Tek TDS744A does not power on

" snipped-for-privacy@my-deja.com" wrote in news: snipped-for-privacy@a26g2000pre.googlegroups.com:

Could be abad power supply.The flashing is indicative of a current-limit condition in the PS.It tries to start,but current-limits.(switcher supply)

Probably bad electrolytics,ESR has gone high;replace them with 105degF,low- ESR caps ONLY. An ESR meter may help here to find just the bad caps,or you could do the whole supply.(or the bad caps could be on the scope boards.) IIRC,the PS in those were purchased suplies,not made BY Tek. no one will have a schematic for them or your TDS scope,either.

Thanks. Some of the other symptoms are consistent with this failure mode, so there is hope that this is the correct diagnosis. I will give it a try.

" snipped-for-privacy@my-deja.com" wrote in news: snipped-for-privacy@j4g2000prf.googlegroups.com:

I've read here of other TDS owners having to replace all the electrolytics on the main scope board because of problems,although different ones from what you described.

innews: snipped-for-privacy@j4g2000prf.googlegroups.com:

If you figure it out, PLEASE let me know. I just picked up a tds744a today. I verified that it worked a day ago (before I picked it up). Brought it home, and plugged it in. It smelled bad, the same blinking/ fan trying to start up once a second. I design supplies for a living, so I'm hoping it's just a burnt cap or something easy to find. Never dug into the innerds of an O Scope before though. Any info would be greatly appreciated.

flyt100 wrote in news: snipped-for-privacy@w5g2000hsg.googlegroups.com:

Was it the *same one* you examined??? no "bait and switch"? ;-)

if it "smelled bad" it's possible that an electrolytic burst or has overheated and blown a seal. THAT you could eyeball. In other TEK instruments,I have seen PS electrolytics turn BROWN from heating due to bad ESR,seen more than a few visibly bulge out,and a few completely explode;spreading foil and goop all over the board.

Without going into details, I will not be able to look under the hood for about a month. If I can confirm the bad PS hypothesis, I will look for obviously bad caps. If it is not too crowded, I will probably just replace all of them.

innews: snipped-for-privacy@w5g2000hsg.googlegroups.com:

I took it apart, and found visible arcing (small fire) under/beside an electrolytic when it was trying to start up. This was on the display driver board (could have been arcing to the coil next door, it was tough to tell from that vantage point.) It's a somewhat tough thing to get at, as it looks like the tube, high voltage supply, etc. have to go out first. I've got the service manual, so I should be able to locate and discharge the dangerous stuff first :)

The cap doesn't look strange from the top/sides, but I'll see if there is a problem on the bottom after removal.

flyt100 wrote in news: snipped-for-privacy@c77g2000hse.googlegroups.com:

you will find that your "service manual" does NOT have any component-level schematics or parts lists. TEK intended that the TDS series scopes be serviced at TEK,or by module exchange",except that module exchange is no longer an option for any TDS that is out of the Long Term Product Support(LTPS) period for that model. TEK never put any schematics in their TDS series manuals,not even for the TEK field offices. Once a model is past the LTPS,TEK disposes of the exchange modules,and only offers cal service for it.

That is the "new" Tektronix...

Looks like a problem in the Main powersupply, a switching type PS.

Besides dried/bad electrolytic caps also do check the rectifier diodes on the secondary of the SMPS transformer. I changed several of these, due to partial /complete failure. Same sympthoms as you mentioned.

hth, Andreas