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Old 13th Jun 2011, 1:04 pm   #1
ENGLISH VICTOR
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Default Tektronix 7613 CRT soft

I have just been given a Tektronix scope type 7613, it functions very well except that the crt is giving al the symptoms of being soft, that is only moderate brightness that de focuses with advancing brightness that cannot be corrected with the focus control. Detail: at low brightness focus pot goes through good focus at higher brightness the pot goes through focus but at best the focus is poor. the pot setting is the same for both conditions, about central.
The storage function is fine also poor focus. Does any one have any experience with this instrument who could pass on some advice please
Victor.
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Old 14th Jun 2011, 7:46 am   #2
maninashed
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Default Re: Tektronix 7613 CRT soft

Hi Victor, first check your crt heater, you will still get a faint trace with the heater out. Also check the cathode eht supply as low eht will give the same problem. Be careful though! I havent got a schematic of the scope but it will be in the region of 3kv.
Bill
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Old 14th Jun 2011, 8:49 am   #3
ppppenguin
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Default Re: Tektronix 7613 CRT soft

There is a story that one of the great analogue engineers (possibly Widlar, Pease or Williams) once had a scope whose trace had gone soft. It took him quite a while to find out that a colleague had inserted some very thin tissue under the graticule.
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Old 14th Jun 2011, 8:53 am   #4
dave cox
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Default Re: Tektronix 7613 CRT soft

Victor,
I don't know this particular scope but I recall that the 7000 series scopes has 'auto focus' that changes the focus voltage with the brightness so you don't have to keep fiddling with the focus control. Possibly you have a fault in this part of the circuit.
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Old 14th Jun 2011, 12:46 pm   #5
audiomik
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Default Re: Tektronix 7613 CRT soft

Victor
full Operator and Service manuals for the 7613 here:
http://bama.edebris.com/manuals/tek/7613/

Loads of step by step info in the Service Manual to check the CRT voltages and adjustments.

hope this assists
Mik
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Old 14th Jun 2011, 2:09 pm   #6
ENGLISH VICTOR
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Default Re: Tektronix 7613 CRT soft

Thank you all for your replies, Mik, I downloaded the service manual, full of all the info needed. Dave, indeed the unit does employ auto focusing, it was however set up correctly. Bill, the heater voltage is correct and sits at -1475V below chassis. The good news is that as the tube has now been running for about 24 hours the brightness is considerably brighter whilst the trace is acceptably focused, I am going to leave the unit run for a few days and hopefully the crt will continue to improve.
Victor.
P.S. no tissue issues here.
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Old 14th Jun 2011, 4:00 pm   #7
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Default Re: Tektronix 7613 CRT soft

Page 2-16 describes the setup for the auto-focus.

Edit
I forgot to add that on the models with concentric brightness / focus it sometimes need a drop of light machine oil on the shaft to stop one rotating the other.

dc
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