Probems with gliching QAUD latch 4043 that stops to latch after a while.

Hello,

I have a Reel-2-Reel tepe recorder using the 4043 (U4) to latch the 3 control momentory buttons Play, Fast Forward and Fast Rewind The problem is only when selecting fast forward spooling only 10.5" and when it is accelerating , when using 5" all ok

Problem description:
Pressing fast forward, Latch output O0 pin 2 of U4 goes high (latches) for some sec. and goes low . It is different each time, sometimes it works for about 5 sec, sometimes 30 sec.... s , it seems that something with high load spooling giving som kind of disturbance (hard to locate).

the 4043(U4) is very sensitive, if I add a decopling capacitor to the +5 supply/gnd on pin7 and 14, it stops to latch! .... if I try to mesure with a probe on the outputs (O2,O0,O3) some times it does not latch on any buttun while I am mesuring.
I checked the supply rail +5 , all ok.

R0,R2,R3 inputs are all low when the latch releases after some sec on fast spool! . R0 R2 R3 goes only high momentary when pressing Stop (normal to release the current command)

Is there a miss in the design or can I add aditional filtering? u3 is a 8 bit priority encoder that encodes the 3 inputs from the U4.

Thanks for any help!

Adding a decoupling capacitor should not make the problem worse so either the IC is faulty or one of the inputs is not at proper logic state and perilously close to the logic threshold voltage. Check all the inputs are either at 0v or +5V, being (almost) unloaded CMOS the voltages should be very close to ground and supply.

Brian.

Hi,

connecting a capacitor to pins 7 and 14? These are not GND and VCC..

Klaus

KlausST said:

Hi,

connecting a capacitor to pins 7 and 14? These are not GND and VCC..

Klaus

Click to expand...

Sorry, I meant pin 8 and 16

betwixt said:

Adding a decoupling capacitor should not make the problem worse so either the IC is faulty or one of the inputs is not at proper logic state and perilously close to the logic threshold voltage. Check all the inputs are either at 0v or +5V, being (almost) unloaded CMOS the voltages should be very close to ground and supply.

Brian.

Click to expand...

I replaced the IC, same problem , the inputs are ok, high (5v) as long as the push button is pressed. The problem is that the latch resets for some reason only when in fast spooling mode and the motors are spinning very fast under the load of large spools.
when the latch resets while motor is spining at high speed, then the system goes to STOP mode decoded by the 4032 (U3) (normal behaviour since EO goes high in U3)
I suspect some kind of interference problem some where. I will try the decopling apacitor again, I dod not know why it stoped to latch , what optimal value should I use ?
Thanks

Entire schemtics for the tape control part

betwixt said:

Adding a decoupling capacitor should not make the problem worse so either the IC is faulty or one of the inputs is not at proper logic state and perilously close to the logic threshold voltage. Check all the inputs are either at 0v or +5V, being (almost) unloaded CMOS the voltages should be very close to ground and supply.

Brian.

Click to expand...

I will check the buttons if when runnin

hafrse said:

The problem is that the latch resets for some reason only when in fast spooling mode and the motors are spinning very fast under the load of large spools.

Click to expand...

Well, that sure sounds like the problem is noise from your motors. Decoupling the power supply is useless if the noise is coming in on the inputs. How are you measuring the voltages, with a meter or an oscilloscope?

Found the reason why the 4043 resets, I detected a short pulse at pin9 U5 which goes low for a short period to reset the latch , that is derived from the spool motors FaultDetection signal at pin1 of U23. FaultDetection is from drive transistor TIP140 on each motor fead by one dioe on each side D2 and D3 (FaultDetection =P6-17) .
That makes sense , since on high load with large spools, the problem is visible only on fast forward. It can be a problem in the take up side (right motor) where the TIP140 cuts of high load? I will try to switch the transistors and see if the problem moves to the left motor since I do not have a replacement

After some close look , its seems that the latch on fast forward output is reseting because of noise from the tape up motor or driver circuit and not a fault condition that is reseting the latch through the logic circuit!


I did this:


1 - Connected fast forward Reset R0(3) input to ground
2 - Disconnected the resistor connected to output O0 in order to be 100% sure nothing is interfering there

Same problem! the latch resets. Tried to use an external +5 supply, same problem!

Connected scope , AC mode and checked the noise at +5 logic supply while activating the fast forward with large spools, I can see lot of noise.
Using smaller spools, less weight, less noise visible on scope and the latch do not reset (=OK). On Fast rewind (which works ok on all type of spools ) alsmost no nose. The motor is used is a Print Motor type , unfortunatly, the left and the right motors are not easy replaceble to see if the problem in the take up motor , i need mechanichal job and equipment in order to detach the central shaft sincs it sticks in the motor assembly system.

Is there a way to add filetring ? scematics of the motor amplifiers in previous post.

you can put RC‘s on the problem inputs.