Strange Resistor network, how to identify its topology - Page 1
Forum section: electronics / projects

author: hitech95
author_rank: Regular Contributor
timestamp: September 25, 2021, 04:02:15 pm
content: 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 2 is ADC_OUT (4.620V)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 ish1>3: 4.5K1>4: 4.5K1>5: 4.5K1>6: 4.5K1>7: 77K2>3: 26.85K2>4: 29.12K2>5: 30.06K2>6: 30.61K2>7: 92.95K3>4: 8.75K3>5: 8.84K3>6: 8.94K3>7: 80.64K4>5: 8.75K4>6: 8.86K4>7: 79.63K5>6: 8.75K5>7: 78.53K6>7: 73.84KSo 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.

author: daqq
author_rank: Super Contributor
timestamp: September 25, 2021, 05:45:46 pm
content: Assuming it's a resistor network only, you can adapt this kind of measurement:https://www.daqq.eu/?p=1333

author: tooki
author_rank: Super Contributor
timestamp: September 26, 2021, 03:20:27 pm
content: 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.

author: tooki
author_rank: Super Contributor
timestamp: September 26, 2021, 03:44:03 pm
content: 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.

author: tooki
author_rank: Super Contributor
timestamp: September 26, 2021, 03:55:44 pm
content: 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 switchopenand the record protect switchesclosedshouldn’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…

author: Terry Bites
author_rank: Super Contributor
timestamp: September 26, 2021, 04:36:50 pm
content: Terrible photo mate. Tryhttp://kirr.homeunix.org/electronics/resistor-network-solver/

author: tooki
author_rank: Super Contributor
timestamp: September 26, 2021, 05:26:28 pm
content: Quote from: Terry Bites on September 26, 2021, 04:36:50 pmThe issue was that they didn’t know the internal topology of the network. That tool can’t help with that.

author: hitech95
author_rank: Regular Contributor
timestamp: September 27, 2021, 10:55:00 am
content: Quote from: tooki on September 26, 2021, 03:20:27 pmYea, i've decoded that too, thats why I asked if someone knew what it was.Quote from: tooki on September 26, 2021, 03:44:03 pmWow you did it! My deck is from a Panasonic Mini System so I searched only on Panasonic Service manuals...SA-PM28ESA-PM28EBSA-PM28EGQuote from: tooki on September 26, 2021, 03:55:44 pmAbout the states I thought that too after writing some more code.TheMODEswitch is closed when the head is in contact with the tape so when the mech in engaged.Quote from: tooki on September 26, 2021, 03:55:44 pmYou're absolutely right, Ive forgot to add the link on top post to the original service manual. I generally add all the possible references.

author: tooki
author_rank: Super Contributor
timestamp: September 27, 2021, 04:47:35 pm
content: Panasonic and Technics are both brands within Panasonic Corp (what used to be Matsushita Electric), so it’s not surprising they’d share designs under the hood.

author: hitech95
author_rank: Regular Contributor
timestamp: September 29, 2021, 11:53:17 am
content: Quote from: tooki on September 27, 2021, 04:47:35 pmGot that, I've updated my thresholds and now it is a bit better, I still have to figure out how the magnetic pickup work. There is some transistor circuit that I have to control for Playback and Recording.At least now I can correctly move the mech to PLAY, FF, REW and STOP positions.

author: tooki
author_rank: Super Contributor
timestamp: September 29, 2021, 08:26:41 pm
content: Are you reusing parts of the existing circuitry, or are you making the audio signal path from scratch?

author: hitech95
author_rank: Regular Contributor
timestamp: October 01, 2021, 04:51:26 pm
content: I'm using the wholle deck circuit. I'm just using my own amp and MCU to control the deck.I don't have experiece with cassette tapes in general I just wanted to make something out of my iold mini system.I'm wondering what REC and BP signals are:

author: tooki
author_rank: Super Contributor
timestamp: October 01, 2021, 06:25:36 pm
content: Do you mean you’re having trouble identifying the audio in and out signals from the tape deck? Or are you asking something different?

author: hitech95
author_rank: Regular Contributor
timestamp: October 02, 2021, 09:53:28 am
content: Quote from: tooki on October 01, 2021, 06:25:36 pmPlayback and Audio signals are on pin 1,2 (rec) and 4,5 (playback) of CN1304.I dont understand what Pin 9 and 10 on CN1303 are for. I think it is to enable the erase head and to apply the bias or something to the playback head.Watching Q1305 Q1303 it sems that thoose are simple digital signals.

author: tooki
author_rank: Super Contributor
timestamp: October 02, 2021, 02:42:49 pm
content: Quote from: hitech95 on October 02, 2021, 09:53:28 amI'm not certain either. It seems very likely that REC enables the bias signal (used both for the record head and the erase head). BP seems to refer to something called "Beat Proof", but I have no idea what that is*. If you follow the outputs of those two transistors, they go to 1) what must be the bias oscillator, plus the switch (IC1000) that enables bias, and 2) the beat proof level switch (Q1317) that does… something.It should be easy enough to observe the REC and BP signals to see when they're enabled. I'd just make sure to test (at minimum) in both playback and record, with regular and chrome tapes. My guess, based on the info below, is that the BP line is controlled (via the MCU) by a setting on the radio tuner, and that if one is not recording from AM radio, that its position is completely irrelevant.*Googling "cassette beat proof" led me to some Panasonic manuals, likeone for the RQA-160portable cassette recorder, which has Beat Proof on a mode knob. It says on page 12:Quote

author: hitech95
author_rank: Regular Contributor
timestamp: October 08, 2021, 10:06:45 am
content: Quote from: tooki on October 02, 2021, 02:42:49 pmUnfortunatly the unit was flooded and the only part that was fine is the deck. So I have no original circuit to test it out and mimic the original signals.

author: tooki
author_rank: Super Contributor
timestamp: October 09, 2021, 06:02:47 pm
content: Unless you plan to record from AM radio, it sounds like you can ignore the BP line. I’d probably just tie it high or low to keep it stable.The bias signal will definitely be needed during recording.