SCHEMATICS - Page 1
Forum section: electronics / general

author: Sagat
author_rank: Contributor
timestamp: December 17, 2012, 04:20:26 pm
content: Hi guys! Can someone help me with the schematic s for Cheetah Masterseries 7P controller keyboard?Im trying to repair it... i ve checked all the caps, rezistors, diodes... but something tells me that the Cmos are the one to blame...Does anyone have the schematic s for it?Thank you!

author: jeroen74
author_rank: Frequent Contributor
timestamp: December 17, 2012, 11:12:37 pm
content: I still remember those, my father used to sell them in his computer shop long ago.Wasn't this just a simple Z80 based system without any fancy ASICs? This being a fairly obscure product I don't think there are any service manuals available.Maybe you can post photo's of the innards?

author: Sagat
author_rank: Contributor
timestamp: December 18, 2012, 02:11:24 pm
content: Yeah. That s the one. A 8bit CMOS Microcomp( M50734SP), a 8bit CMOS STATIC RAM(HM6264P-10) , a NMOS WITH EPROM( M27128AF1). I havent replaced all the bad rezistors yet, the values of VCC, VSS on these chips are all over the place, low input high output and so on. Due to the way that this board was build(lots of vias), and oxides im having a dificult timp without the schematics.BTW:Im having an issue in finding rezistor value: it has only two stripes GREY(larger) and GREEN(smaller). Now i think im seeing a third colour WHITE (im not sure), in this case is simple 85000Mohms. Not sure about the third colour, so what value has it with only GREY an GREEN? THX.

author: alanb
author_rank: Frequent Contributor
timestamp: December 18, 2012, 02:29:08 pm
content: QuoteWhy do you think there are bad resistors?QuoteVery unlikly to be a resistor with such a high value. Either its a different component or a non standard colour code.

author: Sagat
author_rank: Contributor
timestamp: December 18, 2012, 02:47:42 pm
content: I have mesured all the rezistors and i have higher or very low values. I have taken the rezistors out the board for measurement.Another component? The board was manufactured in 1989. Thats why i need the schematics.

author: Astroplio
author_rank: Regular Contributor
timestamp: December 18, 2012, 03:03:26 pm
content: Wait a minute, you decided to replace all the components on the board until you find the problem?That's illogical!Please post a picture (or several) of the board (both sides) so we can see the layout. You can understand a lot just from the placement of the components.Also important is to DESCRIBE the problem! You said you want to repair it, what was wrong with it?If it full of oxidization of the components leads, copper traces etc, then you first have to address that.If you are trying to measure resistors with oxidized leads, you are not going to get any good results, (plus I don't know how much you can trust your meter; what model is it?) You need to get under the oxidization...The whole thing sounds frustrating, better tell us the problem, maybe I or somebody else can point you to the right direction, really there is no point going in blind changing every passive component on the board, that is ridiculous!

author: alanb
author_rank: Frequent Contributor
timestamp: December 18, 2012, 03:09:32 pm
content: QuoteNot only is it illogical, in Sagat's first post he claims to have checked the Caps, resistors and diodes!

author: Sagat
author_rank: Contributor
timestamp: December 18, 2012, 03:34:35 pm
content: Yes i have checked them.. but i didnt say that they are all ok. I find a few rezistors that are out of the range ( where is the problem in replacing them?). Im not replacing all the components ( that is madness). The board was improperly stored, i have found oxides on every pin and pad, not to mention about vias. How do i place a picture here?The problem: the lcd display doesnt start. Is the first step. As for the nmos or cmos.. will see after.

author: jeroen74
author_rank: Frequent Contributor
timestamp: December 18, 2012, 03:38:02 pm
content: The oxides won't affect the soldered connections and vias, but it might affect the connectors of which I'm sure it has a few of them.

author: Sagat
author_rank: Contributor
timestamp: December 18, 2012, 03:52:00 pm
content: Yes. There are a few board to board conectors, but i ve cleaned them. I ve found a few traces missing, i ve replaced them.

author: Sagat
author_rank: Contributor
timestamp: December 18, 2012, 04:19:21 pm
content: Here are a few photos... pure vintage

author: Sagat
author_rank: Contributor
timestamp: December 18, 2012, 04:57:59 pm
content: Larger...i think

author: SeanB
author_rank: Super Contributor
timestamp: December 18, 2012, 05:43:03 pm
content: The battery has definitely dropped it's guts all over the board, you probably will have to trace each track out in that area and check them all for continuity. As the PROM next to it is likely the system prom if it has missing data or address signals to it it will do odd things, you probably will have to change the socket to it.

author: Astroplio
author_rank: Regular Contributor
timestamp: December 18, 2012, 05:58:28 pm
content: To continue what SeanB said, yeah, remove that on-board battery carcass and clean the crime scene.Also for the power up issue, what do you use as a power supply?What voltage do you measure on the input and output pins of the voltage regulators (TO-220 package mounted on the heatsink)?I checked online, it says that this model requires 9V input and 1A, is this true?

author: SeanB
author_rank: Super Contributor
timestamp: December 18, 2012, 06:38:27 pm
content: Anything from 8 to 15v will do ( depending on how big a heatsink you have on the 5V regulator more than anything else) and current will probably be between 600 to 900mA for the amount of TTL and logic family on the board.

author: Sagat
author_rank: Contributor
timestamp: December 19, 2012, 06:27:08 am
content: Under the battery is already cleaned. Was a real mightmare.. traces were simply missing. I ve also changed the socket of the PROM ( real pain).Yes 9v and 1A. The Vin of the voltage regulator is 8.99V and the Vout is 5V ( Vreg also changed).

author: SeanB
author_rank: Super Contributor
timestamp: December 19, 2012, 03:38:45 pm
content: Then you are stuck to checking the Z80 data pins for activity. you care more for the lines all having activity and are not stuck or floating. address lines should have change on the bottom ones (A0 to probably A10) while the higher lines can have little activity. Check the crystal is actually working and has the right frequency, these old crystals are rather prone to getting stuck, though often they will work again if you turn them around........

author: Sagat
author_rank: Contributor
timestamp: December 19, 2012, 03:48:35 pm
content: Well i dont have an osciloscope. And due to the number of vias(few popup under components) is very hard to test for continuity or voltage.The VCC for the Ncmos eprom, microchip and the other cmoc is 5.0V, but for the other components such as 74LS374N is 8.99V(Vcc 5V)

author: jeroen74
author_rank: Frequent Contributor
timestamp: December 19, 2012, 03:58:38 pm
content: Then I guess most, if not all, of the board and components are fried. Maybe just give up and throw it away; I think the device is not anything special worth saving. Now, if this were a TB303

author: Sagat
author_rank: Contributor
timestamp: December 19, 2012, 04:03:31 pm
content: Yeah..i know.. it s only the joy of working on something vintage

author: SeanB
author_rank: Super Contributor
timestamp: December 19, 2012, 04:04:37 pm
content: TTL really likes 5V. If it is higher then you must check the source of this voltage, possibly a pass transistor or a second regulator supplying them that is faulty.

author: Sagat
author_rank: Contributor
timestamp: December 19, 2012, 04:08:45 pm
content: There are a total of 10 tranzistors on this board.. NPN, PNP power amplifier every one is new.. the old ones were passed. I ve checked them on the internet .. they were manufactured in 1989

author: Astroplio
author_rank: Regular Contributor
timestamp: December 19, 2012, 07:29:08 pm
content: Quote from: Sagat on December 19, 2012, 06:27:08 amQuote from: Sagat on December 19, 2012, 03:48:35 pmIf you can't not even trust the traces and vias and check them for continuity then I am afraid it just don't worth the trouble. Even if you find and fix all the cut traces and the vias, you still face the uncertainty of whether the Z80 is functional or not :-/Too bad, you already have invested a lot of time and effort into this, it's sadQuote from: jeroen74 on December 19, 2012, 03:58:38 pmYeah, I agree, I would also give up and move on by now.

author: Sagat
author_rank: Contributor
timestamp: December 23, 2012, 03:00:59 pm
content: Hi guys!Since i ve opened this topic i would like to get on opinion about my MFD2VW display. It decided to become white with vertical stripes. Enyone had this problem? Are the IR3Y26A OFP or the LZ9GH32 to blaim?Thanx!

author: Sagat
author_rank: Contributor
timestamp: December 24, 2012, 03:20:12 pm
content: Merry Xmas everyone! I have found the answer to my problem.Merry Xmas!

author: Unknown
author_rank: Unknown
timestamp: December 24, 2012, 03:27:07 pm
content: Could you post this answer for the benefit of anyone with the same problem stumbling on this thread via Google?

author: Sagat
author_rank: Contributor
timestamp: December 24, 2012, 03:31:23 pm
content: Replacing the Lcd display.

author: Sagat
author_rank: Contributor
timestamp: January 03, 2013, 07:46:24 pm
content: Hi guys! Help me with this one. What is the value of this rezistor? The first line is grey the second is green. THX!

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 03, 2013, 07:52:37 pm
content: The picture is not very good but it could be a diode, not a resistor.

author: SeanB
author_rank: Super Contributor
timestamp: January 03, 2013, 08:02:44 pm
content: Looks more brown, which makes it a 150kohm resistor, which is a preferred value. Probably 1/8W 10% as well. They are rather infamous for going quietly open circuit.

author: Sagat
author_rank: Contributor
timestamp: January 03, 2013, 08:05:01 pm
content: Not brown.. it s grey. Wow!... i ve never seen a diode like this. True it cross my mind..that it could be a diode.. but the colours? Is the older version of the 1N4148?L.E. the board has 1N4148 on it.

author: SeanB
author_rank: Super Contributor
timestamp: January 03, 2013, 08:14:29 pm
content: Remove one, measure resistance both ways and see. Break it and you will be sure, white is a resistor while glass with black shiny inside it was a diode.

author: Sagat
author_rank: Contributor
timestamp: January 03, 2013, 08:30:07 pm
content: Now this is odd... the values oh the rezistance are different: 1.272 Kohm .. from the other end 04,60Kohm. And on the diode meter is 0,222 volts from green to grey... closed circuit from the grey to green. Inside like the 1N4148.. glass with redlike colour... all this under the white paint with grey an green stripes..

author: jeroen74
author_rank: Frequent Contributor
timestamp: January 03, 2013, 08:38:50 pm
content: Not a zener by any chance with the colour code indicating the voltage?

author: Sagat
author_rank: Contributor
timestamp: January 03, 2013, 08:48:03 pm
content: Highly like... a zener with colour code ...1N85 maybe?

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 03, 2013, 09:11:20 pm
content: Quote from: Sagat on January 03, 2013, 08:30:07 pmDid you measure this with one pin disconnected from the pcb? Do they both read the same?And since you were looking to find out what it is, do you have any reason to think they are defective?

author: Sagat
author_rank: Contributor
timestamp: January 03, 2013, 09:15:12 pm
content: The component is remove from the pcb.. they dont read the same.. and im just confused.. is it a zener or a rezistor.

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 03, 2013, 09:29:05 pm
content: If you measure these values when removed from the pcb, and it changes with the direction you measure, it is certainly not a resistor.Looks like itwasa diode in better times, to find out what exactly it was you may have to draw some diagrams on how it is used in the circuit.

author: Sagat
author_rank: Contributor
timestamp: January 07, 2013, 03:33:24 pm
content: Hi guys!Does anyone recognize this component, or have a better program for a replacement unit?Ive this to improvize something with a couple of BC s PNP/NPN, 1k, 10k and 47Kohms rezistors but im not sure that are the proper values.Thx!

author: jeroen74
author_rank: Frequent Contributor
timestamp: January 07, 2013, 04:20:50 pm
content: Looks like an optocoupler to me. These often come in 6 pin packages and your widget being a MIDI device which requires an optocoupler in its receive part it seems quite logical to me.Oh, and you cannot replace it with a couple BJTs

author: Sagat
author_rank: Contributor
timestamp: January 07, 2013, 04:23:36 pm
content: Will a CNX83A or a CNY17/3 do the trick also?

author: jeroen74
author_rank: Frequent Contributor
timestamp: January 07, 2013, 04:29:37 pm
content: You need a fairly fast optocoupler. The usual device is a 6N138 or PC900. I have no idea what the part# of this one is.

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 07, 2013, 08:49:24 pm
content: A 4N25 will work:http://www.circuitbenders.co.uk/forum/index.php?topic=1821

author: Sagat
author_rank: Contributor
timestamp: January 11, 2013, 08:36:28 am
content: Many Thanks.