greetings and help request on Tek 465

[vangheli]

Hi .
I am new as a member on this forum. So ,first of all, let me greet everybody.
I hope not to be posting on the wrong section: I apologize if this is the case .
From today I am the owner of  an old Tektronix 465. I immediately started to examine it. It is really beautiful .The instrument seems  to work correctly.

Apart from one stiff potentiometer , I found that the trace goes out of sight pressing the CH2 INV button (I need to use the vertical position knob to make it visible again). After some head scratching ,I searched on the schematics, and I concluded that one of the to transistors Q228 or Q226 on the channel 2 vertical preamplifier is bad. I found that these two NPN transistors  should be   matched , but I have no idea of what type they are.
Is there somebody who can help me telling 1) if my initial hypothesis is correct , 2) if yes what are the transistors suitable to do a repair
Any idea or information is wellcome
Thank you very much

[med6753]

Don't condemn those transistors just yet. Make sure your channel 2 variable balance and position centering is set properly first. The manual will give instructions. If they are not adjusted properly the trace will go off screen.

[pbarton]

It could also be a faulty ‘Invert’ switch (breaking the ‘normal’ connection but not making good contact in the ‘invert’ position).

[bd139]

Indeed.

A lot of the problems I've had with these scopes is solved with some decent contact cleaner in the channel switches.

[Gyro]

But not the attenuator switch contacts!

[bd139]

Good reminder! Yes don't spray them!

[vangheli]

Don't condemn those transistors just yet. Make sure your channel 2 variable balance and position centering is set properly first. The manual will give instructions. If they are not adjusted properly the trace will go off screen.

Thank you MED6753, I will follow your advice.Actually I forgot to mention the fact that I had an inverted signal (it was just out of sight until I regulated the vertical position knob) . I am wondering if it is  possible to see the inverted signal , if one of the two transistors Q226 or Q228 is inoperative .

[vangheli]

It could also be a faulty ‘Invert’ switch (breaking the ‘normal’ connection but not making good contact in the ‘invert’ position).

Thank you pbarton. I suppose the switch is not faulty because (I didn't mention before ,sorry about that) I actually see the inverted signal ,after regulating the vertical position knob.

[vangheli]

Indeed.

A lot of the problems I've had with these scopes is solved with some decent contact cleaner in the channel switches.

Thank you BD139 . This is an operation I will do as soon I have a suitable cleaning agent available (I am thinking to isopropyl alcoohol)

[vangheli]

But not the attenuator switch contacts!

Thank you Gyro . I will stay away from them!

[med6753]

But not the attenuator switch contacts!

Thank you Gyro . I will stay away from them!

You can use IPA on the attenuator switch deck. That is the recommended cleaning agent as per the manual. For other general switch contacts you can use cleaning agents such as Deoxit.

[Gyro]

Yes, IPA is the only cleaning solvent suitable for the attenuator contacts. You can't just spray it in or use cotton buds though. You have to apply a small amount to a piece of lint free paper (not tissue) and work it horizontally under the contacts with no vertical motion. There are other posts on the forum that probably describe it in more detail if you do a search also in the service manual (actually the first port of call) and the Tekscopes forum.

You should use the same paper method for the invert and bandwidth limit switch contacts with your chosen cleaner. The reason for the difference (IPA only) is the special PCB substrate used in the attenuators, the others are on standard PCB.

[vangheli]

Now  I was able to find  the isopropyl alcohol.I am not able to find here the WL  (what is this subtance?a contact cleaner?).
I still need help due to my lack of experience: the issue is the high voltage inside my scope (15kV) .As a precaution I will work with power disconnected for cleaning the potentiometers, but how can I be sure the big capacitors and the CRT are  discharged? Is there any tricks about this operation? thank you if someone of you can clarify my doubts.

[james_s]

Just don't mess with the anode suction cup and you'll be fine. With the power off and unplugged there is nothing likely to hurt you. The 15kV EHT doesn't feel good to touch but the current is very low and it's difficult to touch by accident.