Re: TDS540 restoration project


dnmeeks
 

I can't speak specifically to the '540 about this power supply problem, but I would suggest that this sort of "hiccup" behavior may indicate an over-current (or shorted) output.

You might want to first go around with an ohm-meter and measure across each voltage rail (without power applied) and see if that points to anything with a very low resistance.

Since you have the PS board isolated from the scope, you should only see minimal load resistances on the outputs. If you measured, say, one ohm on a 24V output rail, that is probably a problem. But 100 ohms or more is probably fine.

If that doesn't help, you could also put a scope on each output as it is powered and hiccupping. Most rails are probably coming up and then shutting down. But if one isn't coming up at all or is well below what it should be (based on the other rails), then that would also give you a good place to start looking. I did this with a '485 once, that was hiccupping like that.

 

When you get the PS working and go on the t'shoot the display problem, I can offer a tiny bit better advice on that, as I purchased a TDS744 with a bad display once.

Hopefully yours also has a VGA output on the back? If so, you should be able to disconnect the power to the high voltage / CRT driver board (if it's built similar to my '744). With that disconnected, you can use the VGA output to an external monitor to check out everything else on the scope. I ran my '744 like this for a while, until I could replace the high voltage board and get it working again.

 

Best of luck -

 

Dan

 

From: TekScopes@... [mailto:TekScopes@...] On Behalf Of roatlasro
Sent: Wednesday, July 13, 2011 11:10 AM
To: TekScopes@...
Subject: [TekScopes] TDS540 restoration project

 

 

Hello everybody,

This is my first post in this group, so hello again :) I'm not experiencied with servicing electronic equipment but I knew a few thing about electronics and software programming. Also please excuse my english.

I recently bought a TDS540 scope with a screen failure. Reading on some forums, the standard restoration procedure for these scopes is to replace all the electrolytic capacitors, clean the digital boards with alcohol and simple green, put everything back together and pray.

When it arrived, I didn't powered the scope, being afraid not to produce other damages, but the seller claims that the front panel lights were ok ...

So the first step was to identify all the capacitors, remove them and then purchase the caps online. Until caps arrived, I removed and dumped the content of NVRAM and ROM.

The first board on my desk is the power supply. I replaced all the caps, including the caps on the two vertical modules (PWM and control). I powered the board, making a short circuit to switch from standby mode between pin 3 and 5 on J5 (or J6 ...) connector but output voltages are not ok. Also I can hear a periodic sound of about one second. Maybe is trying to start up but something is going wrong an the protection turns it off, and then again.

As a schematic reference I use the service manual from TDS520B, but the power supply schematics are not the same. For example PWM on TDS520B is implemented with a dedicated IC MC34262, but TDS540 is using MC14093 (4-NAND). After verifying with another scope, 2465B, in the first stage on C12 470uF/450V there is a pulse with the same period like the sound. On voltmeter, the voltage on C12 is around 430V.

I reached a dead point, I don't know how to continue.
Any help is appreciated.

Thanks,
Atlas

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