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Old 18th Aug 2019, 7:38 pm   #61
M3VUV51
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Default Re: Tektronix 453 poorly.

That wasn't the BC108, just a random transistor to show what the tester displays for a good one, the other pic is the one from the 'scope.
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Old 18th Aug 2019, 8:01 pm   #62
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Default Re: Tektronix 453 poorly.

Ok, cool, had me going there
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Old 18th Aug 2019, 8:21 pm   #63
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Default Re: Tektronix 453 poorly.

The pics were just to show what the one from the 'scope reads compared to a good one, sort of says it's a back to back diode.
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Old 18th Aug 2019, 8:55 pm   #64
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Default Re: Tektronix 453 poorly.

Forgot to say with it set on ext horiz,i connected the cal output to the ext horiz and still no trace,just a dot,but still movable with the horiz position pot.
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Old 19th Aug 2019, 11:13 am   #65
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Default Re: Tektronix 453 poorly.

You have to set the B Trigger switch (top left) to "Ext" and the Horizontal Display to "Ext" - X input is then top left bnc.



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Old 19th Aug 2019, 12:40 pm   #66
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Default Re: Tektronix 453 poorly.

Well, I had a poke around with my ESR meter and found a dead disc ceramic, took it out and it's open. It goes to a pin marked z or n on one end that has a white wire with a grey trace on it, the other end of the wire goes to the A sweep length pot area, only thing is I've no way to measure the value of it, the markings are;-

100
5%
n750

Any ideas? I can't see it on the schematic. Attached pic of the cap and its location.
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Old 19th Aug 2019, 1:03 pm   #67
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Default Re: Tektronix 453 poorly.

C561, 100pF 200V 5% ceramic? I don't have the correct manual but the position on the A sweep board and the markings seem to correspond. Connects to Q564 collector.

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Old 19th Aug 2019, 2:02 pm   #68
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Default Re: Tektronix 453 poorly.

Well just an update,ive changed the cap for a 120pf as it was the nearest i had to hand,good news is its working fine now!,many thanks for all your help,glad i stuck with it,im fond of this scope,cheers all paul m3vuv 73.
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Old 20th Aug 2019, 11:30 pm   #69
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Default Re: Tektronix 453 poorly.

I'm not quite sure how you managed to find a faulty 100pF using an ESR meter, but well done anyway!
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Old 21st Aug 2019, 6:41 am   #70
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Default Re: Tektronix 453 poorly.

I have to admit that when I replied with the details of C561 I couldn't see why an open circuit C561 would stop the normal operation of the sweep generator. The circuitry to the right of C561 is just the manual reset for single sweep operation. If C561 were short circuit it might change the bias of Q575 to the point where the sweep reset multivibrator would not switch. I would be happy to be enlightened on this in case I am missing something obvious.

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Old 21st Aug 2019, 9:48 am   #71
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Default Re: Tektronix 453 poorly.

Quote:
Originally Posted by rambo1152 View Post
I'm not quite sure how you managed to find a faulty 100pF using an ESR meter, but well done anyway!
well on most of the caps i did have about 2 degrees meter deflection that showed they were at least passing some ac,it was not possible to measure the actual esr,the bad one had no meter deflection,i just used it to find which ones to remove from the pcb to test further,mine is analoge,i doubht it would work for a digital meter or one that uses a test signal greater than 300mv peak to peak.

Quote:
Originally Posted by RogerEvans View Post
I have to admit that when I replied with the details of C561 I couldn't see why an open circuit C561 would stop the normal operation of the sweep generator. The circuitry to the right of C561 is just the manual reset for single sweep operation. If C561 were short circuit it might change the bias of Q575 to the point where the sweep reset multivibrator would not switch. I would be happy to be enlightened on this in case I am missing something obvious.
i havent a clue i just know replacing it fixed the scope,tbh tho i think it was more luck than judgement !!
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Old 21st Aug 2019, 9:52 am   #72
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Default Re: Tektronix 453 poorly.

for anyone interested the esr meter i used was from the theread here,word of warning tho use pdiddys pcd artwork,the other is wrong,even the revised one.https://www.eevblog.com/forum/projec...er-design/250/
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Old 21st Aug 2019, 12:43 pm   #73
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Default Re: Tektronix 453 poorly.

Quote:
Originally Posted by RogerEvans View Post
C561, 100pF 200V 5% ceramic? I don't have the correct manual but the position on the A sweep board and the markings seem to correspond. Connects to Q564 collector.
That's what my Service Manual shows, too. For future reference, my manual is 070-0755-00. It was originally printed in Beaverton and covers scopes above S/N 20,000. It has been overprinted in some places, including the front page, where "GSY S/N 100200 UP" indicates that it was originally for a Guernsey-built 453. My 453 is S/N 102671.

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Old 21st Aug 2019, 2:20 pm   #74
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Default Re: Tektronix 453 poorly.

Tek453.
I am as puzzled as Roger Evans about how replacing C561 could make the Time Base work.
Are you sure you didn't move any of the controls. Or maybe manipulating the board to solder the faulty capacitor affected a dry joint or cracked pcb track.
Could M3VUV51 please help us out.
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Old 23rd Aug 2019, 9:03 am   #75
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Default Re: Tektronix 453 poorly.

nope nothing was moved just replaced the cap,all solder joints look fine and no cracks in the boards,as a matter of fact they almost look as new.
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Old 23rd Aug 2019, 9:35 am   #76
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Default Re: Tektronix 453 poorly.

Looking at the second photo attached to post #68 it's clear that the timebase board was released from its mountings and moved around to allow replacement of the capacitor. I'd venture to suggest that this manipulation may have cured a poor connection (flying leads perhaps) which was preventing the sweep from functioning. Like others I can't see how the ceramic capacitor could have been the cause of the original problem. Glad the scope is working again now anyway.

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Old 23rd Aug 2019, 1:19 pm   #77
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Default Re: Tektronix 453 poorly.

i cant see that to be honest as i had pulled all the leads off the pins 1 by one and cleaned them,it was one of the first things i did.
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Old 24th Aug 2019, 2:47 pm   #78
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Default Re: Tektronix 453 poorly.

Quote:
Originally Posted by M3VUV51 View Post
i cant see that to be honest as i had pulled all the leads off the pins 1 by one and cleaned them,it was one of the first things i did.
Just a thought that might sort this quandary. If you unsolder one end of that cap and see if the time-base continues to work/stops working it might well shed some light....

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Old 28th Aug 2019, 11:31 am   #79
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Default Re: Tektronix 453 poorly.

I replaced the iffy transistor today that i had swapped for a bc108,with one the same case style as the original,was an npn,not sure of the specs but it works ans looks original,while doing it i clipped one leg of the cap i changed and hey ho no sweep/trigger,soldered it back up and working fine again,thought id let you know,maybe it works by witchcraft!!
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Old 28th Aug 2019, 12:22 pm   #80
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Default Re: Tektronix 453 poorly.

I very firmly do not believe in witchcraft and will not do so until there is documented, tested and repeatable evidence. Normally electronic circuits work on the basis of the tested and repeatable laws of physics.

So faced with this observation which appears to challenge my (very) fundamental beliefs I had no option but to read the manual!

The manual refers to C572 (47pF) and that its stored charge is necessary to reset the sweep multivibrator (Q575 and Q585) after both transistors are shut off by the end of sweep pulse. C572 and C561 are both connected to the base of Q575 so it must be that the stored charge on C561 (100pF) is also affecting the reset of the multivibrator. Whether this is an accident of design or the result of drifting of component values with age I cannot say.

The witches can return to their cauldron and fenny snake!

Roger
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