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Trying to troubleshoot the TR808
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Trying to troubleshoot the TR808
I spent a lot of time trying to troubleshoot the TR808 and came to the point where I need to ask for help.
Maybe someone can point me in the right direction and has an idea which component might be faulty.

If I power it on, there’s no LED lighting. At one exception. Only a LED lighting if I switch the mode selector to „CLEAR PATTERN“ position. Then I can even click a step and the LED will light. But then it will remain on, even if I turn the mode pattern switch to change mode.
If I press START/STOP, there’s no change in the LEDs area, no sequence running (logically no sound).

If I check the circuit, I see high/low state (start/stop) at Q12 collector. Means start/stop works.
I see tempo clock at Q15 collector. Means clock works (also react to tempo changes via tempo pot).
I see clock running on pin 3 (and also other pins) of IC4.
Power supply all good. +15V, -15V. +5V. GND. All distributed as it should be.

IC4 (the CPU) should look like this:

Under "Drive signals to STEP LEDS":
pin 22, 23, 24, 25 : no signal START ON or OFF same.

Under step numbers":
pin 17, 18, 19 have a kind of pulse signal on START ON.
No signal on pin 16.
Check the "power on/off detector" section in the service notes
makhho wrote:
Check the "power on/off detector" section in the service notes

thanx. I'm in this area snce yesterday.
Found out right now, after at least one hour trying to decypher why I could not get the schematics to match with my 808 PCB, that in the schematics they inverted Q75 and Q64 d'oh!
It's difficult enough to read this kind of schematics where you have to jump in every place, but then with this kind of errors I'm totally lost.
makhho wrote:
Check the "power on/off detector" section in the service notes

Could please someone confirm the measurements are OK.
I unfortunately don't understand what should be expected at the pins of IC4.

My measures at IC4 pins, START ON or START OFF, same.

pin 1 = 0V
pin 2 = 0V
pin 3 = 0V
pin 4 = -14V
pin 5 = 0V
pin 6 = 0V
pin 7 = 0V
pin 8 = +15.3V
batchas wrote:
Under step numbers":
pin 17, 18, 19 have a kind of pulse signal on START ON

Extremely difficult to troubleshoot the unit cause I don't get the same results when I measure. Going back after a few minutes the measures just show +5v.
Then power of and on again will show a kind of pulse train on these.
Here I'm showing all pins on the CPU. Maybe someone sees what is wrong, so I know which part of the circuit I should check.

Unfortunately I'm not much help here. I've repaired an 808 for someone before, but it was mainly jacks, and a couple of analog components. I don't have any experience with the CPU.

I mainly just stopped in here to say I hope you figure it out! I'm more of an Elektron user for my analog drums now, but I love to see people resurrect these machines, and keep them alive.
i feel bad, ive been watching this thread hoping youd find something, but i just dont have the time right now. this sort of thing takes a lot of head scratching and staring at the schematic. when i get time ill have a look. its typically a bad SRAM or transistor that is pulling a line low at the wrong time and confusing the microcontroller. one thing that sometimes helps, is to go through every single mode, and try every button in every mode, and see if anything works right.
Merci les amis.
I appreciate the support. I felt better after I read your comments.

I tried with a midi-to-sync signal coming into the sync input/output, with switch in "sync in" position, thinking the issue might maybe be related, but it did not change anything.

I'll go next through all modes one by one as adviced.
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