Advice of repair or rework... Awesome retro Tennis Ball Serving Machine

[MadNat]

Hi folks

My local tennis club have an incredible retro tennis ball serving machine, which is almost working... we've managed to repair the obvious (HV side had blown parts now replaced), and I'm trying to gauge if its worth trying to repair the original board, or create a new control board for the 2 drive motors, and left/right - up/down stepper motors.

The only problem is that one of the main drive motors is not receiving power (the motor does work).  I think the cause is due to a failed chip on the control board.  What is putting me off a repair?  Well 2 main reasons:

• I've scoured the internet for a schematic, but cannot find any.  The Machine is a Match Mate Coach - manufactured late 1980's early 1990's.  An example pic of one is here: https://www.govdeals.com/photos/7484/7484_8298_6-1.jpg . Image of top and bottom sides of board here https://1drv.ms/u/s!AnCqbGR7fI2ejg5V6YIPQTu28ZFx
• I'm not sure the parts I may need to perform the repair are still available

I might be being far too naive, but thinking a slicker more modern control could be possible using a Raspberry Pi or ESP8266/ESP32 driving the voltage control of the main motors, and controlling left/right control.

What do more experienced/experts thinks as the best approach?

Thanks in advance.

[sokoloff]

Would the device be designed that way if being built new today? Of course not. 

Now, whether it's going to be easier for you to build a new replacement driver system or repair the existing is a more difficult question. For me, as a lifelong software guy with mostly digital electronics experience, I'd be utterly lost trying to do component level repair of the existing board from scratch. So, I'd go modern rip-and-replacement.

[ebastler]

MadNat, you are not giving us a lot of information to go by. A couple of comments anyway:

• Have you taken the unit apart? Have you done a careful visual inspection, looking for loose connections or signs of overheated/burnt-out components or traces? In my experience, a lot of problems are electro-mechanical in nature (switches, connectors etc.), and a lot can be found by visual inspection. You do not always need a circuit diagram to fix such problems.
  
  
• Could you post photos of the PCB(s) and wiring?
  
  
• The "intelligent control" of the machine will be the least of your worries, and probably is not broken anyway. A Raspberry Pi, ESP or whatever should not be your primary concern, before you have figured out what you need to drive the motors, and what sensors the machine might have.
  
  
• You have not mentioned your level of expertise, which will of course strongly influence our recommendations whether it makes sense for you to embark on a repair or roll-your-own project. From you post, I assume that you do not have a lot of experience with electronics projects, in which case I would advise against trying to design your own control board.
  
  
• Another consideration: This machine is mains-powered, and will be used by others in your tennis club. That implies a significant safety responsibility -- you don't want to electrocute a fellow club member, and you don't want the machine to catch fire if someone leaves it unattended and it malfunctions. Meeting that responsibility with homebrew control and power electronics is a challenge; unless you are highly experienced I would recommend that you stay well away from it.

[MadNat]

Hi ebastier - Thanks for your comment and feedback.  For the points you raise:
1. Yes - I have taken it apart, and put it back together several times.   As I mentioned some obvious repairs have been completed on the board, and the control panel to get the machine 90% working again - as I mention there is only one motor not receiving power.

2 I have included a link to the board photos in the original post.  I can label what each connector does.

3. You are correct - but the point of my question is really about time/effort and probability of success.  With such an old machine, is it best to try and repair or start with a different approach?

4.  I have some experience - I am no expert.  I have some good test equipment (scope and a proud owner of a 121GW!), but I am by no means an electronics expert as you have identified.

5.  Very good points!!  To be honest I'm not sure how safe the machine was when new!  When the scissor lift is fully extended, it gets quite a wobble on with the motors at full speed!  But you are absolutely right, I would not want to hurt anyone or inflict damage to property.

[ebastler]

Thanks MadNat -- and apologies for having overlooked the PCB images in your original post. I looked at the picture of the complete machine, but managed to overlook the second link.

What's up with the broken ferrite core (?) in the lower right of the PCB top view, with the smoke trace to its right? The corresponding connector on the bottom side also seems to have yellowed, probably from overheating. Did you fix anything in that area? What is connected there?

[sokoloff]

That's not a ferrite core; it's a bridge rectifier. (and looks like it might have let the smoke out)

[ebastler]

@sokoloff -- you are right, of course. Just downloaded the full-size image and saw the markings in the four corners of the rectifiers. It does not get much clearer than that...  With the broken rectifier still on board, that area looks like trouble.

@MadNat -- another thing that caught my eye is the row of ICs in the lower left of the PCP top view. From the top, it looks like they were all resoldered, and there is also some strange fuzz or oxide around various pins. But the bottom view of the PCB looks clean in that area, without a trace of resoldering. What happened there?

[MadNat]

Hi ebastler - yes the bridge rectifier on the HV end of the board popped, and the complete unit was then dead.  We have replaced that component.   After replacing the bridge rectifier the machine worked 100%. 

However, I believe a short in the long remote control cable caused one of the main drive motors to no longer receive power.

There are 2 main drive motors (upper and lower) the upper motor stopped working.  The motors are identical, I can confirm that the upper motor works when connected to the lower motor power connector, so quite sure its a power deliver to the lower motor problem, not a motor failure.

Understanding how the board delivers power to the motors is beyond my level of electronics knowledge - and with no schematic, I don't really know where to start.  So you can see when I ask the question of rework v repair.  If someone can understand how the power control electronics work on this board - that could help me go via the repair option.

Thank you again for your help and interest in my post.

[MadNat]

The fuzz is tennis ball residue!  I took the pics after the first time I removed the board.  It's had a clean since them

We haven't had the IC's removed or resoldered that I'm aware of.

Any idea what function these chips would serve?

The speed of each motor is controlled via a simple Potentiometer in the control box, so I'm guessing there is a function of changing voltage to the main motor based on a proportional change of low voltage signal returning from the control box.  Can't quite work out how or where that happens.

[fzabkar]

The ICs are 74 series TTL and 4000 series CMOS. Cheap and common as chips. :-)

I would start by measuring the voltages at the optocouplers. This will tell you whether the corresponding motor is getting a run signal from the logic side of the PCB.

[MadNat]

Thanks fzabkar - I will check out the opto isolators.

My gut feeling is that the fault is before the opto-isolator. 

The control connector (from the potentiometer based control box) connects to the 26 pin header at the bottom left, on the top side of the board.  This connector seems to connect to pins on the CMOS's.  What purpose would this serve (showing some of my basic ignorance here). 

Is it possible for a short on that connector to cause a failure on that CMOS chip - and therefore render the opto-isolator useless?

[SilverSolder]

In the middle of the board, there is a row of what looks like 6 power transistors, with the two bottom ones standing up, different from the rest. 

These seem like possible candidates for supplying power to the motors.  Maybe you can trace the PCB tracks back from the connector and confirm which transistor(s) is sending the power to the motor?  The power devices driving the motor have to be on the list of suspects we want to talk to...

The power supply around the bridge rectifier is also suspect.  The bridge rectifier that was replaced might have blown up because something downstream of it shorted out...  it ended up acting as a fuse...   so it would be a good idea to check the voltages there (compare with the other supplies nearby).

[MadNat]

Thanks SilverSilder!

The bridge rectifier that was replaced might have blown up because something downstream of it shorted out...

I should have mentioned earlier, that the bridge rectifier blew because it was powered up without the provided 220-120v transformer! I’ve now made sure that device can’t be removed.

As I mentioned the machine was then 100% functional after the bridge rectifier was replaced - at least for a short while. 

There was a short across some of the 26 header pins which are connected to the remote control panel, via a long cable (short occurred in the cable).  The only fault now is one motor not turning.  There are 4 other motors, and a remote control box all working fine. 

So believe the 8V transformer is working fine.

I therefore am quite sure the problem is related to the single control line to the failed motor. 

Hope that makes sense.

I will check the voltage at the triacs also.

Any pointers as to how the control logic might be working would be very interesting.



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[fzabkar]

I can't speak for others, but it would help me if you could explain what each of the motors do, what type and rating they are, and what else is in that control box besides a potentiometer.

For example, I notice that the logic PCB has a debounce IC (MC14490), so this would suggest that there are switches or pushbuttons of some kind. There is also a 556 dual timer IC and a 74HC193 counter. This would suggest that the device outputs a pulse with a selectable duration (presumably the counter retriggers the timer).

[SilverSolder]

At this point, I would not yet be at the stage of feeling the need to try to understand the control circuit.  The broken component is far more likely to be something simpler (power supply, driver electronics).

Power supplies are notorious problem areas and here we have one that we know has suffered a trauma - I would definitely compare it with its neighbours and see if it is a deviant, or a good citizen.

The other thing to look at is to identify the exact power transistor or triac that is driving the "bad" line - the source of the power to the motor.  By tracing back from the wire that goes to the motor, you will likely find that critical part quite easily.

Power devices are more prone to failure than control logic...  and even if the power device turns out to be OK, this is still the starting point for tracing things backwards towards the control circuit, if necessary.

[MadNat]

Fzabkar - Sure I can explain those features - sorry for focusing on the motor - I guess I’m being rather tunnel visioned on the thing that isn’t working, but appreciate it’s helpful to understand the broader functions as they maybe connected, and will help understand what is there and why.

Here is a pic of the controls:


There are five motors:
2 delivery wheel motors that shoots ball. They spin at different speeds to send the ball with topspin or with under spin.
1 Oscillation motor aims the two delivery wheels left and right It has potentiometer that connect to the oscillation motor to determine it's position
1 Elevation motor that tilts the delivery wheels up and down to control the trajectory-- has potentiometer to read it's position. 
1 carousal motor controls a turning disk with 4 holes (carousal)  -the faster it spins the faster balls are released.

There are 6 positions that can be set on the right: The location of the sliders determines where the ball will shoot.

Program delivery will cycle through the positions (1-7) set on the right.
Random delivery will chose a random position to send to. You can turn on or off the position sliders to use any number of the 7 possible positions.

Ball Feed knob/potentiometer: Turn to the right and the faster the carousal spins.
Speed Control knob/potentiometer: Turn to the right and the faster the delivery wheels spin.
Spin Control knob/potentiometer: Turn to the right and top delivery wheels spins faster than the  bottom delivery wheel creating topspin. Turn to the middle and both motors run at the same basic speed sending a "flatish" ball. Turn to the left and the bottom motor spins faster than the top motor creating under or back spin.

All of these features work as intended.  The exception is that the top motor (controller primarily from the speed dial), as it accepts an additional speed adjustment from the spin dial.

The denounce IC I believe helps the push button that starts/stops The carousel motor (start or stop serving).

Hope this detail helps.




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[MadNat]

It turns out that the answer to the original posted question (repair or rebuild) - is repair.

I’ve managed to identify the part of the board that receives speed control (the 9 pin connector in the lower middle of the top board).  Tracing from there it almost immediately connects to the trim pots directly underneath (row of 6 pots).  It is clear that the pot directly below the 9 pin connector and above the 2 pin pot for the 120v DC motor had failed.

Finding a replacement looks tricky - I think it is now a PT15 range part from Piher.  It’s the 100k ohm version (linear).

I’ll try and source some (any help or advice on uk stockists for obscure Piher parts welcome!).

I’ve proved with a hack that replacing with the right part the machine runs again.

Thanks for all help so far.  Really pleased to see life in the old dog again!


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[SilverSolder]

Nice!  No technical problem is ever so big that it won't yield to sheer bloody-mindedness.

[eKretz]

Good job finding the fault. If you can't find the exact part don't worry about it. Someone will make something that will fit.

[fzabkar]

ISTR seeing the vertical versions of those trimmers in US gear of that period. The only place I could find replacements with the same pitch was at Tandy (RadioShack).