[sdiy] Tr-808 issue

[rsdio at audiobanshee.com]

This sounds a lot like a problem that I had with a TR-808 repair. The wires from the PCB to the switch that selects between internal and external clock might be broken. This is the slide switch on the rear, next to the DIN jack, labeled SYNC IN/OUT. In my case, the TR-808 had been opened by a previous owner or tech to add MIDI, and at that time the wires must have been damaged while soldering new connections.

What happened was that the broken wire was sitting very close to the solder lug, so it looked fine. Every time I disassembled or reassembled the case, this wire would randomly be touching or not. Looking for potential problems, I couldn’t see the issue readily. Eventually, I discovered the broken wire, stripped away a little insulation and re-soldered it. The TR-808 was good as new after that. Once I had seen the broken wire, it was obvious that it was staying basically in the same location but only sometimes touching. Very maddening, but very rewarding once found and repaired.


As for an oscilloscope, you might be able to abuse an audio interface for this task. The 60 Hz transformer output is slow enough that standard 44.1 kHz sampling should be sufficient to see any problems. Of course, the biggest challenge is that the 26 VAC output will fry any audio interface (unless it can handle +22 to +28 dBu). You should be able to build an appropriate resistor divider to drop this voltage into a safe range. The second challenge is that you’ll need an audio interface with a DC input capability. Even though the transformer output is AC, you might miss some problems if you AC-couple the signal to your interface.

In other words, you’re totally on your own here. Don’t complain if you fry an audio interface. However, it is theoretically possible to use an audio interface as a low-frequency oscilloscope if you have the appropriate front end and software (back end).


Let us know how everything turns out!

Brian Willoughby


On Sep 13, 2018, at 2:42 PM, el macaco <elmacaco at hotmail.com> wrote:
> It powers on with no step Leds lit, just bank a and section 1 leds Lot with no change with toggles switched.
> 
> It’s the power protection circuit.  Sometimes switching it on and leaving it on for a while it will come to life with the step led blinking sudddenly.
> 
> One time I got it to live by plugging a Pedal into the start stop jack.  Sometimes with the din sync switch but only once or twice.  
> 
> Maybe a faulty trace on the clock circuit looks just like the protection circuit kicking in?
> 
> Thanks guys.  It’s been months of sometimes, then I thought I fixed it, it worked reliably for weeks, then when I put all the screws back in it didn’t work :/.
> 
> Giving me new fire to crack this problem.
> 
> On Sep 13, 2018, at 3:51 PM, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>> If it's a cheap inverter that outputs a "squarusoidal" waveform, then it's more likely to make the transformer buzz loudly or to damage the smoothing capacitors and rectifier diodes.
>> 
>> Is your unit definitely configured for 120VAC, and not 240VAC?  Supplying the unit with the wrong voltage will obviously result in no operation or possible damage (if over-voltage.)
>> 
>> Is the TR-808 completely dead?  or does the sequencer (CPU) still function, LEDs flash, etc?  It sounds more like a problem with the CPU's power-on reset, or possibly the audio output muting?
>> 
>> -Richie,
>> 
>> -----Original Message----- From: el macaco
>> Sent: Thursday, September 13, 2018 9:33 PM
>> To: Kenny Balys
>> Cc: synth-diy at synth-diy.org
>> Subject: Re: [sdiy] Tr-808 issue
>> 
>> Am I on the right track thinking a squarish 120vac could bugger the transformer?
>> 
>> On Sep 13, 2018, at 3:26 PM, el macaco <elmacaco at hotmail.com> wrote:
>>> Yes.  Thanks!! I have a true sine wave inverter now and it’s solid and even goes through line conditioners etc before the 808.  All the other gear works fine on it.
>>> 
>>> The earlier set up was before I had a proper solar set up.
>>> 
>>> 
>>> On Sep 13, 2018, at 2:42 PM, Kenny Balys <kenny at beatkamp.com> wrote:
>>>> 
>>>> Are you certain of the AC power coming in to the 808?
>>>> 
>>>> I would suspect that as much as the power transformer.
>>>> 
>>>> 
>>>> On 13.09.18 19:30 , el macaco wrote:
>>>>> Hello all,
>>>>> 
>>>>> I’m asking for guidance to fix my Tr-808.  I usually fix my gear myself but this has me stumped.
>>>>> 
>>>>> I live in the amazon far from any reliable techs and have mostly serviced my own stuff over the years with all I have learned from this list since 1999.
>>>>> 
>>>>> My TR-808 has an issue where the voltage protection circuit kicks in and doesn’t let it run.  This happens when the power is unstable in the 808 and is by design.
>>>>> 
>>>>> It first happened after running it from a solar power system with a voltage inverter that wasn’t pure sine wave power.
>>>>> 
>>>>> The power supply was giving uneven power, 15.6v and -14v.
>>>>> 
>>>>> Replacing the parts in the power supply, rectifier diodes, capacitors, regulators, resistors helped some but it doesn’t reliably work although the voltages were even 15/-15.
>>>>> 
>>>>> The only power supply part I haven’t replaced is the transformer, which puts out static 26v AC voltage as required, but my oscilloscope is busted so it’s with a DMM so I can’t see if there’s ripple or other problems.
>>>>> 
>>>>> That’s the next part I need to order, the transformer.
>>>>> 
>>>>> Does anyone know how to test the transformer without a scope?
>>>>> 
>>>>> Or is there something else that can cause this protection circuit to engage?
>>>>> 
>>>>> I also replaced capacitors and transistors in the protection circuit in case that was the issue, it helped some initially but it still only works occasionally.
>>>>> 
>>>>> The service manual
>>>>> Mentions something about malfunctioning IC can cause this circuit to engage.
>>>>> 
>>>>> If the transformer doesn’t help next on the list is replacing the electrolytic capacitors that filter the power at each circuit (25 total I think).
>>>>> 
>>>>> Any ideas greatly appreciated.