A tutorial to check address bits using the timing chart in a service manual? - Page 1
Forum section: electronics / beginners

author: tupinamba
author_rank: Contributor
timestamp: August 01, 2025, 02:40:17 pm
content: Hi guys!I'm kind of a noob when it comes to digital signals...I have a drum machine (Roland TR-707) that has an issue with some sounds (The Low Tom pad/sequence actually triggers the High Tom, while Middle Tom triggers the LT and HT triggers the MT. No other issue with the other sounds which are a match with both pads and sequencer)I have the service manual (which can be downloaded here:http://www.synfo.nl/servicemanuals/Roland/ROLAND_TR-707_SERVICE_NOTES.pdf)Attached is the timing chart, to check the ROM memory reading... But I don't know how to proceed : how do I check the master clock (CLK0) comparatively to the clocks or states I'm interested in? (for instance, according to the timing chart, the Low Tom should be "low" on A (pin 3)... But my scope only shows a square wave, not a "low" or "high" state... (and I guess this is normal because I'm not doing it the right way).My Rigol scope has 2 probes and a trigger.Thank you in advance for your help!

author: Terry Bites
author_rank: Super Contributor
timestamp: August 05, 2025, 05:29:28 pm
content: CLK0 is a squarewave and it looks like CLKn etc will all be a squarewave too.Is this timing diagram illustrating "test mode"?Is it the timing relative to CLK0 that you need to observe, comparing CLKn to CLK0.Transitions occur on falling edge of CLK0 as far as I can see.Connect CLK0 to Ch1, CLKn etc to Ch2.Trigger from CH1 on rising edge to see CLKn transitions.Timebase to suit tempo or click I guess. Volts per divison, 2V or 5V.You might find display "roll mode" helpful.How many waveforms can your scope store? Check the manual.Can you display them together to compare some how?I imagine your scope can export waveforms to a USB stick too.The tool for the job is a "logic analyser" You can get a cheap as 8-ch USB one from Aliexpress.Search for "USB Logic Analyser Microcontroller ARM FPGA debugging tool 24M samples 8 channels Microcontroller development".Or borrow/ hire a Mixed Signal Oscilloscope.If the gate array is stuffed its game over.

author: tupinamba
author_rank: Contributor
timestamp: August 06, 2025, 10:48:07 pm
content: Hey!Thank you for your answer!I don't believe the test mode being relevant for something else that the LED testing (and those are OK).I've checked all clocks (CLK1 to 4, A, B, C and INH0) and they are all fine, as far as I can tell. Unfortulately my scope has 2 channels only so I was just able to compare 2 clocks at a time, but it seemed fine (CLK1 to 4 @ 200Khz with the right interval between each signal, clock A @ 100 Khz, clock B @ 50 Khz, clock C @ 25 Khz).Unfortunately I don't have the Logic analyser extension that could connect to my scope but I just ordered the cheap analyser that you recommended so thank you, I will soon have a new and very helpful tool!In the meantime, I've changed both IC40 and IC41 (MUX and DMUX), as well as the DAC (IC37), with no success...I still don't get how the pad being pressed actually triggers a sound, as I did not notice any alteration on the waveforms I've checked when pressing the pads, so I must miss a very important part of the process...

author: Terry Bites
author_rank: Super Contributor
timestamp: August 07, 2025, 01:39:38 pm
content: Do a very close visual inspection of the switch PCB before doing anything else.Broken tacks and dodgy solder joints are very common failures around switches.The pads take a bashing. This is the most likely point of failure.Have you checked the the pad switches actually work with your DVM?Check all the diodes too. Clean the switches.Next check the continuity of the pad switches to IC303 and to the LCD controller ic.Is the cable linking the pcbs (F401) ok, are its connectors properly soldered on the pcbs?The pad matrix is scanned by the LCD controller and IC303.If the LCD display is ok, IC303 is suspect.

author: tupinamba
author_rank: Contributor
timestamp: August 10, 2025, 08:28:02 am
content: Hi Terry,Thank you for your answer!The pads are OK, so are the cables connecting the PCBs (I've entirely resoldered them properly).Could there be something wrong with the switch PCB when the issue doesn't concern the switches only (LT switch playing, HT, MT switch playing LT, HT switch playing MT) but also the sequencer? (the LT pattern of the sequencer playing HT,...).When I 'Factory reset' the unit, the demo song is loaded, and there's a moment where it's supposed to play a 'classic' Low Tom - Middle Tom - Hi Tom sequence from left to right (on the stereo out), and it plays HT - LT - MT (right, left, center) instead, so I would guess that the issue is on the main board?...Since my last post, I also switched the ROM ICs (IC34 and 35) with some spares I have from another unit. Same with the LATCH (IC27) as well as the Program ROM (IC13). All this with no success...That leaves us with the Gate Array, right?...It means that if I want the sounds to come out of the instrument as they are supposed to do, I have to cut some signal trace and redirect with jumper wires?...

author: Terry Bites
author_rank: Super Contributor
timestamp: August 10, 2025, 11:18:23 am
content: I wouln't write it off just yet.I need to ask if all supply voltages are correct (within 5% say) and present on all boards.Is UM3 ok?Does the tempo clock from Pin2 IC4 look like a nice squarewave, does it respond properly and cleanly to the tempo control?Do the pad LEDs the LCD reflect the activity of the pads? If so, then its fair to assume the microcontroller is ok.Next. Enevope generators. If they arent being triggered then there will be no sound. Do the latch outputs on IC27 and IC28 follow the action on the pads?What can you see on Q21-Q28 bases. IC40, pins A, B, C and INH (11, 10, 9 and 6=HIGH) , all control signals present?I was mistaken in thinking that matrix is scanned from the LCD controller.It is in fact done with the data lines from the voicing boad.D0-D7 and SC1-SC4. Are these all present on the pad pcb? Do the have the correct 5V amplitude and clean edges?If all the pad LEDs work it would be a very good indication that SC1-SC4 are ok.Look at RA301 are D0-D7 visible.Operation.the pad switches are scanned (checked) at high speed by the logic signals on D0-D7 and SC1-SC4.Upon detection of a closed pad switch the timing gernerator ROM forwards an address to the ROMs.Data from the ROMS is fed to the ADCs to create a series of audio analog output voltages.So my next suspect is the cardridge buffer IC10 or its control lines DIR and Gbar.It selects between internal and external data ROM data, is it doing what you expect?The inputs to this IC from the cartridge are a bit vulnerable to ESD fron the outside world.

author: tupinamba
author_rank: Contributor
timestamp: August 10, 2025, 11:50:50 am
content: Thank you very much again Terry!I'm leaving for a week so I won't be able to dig into it until next weekend but I'll then keep you updated!BestGael

author: tupinamba
author_rank: Contributor
timestamp: August 21, 2025, 08:48:24 am
content: Hi !Problem was solved yesterday!The Control surface was OK, I made sure by switching with the CS from another unit I had at hand.It happens that the Voicing PCB had a white label where its name is supposed to be silkscreened... and under this white label reading "TR-707" was the original silkscreen reading "TR-727".For some reason, a TR-727 PCB was used for this unit and there is one tiny difference between the two PCBs, and it regards the data lines for the Toms... :-)I just had to change one connection (IC30 Pin 59) and it was all solved!Thanks for your precious help Terry, I think I understand what's going on in those digital circuits a little bit better now.

author: RJSV
author_rank: Super Contributor
timestamp: August 22, 2025, 05:52:44 am
content: (Hopefully helpful post-script:)Glad to hear the results! Here is my approach, for what it's worth!The most sophisticated trouble-shooting would need a ROM or program SOURCE and assembled listing, for a full trace and scratch head technician worth their salary...often my debug activities had to be highly supported, by the original Engineers. On some occasions, between departments, there were personality conflicts and ego issues, with obtaining original Engineering dept. materialsBut debugging can be hard ..damn hard often. (And also hard enough and needed enough, to mandate the best documentation possible.)We learn some tricks, like using logic pull-downs in places where it causes no damage. (That's where you strap down some TTL output, in limited cases.)Often, in my experience, technicians would cooperate in hierarchy, where a senior level product tech would head up some training, for the 'younger folk' to teach subtle troubleshooting methods.But for here, I would almost always need a program listing, detailed prunt-out line by line, for using a Logic Analyzer.Trigger on various output code lines, by HEXADECIMAL location.Course I'm talking about assembly language debug, not any higher level language. (That's a good question, for C language troubleshooting though !).If you can isolate the problem area, and contemplate what and where to trigger the analyzer that gets you into some better luck. But I've seen lots of stymied techs, ITS HARD TO FIGURE YOUR TRIGGER points...Now, when lacking that diagnostic, I'd be virtually lost (no intended pun)!