Tek TDS700D PSU debugging

[darkstar49]

Hello,

First of all: I'm not an electronics engineer... I'm used to test/work with embedded boards, but PSU design/debugging is definitely not my specialty !

A Tektronix TDS754D of mine recently started to make a very (very !) loud cracking noise, so I pulled to power line immediately. Any attempt to power the scope (even in stand-by) produces the same loud cracking sound...

My first question would be: what the hell can produce such a loud cracking noise i.e. what type of component ?? transformer, capacitor ('don't think so...), inductance, etc...

The problem is of course that debugging a switching power supply is not simple, especially when no load is present (I took it off the scope), and there's no doc from Tek on the subject... (I assume their engineers would know, or did once know..., what jumpers to short in order to 'simulate' some load, but I don't have that info.

Any hints/ideas/suggestions are more than welcome !!

Kind regards
Philippe

[AndyC_772]

Have a search around for "TDS520B Component Service Manual". There's a schematic in there which is probably very similar to, if not exactly the same as, the one in your scope.

If you get completely stuck I do have a working TDS754D for sale - drop me a PM if you're interested.

[nctnico]

I'd start with disconnecting the cable to the monitor unit. Chances are the monitor unit got bad.

[casinada]

You can probably download service manuals from Tektronix directly:
http://www.tek.com/oscilloscope/tds794d-manual/tds500d-tds600c-tds700d-tds714l-service-manual
Just register and download.
Crackling sounds could be High voltage CRT discharge (bad connection) or switching power supply (but that is more like a chirping sound)

[AndyC_772]

The service manual is worth having but doesn't go into component level detail. it's a common complaint about this model, lack of servicing information.

[tautech]

Crackling sounds could be High voltage CRT discharge (bad connection) or switching power supply (but that is more like a chirping sound)

CRO's usually have several critical voltages.
Signal circuit level
HV supply for tube(usually stepped up via switchmode to around 2KV witch in turn is quadrupled or quintupled for UHV)
UHV for PDA is often 10 KV or more and that will make a crackling sound is leaking from around the Anode connection or cracked UHV lead. Be sure to discharge this lead if you have to check it out.
Heavy dust loading and a humid environment can cause this leakage. If nothing has popped just a good clean can fix it. There maybe some carbon from any tracking and this will need to be cleaned up.