Tektronix 7613 CRT soft - UK Vintage Radio Repair and Restoration Discussion Forum

ENGLISH VICTOR · 13th Jun 2011, 2:04 pm

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.

maninashed · 14th Jun 2011, 8:46 am

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

ppppenguin · 14th Jun 2011, 9:49 am

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.

dave cox · 14th Jun 2011, 9:53 am

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.

audiomik · 14th Jun 2011, 1:46 pm

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

ENGLISH VICTOR · 14th Jun 2011, 3:09 pm

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.

dave cox · 14th Jun 2011, 5:00 pm

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

13th Jun 2011, 2:04 pm	#1
ENGLISH VICTOR Hexode Join Date: Feb 2008 Location: Sydney, Australia Posts: 356	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.

14th Jun 2011, 8:46 am	#2
maninashed Rest in Peace Join Date: May 2011 Location: Preston, Lancashire UK Posts: 955	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

14th Jun 2011, 9:49 am	#3
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	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.

14th Jun 2011, 9:53 am	#4
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,062	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.

14th Jun 2011, 1:46 pm	#5
audiomik Retired Dormant Member Join Date: Oct 2010 Location: Bath, Somerset, UK. Posts: 96	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

14th Jun 2011, 3:09 pm	#6
ENGLISH VICTOR Hexode Join Date: Feb 2008 Location: Sydney, Australia Posts: 356	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.

14th Jun 2011, 5:00 pm	#7
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,062	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