Topics

TDS754D CRT


David Wilson
 

The problem I am having is with a cold instrument turn on, occasionally there is no picture. Waiting a few minutes the picture comes in.

During period of no picture the LCD rapidly flickers.

Ever since I got the instrument there is a slight wine from the CRT and the display is a little dim. I opened up the instrument to brighten it slightly though that makes it a little blurry.

I wonder if I have a tired CRT with a tired CRT driver board. If this really becomes a issue I can find parts or get the NewScope display kit.


Tom Gardner
 

On 03/01/17 19:32, david.wilson92@... [TekScopes] wrote:

The problem I am having is with a cold instrument turn on, occasionally there is no picture. Waiting a few minutes the picture comes in.

During period of no picture the LCD rapidly flickers.

Ever since I got the instrument there is a slight wine from the CRT and the display is a little dim. I opened up the instrument to brighten it slightly though that makes it a little blurry.

I wonder if I have a tired CRT with a tired CRT driver board. If this really becomes a issue I can find parts or get the NewScope display kit.
I saw a vaguely similar problem on a 485.

Turn it on and nothing would happen for 5 minutes, then the everything would spring to life.
Turn it off then on, and it instantly sprang into life.
Leave off for 1 hour, turn on, it would take 30s to spring to life.
Leave off overnight, back to the 5 mins delay.

One test per 6 hours made life tedious!

I traced the problem to an electrolytic in the switching PSU that was used to kickstart the switching. Presumably the "electrolyte was almost dry", so the dielectric took a few minutes to form. Once formed, it gradually decayed over 6 hours.


 

I have seen this happen. It is usually a capacitor with high ESR. As the cap heats up due to the high ESR, the ESR drops. Then the scope starts working. Most of the time this happens with switching supplies.

Regards.

----- Original Message -----
From: Tom Gardner tggzzz@... [TekScopes]
To: TekScopes@...
Sent: Tuesday, January 03, 2017 4:09 PM
Subject: Re: [TekScopes] TDS754D CRT



On 03/01/17 19:32, david.wilson92@... [TekScopes] wrote:
>
> The problem I am having is with a cold instrument turn on, occasionally there
> is no picture. Waiting a few minutes the picture comes in.
>
> During period of no picture the LCD rapidly flickers.
>
> Ever since I got the instrument there is a slight wine from the CRT and the
> display is a little dim. I opened up the instrument to brighten it slightly
> though that makes it a little blurry.
>
> I wonder if I have a tired CRT with a tired CRT driver board. If this really
> becomes a issue I can find parts or get the NewScope display kit.
>
I saw a vaguely similar problem on a 485.

Turn it on and nothing would happen for 5 minutes, then the everything would
spring to life.
Turn it off then on, and it instantly sprang into life.
Leave off for 1 hour, turn on, it would take 30s to spring to life.
Leave off overnight, back to the 5 mins delay.

One test per 6 hours made life tedious!

I traced the problem to an electrolytic in the switching PSU that was used to
kickstart the switching. Presumably the "electrolyte was almost dry", so the
dielectric took a few minutes to form. Once formed, it gradually decayed over 6
hours.




[Non-text portions of this message have been removed]


David Wilson
 

I will open it up to check the capacitors, thanks for the advice


David Wilson
 

I took a picture of this power supply, other than being a little dusty nothing looks out of the ordinary so far. See photos in this Tek group to find the picture


Siggi
 

I had a problem with the CRT in my TDS784D that manifested like occasional
horizontal "shearing" or "ripple". I re-capped the entire CRT board, which
cured it for a while, but then it reoccurred.
There was another member of this forum who had the same problem, but much
less patience or forbearance than me, who eventually figured out that for
him, this was cured by changing the contacts to the horizontal yoke.
Eventually the shearing bugged me enough that I opened up the scope, and in
the interest of science I touched nothing but the contacts to the
horizontal yoke. The contacts on the PCB, I scrubbed with DeOxit, and the
connector was flooded with same, then repeatedly jogged against the PCB
connector pins. Since then, I haven't seen any of this annoying shearing.

I remember reading other people's accounts of how the connector to the
horizontal yoke pretty much burns up - apparently there are impressive
currents going through there. Might be worth pulling the connector at least
to see whether it's getting ugly?

On Tue, 3 Jan 2017 at 14:32 david.wilson92@... [TekScopes] <
TekScopes@...> wrote:



The problem I am having is with a cold instrument turn on, occasionally
there is no picture. Waiting a few minutes the picture comes in.

During period of no picture the LCD rapidly flickers.

Ever since I got the instrument there is a slight wine from the CRT and
the display is a little dim. I opened up the instrument to brighten it
slightly though that makes it a little blurry.

I wonder if I have a tired CRT with a tired CRT driver board. If this
really becomes a issue I can find parts or get the NewScope display kit.


David Wilson
 

Good advice there, thanks.

I will check the horizontal and vertical yoke connectors.


I am also in the process of looking at recapping the power supply, any advice on type of capacitors? High temp, low ESR...?? Imagine replacing the electrolytic capacitors with polymer capacitors.


By the way the TDS380 is still sitting around in a box, I really like the roll feature that the TDS754D doesn't have for some reason.


David Wilson
 

uppon taking the 754D apart I had a difficult time removing the front panel and I believe I developed a trick to remove it easier. On the right side remove the panel and carefully work your left to the CRT. A small flat screw driver will help the difficult tabs at the CRT end, gradually work with it and it will eventually come off. Helpful advice that seemed to work for me.


David Wilson
 

I am debating with myself and I think I need your help.

This concerns the CRT and driver board. I'm about to removing this part for inspection. Looking down from the top I don't notice anything broken or burnt visually.

I read that a high pitch sound coming from the fly back transformer is a indication that it is failing. I read that a dim crt usually means it is getting tired, though sometimes it might be the driver board typically it is the crt.

I read on a crt tv forum that a delay before a picture should already come in is a indicator of a dried / bad capacitor. Recapping the board might fix the delay before picture but it might not fix the wining sound or dim picture I seem to have. When I recapped the TDS380 the picture was noticeably sharper.

Since the display still works I could go ahead and recap it and get what I can out of it. Maybe finding a decent CRT is really all I need to get things going like new again. Maybe, don't know about the flyback.

Picture is stable by the way. Noticing no wobble or jitter.

So what is your take on the fly back transformer? Should I recap the board?

Thanks


 

Hello,

On late CRT TVs there was also cathode current measurement that could delay
the time to bring a picture up when the CRT was worn (Philips).

For whining sounds poke around with a chopstick at any ferrite core, the
most common cause is a loose core. Bad capacitors can be a cause too, not
limited to electrolytics.

Measuring ripple across electrolytics should give you a clue before
actually replacing them.
Beware of electrolytics in the flyback primary drive path.

Often a failing flyback will come with other symptoms: Heat, smells,
blooming picture or measurable oscillations on at least one winding.

Does this scope use the same CRT driver board as the TDS 544?

Paul



Le jeudi 5 janvier 2017, david.wilson92@... [TekScopes] <
TekScopes@...> a ├ęcrit :

I am debating with myself and I think I need your help.

This concerns the CRT and driver board. I'm about to removing this part
for inspection. Looking down from the top I don't notice anything broken or
burnt visually.

I read that a high pitch sound coming from the fly back transformer is a
indication that it is failing. I read that a dim crt usually means it is
getting tired, though sometimes it might be the driver board typically it
is the crt.

I read on a crt tv forum that a delay before a picture should already come
in is a indicator of a dried / bad capacitor. Recapping the board might fix
the delay before picture but it might not fix the wining sound or dim
picture I seem to have. When I recapped the TDS380 the picture was
noticeably sharper.

Since the display still works I could go ahead and recap it and get what I
can out of it. Maybe finding a decent CRT is really all I need to get
things going like new again. Maybe, don't know about the flyback.

Picture is stable by the way. Noticing no wobble or jitter.

So what is your take on the fly back transformer? Should I recap the board?

Thanks

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Yahoo Groups Links




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David Wilson
 

Based on my findings both are color driver boards and have a somewhat similar physical layout.

I posted a picture of the driver board if you would like to take a look.

671-2373-589-1344-02

How do you measure capacitor ripple?


Bruce Lane
 

It might be caps, but it might also be the transformer itself starting
to go bad. You might want to (for the long run) consider replacing the
entire CRT assembly with a nice flat-panel LCD.

http://www.newscope.info/home.html/2001/9001.html

No matter what, happy tweaking.

On 05-Jan-17 08:43, david.wilson92@... [TekScopes] wrote:
I am debating with myself and I think I need your help.

This concerns the CRT and driver board. I'm about to removing this part for inspection. Looking down from the top I don't notice anything broken or burnt visually.

I read that a high pitch sound coming from the fly back transformer is a indication that it is failing. I read that a dim crt usually means it is getting tired, though sometimes it might be the driver board typically it is the crt.

I read on a crt tv forum that a delay before a picture should already come in is a indicator of a dried / bad capacitor. Recapping the board might fix the delay before picture but it might not fix the wining sound or dim picture I seem to have. When I recapped the TDS380 the picture was noticeably sharper.

Since the display still works I could go ahead and recap it and get what I can out of it. Maybe finding a decent CRT is really all I need to get things going like new again. Maybe, don't know about the flyback.

Picture is stable by the way. Noticing no wobble or jitter.

So what is your take on the fly back transformer? Should I recap the board?

Thanks

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Posted by: david.wilson92@...
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Yahoo Groups Links



--
---
Bruce Lane, ARS KC7GR
http://www.bluefeathertech.com
kyrrin (at) bluefeathertech dot com
"Quando Omni Flunkus Moritati" (Red Green)


David Wilson
 

I had my eyes on that LCD kit you mentioned. Have you or anyone had experience with this kit?


David Wilson
 

Last question, power supply. This scope was made in 1998 which is 19 years ago from today. Board is very clean with no leaking bulging caps, should I recap due to 20 years of age? Doesn't seem to be giving problems, this would be preventive maintenance


Bruce Lane
 

Not directly, as yet. All I've seen are photos of retrofitted 'scopes.
They look pretty amazing.

From the descriptions: I think anyone skilled enough to do any kind of
work on their 'scope's innards will not have any problems with the retrofit.

On 05-Jan-17 11:22, david.wilson92@... [TekScopes] wrote:
I had my eyes on that LCD kit you mentioned. Have you or anyone had experience with this kit?

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Posted by: david.wilson92@...
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Yahoo Groups Links



--
---
Bruce Lane, ARS KC7GR
http://www.bluefeathertech.com
kyrrin (at) bluefeathertech dot com
"Quando Omni Flunkus Moritati" (Red Green)


Bruce Lane
 

I wouldn't worry about it. Those caps were made long before the
problems with bad chemistry started showing up, and Tek had a thing (at
that time, anyway) of buying pretty decent parts.

On 05-Jan-17 15:37, david.wilson92@... [TekScopes] wrote:
Last question, power supply. This scope was made in 1998 which is 19 years ago from today. Board is very clean with no leaking bulging caps, should I recap due to 20 years of age? Doesn't seem to be giving problems, this would be preventive maintenance

------------------------------------
Posted by: david.wilson92@...
------------------------------------


------------------------------------

Yahoo Groups Links



--
---
Bruce Lane, ARS KC7GR
http://www.bluefeathertech.com
kyrrin (at) bluefeathertech dot com
"Quando Omni Flunkus Moritati" (Red Green)