Tektronix 465B repair

[bd139]

So two weeks after I landed a 475 which was sick, I have landed a 465B which is sick. I am becoming a home for sick Tektronix scopes. This one is in very good cosmetic condition and comes with the probe bag, graticules and the instructions for two P6106 probes. It also came with an HP probe which has been used by a gorilla or something as it's fecked and in pieces.



Current issues:

NO FOCUS / CRT CONTROL SHOT - RESOLVED

In diagnostics now. Shows the gun is working but jammed right and defocused. Could be power supply or intensity shot.

15v rail is down - snipped out tants and it's working better.

15V RAIL SHORTED TANTS - RESOLVED

Replacing all tants on this rail to start with, probably the whole lot. On order.

Received. Replaced with Chinese electrolytic temporarily then proper ones. Some HF ripple now gone.

Have replaced all the ones showing signs of problems. Found a few screws that were loose (this causes grounding problems) and now and the trace is pin sharp.

GUNKY CHANNEL SELECTOR SWITCHES - RESOLVED

Sticky switches - latching action iffy. Will have to yank and clean.

Didn’t have to yank. Managed to squirt enough switch cleaner in some holes to do the job and then exercise the switches.

NOISY FAN - RESOLVED

Dismantle, clean and oil the fan bearing. Not noisy when flat, sucks when vertical. Just clean for now.

Cleaned all the crud out of the rear and the impeller blades and it's fine now.

CH2 ATTENUATOR JUMPY - RESOLVED

Needs cleaning. Have cleaned it and issues resolved. There were a few loose screws which have been tightened and sorted.

CH1 POSITION KNOB BROKEN - IGNORE

Minor. Bit missing off skirt. Ignore this - can hardly notice it

DIRTY - RESOLVED

Aren't they always. Clean it! Cleaned and scrubbed!

[Specmaster]

Another nice scoop,its always best to look for something that is cosmetically good because that will undoubtedly mean that the insides should also be similar and thus eminently fixable. Something that looks like its been dragged through a field certainly has not been cared for and is a prime candidate for major problems. 

[bd139]

Indeed. This one isn't too bad inside. The outside is a bit dirty at the back end.

So 10 minutes with the case off and I've found a few things it's not happy about and got a trace which is a good start.

Turns out the 15v rail was right down. I've traced this to two tantalum capacitors (again!) on the interface and trigger boards. Snipped them out and we have fuzzy traces.  No tants in stock so repairs on hold until they arrive. I've only got shitty chinese electrolytics left now which I'm not putting near this.

Looking good 

[Specmaster]

Ok so shes half the speed of the 475, both in good cosmetic condition, which is your favorite and why?

[bd139]

Not sure yet. I like the 475 because of the bandwidth and because it was designed in an era before LEDs so all the indicators on it are actually incandescent lamps. But I don't like the DM44. It's very difficult to move the DM44 between scopes as well so I'm not going down that route. This 465 looks like it might need some significant work in the capacitor department as well as it doesn't seem to hold focus as well as the 475. I'll get replacing them when the new ones arrived. Oh and leaning all the leaf switches.

I chucked some wun hung low capacitors in the 465B quick to see how it behaves, cleaned the graticule and it's not that great yet so will require a complete refurb. I haven't measured PSU ripple or ESR yet on the main filter caps as this is just an initial check out (and I'm tired! ). They are buggers to replace so I'd like to avoid that if possible. Also found out the channel 2 attenuator switch bank is clunky so needs cleaning. I hate this job!

Post-wun-hung-lo installation, looking better:

[rhb]

They are gorgeous! 

My 465 looks as if it spent it's working life in a seismic recording truck.  Probably doesn't work right now as it's been sitting idle for a long time.

Just in case you  don't know, be careful with the input attenuators.  I made a foolish attempt to clean them with tuner cleaner before I learned that was a no no.  Spent a lot of time washing them with 99% isopropyl alcohol.  But in the end it was successful and it met the rise time spec.

[David Hess]

This 465 looks like it might need some significant work in the capacitor department as well as it doesn't seem to hold focus as well as the 475.

Higher bandwidth oscilloscopes usually provide for a brighter and sharper trace because of a higher CRT acceleration voltage however the 465, 465B, 475, and 475A are identical in this regard with the same cathode and anode voltages.  So if after repairs your 465B (1) lacks brightness and sharpness compared to your 475, it may be that something is wrong or the CRT is worn out.  Maybe check for double peaking.

(1) Do not confuse the 465 and 465B; they have significant differences.

[bd139]

Thanks both. I’ve repaired a few of these in the past so am well aware of all of the problems etc. The big people with the attenuators is people roasting them. I think this one is dirty switches though.

The 475 trace is always quite thick as the vertical preamps they used are quite noisy. I think the IC designers admitted they screwed that one up. 465B should be pin sharp. You can see some 120Hz ripple on it at some trace speeds so I suspect caps are shot. Not too bad yet however so will leisurely replace them. 

How do I check for double peaking. I’ve heard about it but have no idea what the symptoms are.

[David Hess]

How do I check for double peaking. I’ve heard about it but have no idea what the symptoms are.

As you increase the brightness, there will be a point where it decreases and then increases again producing two peaks.  Trace thickening at low brightness is another symptom of a worn out CRT.

[bd139]

Thanks for the explanation. I assume the best configuration to test this should be X-Y mode, dot at center with astigmatism, focus and "working brightness" set for best shape?

[David Hess]

Thanks for the explanation. I assume the best configuration to test this should be X-Y mode, dot at center with astigmatism, focus and "working brightness" set for best shape?

Using the standard baseline sweep is sufficient, easier, and safer.  Higher sweep speeds will lower the total brightness but not affect doubling peaking or blooming if it is present.

Doubling peaking and blooming is more of a problem with CRTs at high sweep speeds where the brightness will be lower anyway.  At slow sweep speeds, it might not even be noticeable at comfortable brightness levels.

The actual problem is lack of electron emission from the cathode.  As the intensity (cathode emission) is raised, the area of emission is suppose to stay small.  On a depleted cathode, instead the center cannot support the required electron emission so the emission area increases in the shape of a donut and the larger area prevents sharp focus.

[Specmaster]

Thanks for the explanation. I assume the best configuration to test this should be X-Y mode, dot at center with astigmatism, focus and "working brightness" set for best shape?

Using the standard baseline sweep is sufficient, easier, and safer.  Higher sweep speeds will lower the total brightness but not affect doubling peaking or blooming if it is present.

Doubling peaking and blooming is more of a problem with CRTs at high sweep speeds where the brightness will be lower anyway.  At slow sweep speeds, it might not even be noticeable at comfortable brightness levels.

The actual problem is lack of electron emission from the cathode.  As the intensity (cathode emission) is raised, the area of emission is suppose to stay small.  On a depleted cathode, instead the center cannot support the required electron emission so the emission area increases in the shape of a donut and the larger area prevents sharp focus.

Do you have any photos or know of a site that actually shows this blooming / double peaking it might be of great benefit to others on here as well?

[Specmaster]

Further to the post by David Hess about double peaks, reading through the manual for my HP scope I came across the following extract "Common CRT problems consists of open filaments,
grid-cathode shorts (uncontrollable beam), and "hollow cathodes", sometimes referred to as "doublepeaking". Hollow cathodes can be detected by increasing intensity. As the intensity knob is rotated clockwise, the beam will get brighter, up to a point; beyond this point it will decrease in brilliance and may defocus."

It might help to clarify things a bit?

[David Hess]

Do you have any photos or know of a site that actually shows this blooming / double peaking it might be of great benefit to others on here as well?

I do not have any such links and I lack a CRT bad enough to demonstrate it.

[bd139]

My two scopes don’t decrease in brightness. However they both defocus. You can control that with the focus control.

[Specmaster]

I think all CRO's will defocus if you turn the intensity up to high, besides who would want to it adds to the screen burn. 

[bd139]

You need to at very high sweep speeds and that's it.

[David Hess]

My two scopes don’t decrease in brightness. However they both defocus. You can control that with the focus control.

That could indicate a problem with the automatic focus circuit.  The 465B gangs the automatic focus potentiometer with the intensity control but the 475 relies on the CRT design.

You need to at very high sweep speeds and that's it.

And at low repetition rates.

[bd139]

Thanks for the tip - will look at that. I suspect there are a few problems with the 465B like this that need resolving. I'm going for an all out tantalum capacitor replacement first as there are several issue so far across two scopes which were solely down to the things.

[bd139]

Ok tants done. Trace very sharp now.  Also fixed the channel selector switches which are perfect after some contact cleaner has been applied.

On to the attenuators but not until I’ve gobbled up my Kentucky Fried Muck.

[David Hess]

It seems odd to me that the solid tantalum capacitors would fail in a way that yielded high noise do to poor decoupling without shorting.  Changing them without evidence of failure would not have been my first suggestion.

Usually all that is required for the big pushbutton switches Tektronix used is contact cleaner and lubrication.  They are a wiping type of switch so be sure to lubricate them as well or the switch contacts will destroy themselves.

[bd139]

There were a couple of open ones/cracked solder joints so no decoupling was taking place. The green ones seem to crack slightly I found and go open or intermittent as well. If you wiggle them all a little bit, if the trace changes, so you just replace those ones.

I've never bothered with lubrication myself. I have had Philips switches, which are the same type, live for a decade or more after just cleaning. In fact I sold the thing in 2014 and it's still going strong apparently (PM3217)

Anyway it's done now and is fully working. Vertical and horizontal cal and compensation is pretty much spot on so don't need to do anything there. All inputs, outputs, switches and functions are working too.

Final outcome picture (trace is pin sharp - just looks crap because of the phone camera)

[Specmaster]

Nice job, looking great.

[scatterandfocus]

I just got my dead 465B up today after running across this thread:  https://www.eevblog.com/forum/testgear/tektronix-465b-help-needed!/msg2699546/#msg2699546  It seems to be functioning ok after getting the shorted tantalum subbed with an electrolytic.  Lucky for me that nomad86 had the exact same problem and posted about it.  I'm wondering how much of a pain it is to replace all the tantalums in these things.

The only real issue I am noticing so far on mine is the horizontal position pot has about a 1/4" of slack in the middle.  But I did notice a lack of razor sharpness after getting it up.  Mine doesn't seem to have any double peaks and has a constantly increasing brightness, so I guess the crt isn't weak.  That is good stuff to know.  Thanks David Heiss.  The sharpness of the trace seems to really depend on the signal and frequency being viewed.  Signals from a function generator are sharper than the calibration signal.   

[bd139]

It's possible but difficult to change them all. Also it tends to do more damage than good I have found.

The horizontal position pot does have a dead section in the middle. That's by design. It's so that when it's in 10X mode you have a fine and coarse control. Although it feels like it's broken, it's not!

They aren't particularly sharp scopes really. First thing to check is power supply ripple as that contributes the most to sharpness. After that, hit the 20MHz bandwidth limit switch. That decreases the noise in the amplifiers considerably which leads to sharper traces. My Hameg, HP and Philips analogues are all considerably sharper than the Tek ones are!