[sdiy] Tr-808 issue

el macaco elmacaco at hotmail.com
Thu Sep 13 21:30:58 CEST 2018

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.


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