Tektronix 564: Continued weirdness – SOLVED

2 years ago, I attempted to troubleshoot and repair an intensity modulation issue on my 564, but ended up giving up after a few unsuccessful attempts. I had replaced a few power supply caps, and all voltages & ripple were within spec. I thought maybe it could be an issue with the CRT? Last month, a spare CRT popped up 30 miles away for $40, so I grabbed it. I made the swap, and the issue remained.

I have since changed out the remainder of the electrolytic caps in all ‘low voltage’ supplies (all but the -12.2V – I don’t have a good way to mount new caps, and the ripple is really non-existent). I swapped the vertical and horizontal plugins, and never saw the issue, but I think this is a red herring, as when I put just the vertical plugin in the horizontal slot and feed it a 60Hz ramp, I do see the issue. I did this 2 years ago, and confirmed it today. The other thing I did to eliminate my suspicion of the horizontal plugin was to pull the transistor that drives the intensifying signal, Q294. When I pulled it, the issue diminished a little bit, but was still present.

Other things that didn’t work:

Disconnecting the intensifying pulse input – had to leave R838 & R839 connected, otherwise the beam intensity was unstable.

Maybe there was some heater to cathode leakage in the HV regulator tube? I swapped it out with a 12AU7 (similar pin out, close enough performance), the same issue occurred.

The ripple of on the primary of the HV transformer seems negligible, and in line with what I’d expect on a simple unregulated supply. Allowable ripple isn’t listed on the manual like it is for the other rails.

May be on to something: I’m seeing at least 5V of ripple on the grid of V800.

I thought the ripple on the 300V supply was in spec, but I just noticed a filter cap I missed on the other side of the regulator, that’s tucked away in the back of the scope. No change after swapping out that cap, but I think we’re getting closer. When I see problem, I’m also seeing ripple measurements jumping around. When the trace is solid, the ripple is rock steady at less than 5mV.

Things to suspect next:

  • V674, the 6AU6 error amplifier
  • V677, the 6AS7 series pass tube
    Damn, this was looking like the culprit at first, but the issue returned not after not too long. Swapped the old tube back in and issue eventually returned.
  • C670 blocking cap – worth a try?

Worth noting that we can still see the ripple on the 300V rail even if we pull the plug-in.

Turning our attention back to the HV section, looking at the input of the HV regulator error amp, before R816. It’s bang-on at -106V. The problem has been intermittent, and hasn’t happened for a few minutes since I gave that area of the HV supply a blast with cold air (compressed air upside down). Yep, OK, it’s back and the ripple is evident here as well – still not clear if we’re getting closer to the cause, or are just seeing the effect on the +300V rail. OK, this kind of sucks, but I’m getting to the point in the program where we just start swapping out caps. Also waiting on a 6CZ5 to swap out V800

  • C841 – .047u – no change
  • C849 – .001u – no change
  • C801 – 0.2u ? just a disc cap
  • C807 – .001u – says it needs to be 600V, but there’s only ~500V across it. Gonna risk a 600V for now.

SOLVED. It was V634. The 6DJ8 in the -150V supply. The supply that has NOTHING TO DO with any of the HV Circuits, and which exhibited NO RIPPLE. F*&k, this was a hard-earned win.

Side note – I nuked Q294 while I was troubleshooting – it runs the intensifying pulse, and I was yanking it and stuffing it back in live. I put in some basic bitch NPN transistor and it works fine.