The AM Forum

Central Electronics MM-2 scopes from '50s are beginning to lose power transformers because the caps in the CRT HV supply have begun to fail taking out the power transformer in the process.

These are big, beefy bathtubs, maybe 2" x 3" x 1", 1 uF at 1000V DC and .5 uF at 1500VDC, current rating unknown. Rather than try to find replacement caps (since mine still test fine), is it reasonable to put a high voltage fuse in transformer secondary? Does that make any sense?

A schematic would help visualize what's going on. I pulled this quickly off the internet:
 

Yes, that's it. Those caps on the left have become problematic. A friend who has worked on five of these said 3 out 5 had problems with the .5 @ 1500V but the 1 @ 1000 were all OK. That's a small sample size but I notice Bob Heil is advertising for a new transformer for his MM-2 even as I write this and others have reported the problem.

Why not a fuse, or maybe new caps and a fuse?

Hi all,

A fuse is a good idea, properly sized, and in the right place. The last two issues are the hard part.

It would not hurt to vacuum impregnate the transformer with insulating varnish, before it fails!

I'm attaching some recap stuff for the MM2. Info from Dave, WI6R.

Jim
Wd5JKO

I agree that protective varnish is a great idea for an old  transformer that is not replaceable.

High voltage film caps are also readily available.

There are fuses available for microwaves, etc that are good up to 5kv, but this looks like a small current, so a short circuit might not be much current.

If one assumes that the Voltage drops at the instant that a short occurs, one possible approach is to put a voltage divider in the circuit at the cathode of the 5Y3 and tap it low and feed it into a comparator that could control a dropout relay in the transformer primary. A simple OP Amp comparator feeding a flip flop driving a pnp BPT powering a small relay would likely work. Put a push button switch on the back to reset it and an RC on the comparator input would allow varying the time constant to get rid of nuisance transient shutdowns, etc.

The most disturbing thing about that schematic is the 1 Amp Slo-Blow fuse. They should really be called “slow death” fuses…
I have always hated those things because it might never blow if a high impedance winding like the 770vac HV winding shorted and it would allow the thing to slow cook to death.
Just a few cheap and dirty ways to protect the old circuits if one can stomach blending in some new technology…😉

Has anyone measured it via the drop across R73 the 100K resistor resistor in the diagram? Does the rectifier provide some inrush control? A fuse for that service would be delicate indeed.

Did some LTspice just now in its schematic and it gave the current from the power supply as 0.984mA

The CRT HV parts - defl plates and 2nd grid near the defl plates (pin 9) seems to use 0.268mA

The main divider starting with R6 the 680K resistor at the 1000V point abnd ending with R10 the 220K resistor attached to the negative 150V rectifier seems to use 0.713mA

LTspice result is probably close enough for hand grenades and horseshoes fuse selection.  Based on experience with many old scopes with the same simple type of voltage dividers and CRT elements.

But as with anything take with a teaspoon of salt!

So a 2mA slow fuse? If that is actually a reasonable thing. I've never actually measured the HV current in my MM-2 or considered inrush etc. Maybe I better change the caps though.

So if I am reading this thread right, there’s a paralleled section 5Y3 for a load of about a single milliamp? And of course if the cap shorts, that rectifier can deliver hundreds of mA without breaking a sweat. Loaded or shorted, it will drop some volts, like all tube rectifiers.  What happens if you replace it with a couple 1N4007’s in series, and a 25-50k resistor? At 1mA the drop would be reasonable, if shorted, the maximum current would be smaller.  Could the resistor be sized to protect the transformer yet still provide the HV?

Ed

Good points, but my shutdown circuit utilizing a voltage divider, comparator and latching circuit would be better yet. Many resist getting rid of thermionic rectifiers, however, even in ridiculous applications like this one….

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The 5Y3 was probably an economical choice at the time. Replacing with solid state plus a 200-500 ohm resitor befitting more of the 5Y3 characteristic would have the advantage of 15 Watts less load on the power transformer.

Even though the filter capacitor is small, the half wave cap input circuit still relies on a small pulse of high current each cycle to top off the cap. A 25K resistor before the capacitor might undesirably lower the voltage on the cap. A guess would be a reduction of 150V based on a 6mA peak current. That's a lot considering the HV is only 1KV plus the negative 150V supply.

The HV winding may have a fairly high DC resistance since it is of very fine wire. This possibly why and what gets hot and burns/shorts when the HV transformer is failing.

I agree an an electronic circuit would be a great way to protect the transformer.

Maybe the AC lead to the plates of the 5Y3 would also be a good place for a sensor. A small current transformer wound on a toroid could provide isolation. Just a different way of looking at an overload.