Telecom? Thick film resistor networks with a weird component in a blob

[Victorman222]

Hello everyone,
Does anyone have information about these resistors? They seem to be thick film resistor networks.

There is a white blob which hides a tiny component. The tiny component measures 0.02 ohm. When removing the potting i noticed that the component is very soft so i thought it might just be a solder blob. I tried desoldering it and it melted furthering the theory that it's a solder blob..

Here are some measurements of the individual resistors of two such networks in ohms. FL means front left resistor, BL - bottom left and so on. Front side is the one with the blobs.
FL: 1)41.268  2)41.272 BL: 1)41.256 2)41.255 FR: 1)41.242 2)41.251 BR: 1)41.272 2)41.280
Seems rather precise.

If anyone knows about what these networks were used for or what kind of performance can be expected of them, I'd love to hear it. Thanks!

[Poroit]

G'day Victorman222,

They may have originated from this company:

https://cmac.com/solution/ceramic-printed-circuit-boards/

[amyk]

They look similar to the ones found in high-precision benchtop DMMs.

The "blob" could either be a fuse or for carefully trimming the final value.

[golden_labels]

They remind me very much of Telpod’s components. Vertical format, pins shape and how they’re mounted, mask color — identical. The company has some lines of products, but also designs and manufactures components according to customer’s specs. There is also geographical proximity between you (Ukraine) and the company (Poland). I don’t know when they started to produce component in that format, but the company exists (as a part of Unitra) since early 60s, and their parts were ubiquitous in Poland and other communist states.

[Victorman222]

Hi, Poroit. Thanks! It does seem that C-MAC is their maker, but sadly they don't have a datasheet for their capabilities on the fancy looking website.
I asked the seller what are these from and they said that it's Lucent stock. Searching ebay for Lucent and there were boards with similar looking hybrid resistor networks even complete with some blobs on the edges. See picture attached. I wonder what they are used for.
From https://www.ebay.com/itm/123048670503

[srb1954]

They could be part of a protection network for a POTS telephone line interface. These resistor networks, in conjunction with clamping diodes or TVSs, limit the peak currents from line surges etc from damaging the line interface circuits. The components under the white blobs might be thermal fuses, with a low melting point alloy, to disconnect the networks from the telephone line in the case of a prolonged high voltage such as might occur if a power cable shorts to an overhead telephone line.

The resistor values have to be reasonably precise as any variation between the resistance value in the 2 legs of a telephone line will result in degradation of the balance of the line leading to increased noise pick-up and cross talk. It looks like the resistor elements have been laser trimmed to achieve the desired accuracy.

[Analog Kid]

It looks like the resistor elements have been laser trimmed to achieve the desired accuracy.

[golden_labels]

The patent, which is so prominently advertised on the circuits on the most recent photo, explains their function.

[Gyro]

The patent, which is so prominently advertised on the circuits on the most recent photo, explains their function.

A good example of when it is a really bad idea to just clean the flux off - when it is an integral part of the 'mechanism'.

[tszaboo]

POTS sometimes have high voltage on the lines. Remember these phones received the power from the phone line, and they had to ring. Ringing old times were solenoids hitting bells with hammers. From the top of my head, 70-100V is easily on these lines. Depending on how these resistors were connected, they might need to dissipate a bunch of power.
And then there were the different fault conditions, for example that are caused by construction workers, 100% of the time reliably cutting through the lines half an hour after they hear where the phone lines are. And random people randomly connecting everything, including 230V to these lines.
Using solder as a fuse on a highly conductive ceramic substrate is... smart I guess?

[golden_labels]

A good example of when it is a really bad idea to just clean the flux off - when it is an integral part of the 'mechanism'.

Hey, at least we now know it works. The solder melted and, as claimed by the patent, cleanly moved to the pads!

That fuse is tested and test passed!