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 Post subject: Tektronix 321a HELP
PostPosted: Apr Wed 01, 2020 11:58 pm 
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Joined: Apr Wed 01, 2020 12:32 pm
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Hi Folks, Post No.1.
I have a Tek. 321a with a problem, trace starts midway across screen on the slowest 6 sweep speeds. The problem is definitely capacitor No. C180A. However I am unable to locate the part. I suspect it is between the 1st. & 2nd. wafers of the timebase switch where it is unaccessable. does anyone know for sure & if so how do I replace it. Looks like I might have to remove the front panel :(
Thanks for any help,stay safe,ian177


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 Post subject: Re: Tektronix 321a HELP
PostPosted: Apr Thu 02, 2020 4:56 am 
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Joined: Nov Mon 02, 2009 7:01 am
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Location: Lincoln City, OR 97367
Greetings to ian177 and the Forum:

I have never seen one of these scopes in person, but in perusing the rather grainy photo of the range switch in the online version of the manual and the schematic, I wonder if you may be counting the wafers backwards. Normally, the no. 1 wafer could be closest to the panel, but there is no law that says it has to be done that way.

In either event, whether the wafer is the one closest to the front panel or the one furthest away, I do not see a stack of capacitors on the range switch in the photo. Since one end of all of the sections of C180 go to ground, I suspect that Tektronix may have run a wire bundle from the switch over to another part of the instrument and located the caps there. I note specifically in the photo a long set of parallel ceramic tie points with capacitors right behind the range switch. C180A may be located there or in some other place in the instrument.

If you can definitely see capacitors nestled inside the range switch, then you will have to dig it out; either by removing the front panel or by removing the switch. If they are not visible between the wafers, then they are somewhere else; you will have to trace the wiring.

Good Luck,

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Jim T.
KB6GM


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 Post subject: Re: Tektronix 321a HELP
PostPosted: Apr Thu 02, 2020 5:24 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18994
Location: Albuquerque, NM 87123
I have recently restored a 310A and I had a problem of the trace starting in the middle of the screen on the 50, 20 and 10msec ranges; it turned out I had installed a .022uf capacitor where a .0022uf cap should have been. It was on the range switch.


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 Post subject: Re: Tektronix 321a HELP
PostPosted: Apr Thu 02, 2020 7:59 am 
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Joined: Apr Wed 01, 2020 12:32 pm
Posts: 5
Thanks for the replies so far. just to clear up a couple of points, firstly my confidence in nominating C180A. It is 1 of only a few components that are switched on the slowest 6 speeds & when a slow speed is selected, connecting another 2 uf. in the circuit cures the problem. the long bank of caps further back are all power supply caps. secondly it is witched by switch bank 1 (as per manual) the other caps in the hold off circuit are all on the front bank. The photo you might have should show a red (dark brown) 0.027uf cap on the switch & hiding under the power switch is the 0.22uf cap. these are the next 2 caps in the circuit.
So it makes sense that the cap I need is further around the switch or anywhere else with a bundled lead to it. I can see a small Sprague electrolytic cap right down under the switch.
The switch will be very difficult to remove, but likewise I don't relish the thought of removing the front panel. While I am posting, is there anything that I should look out for when removing the front panel?
Thanks again & stay safe.


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 Post subject: Re: Tektronix 321a HELP
PostPosted: Apr Thu 02, 2020 9:11 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18994
Location: Albuquerque, NM 87123
Tektronix sometimes used timing capacitors that had three capacitors in one can; they are oval. I've never seen one go bad though. When I replaced caps on mine I just removed a few parts to get to the back components; never had to remove the switch assembly.


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 Post subject: Re: Tektronix 321a HELP
PostPosted: Apr Thu 02, 2020 10:08 am 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 6899
Location: Sunbury, Ohio 43074
Have you tried cleaning the wafer switch? Just about every Tektronix scope I've ever serviced had dirty switches. Before spraying anything, try just exercising the switch until you feel like your arm will fall off. While you're at it, do the same with every other switch and control. Might just clear this up.

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Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


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 Post subject: Re: Tektronix 321a HELP
PostPosted: Apr Thu 02, 2020 10:32 am 
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Thanks Barry, that's a great suggestion, The fault appeared suddenly & it looks like the cap has gone o/c. might just be the switch.
ian177


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 Post subject: Re: Tektronix 321a HELP
PostPosted: Apr Thu 02, 2020 2:08 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18994
Location: Albuquerque, NM 87123
Seriously doubt that switch cleaning will fix the problem. The capacitor (2uf 25 volt) appears to be a single capacitor, but I couldn't find it on any picture.


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 Post subject: Re: Tektronix 321a HELP
PostPosted: Apr Thu 02, 2020 3:18 pm 
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Joined: Apr Wed 01, 2020 12:32 pm
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Hi, you won't find it in any pic. It's right underneath the timebase switch. Took the bull by the horns & removed the switch. A fair bit of work but could get it out enough to replace the cap. Had to disconect 1 cap &1 wire. All back together, switched on & NO trace. Just a dot. I'been around serviceing for 60 years & know that if something works before you disturb it, it's probably something you have done. Past midnight so another day. BTW I definately resoldered all that I took off. Can't be the cap cos no trace any pos.
Ian177


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 Post subject: Re: Tektronix 321a HELP
PostPosted: Apr Thu 02, 2020 3:23 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 6899
Location: Sunbury, Ohio 43074
Isn't that always the way with older gear? One step forward, two steps backwards. Hope it is simple to find.... Scopes can sometimes be a bit of a challenge.... Guessing you'll find a wire popped off or broke somewhere.

If the transistors are in sockets on this scope, that's another good place to look... see if one got dislodged or wiggle all of them... they get oxidized as well.

Once you get a trace back, if it's still just the low sweep speeds that don't work, have a look at C180A 2uF electrolytic also on the time base switch. Or possibly even something in the trig holdoff system. Clean the switches :)...allofthem .... Half my tektronix repairs are just as simple as that.

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Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


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 Post subject: Re: Tektronix 321a HELP
PostPosted: Apr Thu 02, 2020 11:56 pm 
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Joined: Apr Wed 01, 2020 12:32 pm
Posts: 5
Hi all, just a quick post to say all good. Scope performing as it left Tektronix in about 1962. (I purchased 2nd. hand in about 1963.) Original fault was o/c between wiper of sweep length trim pot & element. Second fault was o/c cap C180A (sweep starting at screen centre on 6 slowest speeds.) Third fault was no sweep. When I removed the time/div. switch to replace C180 I had to remove the power switch. On reinstalling it I pinched a wire between it & the front panel :oops: , just need to put battery tray & covers back.
Thanks for all the comments.
ian177


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