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Repairing a TDS684A power supply

 

The power supply in my TDS684A has decided to go into hiccup mode. Researching the topic, I keep finding schematics and advice only for the TDS520 and similar. Does anyone have or know where I can find a schematic for the actual TDS684A power supply? Or for that matter, experience of the most common failure modes/device failures? And, it seems folks are capable of running these supplies stand alone on the bench - which connector/pins need to be shorted/or grounded as would be provided by the front panel power button to get the supply to turn on?

Thanks!
Reinhard Metz
n49ex@...
 

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Hi,

you can check video of Jared repairing such power supply. As he mentioned TDS500/600/700 have similar power supplies. It also includes information about external dummy load construction with all needed information.



-vf-


Dne 23.09.2022 v 4:00 n49ex via groups.io napsal(a):

The power supply in my TDS684A has decided to go into hiccup mode. Researching the topic, I keep finding schematics and advice only for the TDS520 and similar. Does anyone have or know where I can find a schematic for the actual TDS684A power supply? Or for that matter, experience of the most common failure modes/device failures? And, it seems folks are capable of running these supplies stand alone on the bench - which connector/pins need to be shorted/or grounded as would be provided by the front panel power button to get the supply to turn on?

Thanks!
Reinhard Metz
n49ex@...

 

Excellent! Thanks! I'll post progress/findings later.

Reinhard


-----Original Message-----
From: Vladimir Filip <vf@...>
To: [email protected]
Sent: Fri, Sep 23, 2022 4:47 pm
Subject: Re: [TekScopes2] Repairing a TDS684A power supply

Hi,
you can check video of Jared repairing such power supply. As he mentioned TDS500/600/700 have similar power supplies. It also includes information about external dummy load construction with all needed information.


-vf-

Dne 23.09.2022 v 4:00 n49ex via groups.io napsal(a):
The power supply in my TDS684A has decided to go into hiccup mode. Researching the topic, I keep finding schematics and advice only for the TDS520 and similar. Does anyone have or know where I can find a schematic for the actual TDS684A power supply? Or for that matter, experience of the most common failure modes/device failures? And, it seems folks are capable of running these supplies stand alone on the bench - which connector/pins need to be shorted/or grounded as would be provided by the front panel power button to get the supply to turn on?

Thanks!
Reinhard Metz
n49ex@...

 

There are a few variants of the power supply, some have less pins on one of the output connectors (So each power connector is the same size) depending on scope model, and there are other versions with differences in the standby power part of the supply, among other things.

My tester allows you to check voltages, test the on/off function and it will apply a small load to each voltage rail to check that they don't immediately collapse when turned on.
IIRC, a few of the voltage rails are monitored for current draw so if you don't apply a load, the PSU won't work properly (hence my tester)

The C17 capacitor replacement is mandatory (if fitted, some variants don't have it) as they are almost always high ESR, so check that and replace with a new good quality low ESR cap.
 

Thanks Jared! So, where, how do I get one of your testers, or the PCB and parts list? Or at the least, what pins are the front panel on/off button so the supply can be turned on on the bench?

Reinhard
 

Also Jared, in your video you show some TDS544A schematics labeled VintageTek, but their web site doesn't show any way to get these. How did you get them?
Reinhard
 

I've evolved to finding that in fact my TDS684A power supply is good, and is instead cycling due to a 24V supply over current. Further testing has found that it is the CRT display portion that is at fault. Pull the ribbon connector that feeds it and the power supply and rest of the unit come up. Feed the display board directly with 24 volts from my bench supply and it current limits.

So, my question is this, having found nothing on the subject in forums: Is there any experience out there what is a likely failure/part in that section that could be responsible? If this were a 485, I'd quickly go looking for shorted tantalum caps, but those don't seem to be present here. Any thoughts?

Thanks!
Reinhard Metz


-----Original Message-----
From: Vladimir Filip <vf@...>
To: [email protected]
Sent: Fri, Sep 23, 2022 4:47 pm
Subject: Re: [TekScopes2] Repairing a TDS684A power supply

Hi,
you can check video of Jared repairing such power supply. As he mentioned TDS500/600/700 have similar power supplies. It also includes information about external dummy load construction with all needed information.


-vf-

Dne 23.09.2022 v 4:00 n49ex via groups.io napsal(a):
The power supply in my TDS684A has decided to go into hiccup mode. Researching the topic, I keep finding schematics and advice only for the TDS520 and similar. Does anyone have or know where I can find a schematic for the actual TDS684A power supply? Or for that matter, experience of the most common failure modes/device failures? And, it seems folks are capable of running these supplies stand alone on the bench - which connector/pins need to be shorted/or grounded as would be provided by the front panel power button to get the supply to turn on?

Thanks!
Reinhard Metz
n49ex@...

 

Having found no one to respond with experience with this problem, I've forged on and think have found the bad components. For the sake of anyone else who has a TDS5xx, 6xx, or 7xx with a similar power supply cycling problem, here is what I found:

First, the cycling generally means the power supply itself is actually (likely) ok, and it is an over current somewhere else that causes it to cycle. In my case (and I actually am working on two of these - a TDS684A and TDS744A) it turned out that for both scopes pulling the connector to the CRT driver board allowed the supply to come up. It turned out the excess current in both scopes was the 25V supply. Removing the CRT driver board, it can be run on the bench with just 25V input to pin 1 on the ribbon cable. Just be careful with two things - one is that 5V input is on pin 3 adjacent, so you don't want to accidentally put 25V on there. The other is that once the board is rid of the excess current cause, and even before, if you use an external power supply to force the 25V, the high voltage will come up and you can get seriously zapped, if not killed from the loose hanging high voltage lead that normally is attached to the CRT.

So, applying external 25V and forcing an amp or two into the board, I used my IR camera and homed in on an overheating component, which was Q245. However, it turns out Q245 was not failed, just getting very hot. So, tracing backwards for where the excess current from Q245 was going to, I found in one scope Q205 shorted, and D208 shorted in the other. The path is Q245 to CR156 to T150 to Q205/D208. Replacing these parts got both CRT boards to work and the scopes to come up. Again, just be careful, as soon as the 25V gets up to a substantial level, the high voltage is live - 1" arcs to ground are easy!

Hope this helps someone!

Reinhard Metz


-----Original Message-----
From: n49ex via groups.io <n49ex@...>
To: vf@... <vf@...>; [email protected] <[email protected]>
Sent: Fri, Oct 7, 2022 7:56 pm
Subject: Re: [TekScopes2] Repairing a TDS684A power supply

I've evolved to finding that in fact my TDS684A power supply is good, and is instead cycling due to a 24V supply over current. Further testing has found that it is the CRT display portion that is at fault. Pull the ribbon connector that feeds it and the power supply and rest of the unit come up. Feed the display board directly with 24 volts from my bench supply and it current limits.

So, my question is this, having found nothing on the subject in forums: Is there any experience out there what is a likely failure/part in that section that could be responsible? If this were a 485, I'd quickly go looking for shorted tantalum caps, but those don't seem to be present here. Any thoughts?

Thanks!
Reinhard Metz


-----Original Message-----
From: Vladimir Filip <vf@...>
To: [email protected]
Sent: Fri, Sep 23, 2022 4:47 pm
Subject: Re: [TekScopes2] Repairing a TDS684A power supply

Hi,
you can check video of Jared repairing such power supply. As he mentioned TDS500/600/700 have similar power supplies. It also includes information about external dummy load construction with all needed information.


-vf-

Dne 23.09.2022 v 4:00 n49ex via groups.io napsal(a):
The power supply in my TDS684A has decided to go into hiccup mode. Researching the topic, I keep finding schematics and advice only for the TDS520 and similar. Does anyone have or know where I can find a schematic for the actual TDS684A power supply? Or for that matter, experience of the most common failure modes/device failures? And, it seems folks are capable of running these supplies stand alone on the bench - which connector/pins need to be shorted/or grounded as would be provided by the front panel power button to get the supply to turn on?

Thanks!
Reinhard Metz
n49ex@...

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