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TI-99/4A doesn't power on, how to measure voltage


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#1 AmintaAtari OFFLINE  

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Posted Wed Apr 24, 2019 6:28 PM

Hi all from Italy!

 

 

My TI-99/4A suddenly doesn't power up!

 

So I've disassembled the machine to isolate the internal power supply.

 

Now I have, as from the photos, the power supply and the external original transformer.

 

How can I test the voltage with my multimeter?

 

Where have I to insert the red probe and the ground probe to measure the voltage for the transformer and for the power supply?

 

You'll find all the photos attached...

 

 

Many thanks in advance!

 

Davide

 

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Attached Files


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#2 HOME AUTOMATION OFFLINE  

HOME AUTOMATION

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Posted Wed Apr 24, 2019 7:47 PM

For the A.C. transformer, black/red order does not apply...

 

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Attached File  CASE.JPG   2.46MB   1 downloads

 

But, this does not appear to match your P.S. board...

because this transformer's secondaries are isolated...


Edited by HOME AUTOMATION, Wed Apr 24, 2019 8:08 PM.


#3 HOME AUTOMATION OFFLINE  

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Posted Wed Apr 24, 2019 8:26 PM

This one is internally coupled...

 

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#4 AmintaAtari OFFLINE  

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Posted Thu Apr 25, 2019 4:33 AM

Ok, here its mine:

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#5 AmintaAtari OFFLINE  

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Posted Thu Apr 25, 2019 4:34 AM

but it reads only 4

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#6 Stuart OFFLINE  

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Posted Thu Apr 25, 2019 5:21 AM

but it reads only 4

 

 

Looks like you're measuring between pins 1 and 3 - and pin 3 isn't connected to anything (there's no metal pin where you've got the black probe). You're just picking up 4V of crosstalk.



#7 Stuart OFFLINE  

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Posted Thu Apr 25, 2019 5:37 AM

If you measure the voltages at the red/black/white wires on the PSU PCB, you should get:

 

-- across white and black - about 9V AC

-- across white and red - about 19V AC

 

You can measure the DC output voltages on the white 4-pin connector. The voltage on each pin is written in yellow on the PCB. Measure between the GND pin (black probe) and each of the other pins in turn (red probe). Make sure the probes don't slip and short between two pins! Make sure you set your multimeter to DC Volts for these. The voltages may be around 0.2V higher than stated as there is no load on the PSU.



#8 AmintaAtari OFFLINE  

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Posted Thu Apr 25, 2019 6:34 AM

Ok, Ive changed the fuses and now the light on power supply works, here are the measures.Attached File  57E56C33-6D61-4DBD-88DD-C864FFF3EDE7.jpeg   530.46KB   0 downloadsAttached File  FE83739D-F8EE-4D7B-B3E9-5DBFF102093D.jpeg   565.56KB   0 downloadsAttached File  DCB863BE-CCB4-4B92-8BBD-B7711F213F0A.jpeg   607.14KB   0 downloadsAttached File  5140DC41-EE7F-438E-A4BE-E5A8D637BD4B.jpeg   619.03KB   0 downloads

#9 AmintaAtari OFFLINE  

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Posted Thu Apr 25, 2019 6:36 AM

If you measure the voltages at the red/black/white wires on the PSU PCB, you should get:
 
-- across white and black - about 9V AC
-- across white and red - about 19V AC
 
You can measure the DC output voltages on the white 4-pin connector. The voltage on each pin is written in yellow on the PCB. Measure between the GND pin (black probe) and each of the other pins in turn (red probe). Make sure the probes don't slip and short between two pins! Make sure you set your multimeter to DC Volts for these. The voltages may be around 0.2V higher than stated as there is no load on the PSU.


Ok, I get exactly these measures!

#10 AmintaAtari OFFLINE  

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Posted Thu Apr 25, 2019 6:45 AM

Here are the measures on the transformer... why that 27v?

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#11 Lee Stewart ONLINE  

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Posted Thu Apr 25, 2019 6:58 AM

Here are the measures on the transformer... why that 27v?

 

The total voltage across the secondary of the transformer is the sum of the divided voltages—especially with just one center tap.

 

...lee



#12 AmintaAtari OFFLINE  

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Posted Thu Apr 25, 2019 7:00 AM

The total voltage across the secondary of the transformer is the sum of the divided voltages.
 
...lee


Many thanks for the explanation and for your help!!

#13 Lee Stewart ONLINE  

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Posted Thu Apr 25, 2019 7:26 AM

 

The total voltage across the secondary of the transformer is the sum of the divided voltages—especially with just one center tap.

 

...lee

 

I should say that your measurements imply a center-tapped secondary:

18.7v + 8.5v = 27.2v 

...lee






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