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


unclebanjoman
 

Hi all,

I got that dam'd TDS520 to repair. Powered up, it works perfectly.
Then, the screen gradually expands and then darkens in about 20 mins. The process initially is very inappreciable, and gradually accelerates over time.

I dismantled the display board and its associated CRT to make it work out of the oscilloscope (it's connected to the main chassis with a flat cable only).

Searching for some component in thermal runaway, using coolant spray give me no result. Mumble...

I made some voltage measurement. The A20 board uses only the +24 V supply from the main PSU and uses an LM317 to step down to +21 V.
When the screen starts to expand visibly (and darkens) the +21 V decreases rapidly down to +15/16 V and the LM317 becomes damn hot! Surely and excessive current is drawn from it.

I've changed the C308 speed-up capacitor (base of Q205) with no results.
I'll check the Q205 drive signal to see if it changes over time but I'm strongly oriented toward a failure in the flyback transformer T225.

I know already know that this transformer model is practically unobtainable (120-1841-00).
I wonder that an equivalent or similar transformer cannot be found that can be adapted. I don't think it's such a so critical component!

What is your opinion?

Thanks,

Max


Stephen Hanselman
 

What is the pn on the hv board?

Regards,

Stephen Hanselman
Datagate Systems, LLC

On Jun 22, 2019, at 10:05, unclebanjoman <mmazza@...> wrote:

Hi all,

I got that dam'd TDS520 to repair. Powered up, it works perfectly.
Then, the screen gradually expands and then darkens in about 20 mins. The process initially is very inappreciable, and gradually accelerates over time.

I dismantled the display board and its associated CRT to make it work out of the oscilloscope (it's connected to the main chassis with a flat cable only).

Searching for some component in thermal runaway, using coolant spray give me no result. Mumble...

I made some voltage measurement. The A20 board uses only the +24 V supply from the main PSU and uses an LM317 to step down to +21 V.
When the screen starts to expand visibly (and darkens) the +21 V decreases rapidly down to +15/16 V and the LM317 becomes damn hot! Surely and excessive current is drawn from it.

I've changed the C308 speed-up capacitor (base of Q205) with no results.
I'll check the Q205 drive signal to see if it changes over time but I'm strongly oriented toward a failure in the flyback transformer T225.

I know already know that this transformer model is practically unobtainable (120-1841-00).
I wonder that an equivalent or similar transformer cannot be found that can be adapted. I don't think it's such a so critical component!

What is your opinion?

Thanks,

Max



unclebanjoman
 

Hi Stephen,

the board has P.N 671-1271-03.

Cheers,
Max


Stephen Hanselman
 

I’ll look in my box of spares, we do a pile of 540’s and have 3 different HVPS. One should fit in a flat rate box

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 23, 2019, at 03:05, unclebanjoman <mmazza@...> wrote:

Hi Stephen,

the board has P.N 671-1271-03.

Cheers,
Max



unclebanjoman
 

O.K. Sthepen, thank you.

Please note that I'm from Italy, near Venice.

Max


nonIonizing EMF
 

On Sun, Jun 23, 2019 at 01:51 PM, unclebanjoman wrote:


Please note that I'm from Italy, near Venice.
I sent a direct email message regarding a unit I was going to purchase though shipping was too much for me to ship to the U.S.

Maybe you or someone over there can use. If anyone else is interested... feel free to reply.

Kindest Regards


unclebanjoman
 

Thank you, e-mail received.

Max


Stephen Hanselman
 

Max,

I've looked in my box and found several Sweep PCAs. They are all 671-1271 with suffixes of 01, 02, 06. Since these all came out of 540 and 540A scopes I'd suspect that the sweep PCA is fairly universal in application. I have replaced -03 and -04 with -02, etc./

I did look and even in a flat rate box from the US the cost is very close to $100. So I'd suggest looking for a broke 5xx->7xx and scavenging the sweep. If you want one of mine, tested before we ship, it would be $50.00 plus shipping.

Hope this helps

Steve

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of unclebanjoman
Sent: Sunday, June 23, 2019 1:51 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] TDS520B - the screen gradually expands and then darkens in about 20 mins

O.K. Sthepen, thank you.

Please note that I'm from Italy, near Venice.

Max


unclebanjoman
 

Thank you Stephen,
I appreciate your effort. I'm really wondering for the shipping cost. Courier service, FedEx or US Mail?

Anyway, digging into details of horizontal driver IC (MC1391) I found anomalous waveforms at pins 1 and 8.
The output on pin 1, according to the datasheet and the component values should be a 2us (approx.) negative pulse and consequently the output pulse driving the base of Q205 should a 2 us positive pulse.
From pin 1 I got a strange negative pulse+ramp 15 us total time instead, definitively wrong!
Just ordered the replacement (1 $).

I'll keep you updated.

Max


Stephen Hanselman
 

Max, sounds good shipping international is an expensive option. I got my numbers from the USPS web site so they should be fairly good. FedEx and the others are just short of obscene.

I have several of them so availability is not a problem if you need it and want to go that way. Of course it might be a plan to check the “parts only” units. We’ve found that the usual failures are the acquisition and PS PCAs, sweep boards are fairly robust.

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 24, 2019, at 15:30, unclebanjoman <mmazza@...> wrote:

Thank you Stephen,
I appreciate your effort. I'm really wondering for the shipping cost. Courier service, FedEx or US Mail?

Anyway, digging into details of horizontal driver IC (MC1391) I found anomalous waveforms at pins 1 and 8.
The output on pin 1, according to the datasheet and the component values should be a 2us (approx.) negative pulse and consequently the output pulse driving the base of Q205 should a 2 us positive pulse.
From pin 1 I got a strange negative pulse+ramp 15 us total time instead, definitively wrong!
Just ordered the replacement (1 $).

I'll keep you updated.

Max



Chuck Harris
 

I tried ShippingMonkey for a recent very heavy (140#) shipment,
and they were about 1/2 the price of DHL/FedEx/UPS, and did a
very nice job. It wasn't cheap, but it was insured, tracked,
and a better price than the others.

-Chuck Harris

Stephen Hanselman wrote:

Max, sounds good shipping international is an expensive option. I got my numbers from the USPS web site so they should be fairly good. FedEx and the others are just short of obscene.

I have several of them so availability is not a problem if you need it and want to go that way. Of course it might be a plan to check the “parts only” units. We’ve found that the usual failures are the acquisition and PS PCAs, sweep boards are fairly robust.

Regards,

Stephen Hanselman


unclebanjoman
 

Hi all,

I've re-checked signals from U415 (MC1391) and they are all good.
My mistake was not to consider that the horizontal frequency scan was 31.778 kHz for the 640x480 standard VGA mode instead of the usual 15.750 kHz one. Sgrunt...

So, the final culprit is surely the EAT transformer, leaky and/or lossly.

Max


unclebanjoman
 

What do you think about that ?

http://www.simmconnlabs.com/1401/2606.html

Do any of you tried it?

Max


nonIonizing EMF
 

On Tue, Jun 25, 2019 at 06:47 AM, unclebanjoman wrote:


What do you think about that ?
I thought was a little pricey compared to using like say an arcade or other video adapter and LCD that will fit.

More recently, I've been wondering about making something to perform more complicated math functions using an STM Discovery or other SBC and larger LCD like this video notes with a spectrum analyzer:
https://www.youtube.com/watch?v=CwwRvqHGyts

Seems more cost effective and customizable... plus a great learning experience.


unclebanjoman
 

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


nonIonizing EMF
 

Your welcome, thanks for the affirmation.

This is the video that inspired me to consider just making the conversion with an LCD and in my case since not having the VGA outputs on the TDS-520... I considered using an arcade CRT to VGA adapter or SBC so I can add the additional outputs also.

https://www.youtube.com/watch?v=AvXbKfP1dA0

Initially, I thought the CRT was bad in mine... though found the A10 to A11 board connector was upside down, flipped and was able to get the system to display as shown in the image: https://drive.google.com/open?id=1_22JWKqhqzPoAqglQ8TmonyCA5zSxOvP

Looks like I last left off where C1256, C1257 and C1220 need to be desoldered and tested out of circuit.

Thanks for sharing your work also Max. Interesting learning about locations on the board to be concerned regarding and reading about your troubleshooting.


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


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


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




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