Date   
Re: TDS520B - the screen gradually expands and then darkens in about 20 mins

unclebanjoman
 

Hi nonIonizing EMF,

Thanks for your comment.
Yes I know that I could make a DIY HV probe, but I think it's better, as soon as I get the chance, to buy an old but still useful used P6015 probe....

Max

Re: TDS520B - the screen gradually expands and then darkens in about 20 mins

unclebanjoman
 

Hi Stephen,

at this point obviously I prefer to get the complete board, not just the transformer!
I wrote you an email about eight hours ago.
I request a quote for 2 complete and functioning boards (I would like to have one as a spare).

Thanks,

Max

Re: TDS520B - the screen gradually expands and then darkens in about 20 mins

unclebanjoman
 

On Wed, Jun 26, 2019 at 04:39 AM, greenboxmaven wrote:
Do you
have good focus and a reasonably bright image when you first turn the
scope on?
Yes, when the scope is first turned on, brightness and focus are both perfect.
After approx. 10 mins, the screen starts blooming, first fractions of a millimeter, then gradually more accentuated but the display is still well focused and bright.
After 20 minutes the blooming dramatically accelerates and screen darkens to totally black.

I turn off and let it cool for at least 10 minutes. Then turn on the scope and the scene repeats itself.

Max

Re: 7854 with strange intensity/focus problem (to me)

Albert Otten
 

On Wed, Jun 26, 2019 at 03:29 PM, unclebanjoman wrote:


Can be something's wrong with the focus grid.
Hi Max,

I don't think so, but I got even more confused after other measurements in my 7704A. I measured the cathode current by means of the voltage drop across the 100 ohm resistor between supply and cathode. When I suddenly turn intensity to maximum then the voltage jumps to a "high" value (somewhere 1.0 - 1.5 mV) and within seconds drops to a lower value (say 0.8 mV or even less). I tested this also with very defocused trace; the maxima seem higher with trace at the screen than with trace off-screen. The multiplier current sense works different in the 7704A but current got never limited by this sense line.
I'm afraid I can't help you any further.

Albert

Re: 7854 with strange intensity/focus problem (to me)

Chuck Harris
 

In the 2465 family CRT's, I have noticed that tek did
most of the internal element connection welds quite well,
but others, like the control grid, were universally weak.
No real deformation of the steel by the heat of the weld,
and the pressure of the welder's electrodes.

As a result, I have seen a couple of 2465 CRT's that had
open circuit control grids. The weak, barely there, weld
snapped, and the thin ribbon of material used to make the
connection sprung away from the electrode it was "welded"
to. You can see these failures through the glass neck of
the tube.

The effect of this failure is obvious in operation, the
intensity slowly grows brighter and brighter until it is
too bright, and you feel compelled to shut the scope down.

Of course, the intensity pot does nothing.

Our scopes almost all have been shipped, or banged about,
by the time we get them, and weak CRT welds will sometimes
break, ruining an otherwise beautiful looking the CRT.

-Chuck Harris

unclebanjoman wrote:

Hi Albert,

Can be something's wrong with the focus grid.

One month ago I've repaired a 475. It did not regulate the intensity.
After a careful check of all the components of the HV section and after many hours of useless work, I solved it simply by changing the CRT (luckily I had a spare one).
Changed the CRT, everything has started to work properly.
Probably something wrong with the G1. Mystery....

Max



Re: Tek 577 for sale and a note about the display

8850hp@...
 

I have some interest. Please contact me at 8850hp at gmail dot com

Re: 7854 with strange intensity/focus problem (to me)

unclebanjoman
 

Hi Albert,

Can be something's wrong with the focus grid.

One month ago I've repaired a 475. It did not regulate the intensity.
After a careful check of all the components of the HV section and after many hours of useless work, I solved it simply by changing the CRT (luckily I had a spare one).
Changed the CRT, everything has started to work properly.
Probably something wrong with the G1. Mystery....

Max

Re: 7S12 1nS semi-rigid SMA cable bending: WAS Tdr cable 015-1023-00

NigelP
 

Thanks for that heads-up. It doesn't have the exact cable I'm dealing with but it is quite surprising how tight you can bend them. This Tek cable is 4.5mm overall and even a 6.35mm cable on their website can be bent to 9.5mm internal radius..... much smaller than I am anticipating.

Actually I'm beginning to wonder whether semi-rigid is the correct description for this cable; it seems almost certainly a solid copper sheath (it's very stiff) but it's entirely covered in a PVC sheath from connector to connector, so I can't see the outer conductor material at all.

I do have in my possession an example of a solid copper sheath SMA cable which is 3.5mm OD which has already had manufacturing pre-bends of about 10mm radius (just an estimation), so I suppose that should give some guidance. I really only anticipate a single bend of maybe 25-50mm radius.... or whatever makes it convenient not to have it sticking out the front of the sampling head.

I'm just being over-cautious maybe but the item cost serious bucks just for a "bit of cable with connectors"...... but it does have the Tek name and part number printed on it so it must be worth it! I'm not really sure how much stress the bend puts on the centre conductor attachment at either end. Maybe it's just about kinking the cable and making an impedance discontinuity.

Regards

Nigel

Re: 7854 with strange intensity/focus problem (to me)

Albert Otten
 

Hi Max,

So in summary you found in the bad scope that when you increase intensity from CCW to CW:
- Logic board "FX" voltage first increases then drops again. In my opinion "FX" reflects CRT PA anode current.
- TP127 <14> drops monotonically. In my opinion TP127 reflects cathode current.
I might be wrong with my interpretation. If not, maybe someone can explain the discrepancy between currents.

Albert


Drops monotonically.

Max

I re-checked TP127 <14> (I trimmed it three weeks ago). It's fixed to +100V
with less 1 V turning the intensity pot from one end to the other.

Max

The waveforms of my two 7854 at pin FX on logic board <4), they are definitively different.

Settings are as follows:
7854: left vertical with 718 (to move the trace off-screen if necessary), B horz 7B53A set to 1 us/div
Test scope: 10:1 probe, 7A13 set to 100mV/div, 7B53A set to 20us/div

The "good" 7854: turning intensity gradually up, a rough square wave appears, 100mVp-p, period approx. 40us
(approx 25 kHz), baseline about 0V. Increasing more and more the intensity, the square wave (its mean level)
shifts gradually upwards, reaching 600 mV and more. Trace brilliant and sharp always.

The "BAD" 7854: turning intensity gradually up, a rough square wave appears, 100mVp-p, period approx. 40us
(approx 25 kHz) baseline about 0V. Increasing more and more the intensity, the square wave (its mean level)
shifts with difficult, hesitantly upwards, reaching no more than 100/150 mV up. Increasing the intensity pot fully
CW the square wave returns towards the 0V baseline, the trace (as expected) become blurred and less bright.

Max

Re: TDS520B - the screen gradually expands and then darkens in about 20 mins

nonIonizing EMF
 

On Tue, Jun 25, 2019 at 07:39 PM, greenboxmaven wrote:


If you can find someone who had a TV shop, they might have an old self
contained HV probe you could use. Those read up to 30KV, I doubt the anode
voltage is over 12KV on your scope.
A Youtube search of the terms "HV Probe DIY" and "Homemade 40KV oscilloscope probe" returns interesting results for potential DIY HV Probes you might be able to make cost effectively with parts you may even have around already.

Re: TDS520B - the screen gradually expands and then darkens in about 20 mins

greenboxmaven
 

If the 50 and 500 volt supplies are stable, at least part of the flyback looks good. I see it is an integral flyback/rectifier module. About the only way to see if the 2nd anode voltage is sagging is to meter it.

If you can find someone who had a TV shop, they might have an old self contained HV probe you could use. Those read up to 30KV, I doubt the anode voltage is over 12KV on your scope. You might also insert a meter in the cathode lead of the CRT. I am guessing the current should be around 100-200 microamps for a good raster, if the current drops when the image blooms the anode supply is probably weak, if the current rises then the beam current is probably dragging down the second anode supply. If this happens, check the voltages on the G1 and cathode. Do you have good focus and a reasonably bright image when you first turn the scope on?


Bruce Gentry, KA2IVY

On 6/25/19 6:20 PM, unclebanjoman wrote:
Hi Bruce,

I've just uploaded the schematic of the A20 CRT driver for the TDS-520B.
It can be found at:
https://groups.io/g/TekScopes/files/TDS520B_CRT_board.pdf

There are no external multipliers after the flyback transformer, and the voltages (+50V and +500V) are stable.
I cannot track the CRT anode HT voltage since I've only a 80K-6 Fluke probe (6 kV max).

Max


Re: TDS520B - the screen gradually expands and then darkens in about 20 mins

Stephen Hanselman
 

Well two things, first that grey glass is what I did a poor job of describing. And second we could pull one and ship it. Problem is it costs the same as the shipping the complete board.

Regards,

Stephen Hanselman
Datagate Systems, LLC
3107 North Deer Run Road #24
Carson City, Nevada, 89701
(775) 882-5117 office
(775) 720-6020 mobile
s.hanselman@...
www.datagatesystems.com
a Service Disabled, Veteran Owned Small Business
DISCLAIMER:
This e-mail and any attachments are intended only for use by the addressee(s) named herein and may contain legally privileged and/or proprietary information. If you are not the intended recipient, any dissemination, distribution or copying of this e-mail and any attachments is strictly prohibited. If you have received this e-mail in error, please immediately notify me and permanently delete the original and all copies and printouts of this e-mail and any attachments.

On Jun 25, 2019, at 12:17, unclebanjoman <mmazza@...> wrote:

Thanks for your informations Steve.

I confirm that the model I have is the TDS-520B, so no problem should arise, I think. It has a gray glass front panel, no color panel.
I will leave my client the choice on how to proceed, whether to keep the old CRT display or install an LCD panel.

I never thought that finding a flyback CRT transformer was so hard!

Max



Re: 7854 with strange intensity/focus problem (to me)

unclebanjoman
 

Drops monotonically.

Max

Re: TDS520B - the screen gradually expands and then darkens in about 20 mins

unclebanjoman
 

Hi Bruce,

I've just uploaded the schematic of the A20 CRT driver for the TDS-520B.
It can be found at:
https://groups.io/g/TekScopes/files/TDS520B_CRT_board.pdf

There are no external multipliers after the flyback transformer, and the voltages (+50V and +500V) are stable.
I cannot track the CRT anode HT voltage since I've only a 80K-6 Fluke probe (6 kV max).

Max

Re: 7854 with strange intensity/focus problem (to me)

Albert Otten
 


I re-checked TP127 <14> (I trimmed it three weeks ago). It's fixed to +100V
with less 1 V turning the intensity pot from one end to the other.

Max
Hi Max,

But is that 1 V drop going monotonically, or is there a peak similar to what you saw in the multiplier current?

Albert

Re: TDS520B - the screen gradually expands and then darkens in about 20 mins

greenboxmaven
 

The fading out and expanding image is called blooming to old TV repairpeople. Check the second anode voltage, there is almost certainly something failing and dragging it down, most likely a capacitor or possibly a diode in the voltage multiplier after the HV transformer.

Bruce Gentry, KA2IVY



Re: 7S12 1nS semi-rigid SMA cable bending: WAS Tdr cable 015-1023-00

Karin Johnson
 

Take a look at this website. You may have to click down a few pages but I think the information you required is there.
https://www.tek-stock.com

Regards,
Karin

Re: TDS520B - the screen gradually expands and then darkens in about 20 mins

unclebanjoman
 

Thanks for your informations Steve.

I confirm that the model I have is the TDS-520B, so no problem should arise, I think. It has a gray glass front panel, no color panel.
I will leave my client the choice on how to proceed, whether to keep the old CRT display or install an LCD panel.

I never thought that finding a flyback CRT transformer was so hard!

Max

Re: 7854 with strange intensity/focus problem (to me)

unclebanjoman
 

Hi Albert,

I apologize for delayed reply, I was busy with the repair of a TDS-520.
I re-checked TP127 <14> (I trimmed it three weeks ago). It's fixed to +100V with less 1 V turning the intensity pot from one end to the other.

Max

Re: TDS520B - the screen gradually expands and then darkens in about 20 mins

Stephen Hanselman
 

Max,

We have installed several of the Simmconn Labs displays and the customers always say very nice things. even though the scope is monochrome the display puts limited color on the screen. If you decided to go that route I would suggest deviating from the instructions. They seem to tell you to leave the color corrected glass panel off and just have a great "hole" down to the LCD panel. When I did my personal TDS540C I modified the glass support frame and left it on, much nicer looking.

The only thing though it only will work on the "A", "B", etc not the "no suffix" unit.

We just did one on a 8566 and it was spectacular. Not only a crisp display but the alignment pot now moves through 4 display formats inverse, monochrome, limited color, etc

steve

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of unclebanjoman
Sent: Tuesday, June 25, 2019 7:45 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] TDS520B - the screen gradually expands and then darkens in about 20 mins

WOW! Very interesting!

I would be interested for my personal use (I am also a programmer firmware) but unfortunately the TDS-520 has been given to me to repair and the customer wants a professional solution without unnecessary complications and excessive times.

Thank you for your reporting nonionizingEMF. Very very interesting.

Max