Trying to troubleshoot the TR808

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batchas
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Trying to troubleshoot the TR808

Post by batchas » Sat Sep 07, 2019 5:35 am

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:
Image

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.
Last edited by batchas on Wed Sep 11, 2019 9:43 am, edited 4 times in total.
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makhho
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Post by makhho » Mon Sep 09, 2019 1:39 am

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Last edited by makhho on Tue Aug 25, 2020 2:28 pm, edited 1 time in total.

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batchas
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Post by batchas » Mon Sep 09, 2019 10:04 am

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 :doh:
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.
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Post by batchas » Wed Sep 11, 2019 9:22 am

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.

Image

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

IC4
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
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Re: Trying to troubleshoot the TR808

Post by batchas » Wed Sep 11, 2019 9:55 am

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.
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Post by batchas » Wed Sep 11, 2019 2:49 pm

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.

[video][/video]
Link:
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Post by J3RK » Wed Sep 11, 2019 3:05 pm

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.
Are you lemon? Does your head come to a nub?

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Post by guest » Wed Sep 11, 2019 4:00 pm

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.
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batchas
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Post by batchas » Fri Sep 13, 2019 7:26 am

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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makhho
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Post by makhho » Thu Sep 19, 2019 6:19 pm

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Last edited by makhho on Tue Aug 25, 2020 2:28 pm, edited 1 time in total.

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batchas
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Post by batchas » Fri Sep 20, 2019 1:32 am

makhho wrote:Check the driver sequencer LEDs and the transistors they are connected to?
Also maybe check the ribbon bus between main board and sequencer board?
I suspected something wrong with this part when I started troubleshooting the unit, cause the ribbon was wacky.
So I did connect the 2 boards with jumper wires instead.
I'll go through the related circuit one more time, test again and report.

Thx makhho :tu:
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Post by guest » Fri Sep 20, 2019 9:14 am

have you had a chance to to through all modes? does anything work correctly?
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batchas
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Post by batchas » Sun Sep 22, 2019 4:05 am

guest wrote:have you had a chance to to through all modes? does anything work correctly?
Thanx guest :tu:
As I previously struggled a bit too long with the troubleshooting, I'm taking a break to go back on it with a fresh mind.
But I'm not giving up :party:
I will report as soon as I'm back on it.
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