I'm not familiar with the old toobers, but looking at the schematic, I'd guess that the +225V being quite a bit low caused V343B follower to drop a lot, putting the upper V364A near cutoff. This would cause the lower V384A to take most of the available cathode current. Maybe this caused R384 to eventually burn out, so then the current could only go through V364A, until the upper R364 likewise burned out. A chain reaction - and while each resistor cooked, so did its corresponding tube.
Or not...
Ed
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--- In TekScopes@..., "Steve" wrote:
I have spent the past couple of evenings troubleshooting a 541A with a problem in the horizontal amplifier. It has really weird symptoms that caused a lot of head scratching on my part. I finally found the problem and am in need of a couple of special resistors from a parts donor.
The symptom is a trace only on the left side of the CRT, not extending to the left edge, with some non-linear break points. Voltage on the left deflection plate followed the non linear deflection. In Horizontal mode: external 10X, the position control would move the voltage linear for a bit, then stop, then snap back to a higher voltage. The right deflection plate measures +120 V constant, regardless of position control or sweep.
To make a long story short, both of the plate pull up resistors in the output amp stage (before the final CF stage) are open! Both plates are sitting at zero volts. From what I can gather, the output signal is actually coming from the feedback path, where a 400K resistor connects with the grid of the initial CF stage (V343B).
The plate resistors are special high power non inductive. The left side, R364, is 25 kohm, while the right side, R384, is 30 kohm. Both are tapped at the 6k point from the plate side to connect to a small trim cap for HF compensation. The pull up voltage is 500 V.
It appears both resistors have opened on the +500V side, as the 6 k to the plate side still measures OK in both parts. Any idea why these would fail? The tubes are Tek's favorite 6DJ8. They both still work (when swapped in other circuits). The envelopes show considerable blackening. I suspect these are driven pretty hard in this application.
So I am in need of these resistors if anyone is parting out an early 53x,54x series scope. I believe the horizontal amplifier (and quite a bit of other circuitry) is identical in the 531, 531A, 535, 535A, 541, 541A, 545 and 545A. It looks like the 543 (and probably the 533) have some variation to accommodate the 100X horizontal magnifier. I am not sure if they use the same resistors.
Let me know if anyone can help with either parts (of list reply), or ideas of what caused the failure (reply to the group).
BTW, the power supplies were right on except the +225 V was low (about 170 V) due to a weak 12AX7 in the error amplifier. I have since fixed it. Looking at the schematic, I don't see where that would have caused this problem.
- Steve
