Author Topic: Tektronix 2236 Analogue Scope  (Read 3540 times)

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Offline MFinney

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Re: Tektronix 2236 Analogue Scope
« Reply #25 on: May 02, 2018, 04:27:50 pm »
Hi voltage across C292 is +8.07 volts. At the collector of Q231 I have -1.238 volts.

 

Online markce

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Re: Tektronix 2236 Analogue Scope
« Reply #26 on: May 02, 2018, 07:40:05 pm »
Just to be sure, base of Q230 is 0V, base of Q231 is 0V, base of Q284 is 0V?
Also base of Q254 is -0.5V and base of Q255 is -0.5V and  emitter of Q255 -5.6V ?
 

Offline MFinney

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Re: Tektronix 2236 Analogue Scope
« Reply #27 on: May 02, 2018, 09:08:59 pm »
Hi readings as follows
Base q230 +35.8 mv
Base q231 -30.4 mv
Base q284 + 3.0 mv
Base q254 -5.73 V
Base q255 -3.657 V
Emitter Q255 -6.47V

I re checked collector q231 and was -0.825v

Hope this helps,
 

Online markce

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Re: Tektronix 2236 Analogue Scope
« Reply #28 on: May 03, 2018, 10:52:56 am »
Q255 must be defect or bad solder joint (base).
Before you replace/repair it, check the base voltage of Q284, should be close to 0V.
Also check b-e voltage of Q257, should be close to 0.7V.
« Last Edit: May 03, 2018, 11:14:59 am by markce »
 

Offline MFinney

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Re: Tektronix 2236 Analogue Scope
« Reply #29 on: May 03, 2018, 11:12:07 am »
Markce,
thanks for your assistance in getting me this far, Im glad the readings make sense and point to where the fault is. I will re check all readings again tonight to confirm. Are spares/replacements readily available? I note Q255 is a SRF3188 NPN transistor.
 

Online markce

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Re: Tektronix 2236 Analogue Scope
« Reply #30 on: May 03, 2018, 11:35:32 am »
Some of the semiconductors are obsolete after 30 years! The SRF3188 will be hard to get, but the BFR96 which it replaced is available as NOS, at littlediode.com for example.
 

Offline MFinney

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Re: Tektronix 2236 Analogue Scope
« Reply #31 on: May 03, 2018, 01:03:26 pm »
Hi it looks like BFR 96 transistors are still available so good news. For my own interest and education can you explain from the readings I have gathered what indicates to you that Q255 is not working correctly?
Thanks Mark
 

Online markce

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Re: Tektronix 2236 Analogue Scope
« Reply #32 on: May 03, 2018, 02:23:32 pm »
The base voltage of Q230/231 is nearly identical and close to 0V. The plate output is alive (not 0 or 30V), but unbalanced. o the problem must be in the driving of the output stage.
Base-emitter voltage of Q255 is nearly 3V (-3.657 - (-6.47)), should act as a diode and  be close to 0.7V. So if your measurements are correct, I conclude the transistor is defect (or not correctly connected).
« Last Edit: May 03, 2018, 04:04:04 pm by markce »
 

Offline MFinney

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Re: Tektronix 2236 Analogue Scope
« Reply #33 on: May 03, 2018, 05:04:08 pm »
Hi I’ve checked all readings and in summary:

Transistor.     Base volts.    Collector.      Emitter volts
Q230.            +35.8mv.        -5.627V.     + 1.129V
Q231.            -30.4mv.      -0.825V.      +0.766 V
Q254.            -5.73 V.       -2.08 V.        -6.47 V
Q255.            -3.657 V.      +9.29V.      -6.47 V
Q256.            -1.406 V.       +2.91V.    -2.114V
Q257.           +9.82V.         +21.83V     +9.31V
Q284.            +1.8 mV

I’ve located the BFR96 transistors and ordered 4 so hopefully I can get this lovely scope working again.
 

Online markce

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Re: Tektronix 2236 Analogue Scope
« Reply #34 on: May 03, 2018, 05:11:18 pm »
Please check Q230, as you measured it, it is also suspect.

Edit:
Not so in this case (?), looked up the datasheet of the AST4261.
« Last Edit: May 06, 2018, 10:47:00 am by markce »
 

Offline lordvader88

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Re: Tektronix 2236 Analogue Scope
« Reply #35 on: May 03, 2018, 05:54:08 pm »
Grab the service manual here first: http://w140.com/tekwiki/wiki/2236

First thing to test is the voltages of the power supply with a DMM. Steps in the manual above and expected voltages.

This line of scopes have a lot of rather old and dicky capacitors in them which can drag the voltages off. I did a 2225 recently which ate about £20 of capacitors before it worked properly.
What about recapping a Tek2215, I barely looked around inside when I "improvised/fixed" it's power button, I must have been busy.
 

Online markce

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Re: Tektronix 2236 Analogue Scope
« Reply #36 on: May 05, 2018, 12:57:17 pm »
On a 30 year old scope, some preventive recapping of the smps is advisable. I should start with the elco's at the primary side of the power supply. Tektronix uses quality capacitors, so be careful to select a comparable series.
« Last Edit: May 05, 2018, 01:02:12 pm by markce »
 

Offline MFinney

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Re: Tektronix 2236 Analogue Scope
« Reply #37 on: May 08, 2018, 05:35:06 pm »
Hi Markce, just a quick follow up email. I replaced Q255 with the Replacement bfr96 transistor and morning he scope is now drawing a straight line without beam find pressed in!! I still need to do some testing but at least it’s working!! Thanks for your assistance. Mark  :)
 

Online markce

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Re: Tektronix 2236 Analogue Scope
« Reply #38 on: May 09, 2018, 05:51:57 pm »
Hi, good to read that your repair of the Tek 2236 vertical amp was successful.
 


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