Ok, guys, without taking it all apart again, does anyone know a typical "it's nearly always this part" type of cause?
When turning a Tex 7704A on the power supply lets off a faint bzzzzt .. ttzzzt ... brrp ... bzzzzzt .... until after several minutes it come to life. Is the magic neon bulb past its prime? Or something else?
I can still use the scope but I see the day coming when it no longer cranketh over.
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It's cold ;-) ...Jim Thompson
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Remember: Once you go over the hill, you pick up speed
67F in the office. But thanks for the reminder, got to reload the wood stove. My wife ain't gonna be happy when she comes back and the house has dropped to 65F :-)
Anyhow, one minutes ago ye olde 7704A has decided to call it quits :-(
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Do you have the service manual? There is a very good theory of operation on the power supply. It is a combination switcher and linear system.
Most of the time, it's a shorted capacitor. There is current limiting on all the low voltage supplies in the form of current shunt emitter resistors on the series pass transistors.
The good news is there are many parts units available for the 7704(A) mainframes.
You might want to check the "Tekscopes" group on yahoogroups. A lot of help is available there.
tm
Even 62F isn't bad with a few layers of clothing... below 60F the problem gets to be that fingers just don't want to work very well (nimbly), even if overall you're still warm and comfortable!
On the other hand, as I recall, in summer you're all for having the office at something like 94F before you turn on the AC? :-)
Second life as yard art? Or do you plan to fix it?
I do have that. The switcher is one of those "hairball" circuits with self-start, SCRs, and whatnot. If I have to I'll dig in. Was trying to avoid that if someone knows a part that is the culprit in most cases. Like on those AOR scanner receivers where once they are 5+ years old usually the backup battery in there croaks and the thing quits. The battery is soldered onto a board, sort of hidden.
Yes, I saw that. The post-regulators are on the acquisition board. They've cut it quite close, 2V dropout on the 15V rails.
Yes, I could just buy a "new" one and be done with it. But somehow that goes against the grain, from an environmetal POV. And I'd be throwing away a piece of history. It's not quite a 2465 but those 7704A are darn good scopes.
Thanks, I'll have to check that one out.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
In the mornings it can be 58F in the office and I actually like that. Problem is, I don't feel it being too cold but my wife does. So when she is gone for a few hours I set a kitchen timer. It reminds me to look at the thermometer every 1-1/2h and reload the wood stove if too low.
Not anymore. Now we have a small evap cooler which keeps the living area under 85F and the office under 90F. If I could figure out a way to mount one near the office it would both go to less than 80F. But the way this house is built that's nearly impossible. It is one of those Frank Lloyd Wright style houses.
If it won't turn into a major science project I want to fix it. Admitted, I rarely use it these days but when I do it always performs. Or has :-(
Seems like old lab gear isn't immortal. The Wavetek 23 that I really like went thoroughly kaputt half a year ago. But Tektronix did a much better job omn this scope. Nothing gets too hot in there.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
You might also try it with most of the plugins pulled just in case it is a problem there. You can take one known good module (vert) and try it in different slots.
On the backplane there are about five electrolytic, most are tantalums and they are known to short out. You will need to unscrew the boards to see them. That is done much easier if the panels are all removed and the bottom guide rails are pulled.
The power supply can be pulled out the rear and fully exposed while running. Use care not to stress the cables too much.
The primary inverter has a circuit that requires all the voltages to balance out or it will go into a "tick" mode while it tries to restart. The frequency of the tick can give clues to the problem.
There are some voltages in there that can teach a lesson though.
tm
I had tried that. It won't even start with all modules pulled.
Aha, tantalums. That's often bad news. Looks like a job for a rainy weekend then. Thanks for the hint.
Yes, that was fairly easy, had done that one before.
It's not a tick, more like a rattlesnake in the distance. Irregular bzzzzt intervals. It used to start up after doing that for a couple minutes but not anymore. Maybe it didn't like the long periods where it wasn't turned on and wants to punish me for that now.
Yep, it says so in the manual :-)
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Umm, it's the 21st century! Has been for a while now.
Rigol has a 1 GHz digital scope now. They aren't toys any more.
John
Doesn't mean we have to abandon all the good stuff. I won't throw out a nice bottle of wine just because is was from 1999 :-)
I've got a 200MHz and a 1GHz scope but they are digital. DSOs are what I now use maybe 99% of the time. However, in my line of work there is that
1% where DSOs just can't cut it, no matter how fancy they may be. I have been asked by clients "How on earth did you find this tiny random pulse noise?"-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Yeah, but can you plug a curve tracer, spectrum analyzer, logic analyzer, sampling plug in, etc in the Rigol?
In 20 years, can you replace IC-1 in the Rigol?
tm
That's the self-start circuit functioning.
The main converter is resonant (L3037 C3037). Capacitive rectification on prereg outputs.
The only thing that's going to get better as it warms up is electrolytic esr, but these things used wet tantalum. If they're leaking that could cause high ripple and reduced output voltages. The 50V lines are most suspect.
RL
Yes, looks like 30kHz series-resonant. In those days they couldn't go much higher.
Thanks. Then I guess the drill is to get the power supply out, disconnect the various connectors for the output rails. If it still won't start the short must be on the module itself. Which would be good because prying out the backplane doesn't look like fun.
Strange thing is, sometimes when I let it try for a while the scope will power up and work fine. Except that a very faint ticking of a couple Hertz or so can be heard when I press my ear against the side of the scope. But that could be the neon bulbs which are very tired.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
It will not run without a load. Also, there is a supply cable and a sense cable that come together on the backplane. The ticking is the voltage balance circuit shutting the supply down. Come in from the front and pull the modules from the left three slots and you will see the caps. They are just dipped solid tantalums. It sounds like one of them is getting ready to flame out. There is a summing point on the supply that you can connect a scope to and based on what direction the voltage goes too during one of the ticks, it will tell you what supply is failing. For example, if the
+15 shorts to ground, the voltage will go negative.Also, the HF signal from the inverter feeds the high voltage power supply in the display unit and that load must also be there.
tm
Tektronix has a forum on their website where there is plenty of room for talking about component level repairs.
This website may also help to get diagrams:
-- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.) --------------------------------------------------------------
Jim Thompson wrote in news: snipped-for-privacy@4ax.com:
that is the sign of the PS load becoming marginally too high;the supply current limits and shuts down,then restarts again and again. (this series-resonant supply has several control loops,and current limit circuits on some of the regulated supplies,too.) It's probably some electrolytics(ESR),both on the PS itself,and on the mainframe. it could also be HV related.
One more thing;I've seen the Vb-e rise on the heat-sunk flat-pack(TO-126?) power xstrs on the cap-regulator board and cause a current limit condition on it's supply. Those xstrs are the ones that mount to the rear heat sink,several of them in a row. Replacing the xstrs fixed the problem. Of course,I had a curve tracer to check those xstrs......how to troubleshoot that without one,I dunno. the current limit circuit on those supplies is a foldback type,IIRC.
-- Jim Yanik jyanik at localnet dot com
Thanks. Looks like there are some daughterboards that are on the backplane that I'll have to unscrew and remove. Can't see any tantalums to the side of those. This looks like a tear-down job because there's tight-fitting gray coaxes coming off of those "mezzanine boards".
Surprisingly, this morning the scope turned on immediately, like in the old days, without delay and without any rattlesnake hisses. Weird.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
I have also seen the current sense resistors drift higher causing a lower trip point. These are easy to check in circuit. If you measure a higher resistance than the marked value, you have found it.
tm
Thanks, Jim. I could test those by hand. Takes longer but done it before. From what RL wrote the supply won't start without a sufficient load. I do not have many modules because I only use the 7704A as a plain vanilla analog scope. This morning it decided to work normal again, after thinking about it for a night. Beats me why.
Looking into the 7A26 module I see that there's tons of tantalums in there as well. Maybe one of the modules is acting up.
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