Old varistor code interpretation

[RickEev]

While inspecting a oscilloscope, a Leader LBO 508, with a CRT that suddenly went dead, I noticed some cracks in the varistors.  See photo.

I assume this is not a "good thing" and plan to replace them, but with what?  The Marcon part numbers are close to the current varistor codes, but they have their differences.  From my research (I first thought they were ceramic capacitors), the TNR appears to be a MOV type,  681 is 680 volts, K is 10%, and then things get vague.  I assume 015A is some multiple of 1.5 amps, and maybe 70116 is a date.

Some questions that rise to the top are:

• Will any MOV work or is TNR a special type of varistor?
• Is 680V specification for DC or AC?  I assume DC, since that is what is used in the circuit.

The real question is why they went bad.  They are suppose to protect for surges but I don't know if they were protecting anything or just got cranky in their old age.  I don't see any other burnt components.  Any ideas?

[drussell]

From my research (I first thought they were ceramic capacitors), the TNR appears to be a MOV type,  681 is 680 volts, K is 10%, and then things get vague.  I assume 015A is some multiple of 1.5 amps, and maybe 70116 is a date.

The 15 is the series size (in mm) and the type numbers are more often written the other way around these days like this photo of a later Marcon specimen:



The 681 is the specification for "nominal varistor voltage at 1 mA", so in this case 680 volts.  (Well, actually, according to the datasheet 612-748 volts, nominally 680.)

The specs from the datasheet say:

Maximum continuous applied voltage ACrms: 420 V
Maximum continuous applied voltage DC: 560 V
Peak current 8/20 uSec: 4500 A
Maximum energy 2 mSec: 90 J
Rated wattage: 0.6 W
Clamping voltage: 50 A, 1110 V
Typical capacitance @ 1 kHz: 340 pF
Varistor voltage @ 1 mA DC: 680 (612 - 748) V

MOVs generally suffer from degradation over their lifetime due mainly to total surge power clamped over their lifetime, be that slow degradation from repeated small surges or large spikes causing instantaneous destruction.  If these are just in the power supply input circuitry for clamping of the mains input, any similar appropriate MOVs should be suitable.

[drussell]

Here is just a small sampling of potentially appropriate replacement examples:

4.5-10 kA "680 V" MOVs in stock at Digikey

If they are used in circuitry other than just simple power supply surge/spike clamping you may possibly need to consider other parameters (like capacitance, for example) depending on what they are being used for in the circuit and how they are being applied.

[floobydust]

It's confusing you have another thread in repair on this. https://www.eevblog.com/forum/repair/leader-lbo-508-oscilloscope-no-crt/

The crack is from the MOV epoxy aging and shrinking. It does not mean the varistor failed unless moisture got in.

In the LBO-507, 508 the MOV's are used like zener diodes.
MOV D116, D117, are 1000V and used to limit the -2,000V rail.
MOV D118 is 1000V, D119 820V in series and used for control-grid bias.

So the 1mA DCV rating is important to match.

[RickEev]

Floobydust is right about this being a second posting.  I tried posting this in the repair section but got no traffic on it except for a suggestion to check the oil caps. 

As for the voltages used for the MOVs, I get
D116 & D117:  680 + 820 = 1500.  This matches the expected voltage at T5.
D118 & D119:  680 + 560 = 1240.

Zener Diodes and MOVs appear to preform the same function.  Let something leak out when the voltage gets too high.  Is there a reason to pick one over the other?  Durability?

From what I'm hearing about the use and wear and tear on the MOVs, I better start checking those caps for the real problem. 

Thx for the info.

[floobydust]

Zener diodes are common to 200V and practical up to a few hunded volts. MOVS are a way to make a HV shunt-regulator if you don't want to use 10 parts or need precision.
This LBO 508 service manual clipping is where I got the MOV ratings-  different than yours.

Yes those Marcon oil caps don't last:
https://www.eevblog.com/forum/projects/replacement-for-axial-oil-immersed-capacitor/
https://www.eevblog.com/forum/repair/leader-lbo-580-repair/msg1753184/#msg1753184

[RickEev]

I got those MOV values from the board.  I was wrong about the T5 tap being 1500V, it really is 2000V; as specified in the service manual.

My guess is that they are just tightening up the range where the shunting begins; most probably from experience.