Tektronix 465B Trace vertical position moves up as oscilloscope warms up

[nick39]

Hi,

Is it normal that CH1 vertical trace position moves up as the oscilloscope warms up? I noticed the same thing on channel 2 but not so much. Also if both channels are off the trace is 1.2 divisions up. My guess would be that the trace should be perfectly centered vertically when they are off.
I have another analog scope a Hitachi V-212 and it doesnt do this. But maybe its a normal thing on old Tek scopes
Thanks!

[CaptDon]

When both vertical channels are off (don't know when this would ever be a useful mode) does the Ch1 V Position control still move the trace up and down? (It has been a while since I used a 465, loved my old 453!) It sounds like the D.C. Balance controls may need touched up. Also, the drift problem, I see it on many old Tek scopes. The trace may drift up 1/2 of a division (1/2 CM). Gently using a bit of freeze spray may help you narrow it down but after hours spent tracking down the drift problem I just learned to tolerate it. It seems that the 'married' transistors be it two transistors clipped together or one single 6-lead transistor appear to become mismatched with age and cause the drift problem. The replacement must be an exact Tek part which could be impossible to find or priced at insanity. The worst 'drifter' I ever had was a Tek SC502 modular scope and it was the intensity that drifted. Got dimmer over a 5 minute period then stabilized. A handful of Tek sourced replacement parts didn't fix it. Just learned to accept it. Came to the conclusion that the C.R.T. may have been very slightly gassy causing a bit of G1 emission and current flow. Found a couple of P.C.B. mounted parts that appeared to be the cause of the problem but when replaced made no difference.

[nick39]

When both channels are off, the "Vertical Position" control does not move the trace. I'll try to get that freeze spray to see if I can find the problem. Channel 2 moves up very little about 0.2 divisions. It's channel 1 that moves a lot.

[David Hess]

Is it normal that CH1 vertical trace position moves up as the oscilloscope warms up? I noticed the same thing on channel 2 but not so much. Also if both channels are off the trace is 1.2 divisions up. My guess would be that the trace should be perfectly centered vertically when they are off.

Warm up drift is not unusual for some oscilloscopes, especially if the input buffer is open loop like the 465B.  I might expect 1/2 division at most if there is no other specification.

However after warm up, check the input bias current by shorting the input.  Maybe the input buffer or the input buffer's protection has been damaged and has high leakage.

I have another analog scope a Hitachi V-212 and it doesnt do this. But maybe its a normal thing on old Tek scopes

It is a normal thing for any oscilloscope but depends somewhat on the input buffer design.  The Hitachi V-212 is a two path design with input drift being 2 times the drift of an LF411 JFET operational amplifier, so the total input drift could be 1/4 or even 1/2 of a division at 2mV/div.  The Tektronix 2235 has a similar input design but with a TLC271A operating at a gain of 4 instead of 2, but total input drift should be less than 1/4 division because the TLC271A has less drift than an LF411.  The 465B has a dual JFET operating at a gain of 1, so might drift 0.5 to 1 millivolt or less than 1/5th of a division at 5mV/V.  If yours is drifting more than that, then something is likely wrong.

[nick39]

I found the issue its Q1355 in the CH1 Vertical Preamp board. It seems to be a little tired and it needs a little temperature to pull. I swapped it from CH2. Tektronix part number is 151-0434-00 and it seems like the replacement is 2N4261 or NTE395. Both of them are unobtainable here so for now I swapped it with CH2 and I'll see if I can buy them from the US maybe?

[David Hess]

That is too bad.  I might not trust anything off of Ebay, but some smaller distributors carry them for like $4 each.  151-0434-00 aka 2N4261:

PNP TO-72A
12 Vceo
30 mA Ic
2.5 pF Ccb
2 GHz Ft

If you have to, then a 2N5770 or 2N5179 might be acceptable.

[nick39]

I don't know why, but Tektronix removed the thermal coupling between Q1155 & Q1355 on CH1 and Q1655 & Q1855 on CH2. So I improvised my own thermal coupling using a piece of copper wire and now it drifts to the center of the screen much faster, it only takes about 4 minutes. Also it seems to like Q1655 on CH2 better than Q1155 on CH1 because I'm only getting 0.6 divisions instead of 1.2 I had before.

[Wallace Gasiewicz]

Sphere has the transistor for $6  You might want to ask them if they have the Thermal Link (email them), they have lots of Tek and HP parts, Good Company     


https://www.sphere.bc.ca/oldsite/test/tek-parts/tekparts2.html   

I do not know what the shipping would be