7104 out of focus/astigmatism


Francesco Franzini
 

Hi

I would like to have some advice on a strange problem with my 7104 oscilloscope.

When I power on from cold state the oscilloscope is pretty well focused with little astigmatism, during warm up it goes out of focus but much more out of astigmatism, then after 20/30 min it becomes very focused and the astigmatism disappears.

The CRT tube is very bright and the phospor is not worn out, on both HV board and the Z axis board all voltage levels are on spec.

Anyone experienced something like that?

Thank you

Francesco


 

I had to replace a CRT in a 7104 due to an intermittent internal connection. It was temperature sensitive. I would also remove and replace the CRT socket. I have seen poor connections that occur due to corrosion on the pins.


Jean-Paul
 

The CRTs seldom fail like you describe.

I would monitor the HV during the entire cycle.

Could be a temp related HV transformer or cap, or feedback dropping HV resistors in the HV PS.

Jon


Francesco Franzini
 

Thank you both for your advice, i will get back with the results !!!

I realize that is didn't explained very well the issue, the oscilloscope goes out of focus/astigmatism slowly and constantly, and slowly and constantly recover focus and astigmatism, and at any point of this "process" working with focus and astigmatism setting i am able to have perfect trace and readout.

Francesco


Francesco Franzini
 

Just more detail on the issue..

I noticed something strange, especially on readout trace, during the warm up, i see the trace intensity pulsating and moving a little bit (seems modulated), also this issue goes away after the warm up.

Results of the tests:
HV level (all of them) are stable, with no variation during the warm up, unfortunately i do not have any HV probe for oscilloscope to check the waveforms as per service manual, i tried with cooling spray selectively on the HV components with no luck.
All connections seems to be good, everything is clean and in good shape, i haven't found any corrosion or issue on the connector.

Since the issue last almost 30 min I will continue to check the low voltage level to see if i found something wrong.

Francesco


Chuck Harris <cfharris@...>
 

Did you give any thought to measuring the low voltage power
supplies for tolerance, and ripple?

The reason I ask, is your scope is showing symptoms that are
quite similar to a bad filter capacitor. They tend to be
at their worst when they are cold, and improve when they get
warm.

-Chuck Harris

Francesco Franzini wrote:

Just more detail on the issue..

I noticed something strange, especially on readout trace, during the warm up, i see the trace intensity pulsating and moving a little bit (seems modulated), also this issue goes away after the warm up.

Results of the tests:
HV level (all of them) are stable, with no variation during the warm up, unfortunately i do not have any HV probe for oscilloscope to check the waveforms as per service manual, i tried with cooling spray selectively on the HV components with no luck.
All connections seems to be good, everything is clean and in good shape, i haven't found any corrosion or issue on the connector.

Since the issue last almost 30 min I will continue to check the low voltage level to see if i found something wrong.

Francesco






Francesco Franzini
 

The low voltage section and the PSU are perfectly fine, no ripple and everything is very stable from cold to hot, i starded to investigate on A22 HV board, voltage level are ok and on spec. but i found something strange on TP1784, voltage level (+98.6) ok but very high and variable ripple, when the scope is out of focus/astigmatism the ripple is 4 to 6 volt 2.5ms period then, after about 30 min, drop to 1 volt 2.5ms period and everything is working right.
At this point of my investigation i am pretty sure that the issue is on the HV circuit (--2400/-2265) and seems to be related to something that change its characteristic under power, i guess that i need to check avery single components for anomalies.

Any other advice?

Thank you

Francesco


unclebanjoman
 

Check and eventually replace all electrolytic caps on HV board.
The symptom of "pulsating" intensity it is indicative of some ripple in Z axis DC restorer.
It is very likely that the problem in the focus is also from a similar cause.
I had similar problems in two 7603 and one 7904: dry electrolytic caps in HV board. I replaced all for safety and now are O.K.

Max


Mark Vincent
 

Francesco,

Spray the cooler on C1736, C1770 and C1771 when the scope is warmed up. If the trace becomes like it is when the unit is cold, that means that these should be replaced. The first one I replaced in the one I worked on because its ESR was high. You can use a film type there. The last two I had to replace in one because the voltage was low and ripple was high for the 105V supply. I used 5,6 or 6,8mfd, I forgot which value I used, high temperature low ESR types for C1770 and C1771. That made the voltage right and ripple was almost zero. Also cool the ICs and transistors if you have not already done so. You might have one or more that is/are temperature sensitive. I have had this happen in two scopes in the past.

Check the resistance of R1736, R1796 and R1815. If any of these are out of tolerance, that can cause problems. I have had to replace these because the ones I checked were well out of tolerance. These are 1/4W carbons which were replaced with 1/2W 1% types.

I am telling you what I experienced in the one I worked on and what was done to that one. You may or may not have these problems. Let the group know what you find that fixed your problem. Someone else might have the same problem and use what you found as the fix to theirs. We hope you find your problem(s) and enjoy using the scope. That is one great scope.

Mark


Francesco Franzini
 

Following your advices i double checked everything, replacing resistors out of spec and checking carefully capacitors, transistors and IC's with no improvement, after a lot of power on/power off cycles i noticed that suddenly the crt regained right focus and astigmatism without any logic or correlation with anything, thinking about it i left the oscilloscope powered on for 48 hours with no readout and zero intensity (nothing on the crt screen).

After the "treatment" everything is working as expected, no loss of focus or astigmatism, trace and readout very sharp and bright.

I really have no idea on what was wrong on my 7104 but my findings seems to point to the crt, i hope the experts on this forum can have better explanation on this issue.

Thank you for your support and help

Francesco


Michael Perkins
 

Folks,My 7104 behaves the same way.  I don't use it daily, and have put off examining it until I have more time. I am eager to find out if there was a cause on your scope.Regards,Mike
On Wed, Feb 24, 2021 at 11:27 AM, Francesco Franzini<franzini.francesco@gmail.com> wrote: Following your advices i double checked everything, replacing resistors out of spec and checking carefully capacitors, transistors and IC's with no improvement, after a lot of power on/power off cycles i noticed that suddenly the crt regained right focus and astigmatism without any logic or correlation with anything, thinking about it i left the oscilloscope powered on for 48 hours with no readout and zero intensity (nothing on the crt screen).

After the "treatment" everything is working as expected, no loss of focus or astigmatism, trace and readout very sharp and bright.

I really have no idea on what was wrong on my 7104 but my findings seems to point to the crt, i hope the experts on this forum can have better explanation on this issue.

Thank you for your support and help

Francesco


Mark Vincent
 

Francesco and Michael,

Measure the heater voltage first. If low, go to the second paragraph. If you know it sat a long time before you started using it, read below.

You scopes might have been sitting unused for some time, usually some years, and the crt will look weak when it is not. I have done this to TVs many times by removing the socket and putting a CRT checker on it, raising the heater voltage to about 7-8V for 10-30 minutes. That "cracks" the cathode. I was told by TV techs that a crt that sits for a long time unused will do this, new or used and that raising the heater voltage for a short time helps remove gas. I do not know how it get gas that way when it is still good. After the time is up, the original socket was put back on the end, unit powered up and it looked fine. If a checker was used to test it, it showed in the good region.I do run heaters of crts that have not been on for a long time before powering the unit up to make sure the screen lights up properly. Usually the heater is run at the regular voltage for some time, e.g. 30 minutes. I know that some transmitting tubes that have sat for some time need their heater/filament run for a while before any B+ is applied. Those are run at the rated heating voltage.

Another thing that will cause the problems you two have is the heater voltage is too low. Check the heater voltage where it goes to the socket. It should be 6,3V. The resistor from the heater winding could be high causing the heater voltage to be too low. Usually if the high voltage is right then the heater voltage is right. The meter used here should be a good VOM or DVM that will respond to the frequency of the oscillator circuit. I use my Simpson 260 for this test. If the heater reads 6,2 or 6,4V, that is close enough. Something like <6V or 6,9V is too far off. These heaters are 60mA so use Ohm's Law if needed to adjust the resistor value if necessary to change.

Mark


 

On Wed, Feb 24, 2021 at 10:13 PM, Mark Vincent wrote:


You scopes might have been sitting unused for some time, usually some years,
and the crt will look weak when it is not. I have done this to TVs many times
by removing the socket and putting a CRT checker on it, raising the heater
voltage to about 7-8V for 10-30 minutes. That "cracks" the cathode. I was told
by TV techs that a crt that sits for a long time unused will do this, new or
used and that raising the heater voltage for a short time helps remove gas. I
do not know how it get gas that way when it is still good.
That's probably just some errant, wandering ions that haven't yet been captured by the getter or have broken loose covering the cathode.
Anyone of you got Peter Keller's book about CRT's?

Raymond


Roy Thistle
 

On Wed, Feb 24, 2021 at 02:34 PM, Raymond Domp Frank wrote:


That's probably just some errant, wandering ions that haven't yet been
captured by the getter or have broken loose covering the cathode.
Anyone of you got Peter Keller's book about CRT's?
I think Mark is talking about using a CRT restorer/rejuvenator. Peter's book doesn't discuss that/those, AFAIK.
Probably you know about them... there is some discussion beginning in message #166456, in an older thread.
B&K, and Sencore, et. al. made them for TV repair guys.
The idea of increasing the filament voltage, and so the filament/heater current was to overheat the filament... and in so doing overheat the cathode... with the hope that doing that could restore the cathode's lost emissivity.
I wouldn't say rejuvenator's didn't work; but, I would say they stressed the filament... and probably sold more new CRTs for TV repair guys... than they "rejuvenated" weak CRTs... just by burning out the filament.
Anyway, doing this to an oscilloscope CRT seems like a last resort , when turning down the lights won't bring up a dim display.


 

On Thu, Feb 25, 2021 at 04:56 AM, Roy Thistle wrote:


I think Mark is talking about using a CRT restorer/rejuvenator. Peter's book
doesn't discuss that/those, AFAIK.
I thought I read about the recovery process that may happen in long-unused CRT's by just leaving them on for a time, even without temporarily increasing filament voltage in Peter Keller's book. OTOH, I read about CRT rejuvenation recently as well, so I may well be wrong.

Raymond


Michael W. Lynch
 

On Wed, Feb 24, 2021 at 11:05 PM, Raymond Domp Frank wrote:


I read about the recovery process that may happen in long-unused CRT's by just
leaving them on for a time, even without temporarily increasing filament
voltage in Peter Keller's book.
I have Peter Keller's book, but have not reached that part of the book.

I may have experienced this phenominon on my 7623A which had been sitting for a long time. I had stored the scope for about 6 months and thought that I should get it out and run it for a while. When I first started the scope, the trace and the readouts were just a blur. Since I had nothing to lose, I just turned it on and let the scope run. After a couple of hours, I went back and the screen was sharp and clear as ever. My CRT was never dim, intensity control would give a good bright bloom, everything was just completely out of focus.

Could this have been what is being discussed in this thread?

--
Michael Lynch
Dardanelle, AR


 

On Thu, Feb 25, 2021 at 06:38 AM, Michael W. Lynch wrote:


I may have experienced this phenominon on my 7623A which had been sitting for
a long time. I had stored the scope for about 6 months and thought that I
should get it out and run it for a while. When I first started the scope, the
trace and the readouts were just a blur. Since I had nothing to lose, I just
turned it on and let the scope run. After a couple of hours, I went back and
the screen was sharp and clear as ever. My CRT was never dim, intensity
control would give a good bright bloom, everything was just completely out of
focus.

Could this have been what is being discussed in this thread?
It is what I read about and I think I read it in Peter Keller's book, which I haven't with me where I currently am.

Raymond


Harvey Parent
 

Hi Raymond,

Thanks for replying to my CRT Focus problem.
These eBay ‘scopes have been stored in my garage for over a year and possibly stored for who knows how long with the previous owner.
I actually have three 465 ‘scopes and the first two I have powered on are displaying the same issue. I will try your suggestion on one of the 465s this morning.

I gather I can disconnect one filament lead from the HV transformer and apply 7 to 7.5 V DC to the filament directly for 30 minutes (scope off and unplugged from AC outlet.
Working previously as a broadcast transmitter tech, we would once a year put any spare tubes in the transmitter for 20 to 30 minutes with only filament voltage applied. Failure to do this would often cause an internal arc in the tube and trip the HV breaker.

I’ll report back after doing the procedure on the first 465B I am working on.

Thank you,
Harvey


Francesco Franzini
 

New findings....

In few days focus/astigmatism issue is back......

Using boards from a working 7104 i swapped one a time:
1) Power supply
2) HV board
3) Z axis board
Unfortunately i have not seen any change on the behavior.

During the tests i found something interesting, putting the trace off screen and increasing the intensity to max level the readout recover focus/astigmatism (trace as well).
I really don't understand what is happening because everything seems to point to the CRT but i never heard or read anything similar.
Two more board remain to swap, X and Y, but, i don't see any connection from this boards to the issue.

I reaaly need further advice to understand the root cause of this issue

Thank you
Francesco


Jean-Paul
 

Francesco this sounds like the classic HV transformer epoxy failures.

Did you check the CRT HV during the failures?

Be careful, use correct HV probe and follow service manual on HV section.

Both the HV transformer and HV multiplier can become heat sensitive and fail intermittently.

As the CRT HV drops, the traces dim and expand or "bloom".

Chuck Harris is the Forum Maven on all HV transformers symptoms, replacing and rewinding.

Bon courage


Jon