Electronics > Projects, Designs, and Technical Stuff

Strange Resistor network, how to identify its topology

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hitech95:
Hi,
TLDR version:
I have the following circuit, it have a some sort of resistor devider that I'm not able to understand what is it,
and how is it connected inside to properly reverse engineer the circuit.

Long version:
I'm working on a alternative control circuit for a Tape Deck of my old mini system. (ASP/volume control and amplifier, CD, RADIO are all broken, the tape drive is the ony partially salvage part of it)
The tape type, record protect, tape presence is read by some micro switches that are connected to a resitor network and via an ADC I can read the state.

What is driving me crazy is identifiing the resistor network component internal topology.

I've applied 5V and GND to its last pin as by schematic and I have identified some sort of resistor ladder, but after a lot of measurements i cannot figure out how is it topology:

PIN 1 is VCC (4.998V)
PIN 3 is a button (4.956V)
PIN 4 is a button (4.942V)
PIN 5 is a button (4.907V)
PIN 6 is a button (4.836V)
PIN 7 is GND (0V)

Then I've started on measuring its resistance between pins:

1>2: 27K ish
1>3: 4.5K
1>4: 4.5K
1>5: 4.5K
1>6: 4.5K
1>7: 77K

2>3: 26.85K
2>4: 29.12K
2>5: 30.06K
2>6: 30.61K
2>7: 92.95K

3>4: 8.75K
3>5: 8.84K
3>6: 8.94K
3>7: 80.64K

4>5: 8.75K
4>6: 8.86K
4>7: 79.63K

5>6: 8.75K
5>7: 78.53K

6>7: 73.84K

So far I'm simply mapping each of the 16 states to its switch combination inside the mcu, but the threshold from the ADC are so near that sometimes a switch is cofused with another one.
So probly there is a better/simpler way once the resitor magic is resolved.

daqq:
Assuming it's a resistor network only, you can adapt this kind of measurement: https://www.daqq.eu/?p=1333

tooki:
Based on the part number, I’m fairly confident it’s a Murata RGSD series. The “12” means 12 resistors, and the A means “custom”. The 1445 is then an internal design number.

So unless the deck’s schematic tells you what’s inside it, it’s going to be difficult to find out.

tooki:
Ok, I think I found it. By googling “ic971 service manual”, under the assumption that companies often reuse schematic blocks to save time, I found the service manual for a Technics deck that uses the same configuration. Lo and behold, for the same component designator Z971, it has the internal schematic of the resistor network (but no part number!). The signals line up, as does the pinout. Without having done any calculations, I think the values look like they might align with your measurements. And the number of resistors inside the network also agrees with the part number in your deck.

tooki:
As for the values coming out of the AD_OUT pin, bear in mind that some combinations shouldn’t ever exist. For example, the “half” switch (which from other schematics I’ve seen seems to be nothing more than a cassette presence detector) should always be closed when a tape is inserted, so the combinations with the half switch open and the record protect switches closed shouldn’t ever be encountered. I dunno what the mode switch does.

P.S. Why oh why do people share schematic/service manual snippets without sharing what model the device is? With that, one could easily just look up what those switches do…