oscilloscope tektronix

HELP! Someone know the mainframe oscilloscope Tektronix 7000 series? I have some 7704 with power supply broken,with same defect: don't start the oscillator. Some suggestion??

Reply to
giannibo
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If you get some money send it in for repair and calibration. Cost on good test equipment is out of site.

Reply to
David Wright

You can download the manual for the 7704 from

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It should help you figure out the problem. The usual suspects are aging electrolytic capacitors.

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Dave M
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Reply to
DaveM

"DaveM" wrote in news:gcOdnekazMCkC-janZ2dnUVZ snipped-for-privacy@comcast.com:

both in the power supply and the mainframe itself.

just make sure you have a 7704 and not a 7704A. There's a big difference.

although both have complex series-resonant switching power supplies.

usually,you hear a clicking or "chirping" from the PS;it's trying to start,but something is loading a supply,or a diode is shorted. Some failures blow a lot of parts....and then the line fuse afterwards.

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Jim Yanik
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Reply to
Jim Yanik

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Thanks for answer.The model is 7704A. I replace the fast diode.The defect remaining is that try to start the oscillator,but isn't stable.After several test, I found that disconnecting the transistor TO5 on parallel to base-emitter of power transistor, the oscillator start, but isn't stabilized.May be is the small SCR. I checked all components, only the SCR remain... Further the semiconductor indicated on circuit as a element with 2 emitter, what is?This machine remain on stock for several years, so some components are gone.Is a pity , because this instrument is made on beautifull mode, and I try to save. Thanking for help!! Gianni

Reply to
giannibo

good

ls/tek/7704/. =A0It should help you figure out the

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Thanks for suggestions. The items I found aren't good, are the diodes on low voltage circuit. I checked also the capacitor on main frame, but are ok. The defect is that oscillator is on and off continously. Now I take out the TO5 on parallel of base-emitter of power transitor, and remain on firmly, but isn't stabilized. I follow to work on, I want resume this beautifull instrument!! Thanks a lot!! Gianni

Reply to
giannibo

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Thanks, but transport cost, custom, etc.., are out of budget. Further I like repair my self.... HI! Gianni

Reply to
giannibo

giannibo wrote in news: snipped-for-privacy@d4g2000prg.googlegroups.com:

A Diac;it triggers at ~33V IIRC.

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Jim Yanik
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Reply to
Jim Yanik

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Thanks! I will replace it! In the meantime I foun that R120 and R122 ( 1,8 Kohm) of Z axis board A41 are burning, with smoke. Many defect all togheter!! Tomorrow i will check why these resistor are smoking.Anyway I am bidding on ebay a 7704 so I can compare some situations. Thanks. I wil post when I found all defect! Gianni IK4EWH

Reply to
giannibo

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Remember what Jim already said about 7704 and 7704A: Both models are a lot different.

So if you plan to compare circuits and repair faults, go for the same models at least.

hth, Andreas

Reply to
Andreas Tekman

Andreas Tekman wrote in news: snipped-for-privacy@s12g2000prg.googlegroups.com:

7704A uses a TEK-made IC for it's PS control,while the older 7704 non- A uses discrete circuitry. The A model has more control loops.
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Jim Yanik
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Reply to
Jim Yanik

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The model is 7704A and I got diagram. Now is working but isn't active the automatic control. I must replace the DIAC. But rhey are others defects... very hard competition!

Reply to
giannibo

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Infact: all IC are custom, with various models.Now I am arrived to have: trace very weak, but seem trigger and vertical work.There is a problem: the trace is only present on central zone of tube. I never seen a wiew like this. I fund another mainframe so I can replace step by step the circuit, so I can see which one is defected. But really this is on very bad conditions. I repaired a lot of

465-475-7704-7603 and others , but this is very hard!!
Reply to
giannibo

giannibo wrote in news: snipped-for-privacy@e4g2000hsg.googlegroups.com:

read the service manual's circuit description; it's VERY valuable in understanding how the PS works. Make notes on a printout of the schematic.

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Jim Yanik
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Reply to
Jim Yanik

giannibo wrote in news: snipped-for-privacy@l32g2000hse.googlegroups.com:

It could be a HV problem like loss of anode supply(usually bad HV multiplier),or a loss of a power supply rail that supplies the deflection output amps.(much less likely)

TEK series-resonant power supplies used in the 7704A,7904,7104,485 are very complex and hard to repair.Many TEK service techs had hard times with them,often just put in new PS assemblies.(not possible anymore...)

the manual's circuit description is VERY valuable in understanding them.

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Jim Yanik
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Reply to
Jim Yanik

innews: snipped-for-privacy@s12g2000prg.googlegroups.com:

Depending on which version you have, there's a bleeder resistor on the HV board...actually a bunch of 'em in series. They go open, the high voltage goes up, the trace shrinks.

Mine had four of about 7 open resistors in series. I found that improbable... The explanation given was that they're carbon and they crack. Then the first one arcs to keep the current flowing until the second one cracks and opens...which arcs...until finally there are enough of them open that it can't sustain all those series arcs at once and the string starts acting like it's open.

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Reply to
mike

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Thanks for help! Actually the PS work at 80 %. I must replace the diac.I replaced all diodes fast use on PS. Before I used the normal diodes, so doesn't work!! Anyway the mainframe suffered a lot because was stored on wet place. Now I found several mainframe, so I try to replace the whole circuit. I will post the final results. Thanks again. Gianni

Reply to
giannibo

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