X-Y mode the
sweep gen is still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem
as they're not used in all settings. There is also a little cap which
must be part of the timing cct, but even if that was duff, why should
it have a huge effect on the varying speed when in a slower setting as
its capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the
top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.

I would be searching for the manual. Apart from leaving the 'scope
turned on for a while to see if settles down finding the fix is like
shooting in the dark.

This might help:

https://elektrotanya.com/goldstar_os.../download.html

You will also need DjVuLibre or similar app to view the file.

As others have said it could be a noisy pot, but then resistors can
become noisy with age. Look at the wiper on VR601 on page 56 of the
manual, and check how stable the voltage is.

Good luck.

I have the original printed manual which has the schematics.
VR601 sweeper shows a voltage that varies in amplitude in harmony with
the TB frequency drifting.

On 15/12/2020 16:05:01, Grumps wrote:
On 15/12/2020 11:49, Fredxx wrote:
On 15/12/2020 08:22:41, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to
the same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped
various points in the sweep generator circuit. When in X-Y mode the
sweep gen is still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem
as they're not used in all settings. There is also a little cap which
must be part of the timing cct, but even if that was duff, why should
it have a huge effect on the varying speed when in a slower setting
as its capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on
the top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are
more relevant forums available to post to.
Ta very much.

I would be searching for the manual. Apart from leaving the 'scope
turned on for a while to see if settles down finding the fix is like
shooting in the dark.

This might help:

https://elektrotanya.com/goldstar_os.../download.html


You will also need DjVuLibre or similar app to view the file.

As others have said it could be a noisy pot, but then resistors can
become noisy with age. Look at the wiper on VR601 on page 56 of the
manual, and check how stable the voltage is.

Good luck.

I have the original printed manual which has the schematics.
VR601 sweeper shows a voltage that varies in amplitude in harmony with
the TB frequency drifting.

Can you work back to the source of this voltage and when it starts
drifting? You seem very close to a cure.

On 15/12/2020 15:52, Grumps wrote:
On 15/12/2020 08:37, Martin Brown wrote:
On 15/12/2020 08:22, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to
the same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped
various points in the sweep generator circuit. When in X-Y mode the
sweep gen is still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem
as they're not used in all settings. There is also a little cap which
must be part of the timing cct, but even if that was duff, why should
it have a huge effect on the varying speed when in a slower setting
as its capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on
the top when you remove the cover.

There is often a potentiometer somewhere as well with the timebase
only being strictly calibrated when it is at one end of its range. My
first guess would be a dry joint causing trouble.

That particular pot doesn't have the range that I remember when I last
used a scope like this. It does vary the TB a bit though.

If anyone can give further clues that'd be great. Or if there are
more relevant forums available to post to.
Ta very much.

How much is the timebase shifting around by?

About +/- 25%.

My suggestion would be to check that pot and any trimpots there might be
inside for signs of damage or erratic behaviour. It may work OK when at
the lesser used end of its track and be worn or damaged at the other.

If all else fails use it in X-Y mode and build a new timebase sweep.

--
Regards,
Martin Brown

On 15/12/2020 15:54, Grumps wrote:
On 15/12/2020 09:18, Dave Liquorice wrote:
On Tue, 15 Dec 2020 08:54:36 +0000, The Natural Philosopher wrote:

So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to
the
same degree in ALL timebase settings except the X-Y mode.

Is there a pot involved? Most scopes have a fine tune pot for s3eep
speed...if that is scratchy...

That would tend to produce step changes in the time base rather than,
asÂ* how I read the OP, smooth variation in the rate. Also implied by
looking at the timing capacitors, I've not known capacitors to step
randomly step change. Of course if the variations are step changes
then dirty/worn pot or switch or other connection is the likely
source.

It's not a step change. It's smooth.
It can be steady, then fall in frequency, and the next moment it can
rise in frequency - all within the space of several seconds.


HV stability? It may just be a high resistance HV connection near to the
front of the tube. All scopes I've worked on have a square test wave
output. Connect this to the input, select the appropriate timebase and
vertical scaling. Does the vertical scaling also change in the same way
as the timebase variation. If so suspect the HV supply. However if the
HV is varying I would expect the trace brightness to vary as well,
possibly more noticeable if the brightness is turned down.

Warning: bleed resistors across a scopes HV supply have a habit of going
open circuit so just make sure that your hand is not going to hit
anything when you remove it at speed after getting the shock


Once whilst working on an old valve based Tektronic scope I rested my
arm across a valve base. I ended up with two cauterised holes as the
voltage on one pin discharged to another through my flesh.

--
mailto : news {at} admac {dot} myzen {dot} co {dot} uk

On 17/12/2020 15:57, alan_m wrote:
On 15/12/2020 15:54, Grumps wrote:
On 15/12/2020 09:18, Dave Liquorice wrote:
On Tue, 15 Dec 2020 08:54:36 +0000, The Natural Philosopher wrote:

So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to
the
same degree in ALL timebase settings except the X-Y mode.

Is there a pot involved? Most scopes have a fine tune pot for s3eep
speed...if that is scratchy...

That would tend to produce step changes in the time base rather than,
asÂ* how I read the OP, smooth variation in the rate. Also implied by
looking at the timing capacitors, I've not known capacitors to step
randomly step change. Of course if the variations are step changes
then dirty/worn pot or switch or other connection is the likely
source.

It's not a step change. It's smooth.
It can be steady, then fall in frequency, and the next moment it can
rise in frequency - all within the space of several seconds.


HV stability? It may just be a high resistance HV connection near to the
front of the tube.Â* All scopes I've worked on have a square test wave
output. Connect this to the input, select the appropriate timebase and
vertical scaling. Does the vertical scaling also change in the same way
as the timebase variation. If so suspect the HV supply. However if the
HV is varying I would expect the trace brightness to vary as well,
possibly more noticeable if the brightness is turned down.

Vertical scaling and brightness are stable.

Warning: bleed resistors across a scopes HV supply have a habit of going
open circuit so just make sure that your hand is not going to hit
anything when you remove it at speed after getting the shock


Once whilst working on an old valve based Tektronic scope I rested my
arm across a valve base. I ended up with two cauterised holes as the
voltage on one pin discharged to another through my flesh.