Yamaha delay D1500, zener diode broken with no marking

[debidoobs]

Hello,

im actually trying to save a d1500 delay from yamaha. I found the service manual but schematic is missing and BOM list is a bit unclear as components listed have no reference linked to the pcbs... In my case, one zener diode was broken (114 code on the pcb in attached file) and couldn't be repaired : it was a zener type package. In the BOM list i found 3 reference on this board : 1S555 (can be replaced by 1N4148), RD4.7EB2 (approx 5v rectifier), RD7.5EB3 (approx 7v rectifier)...

Actually the cathode goes to VCC of sram M5M5118P, without this diode missing, i have 4.7V at vcc coming from 2x10k resistors in serie, the cathode being connected to this resistors and VCC of the SRAM... so it's easy to say i ll go for the RD4.7EB2 (5v rectifier)... but why the anode of the diode is connected to the + of the CR2032 3V battery in this case ?

Sorry for this stupid question, live and learn.

cheers debi

[R_J]

The 114 diode is likely not a zener diode so it is just a 1N4148, Diode 101,102, 103 and 104 are marked as zener diodes Note diodes are maked with the same identifier even if they are zener or regular diodes, not sure why?
See the service manual for the ddl-3-d-1030, it has a schematic and may be a similar circuit (see B301 with D302)

[debidoobs]

Hello, i just put a 1N4148 but no improvement. The unit still shows Bank 7 Preset 777 right after i switch the device on without kind of loading process. Power rails look correct. I hope it's not dead. I m going to find out.

[R_J]

That diode is likely part of the memory circuit for the backup battery, I would check if the reset circuit is working, I suspect it is not. Is the battery ok?
The schematic I provided looks similar in that circuit ie: diode 114= D302; transistor 111 = Q309; Q112 = Q310 IC181 = IC317 etc.
Try and figure out which ic is the same as IC312 (reset) It may also be a bad elec. cap like C325(35) in the schematic causing the reset not to happen.

[debidoobs]

Hi, do you mean ic311 instead of 312? which is the cpu ? Its a Hd6303 similar to DDL3 unit.. im going to look where the reset pin is. What i can say is there is no frequency out of the ceramic 4MHZ crystal. There is also no vcc on a chip placed on a socket on the middle right of the board : a AM2405PC... but might not be the cause of the problem. (https://pdf1.alldatasheet.com/datash.../AM2504PC.html)

update : found some infos about the reset state.

[debidoobs]

So i made some measurement, "reset has to be low for 20ms at start up"... the only thing i see at reset pin at start up is a dc signal that goes up to 5v and stays like this, so i guess at start up its stay low enough ? No oscillation between xtal and extal at all.. At the flipflop 74HC74 supposed to send the reset signal, the clock input is a bit weird, i attached the picture.


I also tried to set up the clock adjustement from the manual at pin 3 of the IC173 74HC163, but i only can go to 213khz max then it will go down if i keep turning clockwise with a screwdriver, it 's supposed to be set to 512khz.. It was at 27khz from the beginning !..

[debidoobs]

Well don't mind the scope signal i was using x1 probe setting so it's all flat at x10... no clock anywhere.

[debidoobs]

Well don't mind the scope signal i was using x1 probe setting so it's all flat at x10... no clock anywhere.

made no sense, sorry. probe is correctly set up at x1. cheers

[R_J]

You said diode 114 was "broken" was it phisically broken or just shorted?, from what I can tell that diode is the same as used in the DDL-3-D-1030 but in location D302. Since all you have on the display are a bunch of 7's as soon as power is applied it looks like the microprocessor may not be running or not reseting, The reset circuit is in the same location as diode 114 so I thought maybe there was a problem with the reset ic317 which is also connected to IC312 (in the 1030) in the D1500 it might be ic189?

Also is the cr2032 good? does it have 3v? if not try removing it and see if the unit works.

Does the microprocessor have vcc? if not, then it can't be working, If there is missing vcc, find out why the vcc is missing and it will likley work

There is also no vcc on a chip placed on a socket on the middle right of the board

Does this ic have a location?

[debidoobs]

Hello R_J,

The IC im talking about is ic158 but i do have VCC finally..AM2504PC, 12 bits register... so don't mind

One thing is in the manual, they say to set 512khz at pin 3 of IC173 with B10K pot 104, but i only get 215khz at best, it will then decrease if i keep turning left or right the pot. Don't know if it helps.

The diode was broken in half due to corrosion. I did replace the CR2032 socket and put a new battery in it. With it or without, there is no difference when starting up the unit.

The processor has VCC, approx 2V, in the datasheet they say 3 to 6v, so may be a clue. ill try to set up a trigger to see if i do have 20ns low state at reset pin at start up, but my cheap scope is not ideal i guess.

Ps: really interesting info about IC317 transistor system reset linked to ic312, i looking around similarities with d1500 board. Ill let you know

ok found it PST518A don't know yet where it's located on the board as im not in front of the unit and unclear listing in the service manual.. but there is a reset ic !

[R_J]

Your microprocessor should likely have a solid 5vdc, not 2v 2v is too low for it to operate. If there was corrosion on that diode there is likely corrosion elsewere.

[debidoobs]

Your microprocessor should likely have a solid 5vdc, not 2v 2v is too low for it to operate. If there was corrosion on that diode there is likely corrosion elsewere.

Yes i did clean the board it was quite dirty but, then i checked for continuity and i didn't find any broken trace. That's why i didn't mention it. thank you for your help and your time, i will look at it tomorrow morning.

[debidoobs]

hello,

so after better understanding of the pinouts, i do have vcc 5V going to the CPU. "Enable" is supposed to be a system clock and in my case (screen attached), it looks like a sine wave with a +3v offset.
With my one channel cheap scope, i can't trigger standby and reset signal to see if i have at least 20ns low state before standby goes up... I can trigger them one by one but do not see any difference they rise exactly the same way/time on my scope.

Reset ic PST518 provides 5V output.

Also, my cr2032 battery looks like it's drawing current as yesterday it was new 3V battery and this morning it's already 2.5V. Don't know if it helps.

[debidoobs]

Hello,

just an update. I found a broken track just under the RAM IC M5M5118p. I was probing pins when at pin 18 CHIP select input S2/, i could hear my injected sine wave but no effect, this pin 18 is supposed to be connected to the front side of the pcb with a metalized vias, this one is located under the M5M51118p and it was broken !.... I made the connection back but it's far from being fixed now. Right part of the front panel is responding now, from "RATE, TIME,... to MIX, INV", leds turn on when switch button is pushed. Rate is also reacting by using increment or decrement switch...

I might have other broken tracks so im digging it out.

Leds screen does not show 7 777 anymore but it's totally silent now, not showing anything. Vu meter led is working. I have no delay out nor mix signal ouput.

I still cannot set the clock adjusting pot V104 as as soon as i reach approx 200khz it falls to 5khz... so i cannot reach the 512khz which is mandatory page 3 of the service manual.