Somewhat OT: Univox Micro Rhythmer

[inverseroom]

Hmm, forgive me if this is too OT.  I got a busted specimen of this old analog drum machine built into a stompbox, and I got the power leads reconnected & got it running again.  There's only one problem, though--as it plays its preset patterns, it "hangs up" on a note and plays it ad infinitum: TOCK tock tic tic TOCK tock tic tic tic tic tic tic tic tic tic...etc.  The longer you leave it on, the worse the problem seems to become.

It occurs to me that maybe I should try replacing all the electrolytic caps.  Do you think that it "knows" to advance to the next note because of a cap charging and discharging?  There are only about half a dozen.  There are also a small forest of mylar caps and an IC (LM8972).

Also, has anyone ever tried to build something like this?  The idea of having rhythms in a pedal is very appealing to me...

[Nasse]

When I was about 15 or 16 years old I did an analog rhytm box. It used some rhytm pattern generator from National Semiconductor, the pattern generator is obsolete now. I never built a decent power supply for it, and though the box I made for it was quite sturdy (I used plywood) I had to operate it the lid open because of some technical problems, and there was so many trimpots for tweakin and some switches which superglue could not hold on the plastic lid made of nice white 4 mm polystyrene... I zapped the semiconductors at least three times by mistake of supply polarity, and my box is dead now. But I can always tell what not to do...

If the rhytm chip is obsolete type, and anyway, be careful with supply voltage and static electricity, I guess it might be cmos technology, if it has such a chip

Could the problem be like hihat is working but other instruments like snare and bass and bongo drums try to work but hangs on non working state after few beats, could be bad cap or semiconductor trouble

Maybe you can find if there is such rhytm generator chip, and how the drum sounds are generated (they used to be something like bandpass filters tuned near oscillation, and a pulse trough some cap/resistor circuit gave a drum like sound). Sometimes drum oscillator is made to oscillate all the time, and is gated with envelope generator and vca or gate. Snare drum, hi-hats, cabasa, shaker and cymbals etc use most often this kind of circuit. Drum oscillators could be made with transistors, opamps or cmos inverters, for example

[Paul Perry (Frostwave)]

1. yeah, the electros are the obvious first choice for replacing.

2. I wonder if any of those cut down 'home keyboards' with rhythm buttons could be hacked?
No idea if they are just one big chip or in parts.. guess it would have changed ocver the years as technology changed.

[Johnny G]

Paul alot of the little keyboards only have about 2 chips in (one ofor sound generation the other is normally an audio amplifier) however some of the larger ones i think have a seperate chip for drums etc.

maybe you could hack it up but it may be easier just to build a drum machine from scratch with discrete transistors

inverseroom: id try to isolate which part of the circuit is the triggering section.

my logic is that since the sound section for that hi hat is being triggered regularlly it should be fine.

another thought is can you can choose the rythme patterns? if not then it could well be easilly replaceable with a square wave clock generator and some form of decade counter/flip flop assemblly

try replacing the caps like the other guys have said and if its still funkeh then come back. you could always just tryt to trace out the sequencing section so we can have a ood look at how it works

[inverseroom]

1. yeah, the electros are the obvious first choice for replacing.

2. I wonder if any of those cut down 'home keyboards' with rhythm buttons could be hacked?
No idea if they are just one big chip or in parts.. guess it would have changed ocver the years as technology changed.

I know there are some older, analog Casios that have the rhythm section and tone section on completely different boards--I should find one of those and implant the rhythm section into a stompbox!  My other electronic-noodling hobby is circuit-bending, so I have a nose for those little suckers...

To clarify, it isn't just the hi-hat that hangs up.  Interestingly it seems to be the clave most of the time, but the more it warms up the more likely ANY of the sounds is to be the one that hangs.  There is an SP8T rotary switch that selects a particular rhythm.  It is actually a neat little box with a decent sound.  Unfortunately the stomp switch seems to be sort of like the Ibanez 10-series switch.  Works though.

I'll replace the electrolytics and see how it works.  Unfortunately I can't get to Rat Shack today, we're in the middle of an endless snowstorm.  Kids home from school and I can't get electonic parts--nightmare!!!!!

Thanks!
John

[Johnny G]

way hay another circuit bender. i know there are one or two hanging around here. i unfortunatelly picked a bad time to get into it and only managed to pick up a little casio SA-5 before the winter stopped all the car boot sales around here. that little keyboard is gold dust for different sounds tho

other thinking if heat is causing it to happen. it could always be the beginnings of chip death. maybe caused by incorrect polarity etc. are there any op amps that are used in the sequencing section? posiblly one of them is on the way out

[inverseroom]

Only one IC, the LM8972, which is I dunno what the hell it is.  If the caps don't work, maybe I'll start looking for one...