Pace PRC2000 paste dispenser motor repair

[Daveedius]

I recently picked up a Pace PRC2000 soldering station for $40(!!!) on facebook with a non working paste dispenser. Basically, the motor is barely receiving any voltage because the base of the PNP transistor (Q20) which controls it, is not being grounded properly. If I ground the base manually, the motor turns on no bother. Here's the relevant schematic:


All the highlighted passives have been verified as good out of the circuit with my DMM. I've only found one thing that was suspicious so far: when there is an input of 15V to pin 8 of U7C, the op amp, U3C,  is only outputting 5.85V on pin 8. This is well below high for the Nor gate but replacing the Op Amp had no effect   

Here's a table of voltages I measured for when the input is both low & high:




Does anyone have any advice on where to go from here? I've also emailed pace but am awaiting their reply. Thank you everyone in advance!

[gcewing]

It's hard to tell exactly what that circuit is supposed to do. I think it's meant to oscillate and pulse the motor, but I'm not sure.

Do you have a scope? Can you tell whether the op amp output is a steady 5.58V or is it pulsing at a high frequency?

Whatever else happens, it looks like the motor should be turned on for a brief time (on the order of about 0.1 seconds) when the input initially goes low, via C38 and pin 13 of U8D. Can you tell whether that's happening?

Beyond that it's not clear what's going on, because an indeterminate output from U8D produces an indeterminate input to U7C, causing it to produce an indeterminate output, etc. So it's hard to tell what's causing what.

Can you temporarily break the feedback loop from U8D to U7C and connect pin 9 of U7C high? Then a high input should make pin 10 of U7C go low, then the output of the op amp should go high, the output of U8D should go low, and the motor should turn on.

[Daveedius]

Yeah, I'm not entirely sure what it's supposed to do either. Unfortunately no scope (any recommendations??), only a DMM & power supply.

But great idea on breaking the feedback loop! Thank you. I can't break the feedback loop since it's just a trace but I was able to achieve the same thing by desoldering R24 from pin 10 of the NAND gate. As you suggested, a low output of the NAND gate does make the OP amp go high, the NOR gate goes low, and the motor turns on.

So, looks to me like the NAND gate is faulty. You agree?

 

[Ian.M]

Maybe.  Is the gate getting power? I'd check across it between pins 7 (Gnd) and 14 (Vcc)  before condemning it.

[asis]

Hi,

After you switch the Rotary Switch to the PD position, via contacts 10-C7 (J32-24_J32-32) +14V are supplied to start programmable timer U4 CD4541, which at OUT pin-8 gives a command to start the pump with a forced speed increase during the first a few seconds.
Further, due to the feedback and C4 = 0.68M, the rotation speed of the motor decreases and air is njected into the internal receiver in which there is a solenoid valve to release air directly into the dispenser.

After 14-15 seconds the timer U4 CD4541 turns off the motor and periodically turns on the motor
after ~ 7 minutes to maintain the pressure in the receiver if the paste application cycle does not occur.

To operate the Dispenser (PD), at a minimum, you need a foot pedal that plugs into the jack on the back of the PRC (FOOT PEDAL).

The PD has two TIMED / CONTINUOS modes (front panel switch).

In PASTE DESPENSER SWITCH mode (pins 5-4) in TIMED position
(J32-29 -> 32-4-> 31-1-> foot switch -> J31-11 GND) the timer U5 (ICM7555) is started, which, through OUT pin-3, gives a command to the timer U4 -pin 6 (MASTER RESET) to start the SV receiver valve
(Q21 n-p-n Darlington) and the release of a portion of air into the dispenser depending on the position of the PD SECOUNDS knob.

In the PASTE DESPENSER SWITCH mode in the CONTINUOS position (J32-17 -> Q9), everything depends on the time you press the FOOT SW pedal.

All operating logic is visible on the two-color PD LED.

[Daveedius]

Maybe.  Is the gate getting power? I'd check across it between pins 7 (Gnd) and 14 (Vcc)  before condemning it.

Yep, it's gettin power! Any experience with logic gates going faulty? I'm guessing it's just this channel that's gone bad if that's possible; the circuitry that uses the other three channels of this gate are working perfectly fine.

Hi,

After you switch the Rotary Switch to the PD position, via contacts 10-C7 (J32-24_J32-32) +14V are supplied to start programmable timer U4 CD4541, which at OUT pin-8 gives a command to start the pump with a forced speed increase during the first a few seconds.
Further, due to the feedback and C4 = 0.68M, the rotation speed of the motor decreases and air is njected into the internal receiver in which there is a solenoid valve to release air directly into the dispenser.

After 14-15 seconds the timer U4 CD4541 turns off the motor and periodically turns on the motor
after ~ 7 minutes to maintain the pressure in the receiver if the paste application cycle does not occur.

To operate the Dispenser (PD), at a minimum, you need a foot pedal that plugs into the jack on the back of the PRC (FOOT PEDAL).

The PD has two TIMED / CONTINUOS modes (front panel switch).

In PASTE DESPENSER SWITCH mode (pins 5-4) in TIMED position
(J32-29 -> 32-4-> 31-1-> foot switch -> J31-11 GND) the timer U5 (ICM7555) is started, which, through OUT pin-3, gives a command to the timer U4 -pin 6 (MASTER RESET) to start the SV receiver valve
(Q21 n-p-n Darlington) and the release of a portion of air into the dispenser depending on the position of the PD SECOUNDS knob.

In the PASTE DESPENSER SWITCH mode in the CONTINUOS position (J32-17 -> Q9), everything depends on the time you press the FOOT SW pedal.

All operating logic is visible on the two-color PD LED.

Thanks, some great info. Do you know what specifically the U7/U3/U8 was designed for? I'm sure it must have some purpose or they would have just activated the transistor directly like on Q21.

[asis]

Hi,

PACE offered good SNAP-VAC technology.

VAC pump performance (Q17) works great with SX-70, SX-80, SX-90, SX100, TP65 and others.

For dispensers, other characteristics of the compressor are required, taking into account the dependence of the dose volume, the viscosity of the dosing material and degree of filling of the cartridge.

Therefore, you see a diagram with all the components, allowing you to change the parameters for dose adjustments and ensure repeatability of the process.

When operating in TIMED mode, Q21 generates a short pneumatic pulse from the receiver, and the PD 1MOhm potentiometer (J32-14, J32-34) adjusts the time pneumatic pulse.
Q20 (p-n-p Darlington) has a short saturation time and high power dissipation and due to the U7C, U3C, U8D circuitry in a short time (SNAP) fills the receiver with air and continues to operate for 14 seconds, but with less performance if the pedal is released.

Let this technology have drawbacks, but with certain skills, it fully justifies itself.

[Daveedius]

The op amp, NAND gate, NOR gate and PNP transistor have been changed but the problem still persists. Does anyone have any advice on where to go from here?

[perieanuo]

i suspected opamp, after changing the parts, you have the same DC values?
try to pulse manually U7C pin 8 (INPUT) from gnd to high (of course by cutting the input track before junction with C38 node), the motor works at least for the C4 imposed delay?

[asis]

Hi.
Then, it is better to cut the track from U4 pin 8 + R29, R66 to 8 U7C and supply a logic level 1/0, observing the process on the Green / Red LED.
Logic "1" can be supplied from the DMM in ohmmeter mode by changing the polarity of the probes or, directly, from + 5V (!).
Also, by operating the foot pedal, observe the process on the Green / Red LED by switching TIMED / CONTINUOS.
I recommend that you test the operation of the U4 timer when it is disconnected from the 8 U7C.
In a sense, you need about 7 minutes of patience to see the result.
Earlier, I described how U4.U5 works.

[Daveedius]

Did as you suggested (completely disconnected U7-8 from U4 and injected 15V with my power supply) but am having the exact same result and the exact same voltages 

On a side note, it was interesting to see the high impedance of the NAND gate - the pump wouldn’t stop until I then tapped U7-8 with my voltage probe!

[perieanuo]

if you changed u8d, than i repeat, the opamp is to be changed, have you changed it?
imho some little resistor like 47 or 100R may be added between opamp out and gate input
they cut cost on anything, for the new engineering era spikes doesn't exist 

[asis]

Hi,
In your position, a logical "1" is already present ("hanging state", not pulled up to + 5V by any resistor and no logic structure # = "1") and touching the probe you simply create a potential close to the threshold state "0", including the condition discharge capacitor C38.
And, since the inputs of the microcircuits (8-U7C + 1,2-U7A + 5-U7B + C38-0,1nF) are combined, this makes the logic level "1" even more unstable.
You never wrote about the GREEN / RED LED response when feeding "1 \ 0" > "0 \ 1" and 8-U4 by manipulating a foot pedal or some kind of simulator.

[Daveedius]

if you changed u8d, than i repeat, the opamp is to be changed, have you changed it?
imho some little resistor like 47 or 100R may be added between opamp out and gate input
they cut cost on anything, for the new engineering era spikes doesn't exist 

Yes, op amp has already been changed

[Daveedius]

Hi,
In your position, a logical "1" is already present ("hanging state", not pulled up to + 5V by any resistor and no logic structure # = "1") and touching the probe you simply create a potential close to the threshold state "0", including the condition discharge capacitor C38.
And, since the inputs of the microcircuits (8-U7C + 1,2-U7A + 5-U7B + C38-0,1nF) are combined, this makes the logic level "1" even more unstable.
You never wrote about the GREEN / RED LED response when feeding "1 \ 0" > "0 \ 1" and 8-U4 by manipulating a foot pedal or some kind of simulator.

LED seems right. When manually injecting 15V, the led is yellow. And when the foot pedal is pressed, the led is green

[Daveedius]

Update for anyone that may be reading this from the future:

I wasn't able to figure out what was causing the problem, but cutting the trace on pin 9 of U7 and pulling it high has "solved" the issue.   

[perieanuo]

 

[Shock]

Carefully peel the now unused trace off the pcb and put it in your box of spare parts.

I had a sneaking suspicion C53 or the bias to the darlington? Q20 was off, I wonder if the actual fault is between there and -14V.