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The Alternate mode on my 465B does not work anymore. When I switch
over from Chop mode, which works correctly, the channel 1 or 2 trace
dissapears randomly. When I switch several times between Alternate and
Chop, the one or other channel will remain,like it's being randomly
latched. Incidentally, when I switch to Add mode the missing channel
returns, just to dissapear when I turn off the Add mode.

Anyone knows where to begin?

Any help is appreciated.

Thanks,

Dieter

Hi Dieter,

Just started looking at that issue on a 465B scope I'm in the process of bringing back from the dead. I have exactly the same issue with my 465B, the Alt mode (Chop switch out) will only display one channel at a time not both.

Any ideas would be greatly appreciated

Thanks,

Edward.

Hello Dieter,

The behavior of CH1 or CH2 being displayed either one at random when you select ALT mode (better saying VERTICAL ALT mode) is consistent with this logic malfunctioning, because when ALT is selected, which trace is being displayed at any given time is determined by a "sort-of" flip-flop circuit so... yes, it's "latched".

At the end of every sweep, a signal coming from the sweep generator triggers the "sort-of" flip-flop, which changes states from CH1 to CH2 or vice-versa.

Which Vertical Input channel is being enabled to pass, whether CH1, CH2 or TrigView is controlled by the interaction between a lookup table stored on a 256 x 4 ROM chip (U1605) and a quad D-type flip-flop 74LS175 (U1701).
On the other hand, U1805 and U1905 forma a logic that basically controls the difference between ALT and CHOP modes, either clocking the Quad latch above from the CHOP clock, or from the VERTICAL ALT SYNC PULSE, that comes from the Sweep generator logic.

The fact that channels alternate during Chop mode gives clues there's probably no problem between the ROM Lookup table and the latch because, as far as the ROM and the latch are concerned, there's no difference between the CHOP and ALT mode, being that what differentiates those two modes are the CHOP clock logic performed by U1805 and U1905.

In ALT mode, U1905D and U1805B stops the CHOP oscillator at the same time assuring its output level is HI (pin 3 of U1805A), which gives way for the "VERTICAL ALT SYNC PULSE", coming from U1905A to pass through U1805C, providing clock to the LS175 latch.

Problem seems to be somewhere between the generation of the VERTICAL ALT SYNC pulse (coming from Diagram <4>) which I couldn't find on my PDF copy, and its passing through U1905A and U1805C.

Hope this helps,
Rgrds,
Fabio

Hi Dieter,

Thanks to Fabio's detailed and clever reasoning (you're very gifted Fabio), I have found the issue with the Alt mode. The problem turned out to be a defective U4391 (74LS02) that can be found on diagram 8 (Horizontal Display Logic A&B Sweep Generator). When measuring pin 3 on this IC there was no change in state (stayed low) when operating the ALT button in the "Horiz display" section on the front control panel (after checking the switch first). Replacing this IC from a butchered 465B restored the Alt mode and the scope now reliably displays two traces :)

Note that your problem might be caused by a different component failure or break in the Vert Alt Sync path, but if you follow Fabio's excellent diagnostics, you will find the problem. Good luck, and another big THANKS to Fabio, I owe you a drink!

Regards,

Edward Thomas.

On Thu, Jun 14, 2018 at 01:53 am, <edward@...> wrote:

Hi Dieter,

Thanks to Fabio's detailed and clever reasoning (you're very gifted Fabio), I
have found the issue with the Alt mode. The problem turned out to be a
defective U4391 (74LS02) that can be found on diagram 8 (Horizontal Display
Logic A&B Sweep Generator). When measuring pin 3 on this IC there was no
change in state (stayed low) when operating the ALT button in the "Horiz
display" section on the front control panel (after checking the switch first).
Replacing this IC from a butchered 465B restored the Alt mode and the scope
now reliably displays two traces :)

Note that your problem might be caused by a different component failure or
break in the Vert Alt Sync path, but if you follow Fabio's excellent
diagnostics, you will find the problem. Good luck, and another big THANKS to
Fabio, I owe you a drink!

Regards,

Edward Thomas.

Off course you could be right that the IC really was bad. Did you put the original back to confirm ?
Another likely culprit would be bad IC sockets which are known to cause various intermittent problems.
465B from that time are known to have them and if not replaced more problems will probably occur in the future.
To identify the sockets check here:
Note that some of the text and the large picture is incorrect. The smaller pics are right.
/H?kan

On Thu, Jun 14, 2018 at 06:03 am, zenith5106 wrote:

H?kan

Hi H?kan,

Thanks for the response and link - much appreciated. You are absolutely correct but.before replacing the IC I removed and cleaned the IC Socket (with switch cleaner). The original IC was then re-inserted with the same results. The fault only cleared after replacing the IC I obtained from a cannibalised Tek 465B. Note that I have also cleaned and re-inserted most of the transistors and IC's (sockets - took a long time!) due to possible poor contact, plus cleaned all the switches.

Edward Thomas

Hello Edward,
Thanks for the update, and for the kind words... I think I was just in a good day.
In your case, I confess, when I jumped in I was believing the problem could be related to the ROM, only because I had a clear remembrance of reading about many incidents of ROM rots on some oscilloscope model(s) of that vintage.
But then, I decided to double check and learnt that that ROM in particular was not among the ones known to suffer from problems... AND ULTIMATELY, when I got to look a little bit more in detail to how the circuit worked, it wasn't that hard to see that it wasn't likely to be around the ROM chip, because the signal that tells ALT from CHOP doesn't even go into the ROM and doesn't matter to the operation of that circuit... and that was important to notice, because we knew CHOP was working.
Finally, to this particular control logic, if we rule-out the ROM and the LS175, and we also rule out a the Chop clock generator that we knew was working too... there was really very little left to get busy with.

Still, Dieter could have been right all along, because those Texas IC sockets are indeed awful.
They're particularly bad if the mating IC is one of the kind in which the pins are only tinned before the pins are cut-out to form the IC bonding spider (or whatever it's called in English).
On those ICs, the pin faces that were formerly the cross-section of the plate from which the pins are cut, are left exposed to the environment and ugly copper/brass oxides develop.
The evil in those Texas sockets is that they grab the pins exactly in that direction, that is prone to become badly oxidized (on the IC pin)... the IC socket contacts themselves are good quality gold-plated, but it's of little help if they contact the pins on the oxidized surface.

I`m glad that it worked,
Rgrds,
Fabio