Tektronix 564


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?


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.


SUCCESS!  Lets try the vertical plug-in.


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

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