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Old 19th Feb 2018, 6:18 pm   #1
M3VUV51
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Default Tektronix 453A

hi all,im looking for a few pointers with a scope i have,i was given an old tektronics 453a scope,it was rather battered mechanicaly,busted knobs/shafts etc,ive repaired all that,the issue i have is when i got the scope it had a mettal gauze in front of the crt,that was full of muck and gook so removed it,the trace on screen seems way to bright,ive turned the crt grid bias right down but the trace still seems way to bright,i dont realy want to use it like this as feel it will burn the screen,any ideas where to look,just wondering if anyone else has come accross this issue?,cheers Paul m3vuv.
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Old 19th Feb 2018, 6:56 pm   #2
MrBungle
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Default Re: Tektronix 453A

Nice scopes, particularly the A revision. Might be drifted resistors in the HV chain. This is a very common problem as they were carbon composition ones, at least in the original 453. Check all the carbon comp resistors on the Z axis board and HV chain. There's also a little plastic box of trouble which contains the HV transformer and multiplier sections in it somewhere (not sure where it is in the 453A). Need to check those. Look in the service manual under the "CRT Circuit" sheet.

Inevitable disclaimer: crazy high voltages in there even when turned off. Make sure you know how to discharge everything safely before you go poking around in it.
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Old 19th Feb 2018, 10:48 pm   #3
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Default Re: Tektronix 453A

Mr Bungle is right about the HIGH voltages around the tube. I had one 30+ years ago, nearly new, and if you turned the brightness up, you could switch the lights out is was so bright. However, you should be able to turn it right off. I can't remember anything about the circuitry, but many scopes have a 100v zener on the tube base. If that goes short, full brightness results. Now if that scope had a BIG tube, it would be unbeatable in its class, but it is only small (0.8cm/divn.?)
Les.
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Old 19th Feb 2018, 10:48 pm   #4
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Default Re: Tektronix 453A

Have you got the manual? You should find one at w140.com/tekwiki.

Do the intensity and grid bias controls have any effect? Do you see the flyback / retrace lines, these are more obvious at fast sweep speeds.

The Zaxis amplifier diagram shows the waveform you should get at TP1047 on the Zaxis board. It should sit around 20V and briefly pulse down to near 0V during the flyback/blanking period. If you have an analogue meter you should see the average value which is close to +20V. This voltage should change with the intensity control. If this checks out OK the fault is probably in the high voltage section as Mr Bungle says.

Fault finding in the HV box is tricky, as well as drifting resistors you have high voltage mica capacitors which tend to leak after a few decades and dirty surfaces can leak or track under high voltage. The beam intensity depends on the difference between the grid and cathode voltages whcih are separate HV supplies and a small change in either of these makes a large change to the brightness. Being able to measure the cathode/grid voltages is a great help but the equipment to do so safely is not cheap.

Regards,

Roger
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Old 20th Feb 2018, 5:08 pm   #5
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Default Re: Tektronix 453A

Well here's the findings so far. The grid bias only makes the trace brighter, the intensity control does nothing, the voltage at TP 1047 is 88V on my AVO 8 and Fluke, seems solid not varying at all. TP 914 shows -11.98V, TP 1-14 shows -0.8V.

If I remove Q914 the brightness seems about right. I did find R901 had gone from 4.3M to about 1M, changed that but no difference, adjusting R900 pot makes no difference. Q914 NPN checks out OK on a diode test, 0.53ish vf bc be etc.

Any ideas?

Cheers Paul M3VUV.

Last edited by Station X; 20th Feb 2018 at 6:37 pm. Reason: Readability.
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Old 20th Feb 2018, 6:42 pm   #6
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Default Re: Tektronix 453A

As a quick aside, if you want to reduce the brightness while fault finding, turn on the 10x horizontal magnifier and turn on both Ch1 and Ch2 for a further factor of two.

Roger
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Old 20th Feb 2018, 6:44 pm   #7
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Default Re: Tektronix 453A

Quote:
Originally Posted by m3vuv51 View Post
Any ideas?
I'd pull the intensity control out and test it.
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Old 20th Feb 2018, 6:46 pm   #8
barrymagrec
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Default Re: Tektronix 453A

I have seen a number of scopes where the tube goes leaky grid cathode - this makes the trace very bright and largely uncontrollable.
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Old 20th Feb 2018, 6:58 pm   #9
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Default Re: Tektronix 453A

Just checked the x10 horizontal amp was already on, with it off it's blinding.

If the CRT cathode is leaky is there a fix?

Cheers M3VUV.
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Old 20th Feb 2018, 10:05 pm   #10
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Default Re: Tektronix 453A

Well I've checked the intensity pot and it's fine, also had the HV compartment open and checked all the resistors, they are all within spec. I could use it with Q914 on the Z axis board removed - may cure the symptoms and make the scope usable, but doesn't fix the issue!.

Cheers 73 M3VUV.
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Old 21st Feb 2018, 9:50 am   #11
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Default Re: Tektronix 453A

If TP1047 is at 88V there is a fault on the Z axis board. The transistors are socketed, remove in turn and test with a transistor tester. Sorry for the short and late response but away from home at present.

Roger
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Old 21st Feb 2018, 11:55 am   #12
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Default Re: Tektronix 453A

Well finaly sorted it, it was Q023 o/c on the Z axis board, didn't have the same so, as it was a small NPN stuck in a BC108 I had.

Working a treat now, thanks for the help guys. 73 Paul M3VUV.
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Old 21st Feb 2018, 11:36 pm   #13
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Default Re: Tektronix 453A

Just a quick question, the scope seems fine now, but the grid bias is turned as low as it will go, pot fully CCW and the same for R900 the high voltage pot on the Z axis board. Is it OK to leave these set as they are, as it seems fine as is, or will using it like this let the magic smoke out?

Cheers, Paul M3VUV.
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Old 22nd Feb 2018, 9:11 am   #14
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Default Re: Tektronix 453A

It won't do any harm but if the cathode voltage is low it will change the deflection sensitivity of the CRT in both X and Y and the calibration will be a bit out, probably no more than 10%. If you remember roughly where the cathode voltage preset was. initially, then put it back there and adjust the grid bias to suit.

Will send another post later.

Roger
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Old 22nd Feb 2018, 3:12 pm   #15
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Default Re: Tektronix 453A

Paul,

If you are unsure whether changing the CRT cathode voltage has upset the calibration there are two options.

1. Using either the built in calibrator or an external source of voltage or frequency that you trust, set up a stable display and adjust the CRT cathode voltage so that the calibration appears correct on screen. You will need to re-adjust the grid bias afterwards. This is easy but may mean that you are correcting for one error by introducing another.

2. Measure and set the cathode voltage according to the manual. This requires that you have a safe way of measuring the cathode voltage. If the calibration is out then run through the manual to re-calibrate voltage and time. This is a better route but requires more test equipment. You have to make the judgement how accurate the scope needs to be to meet your needs.

I would say to go down the first route and only look further if you end up with the high voltage adjust near its maximum setting. If you want the scope to be accurate to better than a few percent you will spend a lot more money on calibration gear than on the scope!

Roger
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Old 22nd Feb 2018, 5:50 pm   #16
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Default Re: Tektronix 453A

Any idea where I can get a manual for this? I downloaded the manual for the "A" version 453 from BAMA, but can't find one for the 453 basic scope.

73, Paul M3VUV.
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Old 22nd Feb 2018, 5:55 pm   #17
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Default Re: Tektronix 453A

Check right hand side of this page: http://w140.com/tekwiki/wiki/453
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Old 22nd Feb 2018, 10:19 pm   #18
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Default Re: Tektronix 453A

hi cheers for that,i sort of calibrated it using the cal output on my other scope thats in calibration still.Luckily work pay for that,so it should be somewhere near for what i need,73 m3vuv.
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Old 23rd Feb 2018, 12:37 am   #19
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Default Re: Tektronix 453A

Has anyone changed the power pilot lamp on one of these? Was going to change mine for an LED as it's blown, looks a swine to get at, may get left for a rainy day. M3VUV.
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Old 23rd Feb 2018, 8:22 am   #20
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Default Re: Tektronix 453A

Unfortunately no. When I had an original 453 I left mine because it looked like hard work!
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