Inherited this antique as a DOA, so thought I'd try to fix it for some fun.

Immediate obvious problem was that the electrolytics in the PSU had failed,
so on general principle I replaced every electrolytic in the machine, and
now it powers on and I've got the vertical amp calibrated against local
references (no they aren't certified, but have been checked against kit that
*is* in calibration), and the sweep timings appear to be pretty darn close
anyway, so I've not dinked with the sweep unit. +24V, -24V and +105V are
rock stable within seconds of power up.

One remaining problem though, when I power on the scope, the scan line is
excessively bright even with intensity turned right down and this takes
about 20 minutes to settle down to a reasonable level (at which point I need
to turn intensity up to about 50%).

Where should I be looking for the culprit - my initial thoughts are pointing
me to the HT/EHT section or possibly the "bright-up" circuit. Any
suggestions on the likely causes of the problem would be helpful. Could it
be the ceramic caps in the HT supply, or possibly the switch on Thermistor
(TH301)? How best to check?

Thanks
--
Dave Partridge (remove xwy from replyto email address)

David C. Partridge wrote:
Inherited this antique as a DOA, so thought I'd try to fix it for some fun.

....snippetey for bandwith...

Where should I be looking for the culprit - my initial thoughts are pointing
me to the HT/EHT section or possibly the "bright-up" circuit. Any
suggestions on the likely causes of the problem would be helpful. Could it
be the ceramic caps in the HT supply, or possibly the switch on Thermistor
(TH301)? How best to check?

Thanks
--
Dave Partridge (remove xwy from replyto email address)


Try to get a schmetic, or better a complete service manual. According
to your description, this scope is a Telequipment D75 (Telequipment
was buyied once by Tektronix and then spit out again). Schmeatics might
be found on the web (bama.sbc.edu, etc.) or for sale at Nigel cooks,
UK.

hth,
Andreas

Well, I do have the schematic, but that's not helping me much :-(

This is down to lack of experience with debugging this type of problem which
is why I asked.

"tekman" wrote in message
ups.com...
Try to get a schmetic,

"David C. Partridge" wrote in message
.. .
Inherited this antique as a DOA, so thought I'd try to fix it for some
fun.

Immediate obvious problem was that the electrolytics in the PSU had
failed,
so on general principle I replaced every electrolytic in the machine, and
now it powers on and I've got the vertical amp calibrated against local
references (no they aren't certified, but have been checked against kit
that
*is* in calibration), and the sweep timings appear to be pretty darn close
anyway, so I've not dinked with the sweep unit. +24V, -24V and +105V
are
rock stable within seconds of power up.

One remaining problem though, when I power on the scope, the scan line is
excessively bright even with intensity turned right down and this takes
about 20 minutes to settle down to a reasonable level (at which point I
need
to turn intensity up to about 50%).

Where should I be looking for the culprit - my initial thoughts are
pointing
me to the HT/EHT section or possibly the "bright-up" circuit. Any
suggestions on the likely causes of the problem would be helpful. Could
it
be the ceramic caps in the HT supply, or possibly the switch on Thermistor
(TH301)? How best to check?

Thanks
--
Dave Partridge (remove xwy from replyto email address)



Is the CAL waveform or a standard external 1V/1KHz square wave reduced
in size on the screen?

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

"David C. Partridge" wrote in
:

Well, I do have the schematic, but that's not helping me much :-(

This is down to lack of experience with debugging this type of problem
which is why I asked.

"tekman" wrote in message
ups.com...
Try to get a schmetic,



To control display intensity,the CRT grid gets biased more negatively than
the cathode,and to 'unblank' the CRT,the grid is moved in a positive(WRT
the cathode)direction.These are high voltages,perhaps -2KV or more.

So,trace back from the CRT grid to see what sort of supply provides the
grid bias,and to see what circuitry changes the grid bias supply to unblank
the CRT.Between the sweep circuits and the CRT grid bias supply will be a
"Z-axis amplifier" to provide unblanking drive for the grid supply.It uses
the sweep gate (the time the sweep ramps)to unblank the CRT,and the
"intensity control" is another input to the Z-axis amp if not to the grid
bias supply.Often there s a internal coarse grid bias adj. pot,and an
intensity control for fine or front panel adjustment.


--
Jim Yanik
jyanik
at
kua.net

Jim thanks for your post.

It was a combination of items - all in the area you were pointing to! I
was looking round trying to think what was wrong - when I spotted item 1
below (and cursed myself for not spotting it last night).

1. The previous owner had fiddled with the mains transformer jumpering
(presumably to try to compensate for the dying e-caps) and the UN-regulated
+30V line was *rather* high. Setting the jumpers to local supply voltage
brought that back into line (and the regulated lines were still spot on).
The +30V was driving the oscillator in the HT ... which was affecting
cathode AND grid voltages.

2. Dug out HV probes, and HT was now rather low (thinks - someone tweaked
this after fiddling with the trafo jumpers). Re-set cathode voltage
to -2.5kV.

3. Adjusted grid cut-off (the coarse adjustment you refer to below)
according to calibration procedure in manual.

And NOW everything seems to behave properly. Or almost - there's *slight*
drop in trace intensity over the first 20 minutes of use - but I can live
with that.

PS Oh yes. I did re-check the calibration afterwards, and (not
surprisingly) did need to make some minor adjustments on the main X and Y
amps (the ones directly driving the plates on the CRT).

All's well that ends well

Dave
"Jim Yanik" . wrote in message
.. .
To control display intensity,the CRT grid gets biased more negatively than
the cathode,and to 'unblank' the CRT,the grid is moved in a positive(WRT
the cathode)direction.These are high voltages,perhaps -2KV or more.

Etc. - deleted-
Jim Yanik
jyanik
at
kua.net

David C. Partridge wrote:

Jim thanks for your post.



And NOW everything seems to behave properly. Or almost - there's *slight*
drop in trace intensity over the first 20 minutes of use - but I can live
with that.

That seems to be normal, we have a D755 at our HAM Clubstation and
the trace brightness also drops a good amount in about the first 20 minutes.

Jorgen

No they weren't, but I'm interested to know why you ask? Were you thinking
EHT too high?

Dave
"N Cook" wrote in message
...
Is the CAL waveform or a standard external 1V/1KHz square wave reduced
in size on the screen?

"David C. Partridge" wrote in message
...
No they weren't, but I'm interested to know why you ask? Were you
thinking
EHT too high?

Dave
"N Cook" wrote in message
...
Is the CAL waveform or a standard external 1V/1KHz square wave reduced
in size on the screen?




yep, increased EHT decreased deflection smaller/brighter trace


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

"N Cook" wrote in :

"David C. Partridge" wrote in message
...
No they weren't, but I'm interested to know why you ask? Were you
thinking
EHT too high?

Dave
"N Cook" wrote in message
...
Is the CAL waveform or a standard external 1V/1KHz square wave reduced
in size on the screen?




yep, increased EHT decreased deflection smaller/brighter trace



Which also may mean that the CRT filament voltage is higher and thus
shortening the CRT's life.(if the fil V is derived from the HV power
supply)
TEK discovered that 0.1v higher fil voltage drastically shortened their
model 2213/15 CRT lifetime.

--
Jim Yanik
jyanik
at
kua.net

In this case no problem - filament is driven straight if the mains
transformer.

HT now spot on, and don't think EHT too high - it's only a quintupler, and
it HT is right, don't see EHT can be too high - or am I being naive?

David
"Jim Yanik" . wrote in message
.. .
Which also may mean that the CRT filament voltage is higher and thus
shortening the CRT's life.(if the fil V is derived from the HV power
supply)
TEK discovered that 0.1v higher fil voltage drastically shortened their
model 2213/15 CRT lifetime.

--
Jim Yanik
jyanik
at
kua.net

"David C. Partridge" wrote in
:

In this case no problem - filament is driven straight if the mains
transformer.

Good,as long as the xfmr insulation holds up. It has to be insulated for
more than 2-3 KV cathode voltage.
TEK departed from that after experience with the old 500-series tube-type
scopes.


HT now spot on, and don't think EHT too high - it's only a quintupler,
and if HT is right, don't see EHT can be too high - or am I being
naive?

No,that is not critical.
TEK used a 3 or 4 percent tolerance on it's scopes HV supplies,IIRC.

--
Jim Yanik
jyanik
at
kua.net



David
"Jim Yanik" . wrote in message
.. .
Which also may mean that the CRT filament voltage is higher and thus
shortening the CRT's life.(if the fil V is derived from the HV power
supply)
TEK discovered that 0.1v higher fil voltage drastically shortened
their model 2213/15 CRT lifetime.

--
Jim Yanik
jyanik
at
kua.net

Well that'll teach me - I was judging this based on a too short "cool down"
period.

After leaving it off for a day, the original problem returns - sure I could
back off the grid cut off much further so that I needed to crank up the
intensity really high once warm up was complete, but it still bothers me
that I can't work out what the problem might be.

From memory the measured cold resistance across R310/TH301 (Mullard
VA1040)was about 66 ohms (100R for the resistor and 130R +/- 20% nominal for
the thermistor) - which suggests that the thermistor is about right when
cold.

When all is warmed up the resistance is about 39 Ohms - NTC obviously.

I'll quote the relevant part of the circuit description in full:

====QUOTE====
The EHT is derived from a Class C oscillator operating at about 28kHz. The
oscillator transistor TR304 uses transformer, T301, as its load and main
frequency determining component. A feedback loop, including a high gain
amplifier, TR301, TR302 and TR303, regulates the cathode supply, which is
set to the correct potential by adjusting R301, and the grid is set to the
correct tube cut-off potential by adjusting R315. Diodes D304 and D303
provide half-wave rectification from T301 for the cathode and grid supplies.
A five stage voltage multiplier (quintupler) provides the +12.5 kV PDA.
The input to the multiplier is taken from the same tapping on T301 as that
used for the cathode supply.

Thermistor TH301 limits the output voltage at switch on and C204 reduces
E.H.T feddback into the unregulated +30V line.
====QUOTE====

Here's the key parts the circuit in line art monspace font - View with small
font I guess.

C303 (1n0)
-------------||-------------x----------------------
-------------- -2.5kV From Secondary of T301 Cathode supply
| |
|
| R304, R309 (27M) |
|
x----------^^^^^^^^----------
|
| R310 (100R)
T301 |
| +30V ---x---^^^^^^^---x------
|| ---|---x----- To Cathode
| | TH301 | (*) |
|| | D304 |
| ---^^^^^^^---x-||-| |
|| c |
| (130R) c
|| c = 20nF
| TR304 P c
|| c S2 |
| c
|| c |
| E C |
|| | |
| Chassis|----|---------O--------
|| --------x---| Chassis
| D302 B|
||
| |
|| 180K 2M2 grid cutoff
| --------
|| ---|---x--^^^^^^--x--^^^^^^^-x--------x---- To grid
| |
|| | D303 | | | | |
+105V --x------------------------------------------x------------- |
|| | | ----- | |
| | | | c
|| c | |
| | _ Chassis R307 S1 c
|| c S3 = 10nF = 10nF 50M0
| | (120K) R312 c
|| c | |
Set R333 | | R306 TR302 TR303 (15K0) |
|| | | | |
-2.5KV (1M0) | B |C (1K0) B C | B C | |
|| | | | |
R301 --^^^^^--x---x----O---^^^^^^^-----O----x------O------x---x----
|| --------x----------x-----------x--------x
(15K) | | E VE VE |
|
| | TR301 | | |
|
| | | -----x | = C305
(33n) = 4n7
| C301 = _ D301 | | | |
|
R302 (1Mu) | ^ C302 = R308 R311 |
Chassis | Chassis
(22K) | | (47Mu)| (330R) (1K0) _
_
| | |
| | | | |
| | x---------------------x-----------x--- -24V
| | |
_ _ _ Chassis


(*) = 47MuF to chassis

Comments?


"David C. Partridge" wrote in message
...
And NOW everything seems to behave properly. Or almost - there's *slight*
drop in trace intensity over the first 20 minutes of use - but I can live
with that.

Further information.

I measured the cathode to grid voltage at switch on from cold with intensity set to minimum.

Immediately on switch on the grid was average -63.9V DC negative wrt. the cathode as measured by my DMM
with 10Meg input impedance. It has been increasing (in a negative sense) steadily over the
last five to seven minutes and was -80.6V DC when I removed the DMM.

The intensity had been decreasing over that time slowly, and while the DMM was clearly affecting
things as intensity went down quite a bit more when I removed it, it's clearly not fully settled
to "normal state" which will be another 10 or 15 mins.

So clearly something is causing the grid voltage to be low for the first while after turn on.

The character mode circuit diagram got mangled by outlook express wrapping lines, so I'm
sending this message in HTML form (sorry) with it included again.

C303 (1n0)
-------------||-------------x--------------------------------x---To cathode -2.5kV
| | |
| R304, R309 (27M total) | |
x----------^^^^^^^^---------- |
| R310 (100R) T301 |
| +30V ---x---^^^^^^^---x------ || ---|---x
| unreg | TH301 | (*) |||| D304 |
| ---^^^^^^^---x-||-| |||| |
| (130R) c||c = 20nF
| P c||c S2 |
| TR304 c||c |
| E C |||| |
| Chassis|----|---------O-------- || --------x---| Chassis
| D302 B| ||
| | || 180K 2M2 grid cutoff
| -------- || ---|---x--^^^^^^--x--^^^^^^^-x--------x---- To grid
| |||| D303 | | | | |
+105V --x------------------------------------------x------------- |||| | ----- | |
reg | | | | c||c | C306 | C309 R318, R322
| | _ Chassis R307 S1 c||c S3 = 10nF = 10nF (50M Total)
| | | (120K) R312 c||c | |
Set R333 | | R306 TR302 TR303 (15K0) |||| | | |
-2.5KV (1M0) | B |C (1K0) B C | B C | |||| | | |
R301 --^^^^^--x---x----O---^^^^^^^-----O----x------O------x---x---- || --------x----------x----------x--------x
(15K) | | E VE VE | | |
| | TR301 | | = C305 (33n) | |
| | | -----x | | | = 4n7
| C301 = _ D301 | | | _Chassis | |
R302 (1Mu) | ^ C302 = R308 R311 | | Chassis
(22K) | | (47Mu)| (330R) (1K0) | __
| | | From bright-up
| | x---------------------x-----------x--- -24V reg amp
_ _ _ Chassis

(*) = 47MuF to chassis

"David C. Partridge" wrote in message .. .
Well that'll teach me ...

"David C. Partridge" wrote in
:

Further information.

I measured the cathode to grid voltage at switch on from cold with
intensity set to minimum.

Immediately on switch on the grid was average -63.9V DC negative wrt.
the cathode as measured by my DMM with 10Meg input impedance. It has
been increasing (in a negative sense) steadily over the last five to
seven minutes and was -80.6V DC when I removed the DMM.

Yes,a more-negative grid cuts off the electron beam.(more neg WRT cathode)

The intensity had been decreasing over that time slowly, and while the
DMM was clearly affecting things as intensity went down quite a bit
more when I removed it, it's clearly not fully settled to "normal
state" which will be another 10 or 15 mins.

Normally,a scope is run for about 20 min before adjusting CRT bias or any
other cal adjustments.
And you're supposed to let instruments warm up before making
measurements,even 100% solid-state equipment is not accurate "instant-on".

So clearly something is causing the grid voltage to be low for the
first while after turn on.

The character mode circuit diagram got mangled by outlook express
wrapping lines, so I'm sending this message in HTML form (sorry) with
it included again.

an aging CRT(vacuum tube) will take longer to settle down.
It could be that your CRT is near the end of it's life.

Maybe you have a temp-sensitive DC restorer diode?
(DC restorer is one method to shift Z-axis amp signals to HV potentials
needed by the CRT;it essentially is a diode-cap modulator between Z-axis
amp and CRT grid.)Commonly used in TEK scopes.

--
Jim Yanik
jyanik
at
kua.net

Follow up to tell folks the current status and probable cause of the problem
(I think):

I replaced the old 10nF ceramic smoothing caps in the grid bias supply as
they were getting distinctly warm after the scope had been on suggesting
that they were getting rather lossy (on a damp day they would steam gently
for a minute or two after turn on).

Didn't solve the brightness problem but new polyesters do NOT get warm, so I
feel better :-).

Static grid voltage at output of grid cut-off adjustment trim-pot was
drifting up during warm up, while voltage on input side was pretty much
constant. Suggests it might be the trim pot drifting with temp.

The trimpot (R315) is supposed to be a 2M2 linear Cermet 20% 1W according to
the parts list, but looks to me like a standard Piher PTC15 1/2W part.
Measuring the resistance as the scope cools after power off seems to confirm
the hypothesis (10% increase in value over 15 minutes, probably as much as
15-20% by the time it reaches room temp).

If I could source a replacement readily I'd just ram a new one in, but I
suspect that the parts list is right and the original WAS a 1W part, and has
been replaced somewhere along the way with a 1/2W part. Finding a 2.2M
0.5W trimmer of any sort (never mind one that will fit) seems a bit hard,
and as for a 1W part ...!

Before I go totally insane hunting for a suitable part, I'd just like to get
a sanity check on my diagnosis from folks more experienced with this than I.

Thanks
Dave

David C. Partridge" wrote in message
.. .
Follow up to tell folks the current status and probable cause of the
problem
(I think):

I replaced the old 10nF ceramic smoothing caps in the grid bias supply as
they were getting distinctly warm after the scope had been on suggesting
that they were getting rather lossy (on a damp day they would steam gently
for a minute or two after turn on).

Didn't solve the brightness problem but new polyesters do NOT get warm, so
I
feel better :-).

Static grid voltage at output of grid cut-off adjustment trim-pot was
drifting up during warm up, while voltage on input side was pretty much
constant. Suggests it might be the trim pot drifting with temp.

The trimpot (R315) is supposed to be a 2M2 linear Cermet 20% 1W according
to
the parts list, but looks to me like a standard Piher PTC15 1/2W part.
Measuring the resistance as the scope cools after power off seems to
confirm
the hypothesis (10% increase in value over 15 minutes, probably as much as
15-20% by the time it reaches room temp).

If I could source a replacement readily I'd just ram a new one in, but I
suspect that the parts list is right and the original WAS a 1W part, and
has
been replaced somewhere along the way with a 1/2W part. Finding a 2.2M
0.5W trimmer of any sort (never mind one that will fit) seems a bit hard,
and as for a 1W part ...!

Before I go totally insane hunting for a suitable part, I'd just like to
get
a sanity check on my diagnosis from folks more experienced with this than
I.

Thanks
Dave


Hi David,

One posible fix (but a bit clunky) is to find out the values of the
resistance is from the ends of the trimpot to the wiper, by measuring the
existing pot, and replacing it with 2 fixed resistors of a suitable power
rating .
At least you could then tell if your diagnosis is correct.

Bye for now,

Ian.

"Ian French" wrote in
:

David C. Partridge" wrote in message
.. .
Follow up to tell folks the current status and probable cause of the
problem
(I think):

I replaced the old 10nF ceramic smoothing caps in the grid bias
supply as they were getting distinctly warm after the scope had been
on suggesting that they were getting rather lossy (on a damp day they
would steam gently for a minute or two after turn on).

Didn't solve the brightness problem but new polyesters do NOT get
warm, so
I
feel better :-).

Static grid voltage at output of grid cut-off adjustment trim-pot was
drifting up during warm up, while voltage on input side was pretty
much constant. Suggests it might be the trim pot drifting with temp.

The trimpot (R315) is supposed to be a 2M2 linear Cermet 20% 1W
according
to
the parts list, but looks to me like a standard Piher PTC15 1/2W
part. Measuring the resistance as the scope cools after power off
seems to
confirm
the hypothesis (10% increase in value over 15 minutes, probably as
much as 15-20% by the time it reaches room temp).

If I could source a replacement readily I'd just ram a new one in,
but I suspect that the parts list is right and the original WAS a 1W
part, and
has
been replaced somewhere along the way with a 1/2W part. Finding a
2.2M 0.5W trimmer of any sort (never mind one that will fit) seems a
bit hard, and as for a 1W part ...!

Before I go totally insane hunting for a suitable part, I'd just like
to
get
a sanity check on my diagnosis from folks more experienced with this
than
I.

Thanks
Dave


Hi David,

One posible fix (but a bit clunky) is to find out the values of the
resistance is from the ends of the trimpot to the wiper, by measuring
the existing pot, and replacing it with 2 fixed resistors of a
suitable power rating .
At least you could then tell if your diagnosis is correct.

Bye for now,

Ian.




I would sub a straight 2 Meg pot,with 100K resistors on each side of it,if
you could not get the original value in the wattage called for.

Chances are a 2Meg/1W pot would work fine by itself,too.

--
Jim Yanik
jyanik
at
kua.net