Tek 475 Scope Help???

Check the filter caps in the power supply. You've got a terrific scope. With a little patience and a few caps you'll be up and running.

The erratic sweep could be caused bt dirty contacts on the sweep switch.

Steve

Volt

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"looks"

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"RB" wrote in news:RTYnb.170379$ snipped-for-privacy@news-server.bigpond.net.au:

Well,read the circuit description,find the current limit(foldback) parts for the -8V supply,and check to see if the supply is in foldback.If that's the case,then the problem may not be in the PS itself,but elsewhere. I've seen where the series pass xstr B-E v-drop increased and triggered foldback;replacing the series pass(chassis-mounted) xstr fixed the problem.Or a bad part in another circuit could put it into foldback.Like a bad tantalum decoupling cap for another circuit.

Then the unregulated supply for the -8 could have a bad bridge rectifier or electrolytic filter cap,so there is not enough unreg V for the -8 to regulate.I've seen the PS filter caps leak electrolyte all over the top of the motherboard,some corroding right thru the PCB traces.

Gotta have the PS regulated voltages correct before you can worry about the rest of the scope,but you know that!

You're better off than most.

That's a plus.

That is not good, sounds like regulator failure. The -8 to -6 could be too much load on the -8.

If it looks that simple, it might be, but what's the reference for the

-8? If it's too much draw, then there's a problem. The -15 might have used the -8 for a reference. You'd want to check that.

First fix power supplies....

Power supplies in this kind of equipment are critical, because a lot of the timing references and current sources may work off the negative supplies.

Trace out the -15 volt supply, and see that there's a feedback from the power supply output voltage to one part of the op amp. That same op amp may have some sort of reference, which has to be exact. The op amp is acting like a comparator, giving you a difference signal between the reference and the supply output. That goes back to the pass transistor. Trace it out, then look to see what the voltages are, and what they should be... Best I can suggest... if you are lucky, they even have some circuit theory....

Harvey

Thanks to Steve, Jim and Harvey for all the suggestions. Ive spent a few hours on it today and have got both rails working within specs.

-15 Volt rail was a crook 1458 Op Amp (some time was wasted trying to desolder it from the top of the PCB till I realised it was in a socket! :)

-8 Volt rail was a dud electro.

All working now, triggering perfect upto nearly 400Mhz, only fault is trace has some 50Hz "wobble" in it, Id say maybe replace all electro's in psu????

Are the values of these electros important????? ie, across the -8Volt rail Tek have used a 5500uF, at the moment Ive got a 6800uF tacked across the bottom of the board, Id never be able to find a 5500 cap but could probably source a 5600 locally...is it that critical???

Thanks again

probably

Good. Glad to hear it.

Don't you love sockets?

Hint for removing bad chips... Cut off the pins flush with the board, then the solder sucker only needs to get out the tiny little piece in the board. Alternative, cut off the chip body, leaving the lead out, then pull gently with needlenose pliers while heating the connection.

Probably so. You might even be able to use the scope to troubleshoot itself.

Most of the electrolytics are in the range of about +/- 20%. I haven't seen precision ones. So the 6800 would be ok, I'd say, unless there's some sort of charge/surge issue in the power supply. That's roughly within the 20%. 5600 would be just fine. but do not go below the voltage indicated. Don't go too terribly far above it either, since most electrolytics need a certain percentage of the rated voltage to work properly. (15 volt cap on a 5 volt circuit is better than a 50 volt electrolytic).

Harvey