Tektronix 2236 Analogue Scope

[MFinney]

Hi voltage across C292 is +8.07 volts. At the collector of Q231 I have -1.238 volts.

[markce]

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 ?

[MFinney]

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,

[markce]

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.

[MFinney]

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.

[markce]

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.

[MFinney]

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

[markce]

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).

[MFinney]

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.

[markce]

Please check Q230, as you measured it, it is also suspect.

Edit:
Not so in this case (?), looked up the datasheet of the AST4261.

[lordvader88]

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.

[markce]

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.

[MFinney]

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 

[markce]

Hi, good to read that your repair of the Tek 2236 vertical amp was successful.