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Old 15th Feb 2019, 4:33 pm   #1
MrBungle
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Default Tek 465 low vertical deflection

In the process of fixing my 465 (again). Not sure if this is a new issue or one when I got it as I havenít used it for anything yet.

Itís displaying an unusual symptom. The vertical is exactly 1/2 the deflection it should be. This is the same in both channels.

Any ideas where to start looking?
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Old 15th Feb 2019, 5:25 pm   #2
Argus25
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Default Re: Tek 465 low vertical deflection

Since the total deflection is the sum of equal and opposite voltages applied to the deflection plates, then exactly half deflection might imply that the dynamic drive to one plate was zero and the other still working. Its average or DC voltage must be about right with respect to the other plate or the vertical position would be off. Is the vertical position control operating over the normal range ? The problem is probably after the channel switching in the vertical amplifier leading to the deflection plates (because both channels are the same). Scoping that area might show one half of the differential drive voltage missing, or perhaps both still there but equally reduced to 50% level due to another type of fault.
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Old 15th Feb 2019, 5:37 pm   #3
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Default Re: Tek 465 low vertical deflection

Vertical position is absolutely fine. Extends to full range. Also I checked plate bias and it’s symmetrical.

I will trace it through. Annoyingly it doesn’t have expected waveforms in the service manual. Very weird.

I’m tempted to sling it at this point (into the for sale section) as I’m getting fed up of fixing broken teks
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Old 15th Feb 2019, 11:36 pm   #4
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Default Re: Tek 465 low vertical deflection

So it would seem there is a symmetrical loss of amplitude of the signal in the amplifier circuits leading to the vertical deflection plates after the channel switching, but before the vertical centering control circuit.

In these sorts of paraphase direct coupled circuits there are often situations where the drive is applied to the transistor emitters, output from the collectors, and there is an inter-base R-C or R-C-L network that determines the DC gain and the frequency response. Or if the drive comes in at the bases, the gain & compensation components are between the emitters. One of these is a DC gain control pot. There is a diagram of the two configurations, on page 7 of this article I wrote on calibrating vertical amplifiers in Tek scopes:

http://worldphaco.com/uploads/TEKTRO...ILLOSCOPES.pdf

I attached the image from page 7

The grey-top white ceramic base trimmers Tek often used are notorious for going intermittent as they were not sealed types. This can drop the gain when the wiper to track connections goes dodgy. In fact, if the trimmer pot was wired with one end leg and wiper connected, and before the wiper went O/C the pot was roughly at mechanical center, then its resistance would jump to about double, which would halve the gain. You could gently wriggle the pots after marking their position.
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Last edited by Argus25; 15th Feb 2019 at 11:45 pm.
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Old 15th Feb 2019, 11:51 pm   #5
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Default Re: Tek 465 low vertical deflection

Excellent. Thanks for the info. I will digest and approach again tomorrow. I suspected it was something in that space. I already found a cracked resistor in it. Old carbon comp one.
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Old 16th Feb 2019, 7:30 am   #6
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Default Re: Tek 465 low vertical deflection

"I’m getting fed up of fixing broken teks" the joy of vintage scope ownership, look at it as a new challenge every day : )

Andy.
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Old 16th Feb 2019, 12:54 pm   #7
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Default Re: Tek 465 low vertical deflection

A good sleep last night cured the frustration.

Joy indeed. I’ve got three broken ones at the moment. I went to fix the timebase problem in my 475A last week and the bulbs in the front panel disintegrated. Looking at LED replacements this time. Also the DM44 on it has fluffed itself now too.

I really miss my old 465B. That appeared to have had all the nasty design flaws knocked out of it by the time they got to the B revision.
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Old 21st Feb 2019, 6:50 am   #8
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Default Re: Tek 465 low vertical deflection

Quote:
I really miss my old 465B. That appeared to have had all the nasty design flaws knocked out of it by the time they got to the B revision.
It is interesting that Tek kept the same model number for some scopes like the 464 and 466, there are two versions of each that are quite different animals. The latter versions of the 464 for example , serial B200000 and up had got rid of the unreliable lamps and gone to LED's. The vertical boards were much better made (re-designed) and a lot of other improvements internally. I would only every consider the later versions, these can be recognized by the LED's and slightly different panel artwork around the attenuator areas. Ebay sellers don't know and they are just sold as 464's and 466's.
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Old 21st Feb 2019, 10:29 am   #9
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Default Re: Tek 465 low vertical deflection

I think they were so ashamed of the 465 they decided to go HP on it and stick the B on the end.

I’ve actually got my 475A’s timebase out and on the bench at the moment. I am humming and hawing over doing a full LED retrofit as half the bulbs have disintegrated and a new bag from RS is £8. Need to fix the leaf switches first.
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