Re: Tektronix 555 problem

Morris Odell

I assume you have a type 21 timebase in the upper beam slot and you can't
get it to sweep at all. Have you tried adjusting the stability screwdriver
pot? What is the "ready" light and the other neons in the timebase doing? If
you try to trigger it from the line, are there pulses coming out of the
"gate" jack? Let us have some more info.

Coincidentally I have just had to repair my 555 and, as ever, am amazed at
what a great performer it is - lovely sharp traces and easy & flexible
triggering in a nearly 60 year old scope. In my case the symptom was extreme
trigger jitter in both time bases. Checking the power supplies I found right
voltages but enormous ripple on the +500 supply. The fault was C760, a dual
40 mfd 450 volt electrolytic that filters the +150 that's stacked on top of
the +350 to make +500. I had a dual 50 mfd 350 volt cap to replace it and
now it's a good as new. Waiting for the relay to pull in the first time
after replacing a cap in a high energy circuit is not for the faint hearted!


The upper beam on this scope will only sweep unless set to trigger from
the lower beam time base. I can only get a dot with the upper beam set to
the (A) upper time base. Both time bases are set the same for recurrent. I
have tested all the upper time base tubes and all good. The supply voltages
checked as per the Tek schem. for the time base seem to be fine. There is a
"sawtooth out" jack on the front of the upper time base and from what I'm
seeing using the working trace, there is no sawtooth waveform there. I only
have this scope to check that with. Before I dig into it further, thought
I'd ask you fellows here if you've had any experience with this. Been many
years since I've worked electronic problems. I do have a time base extender.
Thank for any time and advice.


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