Schmitt trigger issue

I posted this a while back on sci.electronics.repair, but I had to put this project aside for a while. I'm gonna try here to get a second opinion...

I'd like to fix up this old drum machine. It's mostly working, but there's an issue with the Schmitt trigger at the bottom left of the schematic here:

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According to the service manual, in "write" mode a negative-going pulse from the "Start" key to the base of PNP Q8 should produce a positve-going pulse at the collector of Q15. Similarly, from a negative-going pulse from the "Stop" key.

What I'm seeing is that when the "Start" key is pressed, Q15 latches "high" and the "Stop" key unlatches it.

Wondering what I should check, I'm having a hard time locating the problem. There was some corrosion on the board so there might be a blown trace, somewhere.

Reply to
bitrex
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That looks right, if that FOOT SW plug is not inserted, the logic NAND FF l atches a low voltage thru terminal 6 to the junction of R82 and C45 which l atches Q15 collector HIGH. Then the STOP momentary resets the logic NAND wh ich allows C45 and C43 to recharge and reset the Q18-Q15 Schmitt.

Reply to
bloggs.fredbloggs.fred

Thanks Fred. Here's the service manual, the relevant waveforms are on page 3:

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From the diagram it looks a lot like the collector of Q15 is not supposed to latch when S5 is depressed. Maybe the diagram is wrong?

The symptom is that the "steps" are not being properly latched into the CMOS SRAM chip when in "Write" mode. Instead I'm getting blasts of noise through the output from the noise generator when in "Write" mode, which shouldn't happen.

Reply to
bitrex

I don't think pin 6 and pin 8 of the foot switch jack ever contact, but you could check that with an ohm meter. I think the drawing indicates inserting the foot switch plug switches the contact between pins 7 and 8 which are isolated from pin 6. A connection between FF pin 4 and FF pin

1 makes little sense while connecting it to FF pin 2 does make sense. But the ohm meter will tell you what's what.
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Rick C
Reply to
rickman

Hmm. Thinking shorted contact in that mess of a footswitch jack?

Reply to
bitrex

Thanks, there seems to have been a shorted contact inside the footswitch. Pulling it and connecting a jumper between FF pin 4 and 2 and it's functioning pretty much normally now.

The only remaining problem is that there seems to be quite a bit of clock injection noise into the audio section.

Reply to
bitrex

What does that FOOT SW plug look like-how many conductors? The schematic is pretty clear on showing the FF operation as what you are observing when the plug is removed from the socket.

Reply to
bloggs.fredbloggs.fred

Four wires going to the switch. Two brown, two blue, one black. With the plug removed, the brown wires have continuity between each other, the blue seems to have continuity to nothing. With the plug inserted, there doesn't seem to be continuity between any of the wires.

Reply to
bitrex

I think he meant the wires in the foot switch cable to the plug, not the socket. I expect there are just two.

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Rick C
Reply to
rickman

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c is pretty clear on showing the FF operation as what you are observing whe n the plug is removed from the socket.

The operation of the FOOT SW jack is unmistakable. It shows a sleeve, a mov able plug tip contact (the notched piece), and an internal SPDT switch betw een terminals 8, 7- and why they don't number the friggin pole I don't know ! The bent piece on terminal 7 is meant to show a NC connection with the po le with the plug out of the jack, and it clearly shows the internal pole is connected to tip (6) with the plug out of the jack. With the plug out of t he jack, the operation of the START and STOP switches runs the show, turnin g the clock oscillator on and off and some other stuff. With the plug in th e jack the FOOT SW runs the show, the TIP to internal SPDT pole connection is broken as is 7 to internal switch pole. This prevents the START and STOP switches from latching the FF and it also gives the FOOT SW complete contr ol over gating the clock oscillator, turning it on when FOOT closes and tur ning it off when it opens.

Reply to
bloggs.fredbloggs.fred

You seem to be saying the center pole of the foot switch jack (connected to FF gate pin 4) is electrically connected to the tip contact of the jack. I think this is not right. Rather it is an insulated contact providing an switch insulated from the jack terminals.

Here are some examples of similar jacks that illustrate this.

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Here the use of filled contact symbols and unfilled contact symbols are used in the opposite fashion.

If you consider that the switch pole is connected to the tip contact electrically, the output of the gate at pin 4 is shorted to ground every time the start button is pressed, likely not a good idea.

--

Rick C
Reply to
rickman

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movable plug tip contact (the notched piece), and an internal SPDT switch between terminals 8, 7- and why they don't number the friggin pole I don't know! The bent piece on terminal 7 is meant to show a NC connection with th e pole with the plug out of the jack, and it clearly shows the internal pol e is connected to tip (6) with the plug out of the jack. With the plug out of the jack, the operation of the START and STOP switches runs the show, tu rning the clock oscillator on and off and some other stuff. With the plug i n the jack the FOOT SW runs the show, the TIP to internal SPDT pole connect ion is broken as is 7 to internal switch pole. This prevents the START and STOP switches from latching the FF and it also gives the FOOT SW complete c ontrol over gating the clock oscillator, turning it on when FOOT closes and turning it off when it opens.

"Insulated contact"??? WTF are you smoking?!? There's only one way to inter pret the schematic and I told you what it is. How difficult is it to unders tand that 1) with plug removed, circuit operation is push on/ push off thru START/STOP switches and 2) when plug is inserted into jack, circuit is ON (MOMENTARY)/OFF through the FOOT SW ( a SPST to shield GND).

That little f...king knife symbol on the pole touching the TIP means it con nects!

Reply to
bloggs.fredbloggs.fred

The reference I provided says otherwise.

How do you draw a jack where the SPDT switch is electrically insulated from the jack connections?

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Rick C
Reply to
rickman

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a movable plug tip contact (the notched piece), and an internal SPDT switc h between terminals 8, 7- and why they don't number the friggin pole I don' t know! The bent piece on terminal 7 is meant to show a NC connection with the pole with the plug out of the jack, and it clearly shows the internal p ole is connected to tip (6) with the plug out of the jack. With the plug ou t of the jack, the operation of the START and STOP switches runs the show, turning the clock oscillator on and off and some other stuff. With the plug in the jack the FOOT SW runs the show, the TIP to internal SPDT pole conne ction is broken as is 7 to internal switch pole. This prevents the START an d STOP switches from latching the FF and it also gives the FOOT SW complete control over gating the clock oscillator, turning it on when FOOT closes a nd turning it off when it opens.

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nterpret the schematic and I told you what it is. How difficult is it to un derstand that 1) with plug removed, circuit operation is push on/ push off thru START/STOP switches and 2) when plug is inserted into jack, circuit is ON (MOMENTARY)/OFF through the FOOT SW ( a SPST to shield GND).

e

ry

connects!

The way they've done it, except get rid of that edge touching the tip and s how the pole contact making NC contact with the bottom throw by bending the pole contact and not the throw contact. The reason they bent the throw con tact is because they want that pole connection horizontal and making contac t with the TIP, the usual schematic for a phone plug type jack.

Reply to
bloggs.fredbloggs.fred

These days it almost seems like a more reasonable option to use a regular 2 connector foot switch jack and an 8 pin uC hacked in to handle the switching than deal with this weird and unreliable multi-pin jack.

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Reply to
bitrex

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ve, a movable plug tip contact (the notched piece), and an internal SPDT sw itch between terminals 8, 7- and why they don't number the friggin pole I d on't know! The bent piece on terminal 7 is meant to show a NC connection wi th the pole with the plug out of the jack, and it clearly shows the interna l pole is connected to tip (6) with the plug out of the jack. With the plug out of the jack, the operation of the START and STOP switches runs the sho w, turning the clock oscillator on and off and some other stuff. With the p lug in the jack the FOOT SW runs the show, the TIP to internal SPDT pole co nnection is broken as is 7 to internal switch pole. This prevents the START and STOP switches from latching the FF and it also gives the FOOT SW compl ete control over gating the clock oscillator, turning it on when FOOT close s and turning it off when it opens.

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o interpret the schematic and I told you what it is. How difficult is it to understand that 1) with plug removed, circuit operation is push on/ push o ff thru START/STOP switches and 2) when plug is inserted into jack, circuit is ON (MOMENTARY)/OFF through the FOOT SW ( a SPST to shield GND).

ts

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it connects!

nd show the pole contact making NC contact with the bottom throw by bending the pole contact and not the throw contact. The reason they bent the throw contact is because they want that pole connection horizontal and making co ntact with the TIP, the usual schematic for a phone plug type jack.

But there's nothing wrong with it. Something else is going on with that mem ory unit putting a bunch of noise out, but it's not this part.

Reply to
bloggs.fredbloggs.fred

You can speculate all day, but only the ohm meter will say for sure.

--

Rick C
Reply to
rickman

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e, a movable plug tip contact (the notched piece), and an internal SPDT swi tch between terminals 8, 7- and why they don't number the friggin pole I do n't know! The bent piece on terminal 7 is meant to show a NC connection wit h the pole with the plug out of the jack, and it clearly shows the internal pole is connected to tip (6) with the plug out of the jack. With the plug out of the jack, the operation of the START and STOP switches runs the show , turning the clock oscillator on and off and some other stuff. With the pl ug in the jack the FOOT SW runs the show, the TIP to internal SPDT pole con nection is broken as is 7 to internal switch pole. This prevents the START and STOP switches from latching the FF and it also gives the FOOT SW comple te control over gating the clock oscillator, turning it on when FOOT closes and turning it off when it opens.

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interpret the schematic and I told you what it is. How difficult is it to understand that 1) with plug removed, circuit operation is push on/ push of f thru START/STOP switches and 2) when plug is inserted into jack, circuit is ON (MOMENTARY)/OFF through the FOOT SW ( a SPST to shield GND).

s

are

very

it connects!

nd show the pole contact making NC contact with the bottom throw by bending the pole contact and not the throw contact. The reason they bent the throw contact is because they want that pole connection horizontal and making co ntact with the TIP, the usual schematic for a phone plug type jack.

There's no speculation to it. Some of the jacks get quite complex with mult iple isolated switches. All you need to know is that whatever switch positi on is shown with plug removed, the jack only schematic, all of that gets fl ipped when the plug is inserted: what was open gets closed and vice versa, the double throws get flipped, the NCs go open and the NOs go closed, etc.. .

Reply to
bloggs.fredbloggs.fred

If the drawing is clear in that regard. Obviously the drawing is not clear because I showed you a reference and you still choose to believe the pole of the isolated switch is connected to the tip pin of the jack. The only way to determine who is right is to measure the conducting pins on the jack.

By your opinion when the plug is not inserted the input from the push button switch would short the output of the CMOS gate to ground. This is not a good idea and might damage the chip. The way I think the jack is designed the circuit will still work correctly, but does not short the output of the gate to ground at any time. I think the designer drew the jack intending for the pole to be isolated, but didn't seem to make that clear enough.

--

Rick C
Reply to
rickman

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schematic is pretty clear on showing the FF operation as what you are obser ving when the plug is removed from the socket.

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, the

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eve, a movable plug tip contact (the notched piece), and an internal SPDT s witch between terminals 8, 7- and why they don't number the friggin pole I don't know! The bent piece on terminal 7 is meant to show a NC connection w ith the pole with the plug out of the jack, and it clearly shows the intern al pole is connected to tip (6) with the plug out of the jack. With the plu g out of the jack, the operation of the START and STOP switches runs the sh ow, turning the clock oscillator on and off and some other stuff. With the plug in the jack the FOOT SW runs the show, the TIP to internal SPDT pole c onnection is broken as is 7 to internal switch pole. This prevents the STAR T and STOP switches from latching the FF and it also gives the FOOT SW comp lete control over gating the clock oscillator, turning it on when FOOT clos es and turning it off when it opens.

nected

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to interpret the schematic and I told you what it is. How difficult is it t o understand that 1) with plug removed, circuit operation is push on/ push off thru START/STOP switches and 2) when plug is inserted into jack, circui t is ON (MOMENTARY)/OFF through the FOOT SW ( a SPST to shield GND).

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and show the pole contact making NC contact with the bottom throw by bendi ng the pole contact and not the throw contact. The reason they bent the thr ow contact is because they want that pole connection horizontal and making contact with the TIP, the usual schematic for a phone plug type jack.

multiple isolated switches. All you need to know is that whatever switch po sition is shown with plug removed, the jack only schematic, all of that get s flipped when the plug is inserted: what was open gets closed and vice ver sa, the double throws get flipped, the NCs go open and the NOs go closed, e tc....

That's not what happens. When START is pressed it shorts IC3 pin 4 only for 2 x Tpd until it latches logic low thru the FF preventing C45 from re-char ging. The IC is a 4011UB, output is safely current limited.

Reply to
bloggs.fredbloggs.fred

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