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AmintaAtari

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

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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

 

post-53462-0-61380600-1556151749_thumb.jpg

 

post-53462-0-15995900-1556151842_thumb.jpg

 

post-53462-0-51077300-1556151880_thumb.jpg

 

post-53462-0-65502000-1556151923_thumb.jpg

 

post-53462-0-52709100-1556152041_thumb.jpg

 

post-53462-0-90051700-1556152091_thumb.jpg

 

 

 

 

 

post-53462-0-64762600-1556152004_thumb.jpg

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

 

post-64523-0-22884500-1556156778_thumb.jpg

post-64523-0-92670000-1556156815_thumb.jpg

post-64523-0-86439600-1556157793_thumb.jpg

 

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

because this transformer's secondaries are isolated...

Edited by HOME AUTOMATION

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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.

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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.

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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!

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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

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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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