SM 74 keeps freezing. Please help

Hello Seniors,
I am dealing with a difficult fault and I need your ideas please.
I have been working on a 1998 SM 74-2-P Cptronic. The machine was inoperative for a while because it had several faults. After doing all the repairs and replaced some parts, the machine keeps freezing intermittently.

Sometimes it will freeze with all the icons on the screen still present and then reboot by itself. Other times you would have to switch it off and on before is restarts again.
At times too I would have to unplug and plug the cables on the NTMs before the machine can boot.

This is what I have done so far without any success.
1. I have initialized the machine a few times.
2. I have cleaned almost all plugs and sockets with electronic contact cleaner to eliminate bad electrical contacts.
3. I have swapped the ZSK and NTMs with a similar model with no positive results.

I am out of ideas now. I would be glad to try your suggestions. Please help!

 

Can you tell us the machine age? Is it CPTronic Compact or Modular (low pile or high pile)?

 

It’s a 1998 Compact model with low pile.

 

Good.
Can you measure+5VDC supply voltage on ZSK?

 

It’s currently between 4.8 and 4.9volts.

 

It's quite low. And this is the main reason. You need to repair NTM modules. They should supply higher than 5VDC, around 5.2 to 5.4 on ZSK plug.

 

Hi sir;
I can strongly recommend you first test ALL Regulated POWER Supply NTKS, very well, the TOLERANCE will be 0.7% up or down, otherwise, you must fix it, or replace it the board

2) if everything is o.k , you must clean EDGE CONNECTOR FROM ALL electronicPCB , with Eraser pencil, very well in PCB board, and then cleaned with Electronic Spray contact cleaner. I must indicated, that all this must be done with patience, by other side, this the cause of 75% of failure in ELECTRONIC TROUBLESHOOTING, of course look, very well too if there are ; DRY +COLDER JOINT SOLDER !"!

3) Test very well the Ribbon cable, or flat cable, and clean with Eraser pencil, and then clean with Electronic contacts cleaner. !!!

Let me know

 

I really appreciate your inputs. I will work on them as soon as possible and update you on the results.

 

OK

 

The compacts are notorious for locking up due to low 5VDC on the ZSK, usually wiggling the power connectors on the ZSK and the NTM brings the machine back. May be get an electrician to make a plan to change those connectors? 4.8 to 4.9 is low, you are looking for atleast 5.0 v

 

Hi gentlemen. I really appreciate your suggestions that pointed me in the right direction.
I have resolved the fault now.
I took the 2 NTMs out and sought to repair them. I saw that the voltage filtering electrolytic capacitors on the boards have become weak after years of use. So replaced all. I also tested the major components on the boards which were fine.
But this did not resolve the problem.

After more tracing, I saw that one of the 5v dc power cables connected on plug X1 on NTM1 has become a little loose. Though is was in place, the joint between the male pin and the cable has loosened a little. After tightening it up, I now have a constant 5.167 volts on the ZSK. Machine has been running for 2 days now. Thanks so much.

 

However, my troubleshooting to resolve this fault has led to another problem for me.
I swapped the ZSK of the SM74 with an SM 52 compact belonging to my customer.
After reversing and initializing the ZSK in the SM52, the machine boots alright but the MID does not boot as normal. There are no icons on the MID. Only some flashing lights on the buttons around the MID.

How do I initialize the display of this machine? I will appreciate your help.

 

Ok Great Job!!

Please clean and tight very well, Ribbon or flat cable from MID, use eraser pencil or contacts cleaner Electronic.

Look x bad contacts, corroded pins or contacts or loose wires or connectors