MP-1 3TM not powering on

[jaykay73]

Hi all, been enjoying a 3TM for a long while. Hadn't powered it up for about a year and when I did all the parameters had been reset to zero. Played it a while then it became non-responsive. Powered it off and haven't been able to get it to start up again.

Have been through some of the troubleshooting guides on this site. Discovered C131 had 17 volts across it, which is also providing 17 volts to the tube heaters. Too much!!!

Wondering if there is a simple problem in the power supply I could fix myself.

Thanks,

JK

[MarshallJMP]

17 volt ??? are you sure?

There is a 6 pin red connector going to the mainboard comming from the transformer. You will see 2 green and one blue wire on it. Take a meter, put it on AC volt and measure between the blue and green wire. (green and blue and then the other green and blue) and let me know.

[jaykay73]

Thanks,

Between pins 5&6 I get 28.4VAC

Between pins 4&6 and between 4&5 I get 14VAC

I have a machinator transformer with 117V written on it. The voltage from my wall socket is 125.6V

JK

[MarshallJMP]

Ok your line voltage is quite high which result in high secondary voltages. Did you measure the 17V with the tubes in the MP-1?

Next thing to check is the 5V rail. You can measure this on the eprom (the chip with the white sticker). Measure between pin 14 and 28.
Here you can find the pinlayout  https://www.futurlec.com/Memory/27C128-200.shtml

[jaykay73]

I measured 17VDC across C131 with the three tubes OUT.

My 5V rail check gave me 4.95VDC. Both +15 and -15 rails both also are giving correct voltage.

[MarshallJMP]

Ah ok that (No load on the MDRT) with the high line voltage the 17V could be normal.

Now we need to figure out why the MP-1 does not start. First thing you can try is reseating the grey flat cable going from the main to the tube board. (on both sides). Small chance it will do something but try it anyway.

Next thing the reset pin of the Z80, you should have 5V on pin 17 and 26.

Also measure on pin 6 (clock) and pin 16 (Interupt) and let me know what you get on these pins.

Do you happen to have a oscilloscope or a logic probe?

[jaykay73]

Ok, I got 4.90VDC on pin 17 and 4.92VDC on pin 26.

I have 1.38VDC on pin 6 and 0.06VDC on pin 16.

Unfortunately I do not have an oscilloscope or a logic probe.

I reseated the grey cable at the main board.  I can't see the other end of it because it disappears under the board that houses the midi jacks, pots & other jacks, etc.

Thanks for your ongoing help.

JK

[MarshallJMP]

If you remove the bottom lid you can acces the other connector on the midi board.

I have 1.38VDC on pin 6 and 0.06VDC on pin 16 ==> that's good so it's not in reset state.

It seems it has a clock signal but that doesn't mean it's a correct clock signal. Pin 16 should pulse but it's hard to determine from just a voltage.

Can you get access to a scope or logic probe?

[jaykay73]

Stupid me! Totally forgot the bottom lid was removable! I'll check that connector soon.

I can buy a logic probe for 20 bucks. Do you suggest I do this? I've not used one before but it seems easy to use - I just don't know how to interpret the results. Would you be happy to walk me through how to use it on the MP-1 board?

JK

[jaykay73]

No difference unplagging & replugging that grey connector.

I have noticed when I turn the unit on all 4 clip lights shine for a split second and then when I turn it off they light for about half a second.

JK

[MarshallJMP]

The clip lights, that's normal.

If you don't mind investing in a logic probe I will try to help you.

[jaykay73]

You wouldn't believe this... I just fixed my MP-1!!! I was going to replace all the electrolytic capacitors in the power supply to see if that would work. I took off the bottom cover and peaked under the rubber flap which protects the underneath of the power supply from prying hands. I noticed the four voltage regulators had corrosion between their pins on the undersurface of the board. From watching too many YouTube clips I know corrosion on boards isn't a good thing so I scraped off all the corrosion and cleaned up the gaps between the pins with isopropyl alcohol. Shazzam - it came back to life! I should have taken a "before" photo but here is an "after" showing which pins were caked in corrosion.

I wanted to post this fix as too often there are threads which end blindly without resolution.  I got this fix for $0.00 & very happy.

JK

[rnolan]

 :whoohoo!: glad you got it fixed and with a simple clean up.  Question then is, why the corrosion in the first place 

[jaykay73]

Good question & I don't have an answer.

I posted my repair on Faceb**k & another user posted they had exactly the same problem & used the same solution to fix his unit. Uncanny!

JK

[MarshallJMP]

Was it really corrosion or was it left over solderflux (brown stuff)?