Re: A bad CRT in 465?


Eric
 

Timothy,

Changing the CRT will also require and entire re calibration of the unit. If accuracy of measure is important. Which is a very deep rabbit hole I recently crawled out of my self, but would do it again. I you need a PG506 an SG503 and a TG501 or equivalent to do the cal when you go to sub nS rise time the cal gear can get a little exotic. There is also and RC normalizer at the correct capacitance for the input. But a Type 184 is really useful for the geometry adjustment. This also assumes all cal signal sources are repaired and in spec. A 465 is 100 Mhz so 3.5nS rise time. The PG 506 will get you there with the 1nS outputs. you will also need a 10x 5x and 2x attenuator in BNC if memory serves correctly.

Eric

On 4/27/2020 6:48 AM, Timothy Freeze wrote:
I'm trying to rescue a Goodwill vintage 465 scope. I've repaired all the problems except for one.

The final symptom is the intensity control works normally till it gets to about a quarter turn, and then the display gets no brighter. The display so dim I'd need to put a shade on it to use it.
I'm thinking the CRT is bad. Perhaps the cathode is at the end of life emissivity ?

I got the following from Wiki:
"The emissive layers on coated cathodes degrade slowly with time, and much more quickly when the cathode is overloaded with too high current. The result is weakened emission and diminished power of the tubes, or in CRTs diminished brightness."

What I've done:

I set up my other functioning 465 with the same input signal and measured voltages and waveforms from the unblanker input (J6-10) through the high voltages on the CRT.

At TP1486 the dim scope voltage ranges from 14 to 94 volts as the intensity control is adjusted, and compares to the good scope measured at 16 to 95 volts, (both changing with intensity control position).

The CRT negative voltages are:

good scope Dim Scope

Cathode 2450 2440

Grid 2495 2505

Vkg -55 -35

Grid range
with intensity 2526 - 2461 2526 - 2450
change
(min to max)

High voltage 12,960 V 12,860 V

I've also tied the grid to the cathode and it doesn't get any brighter.
I'm new to the vintage scope repair, but finding it great fun and like solving a puzzle. This is a great group and resource. I've learned
lots from reading the messages.. So I'd appreciate any advice before proceeding with a CRT replacement. I'm planning to put used CRT in it and have
never replaced one. So, I'd appreciate any tips on how to do it. I've found the manual replacement procedure.

Kindest regards,
Tim

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