Tektronix 564 Revisited

I recently picked up a somewhat rare 3A5 auto-ranging plugin for the 560 series, and decided to fire up my 564 to test it out. Since it had been over a year since I powered up my 564, I tried it first in it’s current config, a 3A72 Dual Trace plugin, and a 3B3 Timebase. Two problems were readily apparent:

  1. The varying intensity – I strongly suspect this has to do with AC ripple, since it holds steady when line triggered
  2. The trace is about 2 CM short.

Here’s a table showing measured supply rail voltages & ripple. All voltages are within spec, but the ripple on the -100 V, 125 V & 300 V lines are out of spec.

RailDC VoltageRipple
-100 V-100 V120 mV
125 V125.5 V40 mV
300 V301 V120 mV
-12.2 V-12.21 V2 mV
355 V357 V
420 V435 V1.5 V
475 V493 V280 mV
-3,300 V-3,290 V8 V

Replacements:

  • -100 V: C640, a combo 340uF & 10uF 200 V Electrolytic
  • +125 V: C642, a combo 340uF & 10uF 200 V Electrolytic
  • +300 V: C644, a combo 340uF & 10uF 200 V Electrolytic – this is only a 200V cap since it’s low side is off the 125V supply.
  • +475 V: C646 2x 40uF

So that’s a rebuild of 3 cans. C642 & C644 should take care of the ripple on the +420 V unregulated rail, which feeds the high voltage supply.

Using a fluke 6kV probe into my 7854 scope, I was able to measure about 50 V of high frequency ripple on the 3,300 V supply, however I’m not 100% certain that measurement was accurate. I could pick up about that much just by having the probe within a few inches of the HV supply, so I’m not sure how much of that was inductive pickup through the probe, or even the cable of the probe? I’d love someone to set me straight on that.

Following some recommendations on this old post on the tekscopes group, I gave the ceramic binding posts a thorough cleaning with IPA & cotton swabs. So far, I have not seen the intensity modulation problem return that didn’t seem to eliminate the problem, as it returned the next day, albeit not immediately, but after 30 or so minutes of operation. I did discover that with the cover reattached over the HV transformer & diodes, I was no longer able to detect the HF ripple when measuring high voltage. The AC ripple still disappears as soon as it’s measured.

Replacing C342 & C344 greatly reduced the ripple in the 125 V & 300 V supplies, but did nothing for the ripple on the 420 V unregulated, and hasn’t fixed this intensity modulation issue (I was pretty sure it wouldn’t though). I have confirmed that the intensity modulation is at 60Hz, and isn’t related to any errant signals from the horizontal plug-in by running the vertical plug-in in the horizontal plug-in’s slot, and feeding it a 60Hz ramp. The issue reoccurred.

As for the sweep length on the 3B3 – For now I was able to get the trace to the correct length using the sweep length adjustments pots, however, I had to crank them both (the normal and delayed sweep) fairly far clockwise. Once adjusted to the correct length, I do get sweep waveforms as documented in the manual, so I don’t suspect that I’m compensating for a problem further downstream.

Beam Intensity

There are, I believe 3 distinct different methods to modify the beam’s intensity:

  1. J21-14: Intensifying Pulse – changing the intensity of the beam by altering the ground reference of the control grid supply
  2. J21-13: Unblanking Pulse – deflection blanking
  3. J11-24: Chopped Blanking Pulse –

Tektronix 564 CRT Replacement

I figured it was time to break this off into it’s own post.  After some initial troubleshooting, I’d determined that the CRT was the source of the intensity and focus flickering I was experiencing.
I ordered a replacement I found on eBay, and it arrived yesterday, safely and extremely well packed:  Here it is after unbubblewrapping:
DSC_0207

I proceeded to carefully disconnect and remove the old CRT, leaving a gaping hole in the business end of the scope:
DSC_0224
Note the electrode connectors that press against pads on the neck of the CRT.  in front of that is the trace rotation coil, wrapped around a black plastic bobbin.

I compared the two to make sure they were in fact identical.  Spoiler alert:  They’re not.  The part# is the same, but there are other markings that differ.
DSC_0231

OK though, same part number, we should be good, right?  Hmm.. one of these things is not like the other: DSC_0236
There are 6 pads around the old CRT, and 6 corresponding contacts on the scope.  The new screen is missing the 2nd (or 5th?) pad.  Additionally, it looks like there’s a conductive trace coming out of the left of the 2nd pad on the old CRT that’s attached to the 3rd trace on the new one.  I needed to do a little more investigation before I was comfortable just shoving in the new screen and hoping for the best.

These connections are all a part of the storage circuitry, as seen in the schematic below.

Tek564Storage

After a bit of cold metering in the scope, I was able to determine which connections were which on the CRT, and I marked them with a grease pencil:
DSC_0240

So pins 1 & 6 (counting counterclockwise from the bottom in the scope) are STR1 & STR2, the upper and lower storage targets, while 2 – 5 are CE1 – CE5, the collimation electrodes.  I’ve marked the path of the conductive traces so they’re more visible,  you can see that they each terminate at the connection to their respective grids.

So the new replacement is just missing the last collimation electrode.  That doesn’t seem like such a big deal.  Maybe they realized it just wasn’t needed it in subsequent revisions?   The original has the ‘T5640-200’ in it’s markings, which matches the description in the manual from BAMA (©1964), but the replacement does not.  The replacement has what appears to be a lower serial number, but I’d be surprised if it was the older one;  It seems more likely that they removed an electrode in later models than leaving the space and adding one later.  Build techniques also differ.  The traces and connections on the replacement are finer and cleaner, but I don’t know if that equates to a earlier, smaller, more hand-built batch, or a later, more refined manufacturing process.  Any insight would be greatly appreciated.

DSC_0246

Feeling confident enough, I carefully installed the new CRT and brought it up slowly on the variac.  I got a beautifully sharp trace that didn’t flinch, even with gentle taps on the tube:
DSC_0257

As you can see, intensified mode works great.

One of my other recent purchases was a Heathkit TT-1 Tube tester.  I started out the previous version diagnosing a smoking resistor in the storage section power supply.  Now I had a way to check the tube.  Both sides of the 12AT7 checked out OK on mutual conductance and shorts, but one side failed the grid current test (exhibiting lots of it).  AH HA!!

I suspect the grid current was causing excessive draw on the 475v supply, which is why the resistor was overheating.  I replaced the tube and slowly brought it up on the variac –  low and behold the resistor remained cool.
Wait a minute,  a 12AT7?  The schematic (and nicely silkscreened chassis) calls for a 12AU7.  Huh,  I didn’t realize that upon first inspection.  The 12AT7 has a much higher gain then the 12AU7, and I wonder if that led to the premature failure of the tube.  It now has the correct tube.

Next on the hitlist:

  • Documenting the storage functionality.  It sort of works, but needs some attention
  • Understanding the grid current test on the TT-1
  • Sourcing a replacement handle.  I actually don’t mind going off-brand here, and would dig a custom replacement as long as it made use of the original mounting hardware.

 

 

Tektronix 564

DSC_0004

Picked this up as a part of a larger lot, and was the first I attempted to power up.  I did so stupidly, without a Variac.  Quickly shut down after smelling smoke from R646, a 100Ω 1W resistor that’s a part of the unregulated 475v line.  Metered it after the incident, and it appears to be OK.

Yanked the plugins and grabbed my newly re-wired Variac.  as I rounded 50v, things started to spring to life – around 80v I got a spot on the CRT, and R646 was HOT.  Shut ‘er down.

I think I’m going to yank all the tubes & transistors in the power supply, and check each supply one at a time, starting with the -100v supply.  We start with the -100v because as with most Tek scopes of the era,  the -100v (or -150v) supply was the one that all other supplies were referenced from.  We’ll start with -100v, then +125v, then +300v.  While the 475v supply is unregulated, it comes into close contact with some of the other supplies around the storage circuitry, which is also on the list of suspects.  Just for a chuckle, remember all of these are considered ‘low voltage’ supplies 😉

Pulled the following:
V667     6AS7     +300/150v supply
V674     6AU6     +300v supply
V654     6AU6     +150v supply
V800     6CZ5     -3kv supply
V814     12BH7    -3kv supply
V913     12AU7     storage

With only the tubes in the -100v supply installed, I get -150v on the rail when line voltage is brought up to 115v.  There’s about 90v across the gas regular tube instead of 80v.  I’m not sure if this is because the 125v supply isn’t up?  Furthermore, what’s with the pin 6 connection on the regulator tube?

With V667, V674 & V654 reinstalled, I can get the -100v power supply dialed into -100.0v.

Attempted to dial in the +125v supply but it jumps between 123.7v & 126v as I adjust the pot. 125.9v was as close as I could get it.  Getting the +300v rail dialed in was no problem, same with the -12.2v supply.

420v unregulated reads 427v, 475v unregulated reads 485v with 115v line voltage.

Now I’m suspecting the problem is in the storage circuitry, since that appears to be the only thing powered from the 475v supply.  As a side note, are these tin whiskers?

DSC_0007

OK, with V800 & V814 installed, I get a spot on the CRT!  Focus & astig controls change the spot shape, and location on the screen.  When I get it as sharp as I can, the intensity control also makes the beam move around.  Lets try a horizontal plugin.

DSC_0010

SUCCESS!  Lets try the vertical plug-in.

DSC_0014

It needs a cal and cleaning, but all 4 traces seem to work somewhat.  Even the delayed sweep seems to work:DSC_0015

So the trouble is in the storage circuitry.

After running for a while, the intensity started to jump around, became very bright, and the intensity control no longer functioned.  Stay tuned for more…

10/12 UPDATE

it’s general intermittent intensity issues, stemming somewhere from the HV supply.

C830 & C832 measured around .0026u which correlates to the parts list, but not the schematic, which calls them out at .0068u

R842 ((2)2.7M+(2)3.3=12M) Measured 13M cold, measured open after about 15min operation, then slowly dropped the meter picked it up around 50M, and it’s almost all the way back down.  Some metering in the early minutes after shutdown leads me to suspect one of the 3.3M resistors.

there’s a strong mechanical component to the problem; lightly pressing on the HV portion of the central chassis would drastically affect the intensity of the trace.

metering across the two neon bulbs I’d see anywhere between 50v & 120v that’s directly affected by the intensity control.  In one flicker scenario, the intensity wavers but this voltage remains consistent.  After being warmed up for about 20 min, the flickering would affect this voltage, causing the neon lights to fire

11/1 UPDATE:
I was wrong about the resistors, they read OK when immediately lifted from the circuit after power-down.
I’ve measured the HV supplies, and they seem consistent during the focus & intensity flickering, though measuring them does affect the trace (which I was surprised would happen with a 75MΩ probe impedance).
Screen voltage of V800 is around 65v for a normal trace at .2ms and can jump as high as 90v during a full bloom event.

As per the recommendation of Albert on the forum, I checked the V800 screen w/o the CRT connected, and it was around 65v.
Gently tapping on the chassis above the HV terminal strip does accentuate the issue, but I can’t find a component (either tube or passive) that responds specifically to some more directed tapping. I’ve re-seated the CRT socket, but it seems those wires are particularly sensitive to giggling. I opened up the the socket, and the pins look good.

It turns out tapping directly on the CRT shield or neck causes the biggest change in intensity, so now I’m fearing it’s a mechanical problem with the CRT.  I can wiggle the CRT or leads to get a good trace, but it quickly goes back to incessant flickering.  This is getting really frusturating.  I think the only fix might be:
1) Sourcing a new CRT
2) Hitting it with a brick until I feel better

UPDATE:  There’s a replacement CRT en route, part # 154-0410-00  (P31 phosphor, same as mine).
CRT replacement chronicled here