Tips on SH-101 LFO repair

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Solderman
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Tips on SH-101 LFO repair

Post by Solderman » Thu Dec 27, 2012 1:34 am

Believe it or not, I don't do much troubleshooting and isolation, despite my forum user name. The LFO on my SH-101 has stopped oscillating, just sending out a fixed voltage, causing D36 to stay constantly lit. Ext clock In still works.
Not currently having access to an oscilloscope, I have to guess that the problem is either IC3(IR9022) or TR4. D9 and D10 checked good. I don't know how rare these parts are either. The transistor likely has a suitable modern equivalent, but no idea on the dual op-amp.(TL022 maybe?)

Any tips for isolating a dead LFO on an SH-101? I've already gone so far as to modify an external input to replace it, although this input signal must be pre-buffered, and it doesn't go to pulsewidth. If I knew what test points to use and what voltages to check for(aside from the obvious operating voltage for IC3 and IC4), that would probably be enough to nail it.
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Re: Tips on SH-101 LFO repair

Post by rhino » Thu Dec 27, 2012 3:13 am

Offering what thoughts I can grab:
1) Q: is the LFO locked "on"? That is, does the LFO depth slider cause the pitch to change?
2) check all 12 of the conductors on the flat cable to the control board - from the main board - in particular , #8. This runs from the RATE slider to R15 and on to the emitter of TR4.
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Re: Tips on SH-101 LFO repair

Post by Solderman » Thu Dec 27, 2012 3:28 am

It is indeed locked "ON", but I haven't yet measured the output voltage going to the mod attenuators. I did a modification replacing R15 with two resistors tied to a SPDT switch for hi/lo LFO speed, and I checked continuity to and from the switch to the holes where R15 connected to the PCB traces, checking good there. My understanding for checking transistors is they need to be checked out of circuit, so TR4 is pure speculation at this point. Continuity out of ribbon cables points 10(triangle) and 11(square) to Waveform switch and to bender board point 14 are good.

Seems to be a problem actually oscillating, but as mentioned, I don't currently have a scope to verify this.
I am no longer in pursuit of vintage synths. The generally absurd inflation from demand versus practical use and maintenance costs is no longer viable. The internet has suffocated and vanquished yet another wonderful hobby. Too bad.
--Solderman no more.

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Re: Tips on SH-101 LFO repair

Post by psygoat » Thu Dec 27, 2012 5:46 pm

90% of problems on the sh101 are caused by the rubbish sliders.
Check VR10, LFO rate control, before you do anything else.

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Re: Tips on SH-101 LFO repair

Post by Jabberwalky » Wed Jan 02, 2013 6:41 am

The LFO on my Octave Kitten is also stuck "on". I'm interested in figuring this out as well. I've bombed and replaced every IC chip on the board and still no LFO oscillation.

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Re: Tips on SH-101 LFO repair

Post by schmidtc » Wed Jan 02, 2013 8:31 pm

psygoat wrote:90% of problems on the sh101 are caused by the rubbish sliders.
Check VR10, LFO rate control, before you do anything else.
Agreed. Many problems I've encountered on 101's are the unintuitive result of a slider failing open.

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