ICS Electronics 4861A PCB photo or schematics or some info on missing chips - Page 1
Forum section: electronics / testgear

author: 8008
author_rank: Contributor
timestamp: September 13, 2025, 04:18:14 pm
content: I bought for some bucks a GPIB controled analog/digital interface. It works in parts, but I was not able to read from analog inputs (it has 4 of those). Always, voltages around 5 volts were read, independent what was really on input lines.I have checked the PCB and am 99% sure that this thing has two missing chips.These are U17 and U22. One of those, I suppose U17, is the ADC. The purpose of U22 is unknown to me.I followed the traces from analog input pin. The 4x2 input lines go to an analog MUX (MAXIM DG509AC) and from there to pins 4 and 5 of U17 (if I have counted correctly). U17 has 16 pins. U22 has 20 pins.AD converter has (from 4861A datasheet) 12 bit resolution. There is no other AD converter like chip on the board.My hope is that someone has service manual or maybe a photo of a complete board. To get informationabout these 2 chips. My searches for service manual or schematics or even a PCB photo brought nothing up.Greetings,Dennis

author: Kean
author_rank: Unknown
timestamp: September 13, 2025, 06:20:02 pm
content: Can you check the model number on the bottom of the device? According to the attached datasheet, there were several models available and the 4861A-12 and 4861A-14 did not include the ADC inputs.I can't really help with identifying the missing ADC, but I suspect you are right about U17. There is probably some obvious candidates for a 12-bit ADC in 20 pin DIP, with probably a serial interface.Not sure why the MUX at U21 is populated without U17 & U22, bit it seems likely U22 is also related to the analog inputs.U19 and U20 appear to be opto-isolators for the ADC serial interface, and I'd guess the nearby SIL block surrounded by caps is an isolated DC/DC converter.The 4 x DAC circuits are pretty obvious, using a couple of 8-bit latches, and 4 sets of quad opto-isolators, a DAC IC, and some isolated power supply.DAC Ch 4 shares a ground with the ADC as mentioned in the datasheet, and can be seen by the ground fill.I suspect U40 is an optional MAX232 or similar for the RS232 options, possibly coming out on J4 (datasheet mentions 10-pin connector). I'd guess that the isolated RS232 option used a small daughter board that plugged into U40. The RS485 option was possibly handled by the missing SO-8 at U7, and nearby resistors handling termination & pull-up/down.I see there is one of these currently for sale on eBay with a -22 model number, so that would have the ADC option but only 2 channel DAC. Depending on how much this is worth to you, you could buy that to identify the missing components, and end up with two (or sell it again).https://www.ebay.com/itm/185105145510

author: 8008
author_rank: Contributor
timestamp: September 13, 2025, 06:50:12 pm
content: Ah, interesting info.I have the "P.C. board 114886 Rev 1" and "P.C. Assy 114884-24 Rev 1 ", see first attachment.And at bottom of the module, there is a sticker, I realize, this is the "-14" version.See 2nd image.So your info did exactly explain what's going on. I was confused by that "-24" on the PCB.No analog input, sadThanks for additional info on the board. The mentioned ebay article would be expensive, due to shipping to germany. I often have the situation that something interesting is offered on ebay-USA for lets say 100€, but delivery is 260€ or so.Anyway, if someone has some images on a -24 board, would be interesting.Thanks Kean & Greetings,Dennis

author: Kean
author_rank: Unknown
timestamp: September 13, 2025, 07:35:11 pm
content: I was actually wondering if those two ICs had been de-soldered. The pads looked a little strange in the photo, and all the other missing ICs pads are filled with solder. Likely wave soldered.So your PCB was previously a -24 but at some stage ended up in a -14 housing with those ICs removed. Maybe the result of a repaired or refurbished device.

author: Kean
author_rank: Unknown
timestamp: September 13, 2025, 08:32:59 pm
content: I was curious, so I went searching for an ADC that might fit the time period and footprint. It only needs a slow sample rate with the 200 sps spec (so at least 800 sps for 4 channels).No luck unfortunately. I was mainly looking at old Maxim and Burr Brown parts in DIP-20 with serial interface.I did realise a few additional things:- U16 appears to be another quad opto-isolator, probably for ADC and MUX control signals.- U19 & U20 might actually be logic output, single or dual opto-isolators (e.g. 6N137 or HCPL-2630). These would be faster than U16. U19 is probably data out of ADC. U20 is probably clock signal to ADC.- The TO-92 style transistor-like part near the handwritten "13157" might be an external fixed voltage reference for the ADC.- Jumpers W2, W3, W4 are all set to UNI. These would be for selecting uni-polar or bi-polar inputs.- The full scale ADC setting (10V, 1V, 0.1V) is presumably selected by some of the opto-isolated control signals (and the AR command).- U22 has a lot of capacitors at its pins, and this will be relevant to its function. Possibly involved in the ADC sampling.- I expect that most of the parts in this section wouldn't normally be populated in a -12 or -14 unit.

author: 8008
author_rank: Contributor
timestamp: September 14, 2025, 07:11:29 am
content: I will look closer to the board chips and traces today to get the pinout of the two missing chips, at least in parts.When asking ChatGPT, for a 12 bit pdip with 16 pins ADC that was available in 2006 and before, I got one proposal that might fit:Texas Instruments ADS7812It has Input lines around Pin pin 4 and pin 5. These pins are not AIN+ and AIN- but something else ("R3IN" and "BUF", but BUF is connected to "R2IN" (which is pin 3) in the example figures from datasheet.But if the ADC is in U22, its different. I will try to find out some pins by looking at the board and come back with that information later.Small checks:* U19/U20 are 2630* Close to U19/U20 is the power converter HPR106 creating isolated 5 volts.* U40 is connected to J4, so yes a serial connection chip* The TO-92 style transistor-like part near the handwritten "13157" has "LM293" on it, but is has only three pins, strange* I verified that output of the MUX go to pin 4 and 5 of U17. These are Analog+ and Analog- Inputs then

author: Kean
author_rank: Unknown
timestamp: September 15, 2025, 02:00:11 am
content: Quote from: 8008 on September 14, 2025, 07:11:29 amYes, that is one I also looked at, but it didn't seem like a good match.Quote from: 8008 on September 14, 2025, 07:11:29 amOK, so my guesses seem pretty good.If U19/U20 are HCPL-2630 they will be dual opto-isolators.The TO-92 part might be LM2931 or LM2936?

author: 8008
author_rank: Contributor
timestamp: September 15, 2025, 06:13:24 am
content: I have checked U22. This is not the ADC, maybe it is just a OpAmp like AD743 (also 16 pin, but few are used)Some pins could be identified.U22=====Pin Meaning Connection on PCB1 N.C.2 N.C.3 N.C.4 Input AIN5 Input AIN6 N.C.7 N.C.8 V- connected to pin 3 DG509ACJ=U21 -15.3 volts9 N.C.10 goes to upper groundplane11 connected to 12; goes to UNI jumper W312 connected to 11; goes to UNI jumper W313 V+ connected to pin 14 of MUX DG509ACJ=U21 +15.4 volts14 goes to upper groundplane15 connected to 16; goes to opto coupler U1616 connected to 15; goes to opto coupler U16It looks like U17 with 20 pins is the ADC. And U22 is only divider(?) for 0.1/1/10 volts?LM293...: There is also a "1" so maybe this is a LM2931.2630: There is only manufacturer logo (f - Ferranti?) on it and number "2630". I can check the pins there too, needs some timeU17 pins will be checked next.Dennis

author: 8008
author_rank: Contributor
timestamp: September 15, 2025, 06:48:06 am
content: I tried chatgpt again for 1/2 hour and got one really good guess for an opamp with these pin setting:Burr-Brown / TI PGA204 / PGA205 (programmable-gain instrumentation amplifier) im 16-pin PDIP.Pin Name / Function Description1 NC Not connected / reserved2 NC Not connected / reserved3 NC Not connected / reserved4 VIN− Analog negative input5 VIN+ Analog positive input6 NC Not connected / reserved7 NC Not connected / reserved8 V− Negative power supply (here −15 V)9 NC / Ref? Optional / not used on board10 GND Ground plane11 VO Output12 Sense / Feedback Connect to Pin 11 for proper operation13 V+ Positive power supply (here +15 V)14 GND Ground plane15 A0 Digital address line (e.g., for gain selection)16 A1 Digital address line (e.g., for gain selection)

author: Kean
author_rank: Unknown
timestamp: September 15, 2025, 07:19:33 am
content: Yes, the PGA204 seems a good match for U22 and the unit specs. Good find.Also the position of the capacitors surrounding it match well with voltage pins (tantalum caps) and maybe for input/output filtering (smaller monolithic caps).Are you sure that pins 15 & 16 are connected though? Seems weird they would just allow 1x and 1000x gain.I guess the PGA205 with 1x & 8x might work with 10V, 1V, and 0.1V ranges if they do additional software scaling.Using two opto-isolators for for selecting 1x, 10x, and 100x gain make more sense.I am not quite sure what the UNI/BI jumpers are doing, but it will change the input connections to ADC to allow full-scale 12-bits in unipolar mode.I am curious about the long block between the DAC circuits and J2 62-pin I/O connector. I presume it is a multi channel isolated DC/DC converter. Can you see a part number?

author: 8008
author_rank: Contributor
timestamp: September 15, 2025, 07:56:05 am
content: Kean, you are right. My first table was wrong for pin 15 and 16, these are NOT connected. Correct is:U22=====Pin Meaning Connection on PCB1 N.C.2 N.C.3 N.C.4 Input AIN5 Input AIN6 N.C.7 N.C.8 V- connected to pin 3 DG509ACJ=U21 -15.3 volts9 N.C.10 goes to upper groundplane11 connected to 12; goes to UNI jumper W312 connected to 11; goes to UNI jumper W313 V+ connected to pin 14 of MUX DG509ACJ=U21 +15.4 volts14 goes to upper groundplane15 goes to opto coupler U1616 goes to opto coupler U16

author: 8008
author_rank: Contributor
timestamp: September 15, 2025, 08:08:32 am
content: And first finding for U17, not much so far:U17=====Pin Meaning Connection on PCB12 goes to upper groundplane34567 goes to upper groundplane8 goes to upper groundplane910 goes to upper groundplane11 N.C.121314 goes to upper groundplane1516 goes to upper groundplane1718 goes to upper groundplane19 connected to 20; connected to pin 14 74HCT14=U18 +5 volts20 connected to 19; connected to pin 14 74HCT14=U18 +5 voltsI suppose it is a serial output ADC. And inputs are in area pin 1-3. These are wired in direction of those UNI/BI jumpers...Have added photos of that large block thing, LM2391? 2630 and area of the UNI/BI jumpers

author: Kean
author_rank: Unknown
timestamp: September 15, 2025, 09:18:04 am
content: OK, making progress.The F logo on the ICs is for Fairchild, later acquired by ON Semiconductor. Some of their parts are obsolete, but some still sold under the Onsemi brand.LM2931AZ-5.0 is a LDO regulator. I wonder what it is powered from - maybe the 15V rail from the DAC Ch4. This is probably less noisy than the HPR106 5V output.HCPL-2930 is dual logic-output opto-isolator. These will be handling the ADC clock and data signals. U19 from ADC to CPU (data), U20 from CPU to ADC (clock).That quad output isolated DC/DC converter block is certainly interesting. I've seen single bipolar ones before, but never seen a quad bipolar one. Made by C&D Technologies, acquired by Murata.HPR106 is 12V, 5V 150mA out.HPR211 is 12V in, 4 x +/-15V 25mA out. Very handy for this application.Datasheet for HPR2xx seems to have some errors, such as the column titles in the electrical specifications, plus terrible formatting.Datasheets attached.Edit: Apparently the HPR isolated converters were also made by Burr Brown, which later became part of Texas Instruments - same as the PGA204, and quite probably the ADC will be a Burr Brown part.

author: 8008
author_rank: Contributor
timestamp: September 15, 2025, 09:31:50 am
content: Great infos, thanks Kean. This piece of hardware contains a lot of engineering effort. That Zilog Z180 MPU which controls everything wasn't even mentioned so far :-)I have checked the three features it provides over the weekend and everything works quite well (digital IO, analog output).Maybe analog output should not exist in the 4861A-14, but my special (as you guessed maybe refurbished) unit has it.Meanwhile I sat down and tried to extract the schematics part with the ADC. I am not good in doing this and have failed miserably in the past... so it surely has errors.Schematics in attached image.

author: Kean
author_rank: Unknown
timestamp: September 15, 2025, 09:54:01 am
content: Oh, a Z180 - very nice. I was going to ask about the CPU.And I may have a possible candidate ADC - the Burr Brown/TI ADS7808.It is 12-bit, 100kHz sampling, in narrow DIP20. And it can handle +/-10V and 0-10V input ranges (plus others).Many of the pins make sense against your preliminary schematic, but not all. I'm sure you can review further.The analog input are going to the right group of pins, and the opto-isolated pins match up with the DATA, DATACLK, CS, R/C, and BUSY pins.The ADS7808P is obviously quite an old part, but it is still available from a few sellers on eBay.Edit: Oh, and the PGA204BP (or PGA204AP slightly lower spec) is still available from Mouser or RS but it isn't cheap! Some cheaper ones on eBay from China... but may be suspect.Datasheet attached.

author: 8008
author_rank: Contributor
timestamp: September 15, 2025, 11:18:56 am
content: This looks perfect, Kean. I will check the datasheet with some more depth to make sure. But from first glance, it looks like the one they've used.There is also the ADS8508, with same pin numbers and description, I also will check, maybe there are differences in some details…For replacement, I've checked Digikey and Mouser.Digikey: No ADS 7808, but ADS8508 in 20SOIC package, and PGA204AP (24+13€ + 18 shipping = ~55€)Mouser: ADS 7808 in 20SOIC, PGA204BP (84+28, free shipping, so ~112€)ebay cheapest wan-han-lo brand: 20+12 = 32€(€=~US$)If ADS 8508 would also work, Digikey would be ok. In all cases, I would have to do some soldering to fit the SOIC onto aDIP socket.Thanks so much for looking into this issue, I did not expect to find a solution for these missing chips. Despite it could fail, and require even other chips, it's worth the try.Dennis

author: Kean
author_rank: Unknown
timestamp: September 15, 2025, 12:22:01 pm
content: The ADS8508 looks like it will very likely work, but it is hard to be sure without knowing implementation specifics.The two differences I spotted are the conversion time (& thus max sample rate), and the input impedance.I doubt the timing will be a problem unless the firmware was poorly implemented, and the lower input impedance should be OK as you have a PGA driving the input.If you look at either ADC datasheet, the example input circuits for 0-10V (UNI) and +/-10V (BI) ranges are the same.You could try confirm if the jumpers W2-W4 reconfigure the connections for those three Dale resistors and the ADC input pins in the same way.Edit: I compared your schematic with these input configurations, and they seem to match except I think you may have UNI/BI positions swapped on W3 & W4. I was confused about W3 until I realised you need to move two individual shorting jumpers so as to leave a space at one end or the other of the 5-pin header.

author: 8008
author_rank: Contributor
timestamp: September 15, 2025, 01:13:41 pm
content: You're correct, by flipping the board I was confused.I have attached corrected schematics, fixes are in yellow.The jumpers are for "10 volts unipolar" case.The datasheets schematics for that is also appended.There is one difference that is able to break functionality, if wrong chip is used. This is the size of digital samples. ADS7808 delivers 12 bit samples, ADS8508 delivers 16 bit samples. ADS8508 is sending two unneeded zero bits then for each sample.This can be seen in attached screenshot 3Maybe I start with ADS7808, because my impression is, it is an older design and fits more to the design age of ICS 4861A (unfounded guess).

author: 8008
author_rank: Contributor
timestamp: September 15, 2025, 02:16:41 pm
content: I was able to order PGA204BP from a local distributor for ~30€. Besides Intel and AMD processors this is the most expensive chip I have ever bought.ADS7808 is in progress.Have soldered in already sockets for both chips.There are two small modifications on soldering side of PCB, which may harm or not.* +5 volt rail was opened with some grinder tool between ADC pin 20 and the DAC section of the PCB. Maybe donealready in the factory, or later when removing U17/U22. I guess I have to find out if that will be an issue.* For U19, a HCPL-2930 opto isolator, two pins are bridged. These are pin 7 and 5=GND. Pin 7 is output pin for LED at input pins 1/2, this is connected to /BUSY signal of ADC (pin 17). This means after that change U19 always signals a "BUSY" state to the controller. Maybe this small green wire needs to be desoldered as soon as I have inserted U17/U22.

author: Kean
author_rank: Unknown
timestamp: September 15, 2025, 02:26:01 pm
content: I suspect this is an error, where they have incorrectly copied from the ADS8509 datasheet. The ADS5809 is the 16 bit version, and was first created a year before the first ADS8508 datasheet.Both ADS8508 and ADS8509 datasheets list the data format as 16 bits, but if you look closely at the timing diagrams the former shows 12 clock pulses with D11-D0, and the latter shows 16 clock pulses with D15-D0. Plus two additional clocks when using SYNC, but that doesn't apply.Similarly there is an error in the ADS7809 datasheet which is the 100ksps 16-bit version, where in its datasheet Fig 3a shows a Bit 15 but at the top the clock pulses count to only 14 which is the same as in the ADS7808 datasheet ( again, 2 extra clocks due to use of SYNC). Fig 2 in both is correct and shows a count of 12 of 16 bits.So, I cannot be 100% certain but this does looks like a documentation error (or several). Another error I saw was the formatting of the VANAspec for the ADS7808, but correct in the other three.If you can get a ADS7808AP, BP, P, or PB at a reasonable price, then I would do that as I agree it is less risky.The newer SOIC-20 parts ADS7808U or UB also seemed to be more expensive, and would be a bit painful to adapt to DIP-20 due to the wide SO package.The B versions are going to be more expensive due to improved spec. I am not sure why they changed from AP and BP to P and PB suffixes at some point.For the ADS8508 there only seems to be a B version (actually IB) and SOIC-20.

author: Kean
author_rank: Unknown
timestamp: September 15, 2025, 02:30:19 pm
content: Good thing you spotted those mods.I am not sure about the cut in the 5V trace... it could be a standard "fix" to the design (lower ADC noise using the LM2391 for 5V), or just done when the ADC is removed.I think the bridge on the BUSY signal is to avoid the firmware hanging on ADC reads when the ADC is missing.

author: 8008
author_rank: Contributor
timestamp: September 16, 2025, 11:59:23 am
content: Have also ordered ADS7808P now, from local distributor for few bucks (supposedly chinese brand).For trying it out, I hope it is good enough...Both chips are in required PDIP package, so no complicated soldering.The delivery will take some days and I will follow up here when the parts have arrived.

author: 8008
author_rank: Contributor
timestamp: September 20, 2025, 11:00:49 am
content: The chips arrived, I've inserted them to the sockets already soldered in days ago. Furthermore I've removed the green wire mentioned above.Result looks very good. I can read data from analogue inputs, and range selection also works.It is possible to read about 50 times per second, one acquisition takes about 14ms over GPIB bus.Checking the input OpAmp, I can see that with a frequency of 150 Hz, all 4 analog inputs are read in in a loop. This loop runs even when not accessing the inputs via commands.I've attached a graph showing input voltage drift over 30 minutes, +5.00 volts come in from a Sorensen Xantrex XT7-6 PSU. The voltages delivered are ~10mV too high, compared to an attached HP3455A. But the ICS 4861 device is uncalibrated. It is possible to calculate correction values and store them permanently in EPROM, I saw that in the little documentation that I have.I will play more with the 4861 this weekend to check if the issue is really fixed.