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Tr707 TR727 Negative vibes
Tr707 TR727 Negative vibes
The Roland TR707 and TR727 are almost exactly the same.
One of the many things they share is a particularly hard to find transformer, T1. originally made by the sumida corporation but long since discontinued.
it is unfortunately not indestructible and the only source for a replacement part is a dead tr707.
This has long been an achilies heel for the tr707, and many have given up on tr707s with dead sumida coils.
it even has its own thread on eevblog.
https://www.eevblog.com/forum/repair/10v-dc-dc-converter-repair-advice-needed/25/
harry axten, (who make a great tr707 rom swap mod btw) had a nice idea for an alternative using a single chip negative voltage converter:
https://www.facebook.com/groups/vintagesynthrepair/permalink/987500284726415
so when i had a tr707 arrive for repairs with a dead sumida coil. i knew what to do...
a Replacement for the TR707 sumida coil:
a Replacement for the TR707 sumida coil:
i found that harry's original design didn't quite qive me 10v so i put two icl7660 in parallel as described in the icl7660 application notes. this is probably complete overkill.
there is also a footprint for an extra capacitor to further stabilize the output voltage, but i couldn't measure a difference with this extra capacitor fitted so i just leave it unpopulated.
i've published everything on github, if anybody cares about that kind of thing:
https://github.com/ewanuno/tr707_negative_vibes
they are now available.
send me an email if you need one.
ewanuno@gmail.com
how to install:
how to install:
remove these components:
Q1, Q2, T1, D2, D3, D18, C7, C8, R1, R2
cut some 2.54mm pin headers
1x 1pin
2x 3pin
remove the middle pin on the two 3 pin strips
insert them into the bottom of the pcb, don't solder them yet...
Put it in place on the tr707, use a finger or a bit of tape to keep the single header pin from falling out. and make sure you get it lined up with the edge of the main pcb so it will fit properly in the case.
Solder the pin headers on the top of the negative vibes pcb.
Then turn the pcb over and solder them to the tr707 pcb.
That's it! go make beats! (hopefully that was the only thing wrong with your TR707....)
Make sure you put the pins in the right holes!
Errata: One of the pins is marked "NC" on the silkscreen, but it is infact connected to Vout, this will be fixed in a future revision but for now, just ignore it.
do you think you need one of these?, check these things first:
do you think you need one of these?, check these things first:
Check if you are getting -12v at the opamps, if you see -12v anywhere then the -12v psu isn't your problem.
The tr 707 will still boot with no -12v psu but you wont get much sound, maybe a few clicks at best.
If some of your sounds are working then its not a PSU problem, they all get mixed through opamps which need a +/-12 supply
Some things i've seen causing a tr707/727 not to boot:
Bad fusible resistor , just after the power connector there is a big resistor that goes bad if you power the tr707 with the wrong PSU. you wouldn't get -12v anywhere in this case, or +12v or 5v either.
Bad SRAM, the 6116 sram chip can get corrosion on its power pin due to some galvanic shenanigans related to the battery backup that i won't pretend to understand.
Bad OS mask rom Rom, i have seen this once, but it should be fairly rare. unless somebody had been circuitbending...
Bad Vias (the connections from the top copper layer to the bottom copper layer )on the motherboard, the vias on the tr707 PCB are a weird conductive paste type that quickly went out of fashion due to their habit of turning into little resistors if exposed to humidity. this means that you can't trust any of them, and you will have to test them all. bad vias appear as resistors with a few ohms resistance. this is particularly insidious because just one via doing this would not cause any problems most of the time. but add several to a circuit and things start going wrong in totally unpredictable ways. just one or two vias is common, but i've seen basement dwelling tr707s where nearly all the vias are bad.
there are vias underneath the CPU and the Gate array, that can be fun...
if you only have one or two gone bad, just replace the via with a bit of wire soldered on both sides, otherwise invest in some pcb repair rivets.
Bad capacitors: the electrolytic capacitors on the tr707 motherboard can go bad, and even start leaking and corrode the PCB. just the ones related to the power supply, i have not seen problems with the ones in the audio path.
Damaged flat cables, if you spend too much time trying to fix a tr707 then the flat cables that join the two main pcbs together will get damaged. specifically where the flat cable is soldered into the front panel pcb. On my personal tr727 that i use for developing mods i have fitted crimped dupont connectors.
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