Atari ST ASP34-4 PSU Recap - Bad ouput voltages

[Fabrice Labrousse]

Hi.

I just recap an ASP34-4 power supply for my Atari 1040 STF (also found on some STEs).
I followed the Exxos page and Exxos values for improvment

 

https://exxosforum.co.uk/atari/last/psu/#ASP34

 

C5, C4 : 47uF 350V (33uF 350v replacement)
C15, C14, C7 : 680uF 35V (330uF 35v replacement)
C13 : 4700uF 16V
C12 : 4700uF 16V (2200uF 16V replacement)

 

I also changed transistors Q2 (BU508) and Q1 (2SC1384).

 

Here is the schematic of the ASP34-4
Astec_ASP34-4_PSU_schematic.pdf

 

After reassembly I tested the output power supplies using a 2.2 Ohm (20W) load for the 5v output and a 10 Ohm (20W) load for the 12V output rail. 

 

 

Now when reading the output voltages, I obtain:

 

6.4V instead of the expected 5V
14v instead of the expected 12v

 

Not being an electronics specialist, could you tell me on which components acting to try to bring the voltage rails back to 5v and 12v?
Thank you for your help.

 

[guus.assmann]

Hello,

 

The component Q3 is the voltage reference. It's a TL431. (Datasheet on internet...)

And the resistors around that component determine the 5V reference.

Resistors can be checked with an Ohm meter.

 

Best regards,

Guus

[Fabrice Labrousse]

Thank you for the reply

I tested all resistors R10, R11, R12, R13, R14 et R15 and they are fine.
Q3 (TL431) is also fine.

Output voltages are still 6.28V and 14.23V 
 

[TGB1718]

There's obviously something wrong with the feedback circuit, have you checked the optocoupler

PC1 which is a PC817

 

https://html.alldatasheet.com/html-pdf/43368/SHARP/PC817/124/1/PC817.html

Edited September 15, 2023 by TGB1718

[Fabrice Labrousse]

Yes it's a brand new and tested CNY65B