Newbie with a 466 that won't sweep
I've recently become the proud owner of an early 466 (IC date codes
are all '74) that unfortunately is a bit poorly. I've done bits of
hobbyist electronics repair in the past, but I've never tackled
something quite so complicated as this before. I've been studying the
service manual and the Troubleshooting Your Oscilloscope document, but
I'd greatly appreciate any wisdom anyone would have to share.
My symptoms are:
* No horizontal sweep on any timebase setting
* X-Y mode operates as it should, but the trace is off-screen
horizontally (to the right), and can't be fully centered even with the
horizontal control all the way to the left
* Beam finder shows a bright dot on the screen, offset to the right,
but visible with the horizontal control centered
* Vertical deflection of both channels (with beam finder depressed)
looks correct in all ranges, and is more-or-less centered with the
vertical position controls at midpoint
* Depressing the Trigger View and Beam Finder buttons together shows
reasonable vertical deflection on all A trigger sources, and responds
to trigger level control.
* With a square wave input, the Triggered and/or Ready indicators
never light in any of the trigger modes (Auto, Norm, Single)
* The trigger gate outputs on the back panel are low all the time
The scope exhibited these symptoms when I first powered it on. I moved
it to my workbench and it started working for a few hours (although
with a significant horizontal offset to the trace), but after I
powered it off and put it away for the night, the problems came back
and I haven't seen a sweep since.
Power rail voltages are all within spec, although with about 20mV of
ripple, which I gather is a bit high. I haven't checked the HV and
don't have the equipment to do so safely.
Probing the pins of the sweep IC (U980) suggests that indeed, not much
is happening in sweep-land; the outputs of pins 10 (low) and 17 (2
volts) suggest that it might be stuck in holdoff, but I'm seeing DC on
the holdoff timing terminal. The IC is also noticeably hot to the
touch (although not scorching).
Additionally, the service manual suggests that the READY light
(controlled by U980 pin 11) should illuminate when the single-sweep
reset button is pressed (input on pins 2, 3), which seems to be a
fairly straightforward function of the U980's inputs and outputs.
Since this doesn't seem to happen, should I suspect U980 to be bad, or
is this likely to just be a symptom of it being stuck in holdoff for
I realise that the sweep section isn't my only problem here, but my
rationale is that I'll have a much easier time debugging the
horizontal section if I have a working sweep, and while the power
supply ripple is out of spec, it's not drastically bad and shouldn't
be affecting the sweep section, right? If this is the wrong order in
which to tackle things, please feel free to set me straight.
Any and all help and wisdom would be appreciated, even the really
elementary stuff (I won't be offended!). I realise it's probably a bit
of an ambitious project for a newcomer, but I've always wanted a
vintage Tek scope on my workbench, and I'm quite determined to see
Thanks in advance,
Try cleaning the switches including the time base contacts. Dirty switches can cause a lot of problems. Cleaning IC and transistor sockets may also or should be done.
The power supply should have new electrolytics in it that are high temperature, low ESR and long life types. Increasing the capacitance is fine. See if VR1726 is open (very likely). If so, use BZX85B9V1. The output filters from the series pass can be raised in value. The 3.6mfd on the 65V supply should be 10mfd or larger 100V (Nichicon UHE type). Using something like 27mfd 100V at the 65V filter is fine, this is an example that will work. Nichicon ULD is what I used in the lower voltage areas. A 47mfd 25V on the other 5-15V supplies works fine. C1455, 10mfd, check/replace if electrolytic with a new one. The rest of the tantalums can be checked for shorts or high ESR and replaced if bad. Good ones, leave them in. There are resistors that should be raised in wattage throughout the scope. R1445 and R1547 are likely high (out of tolerance). The 1.5A bridge rectifiers should be replaced with 4A types. Using a higher voltage rating than the original 200V is fine. The current increase is the main concern.
I have this model and have overhauled it. Do the cleaning first then test. Adjusting the Mag Reg. pot may need to be centered to get the trace centered. I did add heatsinks to the horiz. outputs.
The more things you have to debug at once, the more difficult it can be. I'd go to XY mode and debug using that. I'd hope you have a second oscilloscope, because otherwise things get difficult.toggle quoted messageShow quoted text
Your symptoms suggest that the horizontal amplifier has problems. If the dot (in XY mode) cannot go to the left (or right) of the screen, then it suggests that half of the amplifier is bad. Fortunately, the amplifier is symmetrical, so the collectors of the + part should have the same voltage
The horizontal amplifier takes a single (almost a current into a base of a transistor) input. That is part of a differential pair that copies the signal, and also produces an out of phase signal. Those two signals drive an amplifier that has cascaded transistors to produce a + signal and a - signal, each of which drive one deflection plate.
If, for instance, the collectors of the mirror stage don't match the collectors of the other stage (one stage each deflection plate), then start looking there.
On 10/20/2021 10:32 AM, Richard Halkyard wrote:
I don't know if you have actually opened it up to take a look around, but if you haven't, you should check whether one of the deflection plate connections has come loose. I've encountered a handful of scopes with that problem.toggle quoted messageShow quoted text
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford, CA 94305-4070
On 10/20/2021 07:32, Richard Halkyard wrote:
Deflection plate disconnected trumps bad amplifier.toggle quoted messageShow quoted text
On 10/21/2021 10:56 PM, Tom Lee wrote:
I don't know if you have actually opened it up to take a look around, but if you haven't, you should check whether one of the deflection plate connections has come loose. I've encountered a handful of scopes with that problem.
I checked my notes and found the following:
" Tektronix 466 Repair (October 2013):
466 early model, Timing circuit board A7, signal 'A' SWP START LEVEL to Q1002A
C1115 shorted (Tantal, 100µF/20V), replaced by AL-Electrolytic 220µF/35V "
Symptom of the 466 was no horizontal sweep, but vertical deflection was ok.
I chose the capacitor just by looking for a most close value fit in my new parts box.
The solder side of the A7 board is reachable only by dismantling.
As I did not plan to carry the scope around in a car (i.e. no shaking and rattling),
I made the repair process straightforward. The tantalum has been cut just at the body.
I soldered the capacitor to the remaining standoffs.
Thanks everyone for your insight so far! I haven't had a chance to dig much further, but I thought I'd drop in with some more observations and responses to your questions:
As far as diagnostic equipment goes, yes, I do have another scope at my disposal, and a signal generator and multimeter.
The power rail ripple isn't as bad as I first thought - that 20mV figure I quoted earlier was a combination of measurement error and a rather electrically noisy environment. Taking a bit more care about the measurements, I think I'm hitting the noise floor of my cheap modern scope and probes at around 3mV. So it may not be quite within spec, but close. Voltages all seem to be stable and within tolerance.
The connections to the deflection plates appear to be OK. With the the horizontal amp common-moded and X-Y mode selected to un-blank the trace, I see a centered dot, so I think we can sign that section off as OK for now.
I've noticed that the horizontal position in X-Y mode is influenced by the sweep mode, which seems a bit odd to me:
- A Sweep (also A intensified by B): trace is offset far to the right, can be brought partially on-screen with the horizontal position control, but not fully.
- B Sweep: trace is offset by approximately one division to the left with the horizontal position control centered
- Mix: When pressed, the trace starts at B sweep position and then sweeps to the A sweep position at the rate set by the B timebase (and then stays there)
To me this lends credence to the hypothesis that the A sweep generator is 'stuck' - Andreas, thanks for digging that info out of your notes! I'll definitely take a look at that capacitor on the timing board.
However, right now the timing board is out of reach. I'm having some trouble removing the B trigger slope knob; no matter how much persuasion I apply, that grub screw isn't budging. Even with regular dousings of penetrating oil over the past few days, it's resolutely stuck, to the point where I've wrecked a couple of hex keys trying to turn it. Anything else I should try before I do some violence with a left-handed drill bit?
Just a quick follow-up to this; I managed to track the fault down to a dead Q1006 in the A sweep generator, and a possibly-marginal Q1036. Replaced both of these and it's working again! Still got a few minor issues that I'll see to as time allows, but some of them have already cleared up with a bit of use - I suspect dirty switch contacts and pots are a big part of it.
I do have one silly question: I need to turn the horizontal position control about 1/3 of the way to the right to center the trace, but in XY mode with the inputs grounded, the dot is centered with the position control centered. Is this normal? It's been so long since I've used an analog scope that I can't remember if this is expected behavior or not!